Isolation and identification of Starmerella davenportii strain Do18 and its application in black tea beverage fermentation

Chunhi Tu, Wenxiu Hu, Sijie Tng, Ling Meng, Zhihi Hung, Xio Xu,Xiudong Xi, Fidelis Azi, Mingsheng Dong,?

aCollege of Food Science and Technology, Nanjing Agricultural University, Nanjing, Jiangsu Province 210095, China

bInstitute of Agro-Product Processing, Jiangsu Academy of Agricultural Sciences, Nanjing, Jiangsu Province 210014, China

cNanjing Institute for Food and Drug Control, Nanjing, Jiangsu Province 211198, China

ABSTRACT

The objective of this study was to isolate and identify a yeast strain from the kombucha beverage and evaluate its potential as a novel starter in beverage fermentation in vitro. Starmerella davenportii Do18 was characterized for its cholesterol reduction; growth at different conditions such as temperatures (25,30, 37 and 42°C), low pH (1.2, 1.5, 2.0 3.0, and 7.0), bile salts (0%, 0.25%, 0.5%, 1% and 2%) high-sucrose stress (2%, 10%, 20%, 40% and 60%); and in-vitro survival in gastric and intestinal environments. Results showed that the yeast strain has a cholesterol-lowering capacity of 45% ± 2%, grew at temperature of 37°C and is resistant to pH 1.5, 2% bile and 40% sucrose solution, could survive in simulated gastric and intestinal environments. The physicochemical characteristics of the fermented beverages were also evaluated, which indicated that the yeast has pH reduction capacity and can produce organic acids and volatile compound such as 2-phenylethanol. Furthermore, the fermented beverage also has high total phenolics and flavonoids content and showed great antioxidant and antimicrobial activities. Therefore,the findings of this research provide strong evidence that S. davenportii Do18 has good fermentation properties, can be a potential starter in food and beverage fermentation.

Keywords:

Starmerella davenportii

Yeast fermentation

Tea beverage

Aroma

2-Phenylethanol

1. Introduction

Kombucha beverage is a traditional and popular natural fermented beverage consumed across the globe because of its known bioactivity. Kombucha beverage is produced by fermenting sweetened black tea (10% (m/V) sucrose and 0.5% (m/V) black tea, at(28 ± 2)°C with consortium of acetic acid bacteria and yeasts [1–3].The microbial consortium (acetic acid bacteria and yeasts) ferments the sweetened black tea to produce specific metabolites responsible for the bioactivity and aromatic properties of the kombucha beverage. The bioactivity and general health-promoting properties often associated with kombucha tea include cancer prevention,anti-diabetic properties and reducing gastrointestinal disorders[4,5]. However, what is not clear is the microbial genera responsible for the beneficial properties often associated with kombucha tea.

Probiotics are living microorganisms that provide beneficial effects to consumers when administered in adequate amounts, lactic acid bacteria and Bifidobacterium are among the predominantly important probiotic organisms [6]. Although yeasts are widely distributed and important in food, their probiotic potential is still poorly studied [7]. Saccharomyces and Kluyveromyces species have good fermentation properties and are among the few yeast genera that have been accepted as probiotic species [8–10]. This raises the question of whether other genera of yeast might have good fermentation properties to be potential probiotics as well [11,12].Starmerella genera have attracted much research interest in recent time because of their excellent fermentation characteristics [13].Starmerella bombicola and Starmerella bacillaris are type species of genus Starmerella which have been studied in wine fermentation [14–18]. Though there is little research information on the food fermentation of Starmerella genera; preliminary fermentation investigation and fermented-product detection on the S. davenpor-tii strain Do18 produced positive results: produced acids quickly,fermented-product has rich fragrance and taste much better. These properties prompted the selection of the yeast strain Do18 from other isolates from the kombucha tea for this study. Hence the purpose of this study was to isolate and identify S. davenportii strain Do18 and evaluate its application in black tea fermentation, which can generate strong aroma and organic acids, cholesterol lowering,possess tolerance towards various inhibitors like high-sucrose, low pH, bile salts and simulated gastric and intestinal environments.

2. Materials and methods

2.1. Chemicals

The compounds 2,2-diphenyl-1-picrylhydrazyl (DPPH) were purchased from Sigma-Aldrich Chemical Co. (St. Louis, MO, USA).Potato dextrose agar (PDA) were purchased from Shanghai Bio-way technology co., Ltd (Shanghai, China), bile salts, cholesterol, pepsin,pancreatin was purchased from Sinopharm chemical reagent co.,Ltd, (Shanghai, China), HPLC grade methanol, acetonitrile was purchased from Tedia Company, Inc. (Fair field, USA). All other chemicals and reagents were of analytical grade.

2.2. Yeast isolation, identification and 26S rDNA sequencing

Isolation of the yeast from the fermented kombucha drink was done using spread plate method on potato dextrose agar (PDA).The sample solution was prepared using physiological saline, plated out on the medium and incubated at 30°C for 48 h. Each colony morphotype was selected and purified by repeated streaking on potato dextrose agar. The pure cultures were preserved at 4°C until further use.

The identification of yeast strain Do18 was performed by Company (Shanghai, China) with sequencing the D1/D2 region of the 26S rDNA using the forward primers NL1 (5′-GCATATCAATAAGCGGAGGAAAAG-3′) and the reversed primer NL4(5′-GGTCCGTGTTTCAAGACGG-3′). After the result was compared with the National Center for Biotechnology Information gene bank(http://www.ncbi.nlm.nih.gov), the strain Do18 was identified as S.davenportii, and the phylogenetic tree was drawn by MEGA X using the neighbor-joining method.

2.3. Phenotypic characterization of isolated yeast

2.3.1. Assimilation of carbohydrates and sugar fermentation

Assimilation of carbohydrate sources was assessed on nitrogen base minimal medium plates supplemented with 1% standard carbohydrate sources for 2 days at 30°C, except when no growth was observed, in which plates were incubated for 5 days. Draw a line about 3/4 mm wide on a white card. Place the card close to the test tube fermentation broth and look directly at natural light. If the clear edge of the discontinuous line segment can be seen, it is denoted as positive; if the solution is very clear, let’s call it negative,that means there is no yeast growth. Sugars fermentation were carried out in inverted Durham tubes on media containing 0.5% yeast extract and 1% of each tested sugar at 30°C for a week. The presence of bubbles in the Durham tubes is considered, full is strong,more than half but non-full is positive, less than half is weak and the absence of bubbles is considered negative.

2.3.2. Bile tolerance

Test for bile acid resistance was done using the yeast culture activated by two transfers in yeast-peptone-dextrose (YPD): 2% glucose, 1% yeast extract, 2% peptone, 2% (V/V) of this dilution was used to inoculate YPD added with 0.25%, 0.5%, 0.75%, 1% and 2%(m/V) bile salts, and without as control [19]. The culture was incubated at 30°C for 2 days and viable cell colonies were determined by plating 100 μL appropriate diluted samples onto YPD agar.

2.3.3. Tolerance to low pH

Growth at low pH was estimated by inoculating 2% (V/V) activated cultures into YPD with pH adjusted at 1.2, 1.5, 2.0, 3.0 and 7.0 with 1 N HCl, pH 7.0 as control [19]. After incubation at 30°C for 2 days and viable cell colonies were determined by plating 100 μL appropriate diluted samples onto YPD agar.

2.3.4. Growth at different temperatures

Culture activated by two transfers in YPD were inoculated 2%(V/V), which were subsequently incubated at 25, 30, 37 and 42°C for 2 days, 30°C as control. Viable cell colonies were determined by plating 100 μL appropriate diluted samples onto YPD agar.

2.3.5. Cholesterol reduction capacity

The cholesterol-lowering activity of yeast was measured using a modification of the method of Cho et al. [20] with slight modifications. Cholesterol solution was prepared by dissolved in 99% ethanol (1:100, m/V), and 30 μL of the cholesterol solution were added into 3 mL of YPD containing 0.30% bile salt oxgall, and then the mixture was incubated at 37°C. Samples were collected at different time, after centrifugation for 10 min (5000 × g), the supernatant was used to determine the residual cholesterol content.Brie fly, 3 mL of ethanol mixed with 2 mL of KOH (33%, m/V) were added into 2 mL of sample supernatant. The mixture was warmed at 60°C for 5 min in a water bath. Then, the solution was cooled and the cells mixed with 5 mL of hexane while 3 mL of distilled water was added to the solution and allowed to settle for phase separation at room temperature for 15 min waiting phase separation. After this step, 2.5 mL of liquid in the hexane layer were taken and pressurized with nitrogas. The residual solution was added with 4 mL of o-phthalaldehyde/glacial acetic acid (0.5 mg/mL) reagent, finally,after mixed with 2 mL of sulfuric acid, the reaction was kept at room temperature for 10 min. Absorbance at 595 nm was measured with a microplate spectrophotometer.

2.3.6. In vitro survival in gastric and intestinal environments

Overnight cultures were harvested by centrifugation at 5000 ×g for 10 min, washed twice and resuspended in PBS buffer, pH 7.0(PBS g/L: sodium chloride 8.0; potassium chloride 0.2; disodium phosphate 1.44; potassium phosphate 0.24). To test for survival under simulated gastrointestinal stress, a concentration of 6 × 106CFU/mL was inoculated in PBS pH 2.0 with 3 g/L pepsin, while the culture used to test for survival under simulated intestinal was inoculated in PBS pH 8.0 with 1 g/L pancreatin and 0.3 (m/V) bile salt. Samples were collected at the inoculation time and after different times of incubation at 37°C. Aliquots were diluted and platted in YPD agar and incubated at 30°C for 48 h. The survival rate was expressed by the population of viable cells grown in YPD, considering the difference between initial and final time (log (CFU/mL))[6].

2.4. Fermentation properties of the black tea beverage produced using the yeast

Black tea beverage fermentation: Tea with 10% (m/V) sucrose and 0.5% (m/V) black tea was inoculated with 2% (V/V) activated yeast culture and the fermentation characteristics monitored at 30°C.

2.4.1. High-sucrose stress tolerance test

High-sucrose stress tolerance test was estimated by inoculating 2% (V/V) activated culture into tea water with 2%, 10%, 20% and 40% sucrose (m/V). Growth at 30°C was estimated by OD600nm.

2.4.2. pH measurement

Changes in pH of the tea water was evaluated during every 24 h in triplicate using a pH meter.

2.4.3. HPLC analysis of organic acids

Organic acids assays were assessed according to the method of Tu et al. [21]. High performance liquid chromatography(HPLC; Agilent 1100 series, USA) with Eclipse plus C18column(4.6 mm × 250 mm) was used for organic acids determination. After centrifugation (15000 × g, 5 min), samples were filtered through a membrane filter (0.22 μm) and then injected into the HPLC, flowing with 0.6 mL/min mobile phase (20 mmol/L Potassium dihydrogen phosphate (pH 2.4) and methanol at ratio (97:3, V/V)) at a 28°C column temperature and detection at 210 nm.

2.4.4. SPME-GC/MS analysis of volatile component analysis

Volatile components of the tea beverages were determined using headspace solid-phase micro-extraction (HS-SPME) gas chromatography coupled with mass spectrometry (GC/MS) and flame ionization detector (FID), the method described by Chua et al. [22].Brie fly, a 10 mL of the fermented tea water was adjusted to pH 2.5 using 1 mol/L HCl solution, then subjected to HS-SPME for extraction (60°C, 50 min) using a carboxen/poly (dimethylsiloxane) fiber(Supelco, Bellefonte, PA, USA) at 250 r/min. the SPME fiber after extraction was desorbed at 230°C for 3 min in the injection port.Carrier gas (helium) flowed at 1.2 mL/min and the temperature program was set to increase at a rate of 5°C/min from 50°C (5 min) to 230°C (30 min). The volatile compounds identification was done through comparing their individual mass spectra with the NIST08 Library and Wiley275 Library.

2.4.5. Antimicrobial activities

Antimicrobial activity was screened by agar assay according to the method of Battikh, Bakhrouf, and Ammar [23]. Gramnegative bacteria: Escherichia coli ATCC8099 and Gram-positive cocci: Staphylococcus aureus ATCC6538 were used as target bacterial strains for antimicrobial activity test. The suspensions of target strains cultured for 12 h was uniformly spread on the LB agar plates at a concentration of 106CFU/mL, and 9 mm diameter wells were performed with sterile oxford-cups. After centrifugation(12000 r/min, 15 min), sterile sample supernatants (100 μL) were obtained by filtering the undiluted sample supernatants through sterilized micro- filter (0.22 μm), and were then transferred into the wells within the plate spread with target strains. Firstly, plates were kept at 4°C for 4 h to make samples pre-diffuse into the agar,then incubated at 37°C for 48 h. Heat-denatured fermented beverage samples were prepared by boiling for 15 min to denature the active compounds. Neutralized samples of the tea beverages achieved by adjusting the pH using NaOH solution (1 mol/L).

2.4.6. Antioxidant activity

The in-vitro antioxidant activity of the fermented tea beverage was estimated by DPPH assay according to the methods described by Xiao et al. [24], with modifications. At the end of the incubation,the samples were centrifuged at 5000 × g for 10 min at 4°C, and kept in 4°C for test. The absorbance of the sample solution (Asample),the blank solution (Ablank) and the DPPH solution (ADPPH) were read at 517 nm. The DPPH radical scavenging capability was calculated using the following formula: DPPH radical scavenging activity = [1– (Asample – Ablank)/ADPPH] × 100%.

2.4.7. Determination of total phenolic and total flavonoid contents

Total phenolic contents of the tea beverages were determined following the method reported by Xiao et al. [24]. The absorbance at 760 nm was read with a microplate spectrophotometer. The total flavonoid contents were expressed as mg of gallic acid (standard)equivalents (mg GAE/L) from the calibration curve.

Total flavonoid contents of the tea beverages were determined using the aluminum chloride (AlCl3) colorimetric assay following[25]. The absorbance at 510 nm was read with a microplate spectrophotometer. The total flavonoid contents were expressed as mg of rutin (standard) equivalents per L of the sample (mg RE/L).

2.5. Statistical analysis

Experimental results recorded were means ± standard deviation (SD) of three independent experiments. Statistical analyses were carried using Origin 2018 and SPSS version 20. To evaluate the difference between means, and differences among samples were determined by comparison of means using Duncan’s multiple range test at P = 0.05.

3. Results and discussion

3.1. Isolation and identification of yeast strain Do18

After comparing the sequences of D1/D2 region of 26S rDNA extracted from the yeast strain Do18 with those in the GenBank database, the strain was identified as S. davenportii, which was firstly isolated by Stratford et al. [26] and got a basionym Candida davenportii, now reclassified as S. davenportii, yeast cells are small and ovoid, approximately 1.5 μm in diameter and 2 μm in length.The 26S rDNA of the Do18 was submitted to GenBank and get the GenBank accession numbers: MK560159. The neighbor-joining phylogenetic tree is shown in Fig. 1 with Brettanomyces custersianus as an out-group.

Fig. 1. Phylogenetic tree based on 26S rDNA sequence of Do18 strain. The number on the branches indicates the support proportion of each branch.

3.2. Phenotypic characterization analysis of Starmerella davenportii

3.2.1. Assimilation of carbohydrates and sugar fermentation

The assimilation of carbohydrates and sugar fermentation of S.davenportii Do18 have been done, and results show that D-glucose(positive), raffinose (weak) and fructose (strong) are fermented,means this yeast can utilize fructose faster than glucose, a phenomenon known as fructophily, which can keep the fructose concentration of fermented products at a low level. However,galactose, maltose, and lactose are not well utilized by the yeast.Raffinose and lactic acid were assimilated, but maltose, glycerol,raffinose, stachyose, galactose, mannose, lactose and sorbose were not.

3.2.2. Growth in low pH, bile and different temperatures

Microorganisms in food will always go through the human digestive system. Tolerance to low pH and bile salts is seen as a prerequisite for strain survival through the gastrointestinal tract. Do18 strain grew at pH 1.5 YPD medium which demonstrates its unusual tolerance to low pH (Table 1), Stratford et al. [26] also shown that Candida davenportii (Starmerella davenportii) grew well at an initial pH 1.4, but some other related yeasts showed resistance to low pH only around 2.0. In addition to the stomach acidic condition, probiotic microorganisms need to resist bile salt. Thus, bile tolerance is also considered one of the important properties required for high survival of probiotic microorganism. For this Do18 strain, bile had certain inhibitory effect on its growth, but it grew at 2% (m/V) bile(Table 1). Probiotics must also be able to grow at temperature of 37°C (physiological temperature) [27]. Results in Table 1 showed that Do18 strain optimum growth temperature was 30°C, and still grew well at 37°C, but hard to grow at 42°C. Hence, our study demonstrated that S. davenportii Do18 was able to grow at 37°C,pH 1.5 and 2% bile. These results also indicate that this yeast strain can adapt to complex fermentation conditions.

Table 1Growth in low pH, bile and different temperatures.

3.2.3. In vitro survival in gastric and intestinal environments

Besides being stable at high temperature (37°C), low pH, bile salt and having ability to reduce cholesterol, probiotic microorganisms are also required to be resistant to gastrointestinal enzymes[28]. Yeasts have been reported to survive in simulated gastric and intestinal juice [27,29]. The yeast was tested for survival rates in gastric juice (pH 2.0) and in intestinal juice (pH 8.0). Results(Table 2) shows that the percentage cell viability of the Do18 in gastric juice after 1 h is (3.39 ± 0.08) log (CFU/mL) and in intestinal juice after 2 h is (4.13 ± 0.11) log (CFU/mL). This result suggests that the intestinal environment has less damage for the yeast than gastric environment. All these results indicated that Do18 may survive in human gastrointestinal tract.

3.2.4. Cholesterol reduction capacity

Yeasts require lipid derivatives for growth and sterols play an important role in the cell membrane of eukaryotes, which signified that yeasts have ability to assimilate cholesterol [28]. Psomas et al.[30] reported that yeast strains assimilated cholesterol in the oxgall and cholesterol supplemented broth. Results in Table 2 shows that the cholesterol was continuously removed by the Do18, and the reduction capacity was (45 ± 2)% after 24 h. This suggests that the yeast strain has high cholesterol lowering efficiency and may be suitable for lowering cholesterol in high-cholesterol environments.

3.3. Fermentation properties

Fermentation properties were researched during black tea beverage production by S. davenportii strain Do18, these results will provide strong evidence for further application of the yeast.

3.3.1. High-sucrose stress tolerance test

As is well-known, initial high concentration of sugar (≥200?270 g/L, i.e. high-sugar fermentation) is one of multiple stressors present during fermentations, such as grape juice fermentation[31]. Resistant to high sugar containing environment is a good fermentation property of yeast strains, this ability determines the success of fermentation on high sugar food matrices [32].

Results of S. davenportii strain Do18 growing in different concentrations of sucrose are shown in Fig. 2a. The result showed that the yeast strain can even grow in 60% (m/V) sucrose, with the capacity for high biomass production at 10% sucrose as shown by the growth curves. These results con firmed that this yeast strain has high-sugar fermentation property and were consistent with the report of Stratford et al. [26], which reported that S. davenportii was osmotolerant and capable of growth in up to 59.4% (m/V) glucose. De Graeve et al.[13] reported that S. bombicola have an ability to grow in media with up to 50% (m/V) sugar concentration.

3.3.2. pH measurement

Low pH environments are relied on by many foods to prevent the growth of pathogens and putrid organisms [33]. The pH results of S. davenportii strain Do18 fermented black tea beverage were shown in Fig. 2b, results showed that this strain had a pH reduction capacity, and the lowest pH value was 3.02 in 10% sucrose culture solution after two days fermentation. Many putrefaction-making organisms and pathogens cannot survive at such a low pH, this suggests that S. davenportii strain Do18 fermented food and beverage may have a good ability to prevent the growth of pathogens and putrefying bacteria, and can be capable of low pH fermentation.

Fig. 2. Comparison of growth curves (2a) and pH values (2b) in S. davenportii strain Do18 fermented black tea beverage with different sucrose concentrations.

Fig. 3. Changes in total flavonoid (3a) and DPPH free radical scavenging activity (3b) of the black tea before and during fermentation.

3.3.3. Volatile components

The result of the volatile composition analysis of black tea and fermented black tea beverage is presented in Table 3. The result showed that the major volatile compounds of the sam-ples are alcohols, esters, hydrocarbons, terpenes and terpenoids,and aldehydes. After fermentation, almost all indigenous aroma components were metabolized to low or undetectable levels,except for β-damascenone which increased. Some new aroma compounds such as 2-Phenylethanol, α-Pinene, and α-Farnesene,were formed. According to Yang, Baldermann, amd Watanabe [34],2-phenylethanol has a flowery, rose-like, honey-like odor and contributors to the fruity, floral smells of flowers, fruits of food products. In this study, 2-phenylethanol account for the largest relative peak area (78.65%), and its dominant scent in the fermented tea beverage gave the fermented tea beverage a pleasant aroma. βdamascenone though has low human odor perception thresholds,has been reported to significantly contribute to the flavor of black tea [35] and green and oolong teas [36]. However, in green tea,damascenone appeared as glycosidically-bound form, and is less easily released [37]. Therefore, the observed increase in the damascenone content in fermented tea beverages could be attributed to the metabolic activity of the yeast. α-Pinene and α-Farnesene were other flavor-active compounds detected and they have been reported improve the aromatic quality of teas [38,39].

Table 2In vitro survival in gastric and intestinal environment and cholesterol lowering capacity of Starmerella davenportii strain Do18.

Table 3Volatile compounds (mean GC-FID peak area × 108) and their relative peak area (RPA, %) in unfermented and fermented tea beverage.

Table 4Changes in organic acids (g/L) of the black tea before and during fermentation.

3.3.4. Organic acids and antimicrobial activities

Formation of organic acids is the main reason for the reduction in pH during fermentation. Organic acids have been reported to have an important effect on the sensory and functional quality of fermented tea beverage [21]. Changes in organic acids content during fermentation is shown in Table 4. The major organic acids were pyruvic acid, α-ketoglutaric acid, succinic acid and oxalic acid,of which oxalic acid was already present the black tea water and slightly increased after 48 h of fermentation, while all the other acids were formatted by fermentation. Pyruvic acid was predominant organic acid found during fermentation and could be the main substance responsible for the decreased in the pH; it increased continuously and reached a maximum concentration of (5.01 ± 0.20)g/L after 48 h of fermentation.

The antimicrobial activity of the beverages against pathogenic microorganisms (E. coli and S. aureus) is shown in Table 5. The unfermented tea water has no anti-bacteria effect on the two tested bacteria; however, the fermented tea beverage was observed to demonstrate a significant inhibition (inhibition zone diameter(1.89 ± 0.03) cm) against both the two tested bacteria. The neutralized sample has a smaller inhibition ring than the fermented sample, which means that organic acids were the main antimicrobial agents in the beverage. Heat-denatured sample showed even lower antibacterial activities than the neutralized sample, this is an indi-cation that there exist other antibacterial agents other than organic acids in the beverage. Such antibacterial agent could be large proteins and bioactive compounds synthesized during the fermentation process, such as peptides, bacteriocins and polyphenols [40].

Table 5Antimicrobial activity of unfermented and fermented black tea beverages.

3.3.5. Polyphenols and antioxidant activity determination

DPPH free radical scavenging activity assay and tests of total phenolic and total flavonoid contents were performed as measures of antioxidant potential of the tea beverages. The total phenolic and total flavonoid contents (shown in Fig. 3a) of the fermented beverage were significantly (P < 0.05) higher than unfermented sample,and results reached highest after 36 h of fermentation, and there was no significant difference (P > 0.05) between 36 h and 48 h samples. Tea polyphenols contain several phenolics such as catechins,theaflavin, thearubigin and caffein, which in their natural form are combined or bound with other molecules. Enzymes produced during the fermentation process have been reported to degrade complex phenolics into soluble free phenolics. In addition, changes of acids in fermentation environment can also lead the release of binding flavonoids [41–43]. The antioxidant activity, on the basis of the scavenging activity of the stable DPPH free radical, was shown in Fig. 3b. With the development of fermentation, the antioxidant activities of tea beverages increased significantly (P < 0.05) from(51 ± 1)% (0 h) to (72 ± 1)% (48 h), fermented tea beverage exhibited much higher DPPH radical scavenging abilities than that of unfermented tea beverage. This is in keeping with the findings of Lee et al. [44] and Dajanta, Janpum, and Leksing [45], who reported that the total phenolic content and antioxidant activity are strongly positively correlated, and the total phenolic content of fermented sample is higher than that of non-fermented.

4. Conclusion

In this study, S. davenportii strain Do18 isolated from Kombucha was evaluated for its application in black tea fermentation.Results revealed that S. davenportii strain Do18 showed desirable fermentation properties. It demonstrates that black tea water is a suitable substrate for the incorporation of S. davenportii Do18 to produce fermented beverage products with good sensorial characteristics. The fermented tea beverage had greater polyphenols content, antioxidant activity and antimicrobial activities than nonfermented tea water. The yeast also generated aroma components and organic acids, thus improving the aromatic and functional quality of the tea beverage. Findings of this study thus demonstrate that S. davenportii Do18 is a good starter candidate for further tea fermentation and investigation with in-vivo studies to further clarify its potential probiotic benefits is necessary.

Declaration of Competing Interest

The authors declare that they have no competing financial interests.

Acknowledgements

This work was supported by the Jiangsu Agricultural Industry Technology System [No. JATS-2018-296]; and the National Natural Science Foundation of China [Grant No. 31501460].