Changes on physiochemical properties and volatile compounds of Chinese kombucha during fermentation

Kombucha is a traditional fermented beverage in China. In this study, six samples of kombucha from different regions of China were fermented, and the pH, total acidity, organic acid, carbohydrate and volatile flavor compounds at different fermentation time were determined. The results showed that the acidity of the samples increased as fermentation progressed, which was manifested as a decrease in pH and an increase in total acidity. During the fermentation period, the content of all organic acids showed an upward trend, and acetic acid was the most dominant organic acid in kombucha. The content of sucrose showed a downward trend, but remained stable in the middle and late stages. However, the contents of glucose and fructose were increasing throughout the fermentation. Twenty volatile compounds were identified by headspace solid-phase microextraction with gas chromatography-mass spectrometry (HS-SPME-GC-MS), among which the contents of 2-phenylethanol, 2,5-dimethylbenzaldehyde, acetic acid, ethyl acetate, ethyl hexanoate, ethyl 2-phenylacetate, and 2-phenylethyl acetate were greatly increased, and could be used as markers of the flavor quality of kombucha. Principal component analysis and correlation analysis showed that unique quality characteristics of kombucha were formed during the fermentation, and fructose and glucose were related to the production of tartaric acid, malic acid, acetic acid, and other volatile compounds. This study provides a theoretical reference for the quality evaluation of kombucha, and is expected to help in developing kombucha products with high quality.