Roasting improves the hypoglycemic effects of a large-leaf yellow tea infusion by enhancing the levels of epimerized catechins that inhibit α-glucosidase.

Teas contain bioactive polyphenols, such as (-)-epigallocatechin gallate (EGCG), which is not stable during the processing of tea. EGCG can be epimerized into (-)-gallocatechin gallate (GCG), which is present in very small amounts in fresh tea leaves. An infusion made from roasted large-leaf yellow tea inhibited α-glucosidase more significantly than an infusion of unroasted yellow tea, with IC50 values of 76.08 ± 8.96 and 170.17 ± 33.00 μg mL-1, respectively. After roasting, the content of GCG showed about a 5-fold increase, while EGCG showed a decrease of 56.6%. Of the two main α-glucosidase inhibitors, GCG exhibited a higher inhibitory effect on α-glucosidase than its corresponding epimer (EGCG), whose IC50 value was about 3-fold lower. Modeling of molecular docking suggested that GCG preferably binds to the target α-glucosidase protein; this was confirmed by in vitro protein-polyphenol binding, where GCG had a binding rate about 4 times higher than that of EGCG. Comparative in vivo studies using oral starch tolerance tests in mice verified that GCG exhibited lower postprandial blood glucose compared to EGCG. These results suggest that roasting is a simple and effective way to increase the capacity of large-leaf yellow tea to regulate postprandial blood glucose.