An overview of plant-autochthonous microorganisms and fermented vegetable foods

Sebstin Torres,Hernán Verón,Lucin Contrers,Mri I.Isl,b

a Instituto de Bioprospección y Fisiología Vegetal(INBIOFIV)-CONICET,Tucumán,Argentina

b Facultad de Ciencias Naturales e IML,Universidad Nacional de Tucumán,Tucumán,Argentina

ABSTRACT Fermented plant-based foods and beverages constitute foods of high nutritional and functional value with appreciated health beneficial effects.They represent a natural and sustainable alternative to counteract the large wastage of vegetables and fruits due to their short shelf life.Usually,the use of controlled fermentation process using autochthonous microorganisms adapted to their vegetable matrix is preferred instead of traditionally spontaneous fermentation to designing fermented vegetable foods with the desirable sensory,technological,nutritional and functional properties.This review summarizes the autochthonous microorganisms selected as starters for the successful fermentation of vegetables and fruits.The main beneficial properties of autochthonous starters and fermented vegetable products with a focus on human health are revised.

Keywords:Plant-autochthonous Microorganisms Fermented vegetable foods Functional foods Lactic acid bacteria Probiotics Probiotic

1.Introduction

Fermentation of vegetables as a way of preservation and the consumption of fermented vegetables have a long history,practically going along with the development of human civilization [1,2].A clear example is the sauerkraut (sour cabbage) consumed by the inhabitants of the Roman Empire or the traditional sauerkrauts from Asia,such as the Chinese PaoCai or the Korean“Kimchi”[2-6].Other ancient typical fermented vegetable foods from Asia include the Chinese“Jiangshui”(fermented celery,Chinese cabbage,mustard,radish sprouts,and potherbs)or fermented soybeans,such as Japanese“Miso”and“Natto”or the Indonesian“Tempeh”[7,8].

Fermented vegetables have always received attention concerning their health benefits,which is continuously increasing,especially in “health-conscious” developed countries.Vegetables and fruits are per se sources of nutrients,such as vitamins,minerals,prebiotic fibers and bioactive phytochemicals,including phenolic acids,flavonoids,phytoestrogens and bioactive peptides [5].A growing number of scientific research have demonstrated the positive relationship between the consumption of a diet rich in vegetables and reducing the risks of age-related diseases,inflammatory pathologies,or metabolic disorders[9,10].Fermentation of fruits and vegetables,which is the main and simple alternative to heat or chemicals for guarantee the safety of vegetable products,may also lead to positive changes in the concentration or composition of vitamins,amino acids,bioactive peptides,or phytochemicals[11].Fermentation may also contribute to improving the bioavailability of these compounds and sensory attributes of vegetables and fruits.It is due to these considerations,that fermentation represents significant biotechnology to improve the safety,as well as,the health-promoting properties and general attractiveness of fruits and vegetables.

Fig.1.An overview of the technological and functional effects of plant-autochthonous microorganisms and fermented vegetable foods.

Vegetables and fruits have a microbial population whose composition depends on the characteristics of each vegetable matrix as well as geographical origin.This microbiota,composed mainly by beneficial microorganisms,including yeasts (Saccharomyces,Pichia,Candida,Torulasporagenera),fungi (Rhizopusspp.),and aerobic (Bacillusspp.andAcetobacterspp.) and anaerobic (lactic acid bacteria) bacteria,is usually responsible for the spontaneous fermentation of raw vegetables and fruits,contributing to their preservation and stability.Under favorable conditions,fermentation of vegetables and fruits occurs mainly by the instance of lactic acid bacteria with the participation or not of yeasts andBacil-lusspp.This even though lactic acid bacteria constitute a minor part of the microbiota of a vegetable or a fruit [10].The lactic acid bacteria often involved in lactic acid fermentation of vegetables belong toLeuconostoc,Lactobacillus,Weissella,EnterococcusandPediococcusgenera,being the ubiquitous and metabolic versatileLactobacillus plantarumone of the most frequently isolated species.Despite the great utility of spontaneous fermentation for the conservation of raw vegetables and fruits,the chance of fermentation failure,in terms of inappropriate inhibition of spoilage or pathogen microbes,and unwanted sensory traits and nutrients composition,are significant.Therefore,contrariwise with the observed with many fermented foods,a controlled fermentation,using selected starters,is highly recommended [12].In this regard,the use of autochthonous cultures isolated from raw and fermented vegetables and fruits,which are adapted for the specific plant matrix,may guarantee prolonged shelf life and improved functional and sensory properties of the fermented products[12,13](Fig.1).Furthermore,many of these autochthonous microorganisms used as starters in plant-based fermented products are probiotics,contributing to fermented vegetables with health benefits which include the potential to modulate gut microbiota and the immune system,and to improve the functional and nutritional status through the generation of new bioactive compounds or increasing the bioavailability of existing ones[8,10,14-16](Fig.1).

The search for autochthonous microorganisms for the fermentation of vegetables and fruits has relatively few years of practice,starting virtually with the beginning of the 21 st century.Unlike fermented foods usually produced in commercial scales,such as fermented dairy products and alcoholic beverages,for which there is an extensive knowledge concerning the used fermenting microorganisms,as well as the fermentation processes,the knowledge about fermented vegetable products,which in many regions are traditionally-made,is still scarce.In this regard,the selection of the appropriate autochthonous starter microorganisms is crucial to ensure the safety and quality of the fermented vegetable foods.The selected microorganisms should overcome various challenges,such as the ability to inhibit the growth of food-borne pathogens or spoilage microorganisms,ensuring the microbiological safety and quality of fermented food,to positively modify the level or the bioavailability of nutrients and biologically active compounds and diminishing antinutrients as well,or possess probiotic potential.For the latter,besides the known beneficial properties to the host,selected microorganisms need to colonize the fermented vegetable food and dominate its indigenous microbiota under different growth and storage conditions,ensuring the recommended minimum level of viable cells at the end of product shelf-life storage(106-107CFU/ml)[147].The selected microorganisms also should improve the sensory characteristics of the fermented product to ensure its acceptance by consumers.Furthermore,the industry requires that the fermentation process be economically sustainable,so starter cultures must be able to ensure rapid acidification of the food matrix at low temperatures.

Currently,the study of plant autochthonous microorganisms is in an upswing,which can be seen from a large number of publications on the subject.In this regard,efforts have been mainly focused on characterizing new indigenous,chiefly lactic acid bacteria,strains from vegetables and fruits based on their technological and functional attributes [148,149].This article performed an exhaustive review of the autochthonous starters selected for the successful fermentation of vegetables and fruits.Also,the main beneficial effects of autochthonous starters and plant-based fermented products with a focus on human health are reviewed.

2.Plant-autochthonous microorganisms

It is comprehended as autochthonous starters those microorganisms isolated from a raw vegetable or fermented vegetable food with a particular geographical origin,which will be used in the fermentation of the same vegetable matrix [12,17].Many scientific reports highlighted the advantages of the employment of autochthonous starters in the fermentation of vegetables and fruits[9,10].They are recommended in order to obtain fermented products with better technological (prolonged shelf life and improved or special sensory features) and functional properties [10].For instance,autochthonousL.plantarumS-811 showed superior performance in the fermentation of cactus pear juice,in terms of growth and survival of the bacteria,final pH,amount of lactic acid produced,and conservation of the antioxidant activity and vitamin C than the allochthonous strainLactobacillus fermentumATCC 9338[9,18].

In most of the cases,the autochthonous microorganisms of plants carry probiotic properties or produce postbiotics,that allow them to transform the fermented product in functional foods [10,11,19-21].While probiotics are live microorganisms that benefit the health of the host,postbiotics are soluble metabolic byproducts secreted by live probiotic microorganisms or released to the matrix after their lysis,and which can mimic the health effects of probiotics.These byproducts are mostly bioactive compounds that exert a myriad of beneficial physiological effects into the host,which encompass microbiota,metabolism and immune system homeostasis,amongst others [22-25].Fermented vegetables are naturally abundant in postbiotics.Among postbiotics that can be found in fermented vegetables are shortchain fatty acids,polyphenolic compounds,vitamins,enzymes,gamma-aminobutyric acid (GABA),peptides,and endo- and exopolysaccharides[26-28].

The criteria adopted for selection of autochthonous microorganisms with probiotic properties are essentially the same for bacteria,fungi and yeast and have already been explained in detail in previous reviews[12,29].These criteria match with those established by the World Health Organization (WHO),including capability to survive the human gastrointestinal tract conditions(low pH,bile salts and digestive enzymes),epithelial adhesion capacity,antimicrobial activity against pathogenic microorganisms and safety assessments(strain-level identification,antibiotic resistance,and enterotoxin,hemolysin,gelatinase,and DNase production) [30-36].Also,technological and functional properties of the selected microorganisms should be considered.Besides the usual technological traits assessed,such as growth and acidification rates,growth temperatures,tolerance to salts,hydrogen peroxide and exopolysaccharides(EPS)production,and synthesis of biogenic amines,other technological features concerning the adaptation of the starter to the vegetal matrix and the transformations that it produces should also be studied[9,30-35].Autochthonous starters should be able to overcome the adverse conditions that often represent a vegetable matrix,with high acidity,and a high content of hard to digest polysaccharides and phenolic compounds with antimicrobial activity.Therefore,the ability of autochthonous microorganisms to growth in vegetal or fruit pulps and juices should be tested.Also,tolerance to phenolic compounds,the ability to hydrolyze complex polysaccharides and metabolize nutrients and functional compounds present in the vegetal matrix should be evaluated.

The microbial metabolism of nutrients and bioactive compounds present in the vegetable matrixes may lead to the improvement of bioavailability of these compounds,the increase in their functional activities(and so,in their health benefits)or modification of the sensory attributes.Ergo,it is necessary study the effect of isolated microorganisms in the functional properties of the fermented vegetal or their host-associated benefits,such as,antioxidant,anti-inflammatory,immunomodulatory,anti-carcinogenic,anti-cholesterol,antidiabetic or anti-obesity effects(Tables 1 and 2)[14,36-49].Also,synthesis of aroma and flavor compounds and their precursors is another criterion to be considered for selection[50-54].

3.Autochthonous microorganisms isolated from raw vegetables and fruits

The literature reviewed in Table1 displayed autochthonous bacteria and yeast isolated from raw vegetables and fruits and their byproducts.Due to the capability of lactic acid bacteria to metabolize diverse carbohydrates,including galacto- and fructooligosaccharides and resistant starch,to produce high amount of lactic acid,their versatility that allow them to adapt diverse environments,and their probiotic properties,they are the most used microorganisms for fermenting,preserving and improving foods.Therefore,is expected that the searching of novel autochthonous microorganisms from raw vegetables and fruits point to the search of lactic acid bacteria.

Autochthonous microorganisms,mainly lactic acid bacteria,have been isolated from varied vegetable sources,from whole vegetables (fruits,leaves,roots,tubers,flowers or seeds) to pulps,juices and by-products,such as powders and wastes (Table1).Generally,native microbes isolated from fruits and vegetables are,in addition to good starters,potential probiotics.Their adaptations to the conditions of the plant matrix make them suitable to tolerate the adverse conditions of the hosts and allow them to metabolize the phytochemicals to new and more active functional compounds.Besides enhancing the nutritional and sensory features of the fermented vegetable products,in most situations,fermented vegetables showed leastwise preserved bioactive compounds contents and increased antioxidant capacities.

3.1.Functional properties of fermented vegetable products by plant-autochthonous microorganisms

Carrots,French beans and marrows fermented with autochthonousL.plantarumM1,Leuconostoc mesenteroidesC1 andPediococcus pentosaceusF4,or cactus pear puree fermented with wild strains ofLn.mesenteroides(isolated fromOpuntia ficus-indicafruits) had a significantly higher concentration of vitamin C than allochthonous started or unstarted vegetables[45,55].Improvement of antioxidant activities was usually related to the conservation or increment in the levels of vitamin C and other antioxidant compounds(phenolic compounds and betalains)[10,32,45,46,56-60].Verón et al.[10]described the enhancement in oxidative stress tolerance triggered by the cactus pear juice fermented withL.plantarumS-811,with the increment in this juice of derivatives of ferulic acid and caffeic acid,which can act as reducing agents,free-radical scavengers,and singlet oxygen quenchers [10].These compounds are also known to promote in cells the activity or expression of antioxidant enzymes,like glutathione and thioredoxin reductases [10,61].Similar observations were made by Di Cagno et al.[46]and Ye et al.[60]in fermented date fruit puree and broccoli puree,respectively.Di Cagno et al.[46]noted that the increase in antioxidant activity in fermented date fruit puree with nativeL.plantarumT1.3 was linked to changes in the profile of phenolic acids and flavonoids in the puree,that was enriched in protocatechuic,p-hydroxybenzoic acid,and rutin.Other health-benefits reported to fermented vegetables with autochthonous starters are related to their potential to prevent or improve chronic diseases.Fermentation of wheat germ using the mixed starter composed of autochthonousL.plantarumLB1 andLactobacillus rossiaeLB5,conducted to the production of cytotoxic 2-methoxy benzoquinone and 2,6-dimethoxybenzoquinone,which showed anti-proliferative activity against human cancer cell lines[62].Also,antitumor properties were reported in broccolipuree fermented withL.plantarumF1 isolated from broccoli leaves [60].Fermented broccoli puree showed an increment in the concentrations of antitumor glucosinolates (glucoiberin,progoitrin and glucoraphanin),perhaps due to the release of bound glucosinolates from the plant tissue[60].

Table1 Autochthonous microorganisms isolated from raw vegetables and fruits,and byproducts.

Table1 (Continued)

Table2 Autochthonous microorganisms isolated from fermented vegetable products.

Table2 (Continued)

Table2 (Continued)

Several fruits and plant fermented using autochthonous bacteria demonstrated potential to ameliorate or protect against obesity and its associated abnormalities.This protection is mainly related to hypoglycemic and hypolipidemic effects reported in the fermented vegetable foods[10,63,64].Hypoglycemic activity of fermented bitter melon using wildL.plantarumBET003 was associated with the production of aglycones from the degradation of glycosidic momordicoside that displayed anti-diabetic α-glucosidase inhibitory activity [63].Anti-obesity effects of fermented cactus pear juice (L.plantarumS-811) were demonstrated in an animal model of obesity[10].Administration of fermented juice ofOpuntia ficus-indicafruits to obese mice caused a significant drop in the body weight gain and improved the insulin resistance,hyperglycemia,and hyperlipemia typical of obesity[10].It was proposed that probiotic microorganisms and probiotic products can exert their beneficial metabolic effects through metabolically active compounds(short-chain fatty acids,phenolic compounds,peptides),or the modulation of the immune system or gut microbiota composition,what is reflected in an improvement of chronic low-grade inflammation linked to metabolic diseases[16].

4.Autochthonous microorganisms isolated from fermented vegetable and fruit foods

Table2 shows a list of autochthonous microorganisms isolated from traditional and some novel fermented vegetables and fruits,which displayed probiotic properties or health-beneficial functional activities.Fermented vegetable products constitute a large and valuable source of beneficial indigenous microorganisms.Nowadays,this kind of products is routinely explored in search of new autochthonous microorganisms with health-promoting and technological features that can be used as probiotics or starters in controlled fermentation processes of functional vegetable foods.Contrary to the observed in the isolations from raw sources,where the pursuit of autochthonous microorganisms is focusing on lactic acid bacteria,the isolation from fermented vegetables depends on which microorganisms are dominant in the microbiota of the fermented product.In most of the cases,plant-based fermented products are obtained through spontaneous fermentation carried out by various indigenous microorganisms intrinsic of the raw ingredients.Thus,the microbiota of these fermented foods can vary based on ingredients and microbial dynamics during fermentation,but it is also affected by geographical location and the technical procedures used.Accordingly,isolation and selection of microorganisms from fermented vegetable foods are more diverse than from raw sources and includes lactic acid bacteria,Bacillusspp.,yeasts and fungi(Table2).Among these,L.plantarumrepresents the most ubiquitous and most frequently isolated and selected species due to its broad technological and probiotic features(Table2).Furthermore,most of the indigenous microorganisms isolated from fermented vegetables and fruits display as main functional feature the antioxidant activity(Table2).

4.1.Sauerkrauts and pickled vegetables

Fermented plant foods of Asian origin,such as Chinese sauerkrauts and pickles or Korean kimchi,are among the most studied and represent important alternative sources of novel starters or probiotic indigenous microorganisms [65-71].These microorganisms were chosen because of their technological robustness,especially due to their adaptation to the adverse conditions of the fermented product,also transforming them into potential probiotic starters.This ability to adapt to fermented products is mainly due to the metabolic capability of these microorganisms,which also allows them to synthesize diverse postbiotics,transforming the fermented products into functional foods with multiple beneficial effects on human health.Several examples of autochthonous probiotic starters isolated from sauerkrauts,pickled vegetables and kimchi are strains ofLactobacillus,PediococcusandLeuconostocspecies.For instance,L.plantarumS4-1,L.plantarumsa28k,L.plantarumEM,andL.buchneriP2 isolated from Chinese sauerkraut,Indonesian sauerkraut,kimchi and Chinese pickle juice,respectively,showed to possess suitable probiotic properties including the ability to reducein vitroandin vivocholesterol levels associated with the inhibition of bile salt hydrolase(BSH)in some cases[66,72,73].Similarly,L.plantarumHY from Sichuan pickles which showed potential beneficial effects for the control of obesity and diabetes,due to its ability to inhibit α-amylase[74].Furthermore,anticancer activity was reported for some autochthonous kimchi bacteria such asL.acidophilusKFRI342,Lc.lactisKC24,L.plantarumLPpnu andLn.mesenteroidesLMpnu[37,69,75](Table2).

4.2.Other traditional fermented vegetables and sourdoughs

Other of the most significant sources of autochthonous microorganisms are fermented table olives and sourdoughs [76,77](Table2).Both,fermented table olives and sourdoughs constitute traditional fermented food with many centuries of history of human consumption in Europe.Nowadays,they are among the main fermented vegetable foods with industrial value and represent a valuable supply of indigenous lactic acid bacteria and yeast that gather multiple technological and functional properties that allow them to be used as starters or probiotics.The table olives are mainly a source of lactic acid bacteria,beingLactobacillusspp.the most isolated bacteria,and to a lesser extent of yeasts such asCandidaspp.,Pichiaspp.,andSaccharomycesspp.The adaptation of the microbiota of olives to the fermented brine solutions makes that autochthonous microorganisms isolated from these ecosystems also usually have technological and probiotic properties.Technological characteristics of indigenous microorganism from table olives include resistance to salt,pH,temperature and phenolics,high acidification rates (lactic acid production),and esterase activities[76,78,79].Also,some autochthonous isolates,such asL.plantarumB3 and B11,L.pentosusB4 and yeastWickerhamomyces anomalusWa1 can degrade the oleuropein responsible of the undesirable intense bitter taste of raw olives [79,80].Meanwhile,probiotic features of indigenous bacteria and yeasts from table olives include survival to transit through the gastrointestinal tract,adhesion to human intestinal epithelia,inhibition of pathogens,cholesterol removal capacity,enhancement of phenolics bioaccessibility,and presence of BSH and phytase activities [79,81-83].Moreover,some autochthonousLactobacillusspecies isolated from table olives displayed antiproliferative activity against human colon cancer cells (Caco-2 and HT-29),and longevity effects in aCaenorhabditis elegansmodel[82,84](Table2).

Spontaneously fermented sourdoughs are a significant source of indigenous lactic acid bacteria and yeast also characterized by their meaningful biotechnological and probiotic features.Many of these microorganisms are largely reported to enhance the technological and sensory characteristics,as well as,the functional and nutritional properties of the doughs they fermented,which was extensively discussed in various reviews[77,85,86].The efforts are mainly aimed at the discovery of autochthonous microorganisms capable of reducing the antinutritional factors present in cereals,their glycemic index,the reduction of gluten content and other immunoreactive proteins,and the modulation of the inflammatory response in the gut and gut microbiota composition[77,87].Phytic acid is the main antinutritional factor in cereals,which have strong binding affinity to the dietary minerals inhibiting their absorption and also affects the bioavailability of free amino acids and proteins.Phytase,which catalyzes the hydrolysis of phytic acid into the non-metal chelator myoinositol and phosphoric acid,enhances the bioavailability of important micronutrients,such as phosphorus,iron and zinc in fermented sourdoughs[86].Diverse autochthonous strains from wheat sourdoughs,such asL.plantarumCE42 and CE60,Ln.citreumSan9,P.pentosaceusBar4,Weissella confusaNey6,Kazachstania humilisG23Y andS.cerevisiaeD20Y and D24Y,orE.mundtiiCRL 2007 from quinoa sourdough,have shown phytate degrading ability improving the nutritional quality of phytate-rich foods [88-91].Numerous starters and probiotic strains isolated from wheat sourdoughs also showed proteolytic activities and the ability to hydrolyze gluten proteins,including gliadin,a protein of significant clinical importance for its role in the development of celiac disease[49,92-95].Incomplete digested gliadin peptides can pass through the epithelial barrier and initiate in genetically susceptible individuals disease-specific immunogenic cascades in the lamina propria,triggering a chronic autoimmune enteropathy [96].Among these autochthonous strains can find the lactic acid bacteriaLactobacillus alimentarius15 M,L.brevis14 G,Lactobacillus sanfranciscensis7A andLactobacillus hilgardii51B isolated from wheat sourdoughs andEnterococcus mundtiiQAUSD01 and the yeastWickerhamomyces anomalusQAUWA03,isolated from traditional Pakistani Khamir sourdough [49,92-95].Proteolytic activity of bacteria isolated from sourdoughs can further transform proteins in the sourdough matrix into bioactive peptides,like cardiovascular bioactive GABA.This was reported inL.plantarumandLn.mesenteroidesstrains isolated from gluten-free alternative sourdoughs,like quinoa sourdoughs [45,97].A high concentration of GABA was also observed in Tempeh,a traditional Indonesian soybean cake,fermented using autochthonousRhizopus microsporusvar.oligosporusIFO 8631 [98].Like Tempeh,other fermented soybean foods are a source of autochthonous starters and probiotics with functional properties.These strains are mainly belonging to theBacillusgenus,as probioticBacillus licheniformis141,isolated from Cheonggukjang,a Korean traditional soybean paste,which enhances the lifespan ofC.elegansnematode through serotonin signalling [99].Other examples are

Bacillus sonorensisandBacillus paralichemiformisstrains isolated from long-term fermented soybean paste that ameliorated obesity,insulin resistance and NAFLD in a mice model of obesity[100].Fermented soybeans are also a source of probiotic lactic acid bacteria and yeast,like anti-inflammatory strainsL.plantarumLAB12 andZygosaccharomyces sapaeI-6,autochthonous from Malaysian fermented soybean and Japanese Miso,respectively[4,101].Other traditional fermented(and some newfangled)vegetable sources of functional autochthonous microorganisms are presented in Table2,and includes fruits(durian,cucumber,pineapple),seeds and cereals(Prosopis africanaseeds,chia,amaranth,beans,rice),tubers (cassava)and vegetables(turnip,radish),which are produced in diverse countries around the world.

5.Conclusions and prospects

Raw fruits and vegetables constitute foods of high nutritional and functional value with appreciated health-promoting properties.However,due to the short shelf life they have,a large amount of them are discarded as waste generating large economic losses and the accumulation of organic waste.The elaboration of fermented plant-based foods and beverages constitute an alternative for its sustainable use,also transforming them in carriers for delivery of potential probiotics of value to consumers suffering from allergy to milk proteins or lactose intolerance.The utilization of a controlled manufacturing process using beneficial autochthonous microorganisms instead of traditionally used spontaneous fermentation is recommended to obtain foods with the desirable nutritional and functional,as well as sensory and technological properties.Thus,fruits and vegetables can be transformed into fermented foods with specific properties targeted to improve specific pathologies or human health generally.

Besides enhancing the functional features and health benefits of fermented fruit or vegetable foods,it is of the utmost importance that the selection of new autochthonous microorganisms is aimed at improving the sensory quality of the fermented product,and therefore the acceptability by consumers.These aspects can be satisfied through the selection of microorganisms producing key aroma compounds and exopolysaccharides to improve flavor and consistency and overall attractiveness of the final product.These investigations should be complemented with the extensive study of fermentation process parameters to developed a commercially viable fermented plant-based product for the functional food market.Also,the confirmation of the proposed health-claims for the selected autochthonous microorganisms and fermented vegetable products is another significant aim to achieved to increase the commercial appeal for the developed product.

Declaration of Competing Interest

On behalf of all authors,the corresponding author states that there is no conflict of interest.

Acknowledgment

The present review was supported by the grant PICT-2017-4436 from Agencia Nacional de Promoción Científica y Tecnológica.