Characterization of an unexpected bioemulsifier from spent yeast obtained from Thai traditional liquor distillation

Crude biopolymer was extracted from spent yeast, lyophilized and fractionated on Sephadex G-100 to yield two fractions coded as fraction I and II. Fraction I was composed of both carbohydrates and proteins, showing emulsifying activity whereas fraction II consisted of only proteins and possessed no activity. Hence composition and chemical characterization of the purified fraction I (bioemulsifier) was analyzed using various analytical techniques. It was found that the sample contained 96% of carbohydrates consisting mainly of glucose with minor quantities of mannose, and 4% of protein built from 17 amino acids with the highest content of serine followed by alanine. The results also indicated that the sample was protein-bound glucan with the average molecular weight of 1.93 × 105 Da. The functional groups and primary structure of the sample were revealed by FT-IR and NMR techniques. The data demonstrate that the sample comprises a mixture of (1→4)-α- and (1→3)-β-d-glucans bound with protein. Enzymatic hydrolyses using α-amylase and endo 1,3-β-d-glucanase confirmed the presence of both glucans. Therefore, this bioemulsifier was identified as glucan–protein complex which is different from usual mannoprotein emulsifier derived from yeasts.