Effects of Tea Polyphenols on Yogurt Fermentation Quality and Its Antioxidant Activity

Objective: The aim was to investigate the effect of tea polypenols (TP) on yogurt quality and its antioxidant activity. Methods: Milk with different concentration (0, 0.1, 0.2, 0.3, 0.4 and 0.5 g/100 mL milk) of TP was fermented for the preparation of yogurt. The effect of TP on the pH value, titratable acidity, nutrients and water-holding capacity of yogurt was determined by physicochemical analysis. Meanwhile, the effect of TP on the texture properties, color and flavor of yogurt was analyzed with a texture analyzer, chromatic aberration colorimeter and headspace solid phase microextraction/gas chromatography-mass spectrometry, respectively. Besides, the changes of four catechins in TP during the fermentation of yogurt was determined by high performance liquid chromatography (HPLC), and the antioxidant capacity of yogurt with TP was determined by in-vitro antioxidant test. Results: TP had no significant influence on the pH value, titratable acidity and nutrients. Then, the addition of TP slightly reduced the water-holding capacity, hardness, adhesiveness, gumminess and cohesiveness of yogurt, and made the color of yogurt slightly red and yellow. The addition of TP slightly reduced the content of acid flavor substances in yogurt, while the types and contents of flavors (ketones and alcohols) increased. During the fermentation of yoghurt, the esterified catechins (epigallocatechin gallate and epigallocatechin gallate) were degraded, while the non-esterified catechins (epigallocatechin and epigallocatechin) were not significantly changed. In-vitro antioxidant test showed that the addition of TP significantly increased the antioxidant activity of yogurt (including the significant enhancement of DPPH/ABTS⁺ free radical scavenging capacity as well as reduction capacity). Conclusions: The addition of TP has changed the quality of yoghurt to some extent and imparted a unique flavour and antioxidant effect, which could provide a reference for the utilisation of TP in functional yoghurts.