Stability of the mechanical properties of edible films based on whey protein isolate during storage at different relative humidity

Formulations for whey protein isolate (WPI)-based films include low molecular weight plasticizers, which implies certain degree of instability of films properties due to plasticizer migration. The aim of this work was to study the effect of storage time on the mechanical properties of WPI films plasticized with two low molecular weight polyols: glycerol (Gly) and sorbitol (Sor). Films were stored inside cabins at 50% or 75% relative humidity (RH) and at room temperature. Mechanical properties and moisture content were measured at regular intervals for 30 weeks. The effect of plasticizer type and content and RH on mechanical properties right after equilibrium (1 week) was also included in this study. After equilibrium, Gly was found to be a more efficient plasticizer than Sor. Films plasticized with Gly were found to be more flexible and weaker and to have higher moisture content than films with the same amount of Sor. There was a saturation value for plasticizer content (40% for Gly and 50% for Sor), above which the film became weaker, but not more flexible. Storage time did not affect either the appearance or the mechanical properties of films plasticized with Gly. However, films plasticized with Sor became harder and less flexible with time likely due to Sor crystallization.