Enhancing Bioactivity and Bioaccessibility of Mulberry Leaf Tea: The Influence of Pretreatment and Kombucha Fermentation

We applied sodium glutamate and ultrasound pretreatments in the processing of mulberry leaf green and black tea to enhance the bioactive compounds and their bioactivities. Kombucha fermentation was investigated to assess further enhancements in the bioactivity and bioaccessibility of bioactive compounds. Overall, the ultrasound-pretreated group showed a higher amount of 1-deoxynojirimycin (DNJ), phenolics, and flavonoids than did sodium glutamate pretreatment, while γ-aminobutyric acid (GABA) in the sodium glutamate-pretreated group was higher. Kombucha fermentation significantly increased DNJ, GABA, phenolic, and flavonoid content, particularly in ultrasound-pretreated samples, resulting in enhanced antioxidant, antidiabetic, anti-inflammatory, and antihypertensive properties. Furthermore, fermented samples demonstrated up to 25% higher bioaccessibility of phenolics and flavonoids than nonfermented samples. Our results reveal that kombucha fermentation, along with the specific pretreatments, can improve the health effects of mulberry leaf tea for health promotion, which has great potential as a functional beverage.