Water sorption isotherms and phase transitions of sodium caseinate–lipid films as affected by lipid interactions

The water content–water activity–glass transition temperature relationships for sodium caseinate films containing glycerol were analyzed in caseinate based films containing oleic acid (OA) or beeswax (BW). Changes in the lipid phase transitions were also analyzed. Glass transitions of caseinate matrices as a function of water activity were modified by the presence of glycerol that plasticizes at low aw values to a great extent, but which limits the water plasticization effect. Incorporation of oleic acid in the film also induced changes in the water plasticization effect, thus indicating that lipid interactions with polymeric matrix occur, which was also reflected in the lipid melting behavior. OA binding produced a weakening effect in the matrix. When the lipid was beeswax, no notable protein–lipid interactions were revealed from the phase transition analysis. So, different characteristics of film matrix fillers (active or inactive) can be deduced from phase transition analysis and allow us to explain the greatly different impact of these on the film properties.