Evaluating the effect of natural fermentation and select fermentation on the quality of the fermented tea prepared from a local tea variety

Abstract

Fermented tea has gradually attracted attention of tea drinker in Vietnam. However, most of fermented tea products are imported from other countries. There are many different methods of tea fermentation and from there, the quality of the tea also varies. Therefore, in this research, four types of tea fermentation were conducted, including natural and select fermentation (inoculated with aromatic producing Saccharomyces cerevisiae and a tea fermenting mold, Aspergillus niger) in both solid and submerged (liquid) state using Snow Shan tea leaves grown in Ha Giang province of Vietnam as the material. During fermentation, tea samples were taken periodically for the measurement of antioxidant activity, total polyphenols content, liquor color and the presences of microorganisms. The antioxidant capacities in most treatments reached the highest values after 14 days of fermentation. IC50 values for natural solid, select solid and select submerged fermented tea samples were 2.189, 0.401 and 0.847 mg/mL respectively. The only treatment, which had the maximal antioxidant activity after 28 days of fermentation, was natural submerged fermented tea samples with an IC50 value of 0.227 mg/mL at that time. Meanwhile, the total polyphenol contents of all four fermentation methods increased gradually over time, and the highest sample belonged to the select submerged fermented tea with the content of 422,594 ± 24.04 (mg GAE/g) which was recorded at 28 day. However, the results also showed no consistent correlation between the total polyphenols content and antioxidant activity, especially both select solid and submerged fermented tea samples. It could be said that the amount of polyphenols doesn’t necessarily lead to the increase of antioxidant activity. Regarding the color of the tea, the solid fermentation method gave the tea a brighter and more beautiful color than the submerged fermentation3 method, with the L value of 18.79 ± 2.02 for natural and 20.45 ± 2.01 for select fermentation. Finally, several microorganisms involved in tea fermentation were found after isolation, including the inoculated yeast and mold. Based on the morphology, Saccharomyces cerevisiae could be found in all tea samples, while Aspergillus niger was detected only in submerged fermented tea. Throughout the results, select submerged fermentation could be identified as the most potential method to improve the quality of tea, and it could be further studied for the production of fermented tea drink