Production of Kombucha With New Flavour Using Chinese Oolong Tea (Tie Guan Yin)
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Kombucha is a slightly carbonated, sweet and acidic refreshing beverage obtained from the fermentation of tea and sugar with a symbiotic interaction between bacteria and yeasts. Due to its inherent health benefits, it gained strong global market interest over the past decade. From previous studies, oolong tea has many health benefits such as anti-obesity effects and antioxidant capacity. However, oolong tea was rarely used as substrates in kombucha. In this study, kombucha was made with Chinese oolong tea (OT) - Tie Guan Yin (TGY), and sucrose. pH, titratable acidity, colour, ethanol, acetic acid, gluconic acid, amino acids, minor organic acids, sugars, total phenolic content, antioxidant activities and volatile organic compounds (VOCs) in kombucha were analysed. As the fermentation time increased, the pH of kombucha declined from 4.69 to 2.80 in 21 days which was mirrored by a significant increase in titratable acidity. In general, the colour of kombucha became lighter as fermentation progressed. The concentration of ethanol and gluconic acid reached the highest values after 21 days of fermentation. However, acetic acid tends to increase significantly and reached a maximum value of about 7.753 g/L at day 14 of fermentation, followed by a decrease to about 4.675 g/L at the end of the 21-day fermentation period. There were nine free amino acids detected in kombucha in this study which included alanine, valine, leucine, isoleucine, proline, threonine, aspartic acid, glutamic acid and phenylalanine. However, all of them decreased significantly (below 0.0025 μmol/mL) after 10 days of fermentation. In terms of minor organic acids after 21 days of fermentation, succinic acid, malic acid, citric acid, maleic acid and malonic acid were detected. The content of sucrose plummeted from 54.59 g/L to 11.95 g/L in the first 10 days of fermentation, and there was little change as fermentation progressed. Besides, glucose and fructose were detected on day 10 of fermentation, but the content of both decreased to 13.15 g/L and 17.48 g/L after 21 days of fermentation, respectively. This is due to the conversion of monosaccharide into other compounds, such as ethanol, acetic acid, and cellulose during fermentation. Notably, the consumption rate of glucose was faster than that of fructose after 10 days, which indicated that glucose was selected preferentially as the carbon source rather than fructose by kombucha microflora. The antioxidant activity of kombucha were analyzed by three common assays, CUPRAC, FRAP and phosphomolybdenum. All these three methods showed an increasing trend during fermentation, which corresponds to the concentration of polyphenols, and the antioxidant capacity increased more rapidly in the first 10 days. SPME assay indicated that some characteristic aroma compounds of OT were retained in kombucha made with TGY in this study, such as (E)-hex-2-enal and 3-methylbutan- 1-ol. Meanwhile, alcohols, acids and esters were generated during fermentation, such as nonanoic acid, ethyl ester, octanoic acid and decanoic acid, formed the unique aroma of kombucha.