Abstract: The effect of different durations (15, 20, 25 and 30 min) of ultrasound treatment at a frequency of 20 kHz and a power of 800 W on the particles size, zeta potential, surface hydrophobicity and secondary structures of goat milk proteins was evaluated. Furthermore, the effect on rheological properties and microstructure of rennet-induced goat milk gel was studied by rheological measurement and scanning electron microscopy (SEM). The results showed that ultrasonic treatments reduced the particles size and altered the secondary structures of milk proteins significantly, increasing the percentages of β-sheet and random coil and decreasing the percentages of α-helix and β-turn. The β-sheet structure showed a positive correlation (r = 0.74) with the surface hydrophobicity. Prolonged (more than 20 min) sonication resulted in a significant (P < 0.05) increase in the maximum storage modulus (G’max) when compared to unsonicated raw goat milk and pasteurized goat milk, and the rennet coagulation time decreased after 25, 30 min of ultrasonic treatment. SEM observation revealed that the gel microstructure of the sonicated samples was more interconnected. To sum up, ultrasonic treatment could effectively improve the quality of rennet-induced goat milk gel and have potential application value in industrial production.

Key words: goat milk; ultrasonic treatments; rennet-induced milk gel; rheological properties; microstructure