Effect of lipid-mediated moisture status on casein characteristic in two types of milk powders during accelerated storage

This study aims to investigate the relationship between casein oxidation and moisture status in the presence of milk fat to clarify the role of fat on water in milk powder. The results showed that at the same water activity (a_w), the faster oxidation of whole milk powder (WMP) is attributed to the high proportion of free water and immobilized water in them, indicating that the adsorption of milk fat mainly depended on multilayer water and the entrapped water. In the diffusion-limited reaction, changes in water state affected more than radicals on casein oxidation. Glass transition temperature (Tg) was also affected by the water state of lipid adsorption. At low a_w (<0.4), the water content of WMP and skim milk powder (SMP) was similar, but the Tg of WMP was significantly lower than SMP, especially when a_w was 0.33, its Tg was <0. It suggested that water state rather than water content was the main factor affecting Tg. Especially, because the mobile water absorbed by milk fat in WMP reduced Tg, which further led to the acceleration of lactose crystallization, casein oxidative aggregation, and Maillard reaction.