Changes in physicochemical and surface characteristics in milk protein powders during storage

Physicochemical characteristics of whey protein concentrate (WPC35, 35% protein), whey protein isolate, sodium, and calcium caseinate powders were studied by storing them at 25, 35, and 45 °C and 11, 44, and 85% relative humidity (RH) for 21 days. Monolayer moisture content (M_o) in these powders increased with increasing protein content. The glass transition temperature (T_g) of all of these powders decreased with increasing RH at all storage temperatures. Lactose crystallization occurred in WPC35 at 85% RH at all temperatures and 44% RH and 45 °C, and this caused caking. Caking was also observed in whey protein concentrate and isolate, and sodium and calcium caseinate powders at 85% RH and 45 °C, despite having low (0.2–1.5%, w/w) lactose content. Surface fat content was higher in powders stored at higher temperatures and RH. All powders developed browning, which was especially pronounced at 85% RH. These findings help better understand the temperature and humidity dependent behavior of whey protein and casein powders during storage.