Effect of ultra-high pressure treatment on the characteristics of a tea polysaccharide conjugate aqueous solution

Previous studies have shown that ultra-high pressure (UHP) processing technology to extract, prepare, and sterilize tea beverages (tea infusion) significantly increases the extraction rate of tea polyphenols (catechins), free amino acids, caffeine, and other small molecules in green tea, besides sterilizing and inactivating residual oxidases, such as polyphenol oxidase and peroxidase. UHP avoids the deterioration of color and aroma, as highquality green tea infusions were obtained with nutritional value and flavor quality close to the original tea. However, the effect of UHP on polysaccharides in green tea infusions is unclear. The water-extracted tea polysaccharide conjugates (TPC-W) and alkali-extracted tea polysaccharide conjugates (TPC-A) from low-grade green tea were subjected to a UHP treatment of 100, 300, and 500 MPa for 5, 12, and 25 min. The circular dichroism (CD) spectra results showed that UHP transformed the secondary structure of the TPC-W and TPC-A from compact and ordered alpha-helices and beta-sheets to disordered beta-turns and random coils; one component of TPC-A was dissociated into two components. The absolute values of the zeta potential of TPC-W treated with 100, 300, and 500 MPa increased by 2.4-5.6, 5.3-8.0, and 5.5-9.1 mV, while those of TPC-A increased by 0.5-2.8, 3.5-5.5, and 4.8-6.9 mV, respectively. The stability of TPC-W and TPC-A aqueous solutions treated with 300 and 500 MPa was enhanced during storage at 5 degrees C. These findings will contribute to applying UHP for tea beverage processing and in-depth investigations on the physiochemical properties of TPC, particularly the protein moiety.