Antonio Puglielli with Rajesh K. Bund and Dr. Richard W. Hartel

UW-Madison Department of Food Science

In this study, we investigated the changes in moisture sorption and desorption in delactosed permeate (DLP) upon addition of four different proteins: whey protein isolate, sodium caseinate, soy protein isolate, and hydrolyzed soy protein, each in concentrations of 5, 10, 20, and 30 percent by mass. The samples were incubated at 0 or 100 percent relative humidity in desiccators using silica gel and distilled water, respectively. Measurements of mass and vapor pressure ratio were taken each day. The moisture content of the DLP-protein mixtures was reduced compared to the control during both sorption and desorption, with the reduction directly proportional to the protein content. Based on the assumption of ideality, a theoretical isotherm was calculated for each mixture using a mass balance of the isotherms for each pure component. Upon comparison with the observed isotherms, deviations from ideality, which increased with protein content, were observed. Most likely, the interactions between DLP and proteins replace those between each individual component and water, limiting the strength and number of interactions with water and reducing the overall hydration of the solid components, thus decreasing moisture content. Furthermore, the degree of interaction was lowest for hydrolyzed soy protein but approximately the same in the other three proteins, indicating that, for the proteins studied, molecular size and general protein structure are significant in determining the extent of interactions but identity is not. These findings can be applied to improve the drying materials with similar sorption and desorption properties to DLP.