Destabilization of casein micelles can occur on freezing. Crystallization of lactose as a-monohydrate binds up some unfrozen water. Hence, the amount of unfrozen water decreases and thus the Ca2+ concentration increases leading to destabilization of casein micelles. Also, since milk is supersaturated with respect to calcium phosphate, some precipitates on freezing:
3Ca2+ + 2H2PO4- → Ca3(PO4)2 + H+
Production of H+ leads to a drop in the pH of the unfrozen part from 6.7 to ~5.8, leading to further destabilization.
Destabilization can be avoided by:
· Reducing the rate of lactose crystallization (by using very low temperatures or increasing the viscosity of the product),
· Rapid freezing and agitation (this gives faster crystallization and less destabilization)
· Reduce lactose content (e.g., by ultrafiltration or lactose hydrolysis).