Modeling moisture sorption isotherms of milk powders at ambient and elevated temperatures using the dynamic dewpoint isotherm (DDI) method

The water activity of milk powders is a critical parameter for predicting quality and safety, but some retailers in the supply chain may be limited to measuring moisture content, which can be easier and more affordable. Moisture sorption isotherms relate moisture content to the corresponding water activity. In this study, moisture adsorption and desorption isotherms were determined for nonfat dry milk (NFDM) and milk protein concentrate (MPC-85) powder samples at ambient and elevated temperatures via the modernized dynamic dewpoint isotherm (DDI) method. Previously cited models (with coefficients determined through linear regression) were also fit to the data for predictive applications of determining shelf life and the potential changes to quality that thermal treatments or elevated storage temperatures could cause. The adsorption isotherms of NFDM were type II, whereas MPC-85 adsorption isotherms were type III. Both NFDM and MPC-85 exhibited type III desorption isotherms, likely due to crystalline components. Temperature significantly affected the adsorption and desorption isotherms of both NFDM and MPC-85. Hysteresis was observed between the adsorption and desorption isotherms for both powders at all temperatures, but the magnitude was greater for MPC-85. The modified Oswin, modified Chung-Pfost, and Guggenheim, Anderson, de Boer models best described NFDM and MPC-85 adsorption and desorption to different degrees, highlighting that treatment and storage of the powders are influenced by both composition and temperature. These findings provide tools for prediction and assessment of quality of 2 vastly different milk powders, which inform handling, processing, and storing milk powders with different protein and lactose concentrations.