Optimisation of green tea polysaccharides by ultrasound-assisted extraction and their in vitro antidiabetic activities

Tea polysaccharides have attracted scientific interest due to their antidiabetic effects, and lower quality tea leaves have more polysaccharide in their content compared to higher grade tea leaves. The aim of this study was to optimise the ultrasound-assisted extraction (UE) conditions of polysaccharides from low-grade green tea (GTPS) by Box-Behnken response surface design on the desired response (yield). The optimal extraction parameters were determined as follows: extraction temperature (80 degrees C), extraction time (60 min), ultrasound power (400 W), and liquid to solid ratio (22 ml:g). The experimental yield of GTPS (4.65 +/- 0.29%) obtained under these conditions were well agreed with the value predicted by the model. Without applying ultrasound, while the other extraction conditions were the same (CE), the extraction yield was lower (1.83 +/- 0.04%). Fourier transform-infrared spectroscopy (FT-IR) was used for the identification of functional groups present in GTPS and gel permeation chromatography was used to determine the molecular weight distribution of samples. The molecular weight of GTPS obtained by UE was lower, probably some polysaccharide degradations occurred due to ultrasound application. The IR spectrum of GTPS obtained by UE and CE had very similar absorption bands typical for the polysaccharides. Although ultrasound application significantly increased the yield compared to classical hot water extraction, it reduced antioxidant and a-glucosidase inhibitory activity of GTPS.