Conjugation of α-, β-, and κ-Caseins with Propylene Glycol Alginate Using a Transacylation Reaction as Novel Emulsifiers

Protein–polysaccharide conjugates usually have better emulsifying properties than their constituent biopolymers. In this study, casein–alginate conjugates were prepared using a transacylation reaction between different types of caseins and propylene glycol alginate (PGA) at pH 11.0. The purified conjugates had the combined molecular structure of casein and hydrolyzed PGA (alginate), according to Fourier-transform infrared spectroscopy, and α-, β-, and κ-casein–alginate conjugates had 39.17%, 37.78%, and 23.14% protein content, respectively. Emulsions were prepared with all the conjugates at a surfactant-to-oil ratio of 1:100 (w: (v), which is much lower than emulsions stabilized by Maillard-type protein–polysaccharide conjugates. The emulsions had high stability during storage and at wide pH (3.0–11.0) and ionic strength (0–450 mM) ranges. The interfacial tension of conjugate dispersions and therefore the droplet size were dependent on the protein content, not the casein type, while the polysaccharide moiety was critical to the emulsion stability. The present findings suggest a transacylation reaction can be used to prepare protein–alginate conjugates with novel emulsifying properties.