Purification, characterization, and emulsification stability of high- and low-molecular-weight fractions of polysaccharide conjugates extracted from green tea

Tea polysaccharide conjugates (TPCs) are macromolecular polymers comprising polypeptides covalently bound to polysaccharides with a wide range of molecular weights that are found in tea leaves. We previously found that TPCs had emulsifying properties, but the difference in the emulsification stabilities between the high- and low-molecular-weight TPCs was unknown. In this study, the total polysaccharide conjugates (gTPC-T) with average molecular weight of 258,900 Da were extracted from low-grade green tea, from which two separate fractions with average molecular weights of 989,600 Da (22.43%) and 47,780 Da (77.57%), which were referred to as gTPC-1 and gTPC-2, respectively, were separated using an ultrafiltration membrane. The polypeptide chains of the three TPCs (gTPC-T, gTPC-1, and gTPC-2) contained 17 out of the 20 canonical amino acids. In addition, gTPC-1 had the highest acidic monosaccharide content. Circular dichroism (CD) an analysis showed that the secondary structures of the protein moieties of the three TPCs were dominated by α-helices. The nano-differential scanning calorimetry (Nano-DSC) results showed that the transition midpoints (Tm) of the three samples were 86.69, 59.53, and 68.59 °C, respectively. The zeta potential of gTPC-T at weakly acidic or neutral pH (4.0–7.0) was the highest, followed by gTPC-1 and then gTPC-2. Of all three gTPCs, gTPC-1 had the smallest interfacial tension, the highest interfacial pressure, and the best interfacial activity. In addition, gTPC-T slightly better-stabilized the MCT oil-in-water emulsions than gTPC-1, while the emulsification stability of gTPC-2 was the worst. This trend was likely attributed to the synergistic effect between gTPC-1 and gTPC-2.