Effect of combined treatments of hydrolysis and succinylation on the solubility and emulsion stability of rennet casein and micellar casein

Casein is the most abundant protein in milk with good emulsifying properties and bioavailability. However, the tight micellar structure of casein results in poor solubility. In the case of soft solid materials such as processed cheese, imitation cheese, yoghurt and protein-based oil-in-water emulsions, poor solubility directly affects the homogeneity and stability of the texture structure of such products, leading to a poor user experience. In this study, two protein modification techniques, hydrolysis and succinylation, were combined to improve the solubility of casein and the stability of its emulsions. The individual and combined effects of enzymatic hydrolysis and succinylation modification approaches on the stability of rennet casein (RC) and micellar casein (MC) emulsions were further explored. After double-treatment of casein with enzymatic hydrolysis and succinylation, the solubility of RC and MC was up to about 95 %, which was superior to that of single-treatment. Fourier transform infrared spectroscopy showed that the characteristic wave signals of the double-treated samples were located between the two single-treated samples, and that there may be an opposite effect between the two modifications. After 21 days of storage, the emulsions prepared from double-treated caseins still remained stable. The salt ionic stability and freeze-thaw stability were significantly improved, and the physical stability of MC was increased by nearly three times. The results explained the effects of enzymatic hydrolysis and succinylation on the functional properties of casein, provided a reference for the development of food systems based on oil-in-water emulsions, and offered a new idea for the wide application of succinylated casein.