Formation and structural properties of acid-induced casein–agar double networks: Role of gelation sequence.

Agar is widely used in yogurt manufacture to improve the stability of milk gels. However, the formation and structural properties of acid-induced casein–agar gel have not been elucidated. In this study, different acidification temperatures were used to evaluate the evolution of acid-induced casein–agar double networks and their microstructure and penetration properties. Agar (0.25%, w/w) was mixed with a micellar casein dispersion (3%, w/w), acidified with glucono-δ-lactone at 30, 37, 42, or 45 °C, respectively, and then cooled to 10 °C. Compared with casein gel alone, the casein–agar gel had a higher firmness and water holding capacity (WHC). Moreover, these acid-induced casein–agar double networks formed different structures depending on the acidification temperature. When acidified below the agar transition temperature (at 30 or 37 °C), the agar formed a weak gel prior to casein network formation that hindered the movement of casein micelles. Therefore, the casein–agar mixtures formed double networks during the acidification stage with relatively low elasticity indexes (EIs). In contrast, casein–agar mixtures acidified above the agar transition temperature (at 42 or 45 °C) formed double networks during the cooling stage that exhibited higher EIs. Cryo-scanning electron microscopy showed denser networks in casein–agar gels acidified above the agar transition temperature. Furthermore, casein–agar gels acidified above the agar transition temperature showed a significantly higher firmness and WHC than those acidified below. Overall, acidification temperature significantly affected the gelation sequence and, thereby, the formation and structural properties of the acid-induced casein–agar double network. [ABSTRACT FROM AUTHOR]