Water Relationships of Whey Permeate Powders

Water relationships are developed for whey protein permeate powders based on an analysis of the composition, structural organization, and thermal properties of three commercial whey permeate powders (WPPs). On the basis of microscopic analysis, a two-phase model of permeate powders with two types of particle morphologies is proposed: (1) particles consisting of small lactose crystallites dispersed in a continuous amorphous phase consisting of lactose and all non-lactose constituents and (2) particles consisting of one or a few large lactose crystals with a thin, discontinuous amorphous surface layer. The amorphous fraction of the WPPs is characterized by a low glass transition temperature even at very low water activities and is furthermore strongly plasticized by water. The amorphous fraction of permeate powders was determined from the change in heat capacity associated with the glass transition. The fraction of the amorphous phase and its physical properties were found to vary significantly between the three powders and were influenced by both composition and processing. The results of this study provide a deeper understanding of the physical structure, water relations, and thermal properties of WPPs. PRACTICAL APPLICATIONS: We introduce a novel structural model for whey permeate powders (WPPs) consisting of a crystalline 𝛼-lactose monohydrate phase and an amorphous phase that is composed of all other permeate constituents as well as some residual lactose. The amorphous phase of the WPPs, which is present at the surface of the powder particles, is characterized by a low glass transition temperature (T_g). The low T_g explains the instability (caking and browning) of commercial WPPs. Our studies confirm the high sensitivity of WPPs to water and the limited potential to stabilize commercial WPPs by further drying: even when fully dry, which is difficult to attain in industrial drying, the amorphous fraction of WPPs is still in the rubbery state at ambient temperature and WPPs remain thus prone to caking and color formation. Thus, stabilizing the WPPs by drying them to lower water contents is expected to lead to only very limited improvement of the powders' stability due to the inherently low of the T_g of the amorphous phase in WPPs. We argue that the way to effectively improve the stability of whey permeate powders is to increase the T_g of the amorphous fraction powders.