Fermentation-enabled wellness foods: A fresh perspective

Hun Xing, Dongxio Sun-Wterhouse,b,?, Geoffrey I.N. Wterhouse,b, Chun Cui,Zheng Run

Keywords:

ABSTRACT

1. Introduction

Fermentation has a long history in human food production and consumption. Fermented foods have been an integral component of the human diet since 8000 BC and account for nearly a third of the world’s food consumption (up to 40% for some populations). The term “fermentation” comes from the Latin word “fermentum”, and is defined as a natural decomposition process which involves chemical transformation of complex organic substances into simpler compounds by the action of intrinsic organic catalysts generated by microorganisms of plant or animal origin (“microbial factories”,either naturally occurring or added) [1]. Fermentation is a traditional method for food preservation (alongside drying and salting),and for food quality modification and culinary enjoyment (owing to the distinct flavors, aromas and textures of fermented foods).The microorganisms used in the production of fermented foods and beverages include bacteria (e.g. lactic acid bacteria (LAB) such as Lactobacillus, Streptococcus, Enterococcus, Lactococcus and Bifidobacterium) and molds (e.g. Aspergillus oryzae, Aspergillus sojae,Penicillium roqueforti and Penicillium chrysogenum), and yeasts (e.g.Saccharomyces cerevisiae, Andida krusei and Candida humilis).

Considerable effort has been devoted to developing better fermentation processes, fermentation equipment, fermented food products and an associated scientific basis since the 19th century.The diversity of microbiota and raw materials, as well as the different types of production processes, lead to a wide range of fermented foods and beverages being available in today’s global food markets(i.e. more than 3500 fermented dairy-, cereal-/pulse-, vegetable-, tea-, fish- and meat-based products, with some representative examples given in Table 1). Many of these fermented foods are produced from local food sources and cultural preference, thus possessing distinct organoleptic properties such as doenjang, douchi,kimchi, kombucha, lambanog, leppetso, miang, narezushi and tempeh from East and Southeast Asia; Surstr?mming, mead, rakfisk,sauerkraut, salami, kefir, filmj?lk, prosciutto, quark, smetana and crème fra?che from Europe; Boza, kushuk, mekhalel, torshi and lam-oun makbouss from Middle East; Garri hibiscus seed, hot pepper sauce, iru ogiri, laxoox, injera and mauoloh from Africa; Kaanga pirau, poi and sago bean-based fermented foods from Oceania.

Table 1 Examples of global fermented food products.

Improving the safety of fermented foods is an ongoing global effort, with the health-promoting benefits of these foods recently attracting growing scientific interest because of consumer awareness of diet-disease relationships. Raw material(s)can be transformed through fermentation into new products with increased nutritional value (due to the generation or enrichment of certain bioavailable nutrients during fermentation), enhanced gut health properties (due to the involvement of probiotics in fermentation), as well as specific biological functionalities (due to the high diversity and amount of bioactive substances created during fermentation) [2,3]. Most recently, fermentation has been considered a sustainable approach for maximizing the utilization of the bioresources to address the recent global food crisis [4]. Fermented foods containing different numbers and species of live microorganisms are listed as one of the top 10 superfoods in 2017.Fermented foods of traditional or innovative nature are produced to possess nutritional and quality advantages, offering not only better nutrition to the general population, but also health functionalities to specific groups of consumers (including vegans, or those on lactose-intolerant or cholesterol restricted diets) [5]. The microorganisms used to initiate fermentation, along with the probiotic microbes supplemented in fermented foods, can function as “microbial factories” for the production of desired nutrients and bioactives while consuming undesired substances. For example, bacteria containing β-galactosidase in fermented milk enable the production of lactose-free/lactose-reduced products, since this strain breaks down lactose during fermentation. Different types of hydrolysis reactions may be induced by the inherent enzymes of microorganisms, which can release nutrients and bioactives with desirable molecular sizes and bioavailability (e.g. peptides and amino acids) from the raw materials [6]. Compared to the nonfermented counterparts, fermented foods can confer multi-level benefits to humans, such as improvements in digestibility, increase of glucose tolerance [7], inhibition of pathogenic bacteria growth and bacterial toxin formation, reduction of gastrointestinal disorders, degradation of plant toxins (e.g. cyanogenic glycosides), as well as decrease the risks of various illnesses and diseases including cardiovascular disease [8], arthritic disease [9], type 2 diabetes [10],periodontitis [11], respiratory problems [12], bladder disorders[13], bone problems [14], liver problems [15], and skin problems[16].

There exist a variety of books and review papers covering specific fermentation-related topics in great detail. This review will focus on fermentation-enabled foods with nutritional advantages,rather than traditional fermented foods and their physicochemical characteristics. Only a modicum of information on fermentationinduced sensory and quality attributes is provided, sufficient to allow the demonstration of multiple beneficial properties of a fermented product.

2. Classification of fermented foods

2.1. Fermented dairy products

Fermented dairy products represent one category of highend fermented foods. They gain high popularity owing to their high contents of lactic acid, galactose, free amino acids, fatty acids and vitamins (especially B complex), and their favorable properties such as anti-inflammatory effects [17], anti-stress[18], memory-improving [19,20], neuroprotective and cognitionenhancing effects [21–23], improvement of lactose tolerance,enhanced absorption of nutrients (including minerals) and gutassociated immune response, decrease of cholesterol levels,shortened duration of diarrheal bouts, and incidence of arrhythmias, ischemia and cancer [24–26]. Fermented dairy products (such as yogurt, dahi, shrikhand, Bulgarian butter milk, acidophilus milk,kefir, koumiss, and cheese) are commonly manufactured throughout the world, especially using LAB [27]. Cheeses and yoghurts are well known vehicles for health-promoting microorganisms including probiotics, offering advantages in nutritional value and health benefits over non-fermented milks [28], An example includes Iranian ultrafiltered Feta cheese containing Lactobacillus casei with NaCl partially replaced by KCl [29]. Cheese is a rich source of essential nutrients including proteins, lipids, vitamins and minerals that have beneficial effects on human health e.g. calcium for osteoporosis [30] or dental caries [31], conjugated linoleic acid (CLA) with anticarcinogenic and antiatherogenic properties[32,33], and peptides with various biological activities. Yogurt, as a nutrient-dense fermented food, contains bacterial cultures and various macro-/micro-nutrients and bioactive metabolites derived from fermentation, thus, exerts health benefits beyond its initial raw material against health problems such as type 2 diabetes and cardiovascular disease [34]. Kefir is a self-carbonated slightly foamy alcoholic beverage made with milk from animals (e.g. goat, cow and camel) or plants (e.g. soya, rice and coconut), offering unique sensory and nutritional properties [35].

2.2. Fermented staple crop products

Fermented staple crop foods represent a large category of solid and liquid products, and these foods are welcome alternatives for certain populations, especially vegans, vegetarians or individuals with lactose intolerance. Cereals such as barley, maize,millet, oats, rice, rye, sorghum and wheat have been used as the substrates for fermentation. Compared to unfermented cereals, fermented cereal foods tend to be more palatable and have lower anti-nutritional effects, and higher bioavailability of minerals [36].The health benefits of fermented cereals can be doubled, if probiotic microorganisms (e.g. Lactobacilli and Bifidobacteria) and raw materials containing prebiotic carbohydrates (e.g. beta-glucan, arabinoxylan, galacto-/fructo-oligosaccharides and resistant starch)are used. Rice and rice bran fermented with microorganisms like Saccharomyces bouldardii and Monascus purpureus were found to contain elevated contents of sterols, unsaturated fatty acids and phytochemicals (e.g. vitamins and phenolics), and possess health-promoting properties such as those against type 2 diabetes,cardiovascular disease, Alzheimer’s disease, cancer, and human B lymphomas [37]. Vinegar and beer are two common fermented cereal products. Vinegars such as Shanxi aged vinegar, Zhenjiang aromatic vinegar, and Kurosu vinegar are fermented from sugar and alcohol [38], and exhibit a range of functionalities such as antioxidant, antibacterial, anti-infection, blood glucose-lowering, lipid metabolism-regulating, appetite-improving, fatigue-reducing and osteoporosis-fighting effects [39]. Other popular fermented cereal products include Amazake (a sweet fermented rice beverage consumed in Japan), Ogi (a fermented cereal porridge consumed in Nigeria), Pozol (a traditional beverage of fermented maize dough originated from Pre-Columbian Mexico), and Kvass (is a lactofermented alcoholic beverage from rye grain originated in Ukraine,Russia and Belarus) [40]. Beer is a low alcohol drink containing nutrients and bioactive substances such as nitrogen compounds,melanoidins, organic acids, vitamins, phenolic compounds and mineral salts [41]. Fermented sourdough products have been used as alternatives to gluten-free products, as these foods possess desirable textural, flavor and nutritional characteristics induced by proteolytic lactic acid bacteria and wheat flour endogenous enzymes [42].

2.3. Fermented soy products

Soybean is a popular food that can be easily grown, being rich in protein (40% on a dry basis). The FDA’s approval of a health claim for soy in 1999 (“Diets low in saturated fat and cholesterol that include 25 g of soy protein a day may reduce the risk of heart disease”) has encouraged the development and improvement of soybean products. Fermented soy products have unique flavor and high nutritional value associated with their significant amounts of amino acids and fatty acids [43]. Various soybean materials are subjected to fermentation with a yeast, bacteria, mold, or their combinations to yield fermented soy products such as soy or tamari sauce, miso, tempeh and natto. For example, tempeh is a traditional fermented food produced via fermenting soaked and cooked soybeans and/or cereal grains with a mold (usually the genus Rhizopus). The enzymes in the mold catalyze the hydrolysis process(by which soybean constituents are broken down to create distinct and desirable food texture and flavour while reducing or even eliminating antinutritional substances) [21]. Some fermented soy products have been used as seasonings to provide salty and umami flavors such as Ganjang/soy sauce (brown-colored liquid), Doenjang (brown-colored paste made by Meju), Gochujang (red pepper paste, a fermented spicy condiment), and Cheonggukjang (soybean paste fermented by Bacillus at 40–43°C). In recent years, a focus has been placed on improving the health properties of fermented soy foods e.g. the increase of vitamin K2 (menaquinone), which is important to bone and cardiovascular health and present only in the fermented soybean and animal products [44]. Certain fermented soy products have demonstrated abilities to reduce the risks of age-/hormone-related diseases, inflammatory diseases, infection,asthma and cancer [45].

2.4. Fermented vegetable products

Fermented vegetable products can positively influence human health, because they are rich in substances beneficial to humans(e.g. dietary fiber, minerals, antioxidants and vitamins). Fermented vegetables such as kimchi, sauerkraut, carrots, cauliflowers, tomatoes, olives, green peas and peppers are traditional foods made at home or produced industrially. Sauerkraut, a popular fermented vegetable is made via lactic acid fermentation of salted white cabbage (Brassica oleracea var. capitata) [46]. Fermented cucumbers are produced via fermentation in brine (ususally 5%–8% NaCl) in open-faced tanks to convert saccharides into acids and other products [47]. Korean kimchi products include ordinary (without added water) and mul-kimchi (with water), beachu kimchi (diced Chinese cabbage), tongbaechu kimchi (whole Chinese cabbage), yeolmoo kimchi (young oriental radish), kakdugi (cubed radish kimchi), as well as baik kimchi (beachu kimchi with water), dongchimi (whole radish kimchi with water) and nabak kimchi (cut radish and Chinese cabbage). These kimchi products are produced via LAB fermentation of baechu cabbage along with other vegetables, thus contain significant amounts of LAB (107–109CFU/g), dietary fibers, vitamins and minerals. Kimchi products have probiotic, antioxidant,anti-aging, anti-inflammatory, anti-bacterial, anti-obesity and anticancer effects [48].

2.5. Fermented fruit products

Fermented fruits can be produced using whole fruits (e.g. apples,lemons, mangoes, palms, papaya and pears) or fruit pulps/juices(e.g. banana and grape), via “spontaneous” fermentation by natural lactic bacterial surface microflora (such as Lactobacillus spp.and Pediococcus spp.), or “controlled fermentation” by starter culture(s) (such as L. plantarum, L. rhamnosus and L. acidophilus) [49].Wine, cider and vinegar are fermented fruit drinks made via fermentation of grape, apple and/or pineapple with a combination of microorganisms involving yeasts and bacteria. A new generation of fermented fruit products are being developed to possess specific biological properties, in addition to distinct organoleptic characteristics. For example, red wine is developed to contain higher amounts of antioxidants especially certain polyphenols with specific health peoperties [50]. Fermented papaya preparation has been produced to possess antioxidant potency including free radical scavenging capacity [51]. Apple cider vinegar has the ability to lower the levels of cholesterol and triglycerides and protect LDL cholesterol particles from oxidation [52].

2.6. Fermented seafoods

Fermented seafoods are made through different processes involving fermentation of whole fish or derived by-products. Lacticacid fermented fish is normally produced via fermentation with gram-positive cocci including LAB such as Lactobacilli, Pediococci and Streptococci. For example, Nare-zushi in Japan is a fermented sushi made from raw fish (e.g. mackerel, salmon, crucian carp or sandfish) along with rice or other carbohydrate-based materials [53]. Fish sauces and seasonings are produced, via long-term fermention, from fish mashes of anchovy, small shrimp, icefish and sand lance together with the by-products of tuna, sardine and squid. In Korea, Jeot-gal is served as a side dish or appetizer, and used as an essential seasoning of Kimchi. Jeot-gal is produced through fermentation at temperatures > 25°C using marinated raw fish and microorganisms with proteolytic, lipolytic,and amylolytic activities. Through the optimization of processing conditions, Jeot-gal can be tailored to contain high contents of minerals (e.g. calcium, iron and phosphorus) and proteinbased compounds (e.g. amino nitrogen, soluble nitrogen and volatile basic nitrogens) and possess antioxidative, angiotensin-1 converting enzyme (ACE)-inhibitory, gamma-aminobutyric acid(GABA)-producing, cholesterol-reducing and antitumor effects[53].

2.7. Fermented meat products

There are a wide range of fermented meat products worldwide,because fermentation has long been used to preserve meat. Fermented meat products (e.g. sausage and ham) are produced by utilizing naturally occurring microflora, or by using one or more species of commercial preparations of bacteria, yeasts and molds(such as LAB, Micrococci and Staphylococci). Meat fermentation is a low-energy acidulation process involving complex physical, chemical and microbiological events, resulting in products with unique color, flavor, texture and nutritional value [54]. Acids play important roles in monitoring microbial activities and consumption of sugars in the production of fermented meat. Oxidation of meat components such as lipids and proteins (including protein carbonylation), as well as the interactions between meat components (e.g.protein-lipid or protein-protein interactions), all take place during meat fermentation [55]. Traditional fermented meat products include Italian salami, Spanish salchichon and chorizo, Icelandic Slátur (blood sausage), Irish pig-blood derived black pudding(blood sausage), beef sticks and pepperoni. Fermented sausages produced through prolonged ripening and drying processes likely have a low moisture content, more concentrated flavor, firmer texture and higher amounts of nutrients.

2.8. Other fermented products

Besides the above-mentioned fermented foods, a broad spectrum of other fermented products can be found worldwide. Among which, fermented tea has gained wide popularity, due to the feasibility of modifying the contents of organic acids, vitamins,caffeine and polyphenols in teas for taste and helalth purposes using different strains of bacteria, yeasts and/or fungi [56]. These contents can be tailored via the fermentation step and associated processes, in order to enhance the tea’s properties against undesirable microorganisms such as Salmonella and Staphylococcus[57] and contribution to the prevention and treatment of gastrointestinal infection, diabetes, cardiovascular diseases and cancer[58]. Fermented ginseng has been developed to improve behavioral memory function through reversing memory impairment and reducing β-amyloid accumulation in Alzheimer’s disease mice [59].Fermented olives are marketed as probiotic foods, as they can provide many vital nutrients and bioactive substances such as phenolic compounds that promote the performance of the human body while enhancing the protection on humans against a number of diseases [60].

3. Advantages of fermented foods

3.1. Changes in food composition and properties induced by fermentation

Fermentation doesn’t require sophisticated equipment for the fermentation process or the subsequent handling and storage of fermented products, though advanced devices including on-line sensors and in situ computer visualization and simulation programmes have recently been installed into bioreactors to facilitate real-time measurements for precise control of the production of fermented foods. The initiation and progress of fermentation depend on its nature (spontaneous or evoked intentionally) and the applied and natural occurring microflorae. The fermentative changes directly influence the physical, chemical, biological and sensory properties of final fermented products.

During fermentation, food components in edible and sometimes inedible raw materials are enzymatically and chemically broken down and then modified via biotransformation reactions(e.g. the removal of glycol residues). As a result, the sensory quality, nutritional vaue and health-promoting properties of the fermented products are improved in a safe and effective manner: 1)Fermentation enables unique flavors, aromas and textures (by generating small molecule flavor compounds and texture-modifying substances like exopolysaccharides (EPSs)); 2) Fermentation can increase digestibility and accessibility, whilst reducing cooking times (allowing the nutrients, including initially nondigestible carbohydrates, to be digested and absorbed easier); 3) Fermentation facilitates the enrichment of nutrients (including macroand micro-nutrients like essential amino acids, fatty acids, vitamins and minerals); 4) Fermentation can make foods with specific health benefits (through releasing target bioactives or transforming parent substances into into these bioactives (e.g. probiotics,prebiotics, and non-nutrient bioactives like phenolic antioxidants);5) Fermentation can help improve food safety through suppressing the growth of pathogenic microorganisms (e.g. Gram-positive and Gram-negative bacteria), and promoting the degradation of toxins (e.g. mycotoxins), via producing natural preservatives,antimicrobial compounds and bacteriocins such as butyrate, promoting mucosal cell differentiation and immune barrier function of the epithelium, and removing/reducing anti-nutritional factors in raw materials (e.g. metabolic detoxification of mycotoxins and other endotoxins by Rhizopus oryzae) [61–66]. The impacts of fermentation on foods are microorganism-specific and raw material-dependent, and can be further modified by external environmental conditions, and influenced by other processes coupled with fermentation e.g. pre-roasting buckwheat groats prior to solid-state fermentation with Rhizopus oligosporus [64,67]. It is worth noting that physical properties including rheological attributes of foods (e.g. thermal dynamics, viscosity, consistency,hardness and adhesiveness) may change after fermentation, and these changes, in turn, influence the chemical, biological and sensory properties of final fermented foods.

Anti-nutritional factors may occur naturally (e.g. enzyme inhibitors, flatulence factors, glucosinolates, lectins, tannins,polyphenols, phytic acid and saponins found in mustard, rapeseed protein products, grains and legumes), and can harm humans by impairing intake, digestion, absorption or utilization of other foods and feed components e.g. formation of complexes with essential nutrients or inhibition of enzymes [68]. A number of technological processes have been established and used singly or in combination to reduce the anti-nutritional factors, such as mechanical, thermal, chemical, and biological processes involving germination, fermentation, enzymatic treatment, soaking, cooking, canning, irradiation, selective extraction, fractionation and isolation. Fermentation can enrich nutrients and bioactives (e.g.amino acids and GABA), increase food digestibility and nutritional value, and remove unwanted substances (e.g. antinutrients), without deteriorating food quality (as compared to thermal, chemical and mechanical processes) [69]. Fermentation can also minimize or remove certain intrinsic substances that are undesired (such as protease, amylase and other enzyme inhibitors) and which might affect the rate and extent of important bioconversions [70]. In some occasions, fermentation and germination are combined to generate synergistic benefits. For example, the fermentation of germinated rice with probiotic organisms would enrich natural fibers, GABA and inositol hexaphosphate [71].

3.2. Fermentation and production of the “biotics”

The emerging ‘biotics’ including probiotics, prebiotics, synbiotics, postbiotics, oncobiotics, paraprobiotics, pharmabiotics and psychobiotics have gained much attention in the recent years.Fermentation is increasingly being used for the production of probiotics, prebiotics and synbiotics with high viability and functionality.Most recently, fermented foods and ingredients have attracted growing interest because they can facilitate healthy gut microbiota and promote human well-being.

Probiotics were defined in 2006 as “l(fā)ive microorganisms that should be alive by the end of the shelf-life of the product, and when administered in adequate amounts, confer a measurable health benefit on the host at a defined dose” [72]. Most of the currently commercialized probiotics for human consumption are LAB and Bifidobacteria, with LAB mainly composed of Lactobacillus spp.and Streptococcus thermophilus encompassing ?200 different species.Probiotics should be maintained in foods at levels above 107viable cells per gram or milliliter until consumption. For fermented foods,probiotic functionalities can be introduced through either the addition of probiotic cultures to fermented foods, or enhancement of the probiotic potential of the microorganisms present in fermented foods. The latter approach may have advantages in terms of the resistance to acidity and bile salts, adherence to colonocytes, generation of antimicrobials and/or lactase activity (since microbes have already adapted to the food environment) [73,74].

The food safety of probiotic fermented foods is enhanced, owing to the inhibitory effects on pathogenic bacteria of the antimicrobial bacteriocins produced by probiotic strains like Lactobacillus species.Further, the nutritional and health benefits of probiotic fermented foods can also be enhanced, because of the generated beneficial enzymes (e.g. β-galactosidase, which facilitates lactose hydrolysis and compensates for the reduction of human lactase activity over time), and nutrients or bioactives (e.g. enzyme cofactors, GABA,ACE peptide inhibitor, antihypertensive peptides, poly-unsaturated fatty acids, isoflavone aglycones and vitamins). The probiotic fermented foods have demonstrated positive roles in relief of lactose maldigestion symptoms, shortening of rotavirus diarrhea, immune modulation, regulation of serum cholesterol levels, treatment of irritable bowel syndrome, urinary tract infections, bladder cancer,and allergies associated with skin, gut and respiratory tract (including food allergies in infants) [75–77]. Although a large number of publications report the health benefits of probiotics or probioticscontaining fermented milk, only one health claim in relation to live yogurt cultures and their ability to improve lactose digestion has been approved under the current EU regulation of health claims on food. The limited number of efficacy claims is largely due to the complexity of the gut microbiota and effector strains affecting human health [77]. The health claims of probiotic fermented foods result from the interactions of ingested live microorganisms, bacteria or yeast with the host (probiotic effect), and/or the actions of ingested microbial metabolites formed during fermentation (biogenic effect) [78].

Another category of fermented foods in microflora management are fermented products with synbiotics (in which probiotics and prebiotics are used in combination to improve the viability of probiotic bacteria while conferring the benefits of both the live microorganism and prebiotics) [79]. Prebiotics, as substances linking gut health and probiotics, are defined as non-digestible food ingredients that affect the host beneficially through stimulating selectively the growth or activity of one or a limited number of bacterial species residing in the colon [80]. Prebiotics, such as fructo-oligosaccharides, lactulose and lactitol, inulin, β-glucan and galacto-oligosaccharides, must neither be hydrolyzed nor absorbed in the upper part of the gastrointestinal tract, instead, serve as a selective substrate for one or a limited number of potentially beneficial commensal bacteria in the colon. The health effect of a prebiotic resembles that of a probiotic i.e. improve the host health through modulating the gut microbiota (via increasing the number of specific microbial strains, or altering greatly the composition of the gut microflora). Moreover, prebiotics may be added together with other bioactives such as organic acids (e.g. butyrate), active fatty acids (e.g. CLA), B-group vitamins bioactive peptides (e.g. ACE inhibitory peptides), as such combinations can enhance the survival of probiotic strains and generation of desired bioactive substances e.g. the increased production of ACE inhibitors upon the addition of Aloe vera succulent plant powder [81].

4. Bioactive substances in fermented products

Fermentation has been used for “manufacturing” simple compounds (e.g. ethanol) and highly complex macromolecules(e.g. polysaccharides, proteins and enzymes), including bioactive metabolites (e.g. lactoferrin and flavonoids), from raw materials containing their precursors (Table 2). The newly generated substances can not only extend food shelf life and ensure safety,but also improve the sensory properties, nutritional value, and biological activities of foods against chronic diseases (e.g. via signalregulating, lipid-modulating, immunity-boosting, anti-microbial,anti-parasitics or anti-cancer effects) [82,83].

During properly controlled fermentation, physico-chemical events (including the enzymatic and non-enzymatic reactions involved in microbial metabolism) can lead to desired hydrolysis and solubilisation of macromolecules present in raw food materials (such as proteins and cell wall polysaccharides). As a result, the macro- and micro-structure of substrate materials is beneficially altered, which further influence the retention, release and absorption of the nutrients and non-nutrients in the substrate material. The final fermented foods are therefore endowed with target health-promoting properties derived from the released nutrients and bioactive substances as both reaction intermediates and end products. Cereal foods are good examples. Cereals are rich in bioactive phytochemicals such as vitamins (e.g. thiamine, vitamin E and folate), phenolics (e.g. lignans and phenolic acids) and phytosterols (e.g. sterols and stanols). However, the development of cereal foods using raw material(s) high in fibre and whole grains frequently encounter challenges associated with sensory acceptability, nutrient digestibility (e.g. starch) and bioaccessibility (e.g.minerals), and anti-nutritional factors. Fermentation technologies are particularly useful to resolve these issues while increasing the content of nutrients and bioactive substances in the final product.Under tailored conditions, fermentation can soften plant tissues,loosen and break down cell walls; induce enzymatic degradation of macromolecules and anti-nutritional factors (like phytate); and solubilize minerals and decomposed carbohydrates/proteins [84–86].Fermentation of wheat and rye flour matrix with LAB was found to decrease glycemic index (GI) and insulin index (II) of the obtained breads [87,88], probably through decreasing the degree of starchgelatinisation and generating bioactive peptides, amino acids and free phenolic compounds [89,90].

Table 2 Bioactive products generated or transformed during fermentation.

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During fermentation, unwanted substances such as sugars, antinutritional factors or even toxins may be reduced or eliminated simultaneously and efficiently. Black tea dust, a waste material with the same composition as its corresponding commercial tea,can be utilized through fermentation with the yeast S. cerevisiae.Fermentation for 6 h could convert over 80% of sugars (including sucrose), without decreasing the contents of total phenolics and the beneficial amino acid, L-theanine [91]. The alcohol-soluble proteins (prolamins) in barley and rye and the gliadins in wheat gluten have been associated closely with the damage of the small intestinal mucosa and the incidence of chronic inflammatory disorder (e.g.the Celiac disease) [92]. Fermentation produces foods with a lower risk of causing gluten intolerance, because fermentation facilitates desirable proteolysis reactions needed to break down the proteins(e.g. in sourdough breads) [89].

4.1. Bioactive peptides

Bioactive peptides are inactive sequence fragments within the precursor protein, and may exhibit potent bioactivities once being released from the precursor via proteolysis during aging and fermentation. During fermentation, large proteins are broken down via enzymatic hydrolysis into active peptides e.g. the milk-derived peptides with anti-oxidant, anti-hypertensive, antimicrobial, immunemodulatory and mineral-binding effects [93],and the soybean-derived taste-active peptides (e.g. umami and bitter peptides, with/without anti-hypertension and hypocholesterolemic activities) [94]. Antihypertensive and ACE-inhibiting and antiproliferative peptides as well as opioid peptides have been found in cheese [95] and other fermented products such as soy sauce and fish sauce [96,97]. Peptides of marine origin have demonstrated bioactivities such as cardio protective effects, antioxidant activity, and abilities to promote general well-being and mental health [98]. Proline-containing peptides, Lys–Pro and Ala–Pro,from anchovy sauce, as well as Ser–Val and Ile–Phe dipeptides in kapi, were found to possess significant ACE-inhibitory capacities[99]. The peptides obtained through fermentation of goat placental proteins by Aspergillus Niger exhibited strong antioxidant activity and immunoactivity (especially those with a molecular weight<10 KDa) [100]. In fermented meat products such as fermented sausages, bioactive peptides with high ACE inhibitory activities were also found [101].

4.2. Amino acids

Amino acids are well known for their central roles in human metabolism, performance and health. Amino acids contain carboxyl(-COOH) and amine (-NH2) functional groups, and exhibit different tastes (sweet, sour, umami or bitter). The building blocks of natural proteins are exclusively L-amino acids. Thus, L-amino acids are of importance in nutrition. The amounts of amino acids required by the human body vary with the age and health status of the individual. Nine amino acids (His, Ile, Leu, Lys, Met, Phe, Thr, Trp and Val) are essential for humans (as the human body cannot synthesize them), while Arg, Cys, Tyr and Tau (though taurine is not technically an amino acid) are semi-essential for children [102,103].

In addition to normal chemical synthesis and biosynthesis, fermentation is another approach to produce amino acids. The profiles of amino acids in fermentaed foods are greatly affected by factors such as the nature of raw material(s), type and quantity of starters used for fermentation, and conditions of the fermentation process.Sulfur amino acids, methionine and cysteine were found in fermented soybean products [104]. In salt-fermented shrimp paste or blue-mussel fermented sauce, biologically active taurine can be found (which can fight against oxidative stress) [105]. Tasteactive amino acids are abundant in fermented foods especially fermented seafoods, such as sweet amino acids (e.g. lysine, alanine,glycine, serine and threonine), bitter amino acids (e.g. phenylalanine, arginine, tyrosine, leucine, valine, histidine, methionine and isoleucine), as well as umami- and sour -tasting glutamic acid and aspartic acid [106]. The amino acid profile of kefir was found to change with the fermentation process and storage time, with higher contents of lysine, proline, cysteine, isoleucine, phenylalanine and arginine were found after fermentation [107]. Fermented anchovy and shrimp tend to have higher contents of free amino acids compared to their unfermented samples [108].

Microbial strains have been used to produce amino acids via fermentation e.g. E. coli strains for the production of amino acids like phenylalanine [109]. Strategies have been set to transform natural microbial fermentation towards metabolic engineering to achieve more precise and efficient production of amino acids [110].Branched-chain amino acids are good examples. These amino acids are among the nine essential amino acids for humans and contain an aliphatic side-chain with a branch (L-valine, L-leucine and Lisoleucine). The branched-chain amino acids can be manufactured via fermentation with microorganisms such as C. glutamicum, E. coli and S. thermophilus, with those produced using S. thermophilus exhibiting positive functions in protein synthesis, muscle performance and maintenance of lean body mass [111]. GABA (a major inhibitory neurotransmitter) deserves extra attention here. GABA is abundant in fermented foods like kimchi, cheese and fermented seafood products, but is low in unfermented foods including fruits(grapes and apples), cereals (maize and barley), and vegetables(asparagus, broccoli, cabbage, potatoes, spinach and tomatoes)[112]. Some LAB strains such as Lactobacillus buchneri, Lactobacillus brevis, and Streptococcus salivarius are especially capable of catalyzing the decarboxylation of glutamate and converting glutamic acid to GABA [113,114]. It is worth noting that D-amino acids (DAA)may occur in significant amounts in fermented foods. L-amino acids are the dominant natural amino acids and more utilized by the body than D-amino acids with the same peptide chain sequence.However, food processing may allow racemization of L-amino acids to their D-isomers. Certain microorganisms enable enantiospecific modulation of amino acids during fermentation and subsequent storage of the obtained fermented foods (e.g. wine, yoghurt and cheese) [115,116]. The utilization of any DAA may be affected by the presence of other DAA in the diet. There is a need to investigate the roles of D-amino acids in human nutrition and their different effects on food properties as compared with their L-forms.

4.3. Enzymes

Fermentation and enzymes are closely related. Fermentation breaks down food molecules and produces new substances throught the action of enzymes contained within microorganism(s). Futher, fermentation is a technology used to produce enzymes and their cofactors from microorganisms, such as yeast and bacteria, in industrial settings. Two fermentation methods are mainly used: Submerged fermentation (SMF, which involves a liquid nutrient medium) and solid-state fermentation (SSF, which is performed on a solid substrate). Amylase is a commercial enzyme widely used in the industry and can be produced via SSF of agroindustrial wastes such as mustard oil seed cake as the substrate using Bacillus sp. The amylase obtained after a 72-h fermentation at 50°C and pH 6 exhibited an activity of 5400 units/g and thermostability at 70°C for about 2 h in the absence of salt [117]. Coenzyme Q or ubiquinones as endogenous lipophilic enzyme cofactors can be generated in Jeotgal, and its concentration in final fermented foods can reach 297–316 mg/kg [118]. Purified phytase can be produced via fermemtation with Aspergillus oryzae NRRL 1988,before purification by fractionation with acetone, gel filtration, and chromatographic separation [119]. Glycosidases can be obtained through malolactic fermentation with Oenococcus oeni strain Lalvin EQ54 in a wine medium [120].

4.4. Lipids

Natural resources or industrial waste products have been utilized to produce biofuels and biochemicals including lipids and desirable fatty acids. Carbohydrate- and lignin-rich materials have been used as substrates for this purpose. Bioconversion via fermentation has been proven efficient, when natural or engineered Rhodococcus strains (e.g. R. opacus PD630, R. jostii RHA1, and R. jostii RHA1 VanA?), especially in the form of their co-culture, were used for lipid production (by which glucose, lignins and their derivatives in the raw material were simultaneously converted into lipids)[121]. CLA is well known for its abilities to decrease blood LDL cholesterol level, promote immune function and bone formation,and prevent hyperinsulinemia, atherosclerosis, gastrointestinal and colon cancers. Certain Bifidobacteria and Lactobacilli strains can produce CLA isomers in fermented milk and derived dairy products, thus imparting the final fermented products with enhanced health properties [122]. Value-added functional lipid products can be produced via fermentation from industrial processing wastes such as seafood waste streams (such utilization of waste materials also helps resolve environmental pollution problems). Omega-3 polyunsaturated fatty acids, such as eicosapentaenoic acid (EPA)and docosahexaenoic acid (DHA) (both are beneficial for fighting against neuropsychiatric disorders and cardiovascular diseases),can be produced through fermentation of the wastes from fish processing industries using a LAB culture such as Pediococcus acidilactici NCIM5368 [123].

4.5. Carbohydrates including polysaccharides and oligosaccharides

Raw materials contain more or less carbohydrates including monosaccharides, disaccharides, oligosaccharides and polysaccharides, with most monosaccharides and disaccharides being fermentable. The profile of carbohydrates undergoes dynamic changes during fermentation. The pattern and rate of such changes depend on the type and proportion of carbohydrate constituents in the substrate(s) for fermentation. EPSs with different structures and viscosity, such as glucan, fructans and gluco-/fructooligosaccharides, are produced as extracellular polysaccharides of microorganisms during fermentation. These EPSs either bind to the surface of cells or enter into the extracellular environment[124]. The EPSs produced by LAB exhibit prebiotic effects including the promotion of gut health [125,126] and enhancement of immune response [127]. Beer is rich in fibers that contain monosaccharides (e.g. arabinose, fructose, galactose, glucose and xylose),disaccharides (e.g. isomaltose and maltose) and trisaccharides (e.g.isopanose, maltotriose and panose), and exhibits high colonic fermentability (up to 98%) [128]. The content of total dietary fiber and the proportion of insoluble and soluble fibers in fermented legumes (such as black beans, cowpeas, lentils, bengal or green grams) depend greatly on the type of plant material, the type and activity of microorganism, as well as fermentation conditions. For example, the lignin content may be doubled in fermented lentils,whereas the contents of cellulose and hemicellulose decrease [129].Rare sugars such as L-ribose [142] and low-calorie sugar substitutes such as sugar alcohols (e.g. xylitol) can be produced from hemicellulose hydrolysate via fermentation with an E. coli strain[130].

4.6. Polyphenols

Phenolic compounds are well known for their antioxidant properties and contributions to the prevention and treatment of various health problems. Fermentation has been proven feasible for releasing phenolic compounds from a wide range of natural food resources, and facilitating biotransformation of these compounds (which may increase their bioactivities and bioavailability).As a result, fermented foods and beverages contain different concentrations of simple phenols (e.g. phenolic acids), more complex phenolics (e.g. flavonoids, including monomeric (catechins) and oligomeric flavonols (proanthocyanidins)), chalcones (e.g. xanthohumol), lignans and ellagitannins [131]. Fermentation of Aronia(Aronia melanocarpa, a phenolic-rich native berry of the North America) was found to produce phenolic metabolites with higher antioxidant capacity, bioavailability and α-glucosidase inhibitory activity [132]. Fermentation improves the utilization of milling by-products such as wheat bran. The use of starter cultures L.brevis E-95612 and Candida humilis E-96250, together with the addition of cell-wall-degrading enzymes, was found to release free amino acids and phenols (e.g. hydroxycinnamic acids) and improve protein digestibility [133]. Fermentation of rice bran with

Rhizopus oligosporus and Monascus purpureus, singly or in combination, increased the phenolic acid content and antioxidant activity(e.g. ferric reducing ability of plasma and DPPH radical-scavenging activity), with the combined use of the two strains resulting in the greatest release of ferulic acid [134].

Yeasts such as Saccharomyces strains may significantly affect the polyphenol composition of the obtained fermented product,because the metabolites generated during fermentation such as pyruvic acid and acetaldehyde can react further with the phenolic compounds [135]. Both alcoholic and malolactic fermentation can affect phenolic compounds, with their influence depending on the type of microorganism and fermentation conditions. Malolactic fermentation is often carried out after alcoholic fermentation to convert malic acid into lactic acid, causing the production of novel compounds e.g. trans-ferulic acid and some flavanols (which do not exist initially in wines before fermentation) and increasing the levels of certain bioactives (which already occur after alcoholic fermentation e.g. trans-resveratrol, epicatechin, catechin,caffeic acid, coumaric acid, myricetin and quercetin) [136,137]. The β-glucosidase occurring in microorganisms can contribute in different ways to malolactic fermentation, depending on the type of microorganism and activity of β-glucosidase [138]. The absence or presence of grape skin during fermentation also influences the impact of fermentation on the phenolic profile of wine [139].

4.7. Alkaloids

Alkaloids, a group of natural compounds with basic nitrogen atoms and bitter taste, exhibit significant biological activities including analgesic (e.g. morphine), antiarrhythmic (e.g. quinidine), antibacterial (e.g. chelerythrine), antimalarial (e.g. quinine),anticancer (e.g. homoharringtonine), antiasthma (e.g. ephedrine),antihyperglycemic (e.g. piperine), cholinomimetic (e.g. galantamine), and vasodilatory (e.g. vincamine) effects. Alkaloids can be produced by a wide range of organisms including plants, animals and microbes like bacteria and fungi. Alkaloids have been extracted from the leaves (e.g. black henbane), fruits or seeds (e.g.sour passion), root (e.g. Rauwolfia serpentina) or bark (e.g. Cinchona). Fermentation technologies allow the low-cost production of various alkaloids, such as those from the fungus Trichoderma harzianum via SSF [140]. An Escherichia coli fermentation system with a growth medium containing simple carbon sources but no additional substrate has been created to produce alkaloids (e.g. (S)-reticuline, with a yield of 46.0 mg/L culture medium)[141]. De novo production of the anti-cancer alkaloid, noscapine, was proven feasible via microbial fermentation with a single engineered yeast strain [142]. Producing ergot alkaloids via SSF with Claviceps fusiformis (an ascomycetous fungus) appeared to be advantageous (3.9 times higher contents of ergot alkaloids) over SMF [143].

4.8. Organic acids

Organic acids have pKa values in the range from 3 (carboxylic acid) to 9 (phenolic acid), and can be used as an acid, substrate, solvent, source of protons, and/or ligands for complexing metal cations such as aluminium and iron ions. Organic acids can preserve foods through penetrating the cell walls and disrupting normal physiology of pH-sensitive strains such as Salmonella spp., Clostridia spp, E. coli, Listeria monocytogenes, C. perfringens,and Campylobacter species [144]. The organic acids in fermented foods vary enormously, depending on the type of strain, nature of raw material, and fermentation method and conditions. The organic acids generated in fermented foods can inhibit the growth of spoilage and pathogenic microorganisms [39]. Fermentation with E. coli strain can produce organic acids and their derivatives from substrates such as cellobiose, cellulose glycerol and glucose[144,145]. In chungkukjang, the concentrations of volatile organic acids increase during fermentation, and branched-chain organic acids (2-methypropionic acid and 3-methylbutanoic acid) tend to have much higher contents than straight-chain organic acids[146]. Among the organic acids in fermented fish sauces, lactic acid has the highest content as it accumulates during fermentation [147]. In addition to suppressing undesirable microorganisms,the organic acids generated during fermentation also exhibit health benefits. For example, lactic acid generated helps regulate the glycemic response through lowering the rate of starch digestion[148], whilst acetic acid and propionic acid can slow gastric emptying [149].

4.9. Minerals

Minerals exist in the body as nutrients that are essential for human body’s metabolic processes and sustaining life. Essential minerals (including calcium, chloride, copper, chromium, fluoride,iodine, iron, magnesium, manganese, molybdenum, phosphorus,potassium, selenium, sodium, sulfur and zinc) must be incorporated through the diet. Meat and meat products are a good source of iron (Fe), with fermented sausages having a Fe content in the range of 1–2.5 mg/100 g [150]. Fermented dairy products are rich in highly bioavailable minerals e.g. ripened cheeses have abundant minerals especially calcium, chloride, sodium, phosphorus, potassium and zinc, while yogurt is enriched with calcium, potassium,phosphorus and zinc [151]. The profile and bioavailability of minerals are modified during fermentation. The availability of calcium,phosphorus and magnesium was found to increase when cheese was produced using probiotic cultures, mainly due to the proteolysis and lipolysis processes involved [152]. The bioavailability of Zn and Fe minerals in fermented pulses can be increased by 50%–70%and 127%–277%, respectively [153]. A higher absorption of iron was reported for individuals who often consumed lactic fermented vegetables compared to those who tend to eat unfermented vegetables,probably due to the conversion of iron into the more absorbable ferric (Fe3+) form during fermentation [154].

4.10. Vitamins

Fermented foods generally contain significantly higher vitamin contents than their raw counterparts [155]. Kefir is rich in vitamin B12 while yogurt has high concentrations of vitamins A, B and D[156]. Tempeh (a fermented soybean cake) has greater amounts of B group vitamins than unfermented soybean products. Natto (fermented soybean) produced with Bacillus subtilis is high in vitamin K (especially vitamin K2) [157]. The different contents of vitamins in fermented products result from the differences in raw materials, microbial strains and fermention conditions. Fermentation can improve the bioavailability of vitamins such as biotin, folate,riboflavin, pantothenic acid, pyridoxamine, pyridoxine, pyridoxal and thiamin, because of the actions of microorganisms [158]. Fermentation with Lactobacilli can decrease the content of vitamin B1 in fermented foods, whilst fermentation with yeast raises its level[159,160]. However, a reduction of vitamin content after fermentation is also possible. For example, a significant decrease of vitamin B1 content was found after the fermentation of chickpeas and cowpeas using L. casei, L. leichmannii, L. plantarum, P. acidilactici and P.pentosaceus [161].

4.11. Flavor or aroma-active compounds

Fermented foods are well known for their unique flavors. Fermentation leads to the production and accumulation of volatile and non-volatile aroma or aroma-active compounds including those related to bitter, umami, sweet, sour and salty tastes. The flavor or aroma-active compounds are mainly alcohols, aldehydes, amines, esters, fatty acids (especially those volatile species),ketones, organic acids, phenols, thiophene, sulfur compounds and other nitrogen-containing compounds (Table 3) [162]. The microbial strains used greatly affect the profile of flavor compounds in the final fermented foods. Yeasts mostly produce alcohols, aldehydes, esters, ketones, lactones, organic acids, terpenes and sulfur compounds, whereas, molds can generate a wide variety of flavor compounds such as alcohols, esters, ketones and pyrazines[163]. The macromolecules (i.e., proteins, carbohydrates and lipids)in raw food materials can contribute directly to the flavor of fermented foods, and may also serve as or deliver precursors of the flavor or aroma-active compounds e.g. sweet, sour, umami and bitter amino acids, bitter oligopeptides and organic acids [164]. For example, methyl methanethiosulfinate, methyl methanethiosulfonate, acetaldehyde, ethanol, ethyl acetate, methanol, n-propanol and 2-propanol are responsible for the characteristic aroma and flavor of sauerkraut [165,166]. Free amino acids, alcohols (mainly ethanol), aldehydes, esters, ketones, hydrocarbons, hydrocarbons,as well as sulfur compounds (such as α-copaene, α-farnesene,germacrene B or D, β-myrcene, α- or β-phellandrene, and βsesquiphellandrene), are all important contributors to the flavor of kimchi [167].

5. Consumer perception, cultural impact and safety concerns

The success of new food development is highly dependent on consumer behaviors and attitudes associated with foods including food perception, acceptance, preference and choice [168]. Many factors affecting consumer food choice include 1) biological factors (e.g. hunger, satiety and palatability); 2) economic factors(e.g. cost, accessibility, income, education and knowledge); 3)physical factors (e.g. such as time and skills); 4) social determinants (e.g. culture, family, peers and family norms or setting); 5)psychological determinants (e.g. mood, stress, guilt, family and history influences); 6) meal attributes (e.g. patterns, convenience and familiarity); 7) other factors (e.g. belief, optimistic bias, and previous experience and knowledge about foods, dietary preference and restriction) [169–172]. Among these influencing factors,sensory properties are one of the most important determinants.Recognition of the interplays among individual preferences and cultural/social influences will promote the innovation associated with fermentation processes and fermented foods.

5.1. Tradition and consumer beliefs

Given the long history of fermented foods and their widespread distribution across the globe, high famililarity, popularity and diversity are key characteristics of traditionally-consumed fermented products. Familiarity strongly influences the liking and perception of a cross-cultural ethnic food [173]. This is probably one major reason why the global fermented food segment continues to grow and spread all around the world. During the development of fermented foods, cross-cultural differences in the appreciation of fermented foods, motivational differences between trying and regularly eating, along with the differences between the pre-existing consumer perception/expectation and the actual consumer acceptability, all deserve attention [174,175]. Certain western consumers like Americans do not appreciate or even dislike the fermented flavor of kimchi [176]. A cross-cultural study involving American and Korean consumers using kimchi revealed that the difference in food preference between the two groups of consumers was associated with the consumption frequency and fermentation degree of kimchi [177]. Moreover, consumers may have negative or neophobic reactions to unfamiliar foods (e.g. fear and doubts), which is likely due to insufficient background information about the foods [178].This may also apply to fermented foods. Some individuals tend to associate the fermented flavor with signs of microbial spoilage(a negative response). Therefore, alleviating consumer concerns about microbial safety (through education and resources) and the stepwise introduction of new fermented foods would increase the likeability and acceptance of these foods [179].

Consumer perception on the typical sensory characteristics of certain fermented foods, (such as the perceived “gu-soo flavor”,“well-aged flavor”, “well-fermented flavor”, and “high in bean-topaste ratio” of traditional Doenjang), do not necessarily agree with the sensory attributes that drive the liking for commercial products (e.g. the preference for strong sweetness with umami taste of commercial Doenjang) [174]. Besides product characteristics, consumers’ liking of fermented foods (e.g. commercial rice wines [180]and lager beer [181]) also depends on external factors such as brand,price, and nutritional label. Consumers around the world share almost the same definition of “traditional foods”, but tend to perceive and respond differently to the concept of “innovation” [182].Thus, it remains challenging to combine the two concepts, “tradition” and “innovation”, in one single food product (as for fermented foods).

Modern consumers tend to enjoy new taste experiences and prefer convenient foods (due to their fast-paced lifestyles andabundant food options), while also demanding foods that promote their wellbeing naturally (owing to their increasing awareness of close relationship between diet and health). The wellbeing-driven demands of consumers motivate food industries to develop foods enriched with multiple nutrients and bioactives. Food fermentation enables in-situ enrichment of a range of health-beneficial substances, thus fermented foods and carry various essential nutrients and bioactive substances (e.g. probiotic microorganisms,prebiotics, enzyme(s), proteins, polysaccharides, bioactive lipids,antioxidants, minerals and vitamins). Further, consumers tend to equate fermented foods to probiotic foods and believe that the living microorganisms and their metabolic products in fermented foods have “positive effects on human health”. Interestingly, in pursuit of fermented foods with health-promoting properties, there is no large difference between the Western and Eastern countries, and also between the rural and urban societies.

Table 3 Characteristic flavours generated during fermentation.

Table 3 (Continued)

Table 3 (Continued)

Table 3 (Continued)

5.2. Safety concerns related to fermentation and resulting fermented foods

Fermented foods can be produced from all types of raw food materials including those of plant and animal origins. For most fermented foods, the processes involved in their production are inhibitory to many microorganisms, especially as fermentation can decrease the pH to below 4.0 through the conversion of carbohydrates into lactic acid. However, in some instances, pathogens may survive in the optimal environment for fermentation cultures and present significant food safety hazards. The pathogenic cultures potentially existing in fermented foods include Listeria monocytogenes (which can proliferate at refrigeration temperatures and grow at relatively high NaCl concentrations), Escherichia coli (which may occur in unpasteurized fermented foods and can be eliminated through heating ≥ 60°C and by reducing water activity), Clostridium spp. (which have an optimum growth temperature at 60–75°C and produces spores resistant to heating and hydrolases), Salmonella spp. (which can reside in the human intestinal tract, though are mostly killed under normal cooking conditions), and Staphylococcus spp. (which occurs on the skin and mucous membranes of humans.The failure of the starter culture during cheese making may allow S. aureus to grow) [183,184].

Contamination of mycotoxins represents a major food safety issue for fermented foods. In particular, indigenous fermentation(which involves old-fashioned processes and improper working conditions), or uncontrolled fermentation in the tropical and subtropical regions (where offer ideal conditions for the growth of fungal pathogens or molds), introduce increased risk of contamination by food borne pathogens and associated mycotoxins(including aflatoxins) [185]. For example, aflatoxins, a group of toxic fungal metabolites produced by some species of the genus Aspergillus (e.g. Aspergillus flavus, Aspergillus parasiticus and Aspergillus nomius), can cause severe harm to humans. Therefore, the main challenges with fermentation process are associated with the predictability of contamination caused by undesirable or even toxic microbes in fermented foods. The large scale and high diversity of commercial fermentation processes make the challenges more severe, due to the difficulty to monitor precisely the complex and interdependent/interactive metabolic pathways as well as the balance between synthesis of target substance(s)and innate cell physiology [186,187]. For the age-old practice of “spontaneous fermentation”, uncontrolled fermentation caused by the developed epiphytic microflora is possible (which causes undesirable organoleptic properties or even detrimental microbiological/toxicological consequences) [188]. Accordingly, it is crucial to examine the physiological and metabolic properties of the intended microorganisms prior to their use [189].

The potential presence of nematodes and other species of worms(e.g. Anisakis L3 larvae and trematodes) in the raw seafood materials for fermentation, deserves extra attention, because fermented seafoods are mostly produced without thermal processing. Therefore, these parasites are food-safety hazards of these fermented products. Salting (≥ 15% NaCl for 7 d) alone, or in combination with freezing (e.g. ?20°C for 48 h or ?40°C for 24 h) or irradiation can inactivate parasites effectively [190]. Standardizing the fermentation protocols and proper control over the fermentation process (especially the microorganisms involved) have been the important approaches for ensuring and improving the safety of fermented foods. Like other types of foods, fermented foods must meet all the international standards and requirements, including good manufacturing practices (GMP) and good hygienic practices(GHP) as the basic principles, hazard analysis & critical control points (HACCP) as the practical guidelines outlined by the Codex Alimentarius Commission (Alinorm 97/13A) and the European Parliament (Regulation EC no. 852/2004), and optional certifications like the ISO 22000 developed by the Codex Alimentarius Commission, and the food safety scheme British Retail Consortium Global Standard for International Food developed by food industry experts from retailers, manufacturers and food service organisations. Upgrading indigenous fermentation process is necessary to resolve manufacturing hygiene issues and uncontrolled reactions caused by unspecific microflora (individual or combined action of bacteria, yeast and fungi). Efficient and accurate methods for analyzing mycotoxins in various fermented foods are important for food safety. Chromatographic techniques such as gas chromatography (GC), high-performance liquid chromatography (HPLC) and thin-layer chromatography (TLC), as well as fluorescence-based detection methods, have been used for this purpose [191].

As for the safety assessment, it is important to consider the food matrx effect. Raw food materials used for the production of fermented foods naturally contain various species of microorganisms(which may be desired or undesired e.g. LAB, Bacillus cereus and Vibrio parahaemolyticus) [192]. Inhibition and elimination of food pathogens and other undesired microbes are critical to improving the hygiene of final fermented foods. Fermentation may degrade aflatoxins [193] due to the action of certain microorganisms e.g. LAB and Saccharomyces cerevisiae (which bind to aflatoxins) [194,195].Moreover, during the risk assessment regarding food safety, the host function and barrier effect of a specific fermented food matrix on the bioactivation and biological effects of both the existing toxic or carcinogenic substances, and the newly generated (unstudied)substances must be carefully considered. In 2013, the U.S. Food and Drug Administration (FDA) released the final rule to address the uncertainty encountered during interpreting the results of conventional gluten test methods for fermented or hydrolyzed foods in terms of intact gluten and gluten cross-contact for gluten-free labeling. Communication on both the benefits and risks of fermentation food intake is imperative, to provide consumers with full story about the positive and potentially negative impacts of fermentated food consumption.

5.3. Microorganisms, raw materials and the safety of fermented foods

There are different types of microorganisms involved in fermentation processes, and these microorganisms may decrease or improve the safety of fermented foods. The safe use of microbial food cultures concerns not only microbial culture preparation,but also about their characteristics in different applications (strain levels, processing conditions and fermented food matrices). For the newly discovered or developed strains, safety assessments should be performed to examine metabolism, carcinogenicity/mutagenicity, and toxicity (including short-term, long-term,developmental and reproductive toxicity, immunotoxicity and neurotoxicity along with toxicokinetics/toxicodynamics). These assessments should also be applied to the new strains derived from the organisms that may already have a long history of safe use in food fermentation (e.g. LAB). As foods for human consumption,fermented foods should be subjected to routine microbiological assessment e.g. the control of E. coli O157:H7, Bacillus cereus,Salmonella enteritidis, Staphylococcus aureus, and Listeria monocytogenes in fermented vegetable products like kimchi [196,197].

Various chemical, physical, and biological methods have been used to prevent, reduce and eliminate aflatoxin contamination of fermented foods. Effective approaches include improvements in production, packaging and storage conditions, the application of thermal and nonthermal treatments (e.g. heating, drying, roasting, microwaving or high pressure processing (HPP)), and the use of organic solvents, ozone, charcoal, vitamin C, fungicides, or antimicrobial-containing plant-based extracts/oils) [60]. Amongst these, biological control has attracted special attention, owing to its perceived “naturalness” and effectiveness (especially when certain aflatoxins exhibit high temperature resistance). Biological control approaches suppress the growth of pathogenic or toxigenic microorganisms, via introducing bioprotective microbes with potent antagonistic properties to the fermented food system(such as fungi and bacteria e.g. Lactobacillus, Bacillus, Pseudomonas,Ralstonia and Burkholderia) [198,199]. Probiotic strains such as Lactobacillus, Leuconostoc, Lactococcus, Pediococcus, and Bifidobacterium can assist the breakdown of some toxic chemicals that have been ingested together with food [200]. LAB strains are inhibitory to many other microorganisms including the spoilage organisms.Thus, the use of LAB as part of the co-culture can improve microbiological safety and the shelf life of fermented foods [201].

5.4. Biogenic amines and other harmful or toxic compounds

Toxic compounds such as biogenic amines (BAs) and carcinogenic molecules (e.g. ethyl carbamate) may be formed during food fermention by microorganisms. BAs are a class of compounds generated through microbial decarboxylation of amino acids, or amination and transamination of aldehydes or ketones during fermentation. BAs vary greatly in their chemical structures (e.g. number of amine groups), biosynthesis pathway and physiological functions. BAs are often grouped in aliphatic amines(e.g. putrescine (Put), cadaverine (Cad), agmatine (Agm), spermine (Spm), and spermidine (Spd)), aromatic amines (e.g. tyramine(Tym), and phenylethylamine (Phem)), and heterocylic amines (e.g.histamine (Him) and tryptamine (Trm)). BAs can be found in fermented meat (e.g. sausages), seafood (e.g. fish), dairy products (e.g.cheese), fruit and vegetables (e.g. sauerkraut), soybean products(e.g. temph), and alcoholic beverages (e.g. wine and beer) (Table 4).Factors favoring the formation and accumulation of BAs include the presence of decarboxylase-positive microorganisms such as those naturally occurring in raw materials (e.g. in spontaneous fermentation), starter cultures used for controlled fermentation (especially strains of Lactobacillus species), availability of free amino acids, raw material composition, substrate formulation (e.g. presence of salts,sugars or nitrites), and processing and handling conditions (pH,water activity, fermentation time and temperature) [202–204].Higher amounts of BAs are normally found in the fermented foods produced, handled and stored under poorly hygiene conditions,even though initial raw materials are low in BAs.

Table 4 The biogenic amines in the fermented products.

BAs at low concentrations may be required for certain physiological functions and normally do not cause harm to humans (e.g.the self-detoxifying ability of amine oxidases like mono- and diamine oxidases inside the human intestine). However, intake of BAs at high concentrations can be toxic for humans. The toxic dose of a BA depends greatly on the efficiency of an individual’s detoxification. In addition to routine raw material quality control and assurance of sanitary conditions for handling and processing [205],other approaches are used to minimize BAs in fermented foods:1) The use of nonamine forming (amine-negative) or amine oxidizing starter cultures and probiotic bacterial strains alone or in combination for fermentation; 2) The use of enzymes (di-amine oxidase), or bacteria containing this enzyme like Arthrobacter crystallopoietes KAIT-B-007 to oxidize the formed BAs [206–208]; 3) The application of non-thermal treatments e.g. high-pressure processing (HPP) [209], or low-dose irradiation [210,211] to reduce the BAs formed during fermentation. It was found that some strains could degrade BAs by up to 60%, with/without producing bacteriocin-like substances, such as L. plantarum, S. xylosus N°.0538, B. amyloliquefaciens FS-05 and S. carnosus FS-19, S. intermedius FS-20, B. subtilis FS-12, Natrinema gari, yeast strain Omer Kodak M8 [212–215].The amounts of BAs indicate the degree of freshness/spoilage of a fermented product. The European Food Safety Authority (EFSA) regulates the safety of starters via a premarket safety assessment based on the “Qualified Presumption of Safety” (QPS) status. The EFSA pointed out that Him and Tym are likely the most toxic amines, and set a maximum Him amount of 200 mg/kg for products associated with Clupeidae, Coryphaenidae, Pomatomidae, Scombreresocidae and Scombridae families [216]. The European Union (EU) regulations allows a maximum Him amount of 100 mg/kg in fresh or canned fish, whilst 200 mg/kg in fermented fish or other enzymatically ripened foods [217]. The U.S. FDA defines a food as spoiled if the Him level is 50 × 10?6, and set 50 mg/kg as the upper Him limit for most fish products [218,219]. Canada, Switzerland and Brazil set the maximum legal Him limit of 100 mg/kg for fish and fishery products. The Australian and New Zealand Food Standards Code does not allow the Him level of fish or fish products to be above 200 mg/kg [220]. For wine products, the European countries set different upper Him limits on wine: 2 mg/L in Germany, 3.5 mg/L in the Netherlands, 5–6 mg/L in Belgium, 8 mg/L in France, 10 mg/L in Austria, Hungary and Switzerland [206]. Due to the low volatility of BAs, lack of chromophores for most BAs and the low BA concentrations in complex food matrices, ultraviolet and visible spectrometric or fluorimetric techniques are not suitable for the detection and analysis of Bas. Instead, enzymatic methods along with TLC are used for qualitative or semiquantitative evaluations of BAs, with quantitation of BAs requiring capillary electrophoresis(CE), GC, HPLC, Ultra-HPLC (UHPLC) and/or ion-exchange chromatography (IEC) [221,222]. Some rapid analysis methods have recently been established including commercial test kits based on selective antibody and immunoassay methods [223] and enzymatic sensors [224].

Other harmful or toxic substances may also occur in fermented foods. Besides Him [225], fermented fish products may also contain N-nitroso compounds and genotoxins (which contribute to the incidence of cancer) [226]. Soy sauce made under improper conditions may contain ethyl carbamate (a Group 2A carcinogen) [227],or 3-MCPD (3-monochloropropane- 1,2-diol) and 1,3-DCP (1,3-dichloropropan-2-ol) (Group 2B carcinogens) [228]. The level of carcinogenic 3-MCPD (3-monochloropropane-1,2-diol) is deemed safe at 0.02 mg/kg by the EU and at 1.0 part per million by the Health Canada [228,229]. Furthermore, the removal of contaminated heavy metals is particularly important for fermented products such as fermented seafoods, as heavy metals such as arsenic, cadmium, lead and/or mercury are often found at high levels in certain seafoods. The uses of halotolerant bacteria (genus Staphylococcus and Halobacillus), tannins and/or cation-chelating resins may reduce or efficiently remove heavy metals in fermented foods, without changing the profile of nutrients and associated biological activities [230,231].

6. Strategies for creating fermented food products beneficial to human well-being

Successful food products in the modern marketplace often possess multiple desirable features related to sensory attributes (e.g.taste and texture), quality attributes (e.g. freshness, naturalness,safety and traceability), health attributes (e.g. nutritional value,biological value, clear health claims, or claims of toxin/allergen elimination), emotional attributes (e.g. happiness, enjoyment,communication and food experience), processing (e.g. spontaneous or controlled fermentation), and handling (e.g. ease of access, transport and disposal; visibility; resealability; labeling). Like other foods, consumers expect all these features to be included into a fermented product.

The development of fermented foods for human well-being“fermentation-enabled wellness foods” begins with the inclusion of cerain microorganisms (e.g. probiotic strains) in appropriate amounts. Then, food formulation and processing methods should be optimized to improve consumer acceptance of fermented foods [172]. Traditional fermentation methods based on empirical knowledge are progressively being replaced with science-based fermentation processes, advanced fermentation technologies and equipment, and modern industry safety practices. Science-based fermentation involves tailored microbial metabolism and enzymatic actions [232].

6.1. Fermented foods for human well-being

Scientific evidence has demonstrated that diet has an essential role in the prevention and management of chronic problems such as diabetes, obesity and cardiovascular diseases [233]. Popular fermented foods can make a huge contribution. For example,fermented papayas exhibits relatively high antioxidant activity[234]. The importance of fermented foods in human health can be enhanced and tailored, through increasing the amounts and bioactivities of nutrients and bioactive compounds, whilst removing undesired substances. Microbial fermentation provides an attractive alternative to chemical synthesis of nutrients and bioactives,and represents an efficient, convenient and safe synthesis process that can utilize different types of food materials to produce various macro-/micro-nutrients and bioactive substances such as flavonoids, isoflavones, terpenoids, alkaloids, polyketides and nonribosomal peptides (Fig.1) [235,236]. Unlike primary metabolites(which are essential for the growth of living organisms), secondary metabolites vary widely in chemical structures. The biofunctionalities of a fermented food are the sum of the independent effects of each co-existing active substances in the fermented food, evidenced for example in the anti-diarrhoea effects of fermented soya bean which realte to the multiple components in the product[237,238]. More details about “microbial chemical factories” will be presented in next section.

The consumer demand for healthier foods has lead to widespread efforts to reduce salt intake. To produce fermentationenabled wellness foods or functional fermented foods, reducing the content of salt (especially sodium salts) in fermented foods is an essential pre-requisite. High salt intake can cause health problems such as heart disease, high blood pressure and stroke. The sodium salt content of many traditional fermented products (e.g. some condiment pastes, fish sauces and fermented sausages) remains very high. For some fermented foods (e.g. raw milk, specialty and artisanal cheeses), lowering the quantity of sodium salt is challenging, because the salt is involved in the physical and chemical interactions among food components, and influences the microorganisms and enzymes involved, food structure and flavor [239].Three major approaches are adopted to decrease the salt content in final fermented foods: (1) Reduce the quantity of salts through the thorough washing of raw materials (like washed seafood muscle), partial replacement of NaCl with KCl or flavor enhancers, and addition of fish juice, koji or other umami microbial extract, and/or enzyme(s) (e.g. flavourzyme), accompanied by a fast fermentation process [240–243]; (2) Use certain starter cultures e.g. a high proteolytic starter culture for hard cheese [244], or probiotic strain L. plantarum L4 in combination with Leuconostoc mesenteroides LMG 7954 for sauerkraut [245]; (3) Remove or reduce the salts through post-fermentation steps such as various extraction and separation processes (e.g. electrodialysis, reverse osmosis, nano-/ultra-filtration and chromatography) without changing the typical characteristics of target fermented products [246]. More research should be directed towards the reduction of salts in fermented foods, as the currently available approaches still have limitations.For example, fish sauces with a high potassium content made through replacing NaCl partially with KCl may not be suitable for patients with kidney disease; the use of CaCl2to replace sodium salts demands care that the residual CaCl2concentration in final fermented product is lower than the legal limit e.g. ?36 mM for fermented vegetables (21 CFR 184.1193), and to avoid bitterness issues (i.e. which are detectable at 36 mmol/L CaCl2).

6.2. Improve the performance of strains during fermentation

Many starter cultures (e.g. commercial organisms: Aspergillus spp. fungal culture, Saccharomyces cervisae yeast, and Lactobacillus, Streptococcus and Bifidobacterium bacteria) have been isolated from the nature. With the breakthroughs in cell biology, recombinant DNA technology, biomolecular engineering and metabolic modeling techniques, novel strains with enhanced productivity and tailored functional properties have also been developed. The strains used in the production of fermented food (Table 5) must be live,with GRAS (generally recognized as safe) safety status, and defined metabolic dynamics.

In many organisms, metabolic pathway components and pathway-specific regulators are tunable. The microbial production of desirable metabolites can be enhanced considering the balances in carbon flux and enzyme levels. Many aspects should be considered when one selects the pathway(s) for biosynthesis of nutrients and bioactives as metabolite products of fermentation.Accessibility is one major hurdle for such biosyntheses, because the substances are naturally generated in low yields in the native organisms and often exist in multiple forms. For example, the availability of a NADPH pathway is a factor that restricts a high production of (+)-catechins in E. coli [247]. It becomes more challenging when multiple strains are involved in fermentation. The co-existing strains produce their own products, which may be in competition for building blocks or interfere with the targeted pathway.In order to increase the efficiency of the fermentation system and obtain higher amounts of target metabolite products, it is important to remove (at least reduce) the competing pathway(s), delete the unwanted catabolism pathway(s) and selectively shift towards the desired pathway(s) while upregulating the rate-limiting enzymes to enhance precursor supply.

Fig.1. The safe and sustainable production of beneficial microbes and fermentation-enabled wellness foods via precise control of microbial fermentation.

Table 5 The microorganisims used or detected in the fermentation process.

The selection of microorganisms (“microbial factories”) for producing probiotic fermented foods (i.e. products containing probiotics, or products with desired probiotic functionalities) should also consider carefully the nature and performance of the microbes(e.g. their productivity, viability, stability and metabolic characteristics). The different enzymes potentially produced by starter cultures (e.g. proteases and glycoside hydrolases) should also be considered, as these biocatalysts can modify both the ingested food components and the metabolite products yielded by the existing microorganisms. Such modifications may be beneficial or undesirable. The interactions between the starter cultures and other co-existing substances in food matrix, or between the microbes and digesta biomolecules in the gut, may become more intensive, when probiotics are used as a component of the starter culture. Among all the known metabolite products generated by microorganisms, bacteria account for approximately 70%. The most common heterologous hosts for bacteria-derived metabolite products are Streptomyces hosts and E. coli. Fungi are responsible for about 30% of the microbial metabolite products, and their share increases rapidly owing to low-cost carbon sources required for their growth and value-added metabolite products such as polyketides terpenoids, peptide-based compounds (e.g. nonribosomal peptides), and their combinations [248]. The heterologous production of beauvericin in E. coli [249] and the reconstruction of the four-gene pathway to produce tenellin in A. oryzae [250] represent two recent highlights in this field.

The strain selection for biosynthesis of nutrients and bioactives that exert health benefits to the humans must consider the presence of human gut microbiota as “micro manufactories and processors” (which can modify further the food components in the ingested fermented foods) [251]. LAB like L. casei, L. acidophilus,L. paraplantarum, L. rhamnosus and Bifidobacterium spp.) are commonly present in fermented foods such as kimchi, fermented olives,fermented cucumber, cheeses and salami. They may be used as primary starter cultures (e.g. the mesophilic and thermophilic LAB species for blue cheeses), or as part of the selected secondary microbiota (adjunct species) specifically for certain steps of the entire manufacturing process of fermented foods (e.g. flavor development, and acid or gas production) [252]. The studies on LAB have not been limited to the search for ideal starters, but also their uses as probiotics. LAB-containing fermented foods may possess health benefits associated directly with LAB (e.g. the well known protective effects of LAB as the probiotics on the host against detrimental microbes), and other properties such as antioxidant antiinflammatory and hemolytic activities, anticholesteremic and immunostimulatory effects, enhancement of the host’s gut health and immune system, improvement of digestibility and bioavailability of essential nutrients, and suppression of antinutritional effect,allergy reactions, inflammatory symptoms, lactose intolerance and incidence of certain cancers [253–255]. Also, the health benefits of LAB-containing fermented foods may result from the released metabolites upon the action of LAB (e.g. polyohenols and alkaloids)[256].

Acetic acid bacteria have seen increasing interest in recent years,as more evidence becomes available on the health benefits associated with their fermented products such as vinegars and ciders.More than 40 acetic acid bacteria (including Acetobacter, Gluconacetobacter and Komagataeibacter species) can convert ethanol into acetic acid (a process known as “acetification”, with strict requirements for oxygen) [257]. The oxygen level plays a critical role in the metabolism and performance of acetic acid bacteria: Aerobic respiratory metabolism is favored for most species when oxygen is the final electron acceptor, whereas, under nearly anaerobic conditions, survival and metabolism by some species is possible (other compounds act as final electron acceptors), causing wine fermentation [258]. In addition to the processing method and nature of strains, the origin of raw materials (e.g. apple, cherry, grape, oak,chestnut or strawberry) for vinergar fermentation determines both the sensory properties and the bioactive profiles of vinegar products, such as the type and amount of flavanols (e.g. catechin),hydroxybenzoic acids (e.g. gallic acid), hydroxycinnamic acids (e.g.caffeic acid), and tartaric esters of hydroxycinnamic acids (e.g. caffeoyl tartaric acid) [259,260].

Yeast has been used in various fermented foods and beverages (e.g. kimchi, bread, salami and cheese), especially as a key strain in bread making and alcoholic fermentation through metabolizing substrates like maltose and/or glucose. The species involved are mostly Brettanomyces, Candida, Cryptococcus, Debaryomyces, Galactomyces, Geotrichum, Hansenula, Hanseniaspora,Kluyveromyces, Lodderomyces Metschnikowia, Pichia, Rhodotorula,Saccharomyces, Saccharomycodes, Saccharomycopsis, Torulopsis, Trichosporon, Yarrowia and Zygosaccharomyces. These species can be grouped differently based on the strain nature, food application and product characteristics, for example, the bottom-fermenting S. carlsbergensis and top-fermenting S. cervisae for Saccharomyces yeasts. Fermentation with yeast(s) not only generate or modify food flavor through influencing volatile compounds, but also improve the nutritional quality and health properties of the food product via the production and/or bioconversion of nutrients and bioactives such as dietary fibers, proteins, purines and vitamins [261].The nitrogen-containing substances greatly influence the performance and metabolism of yeasts, including metabolic pathways of yeast, redox status of yeast cells, rate of fermentation, production of biomass during fermentation (e.g. ethanol, acetic acid,glycerol and succinic acid) [262]. The use of yeast in fermented meat and dairy products such as salami and cheese is to improve product flavor through the actions of a number of enzymes from the yeast (e.g. endo-/exo-peptidases, amino-transferases, alcohol dehydrogenases, α-keto acid decarboxylases, aldehyde oxidases,NADP-glutamate dehydrogenase and/or glutaminase) [263–266].Some yeast strains are of special interest, because of their distinct characteristics such as the ability to inhibit the growth of other microorganisms for the Candida lusitaneae, Kluyveromyces marxianus var. bulgaricus, and Saccharomyces cerevisiae strains isolated from aguamiel [267], and ability to produce inulinase of the Kluyveromyces lactis var. lactis strain from pulque [268]. In fermented foods like bread, symbiotic interactions are often observed between yeasts and Lactobacillus spp., e.g. as yeast induces dough leavening by generating carbon dioxide, Lactobacilli acidify the dough by the releasing lactic and acetic acids.

Molds can secrete hydrolytic enzymes that degrade natural materials including complex biopolymers such as starch, lignin and cellulose into simpler substances. While many molds are known for causing food spoilage, some play important roles in the production of fermented foods (e.g. mold-ripened cheeses, soy sauce and Katsuobushi). The koji molds, the Aspergillus species (especially A. oryzae and A. sojae), have been cultured in Eastern Asia for centuries to ferment a mixture of soybean and other raw materials(e.g. wheat) for the development of various soybean paste and soy sauce products. Koji molds can initiate a process called “saccharification” (by which the starch molecules in raw materials are broken down). Most recently, efforts have been undertaken to optimize the manufacture of soy sauce with A. oryzae e.g. the optimization of SSF and proteolytic hydrolysis to overcome the limitations related to defatted soybean meal [97], and the temperature for initial moromi fermentation [269]. Mold starter cultures have been incorporated into the production of fermented sausage products(e.g. salami) to improve sausage’s flavour and color while reducing nitrites and bacterial spoilage [270,271]. Molds (e.g. SP-01, barley koji and kome miso) have been used to to prepare ferment fishproducts with improved sensory and nutritional qualities e.g. Chum salmon sauce mash and fish pastes [272,273]. Furthermore, molds can inhibit unwanted yeasts, molds and bacteria such as Listeria monocytogenes [274].

Table 6 A nonexhaustive list of recent studies on the development of fermentation-enabled wellness foods.

Table 6 (Continued)

Table 6 (Continued)

6.3. Fermentation process and its interplay with other processing technologies

Fermentation process is induced by the sole or joint effort of bacteria, yeasts and molds. The production of a fermented food may involve different fermentation techniques, and treatments before and after fermentation. Therefore, selecting an appropriate fermentation process and optimizing the interplay between fermentation and other associated processing treatments are both important for producing safe fermented foods with high nutritional value and specific health benefits (Table 6).

Using a short-term or long-term fermentation process depends on the specification of the target fermented product. The molecular weight and quantity of macronutrients and bioactives in the fermented foods may change with fermentation time. Fermented soy foods made after a short-term fermentation (< 72 h) with Bacillus and Aspergilus species contain higher quantities of large carbohydrate and isoflavone molecules, as compared to those produced via long-term fermentation (> 6 months) e.g. chungkukjang versus meju and doenjang [275,276]. A sausage product made within a short ripening period tends to contain more Lactobacilli in the early stages of fermentation, whilst a sausage product obtained after a longer maturation period would have more Micrococcaceae species[277].

Fermentation can be an aerobic or anaerobic process, or involves both. SMF and SSF represent two different approaches for fermentation. SMF is employed when microorganisms require a high moisture (e.g. bacteria) and allows the utilization of free flow liquid substrates to produce enzymes and bioactive compounds. During SMF, a microorganism with a high-water activity is basically cultivated in a liquid medium containing nutrients, and it consumes substrate(s) in a rapid manner to release metabolite products (e.g.decomposed nutrients and bioactive constituents). SMF requires constant supply of substrate(s) but allows efficient separation and purification of the metabolite products [140]. In comparison, SSF is a solid-liquid-gas three-phase process that utilizes solid substrates including nutrient-rich waste materials. SSF typically employs fermentation with bacteria before fungal fermentation. During SSF,the microbial growth and the formation of product take place in the absence of water, and substrates are converted slowly but steadily into more digestible and more bioavailable bioactive compounds.SSF requires less effluent generation and energy consumption, and leads to less waste water production [278,279].

Fig.2. The importance of optimizing the interplay between fermentation and other thermal or nonthermal processing technologies in the development of fermented products with desirable properties.

After selecting microbioal strains and fermentation process,one still needs to choose appropriate treatments before and after the fermentation step in order to produce fermented foods and derived metabolite products i.e. pre-treatments (such as overnight soaking and heating), and post-treatments (such as filtration, sterilization and packaging). These pre- and post-treatments may involve thermal and non-thermal processes (e.g. HPP, highpressure homogenization, ultrasonic processing, pulsed electric field, X-ray irradiation, microwave heating and ohmic heating).These processes can influence the microstructure of raw materials(which further affects the heat and/or mass transfer in subsequent processing steps), as well as the activation/inactivation, survival,growth, metabolism and performance of desired or undesired microorganisms [280,281]. These pre- or post- treatments may impart additional effects on the final fermented products (Fig.2)e.g. A 30-s microwave pretreatment was found to increase the production of target metabolite products, via SSF of wheat by A. oryzae[282], and a HPP treatment of green table olives at 500 MPa for 30 min was effective in extending the shelf-life (5 months at 20°C)of fermented olive products [283]. Furthermore, one may protect the selected strains, and control the growth and target activity of microorganisms as well as enzymatic conversions through encapsulation technologies [284]. Such an encapsulation pretreatment would exert additional effects on the fermentation process and the characteristics of fermented foods [285].

Modifications of fermentation processes and derived fermented foods should be carefully conducted, with considerations on the special legislative or cultural requirements of the country or region where fermented foods are intended to be sold. Altering a fermentation process (including the starting material(s), microorganism(s),fermentation conditions, processing steps, handling approaches and storage methods) may introduce new safey challenges in relation to legal barriers e.g. the legal limit of certain substances, and the newly generated substances as new hazards (e.g. the metabolites derived from the strains) [286]. Accordingly, efficient and effective detection and analysis techniques are required and should be advanced to keep up with the progress in fermented food development. Considerable progress has been made on the development and advancement of analysis methods for determining desired substances (e.g. nutrients and bioactives) and unwanted substances (e.g. BAs and mycotoxins) in various fermented food matrices, and examining the diversity and dynamics of microflora and enzymes, using restriction fragment length polymorphism(RFLP), random amplified polymorphic DNA (RAPD)-PCR), repetitive element sequenced-based (Rep)–PCR, single-strand conformation polymorphism–PCR (SSCP-PCR), multilocus sequence typing (MLST), pulsed field gel electrophoresis (PFGE), denaturing gradient gel electrophoresis (DGGE), immunological assays(radioimmunoassay and enzyme-linked immunosorbent assays),chromatographic techniques (e.g. HPLC and GC) coupled with mass spectrometry [287].

The nutritional guides on fermented foods vary by country. For example, probiotics are listed as particular health-promoting substances in Japapanese “Food for specified health uses” (FOSHU).Yogurt and kefir are listed as recommended items under the dairy products section with no appreciation of their nature as fermented foods or as a healthy category in the food guides of Canada and the USA. The food guide of the United Kingdom has emphasized the consumption of fruits and vegetables without specifying their derived fermented products as a category. In Asia fermented foods are generally not considered as a separate category in food guides,except for India (which classifies fermented foods as a product category) and China (which specifies yogurt as a regular food for the populations who do not tolerate milk). The Swedish healthy eating food guide recommends foods low in fat and high in fiber without indication to fermented foods. In the EU, restrictive legal boundaries have been set for probiotic foods [288]. As with any functional foods that targets specific health claims, functional fermented foods(including those for gut health) will need to comply with existing regulations for full official endorsement of their health claims[289,290].

7. Conclusion and future outlook

There exist many substances in fermented foods, including health promoting nutrients, bioactives and enzyme microorganisms, as well as some other undesired substances. These substances may exert positive or negative impacts on the well-being of specific populations and individuals, and their effectiveness and safety requires case-by-case examinations. Further, the microorganisms in the daily consumed fermented foods are considered as “microfactories” to produce and enrich nutrients and bioactives with specific nutritional and health functionalities.

Advances in molecular microbial ecology and characterization techniques, together with detailed knowledge on the microbial interactions during food fermentation and interplays between fermentation and other processing technologies, are anticipated to take the fermented food segment to a higher level in the coming years. More effort should be directed towards the production of beneficial microbes and fermentation-enabled wellness foods to help address the escalating public health issues. To achieve this goal, fermentation-enabled wellness foods should be sustainable and specially designed for various populations and cultural groups.Precise control of the production of metabolite products via tailoring microbial fermentation, and monitoring the interplays between the fermentation process and other pre-/post-treatments (especially those involving emerging processing technologies), represent two major challenges. Any new fermented products including those with high nutritional value and specific biological functionalities(fermentation-enabled wellness foods and functional fermented foods) should be subjected to full and rigorous safety assessment as a novel food before any validation of their nutritional and health properties.