Plasticizer effect on mechanical properties of β-lactoglobulin films Original Research Article

The mechanical properties (elastic modulus, EM; tensile strength, TS and % elongation, %E) of β-lactoglobulin (β-Lg) films plasticized with different plasticizers were determined. Six plasticizer types were studied over a range of concentrations. Propylene-glycol-plasticized β-Lg films were the most brittle, with mechanical properties independent of plasticizer content. Films with other plasticizers studied (glycerol, Gly; sorbitol, Sor; polyethylene glycol, PEG 200 and PEG 400 and sucrose, Suc) exhibited negative exponential dependence on plasticizer concentration for EM and TS, while they exhibited linear dependence on plasticizer concentration for %E. The EM and TS data for each plasticizer were fitted with an exponential model, while %E data were fitted with a linear model to quantify the plasticizer effect. The EM0, TS0 and %E0 of β-Lg films without plasticizer determined from the fitted equations were 1500, 37.28 MPa and 0, respectively. The kEM,kTS and kE values determined from the fitted EM, TS and %E data, respectively, reflect the efficiency of plasticizers. The kEM,kTS and kE values indicate that plasticizer efficiency generally decreased in the order Gly, PEG 200, PEG 400, Sor and Suc, on the bases of mole plasticizer-oxygen-atom/mole β-Lg and mass plasticizer/mass β-Lg. These results reflect the effect of plasticizer composition, size and shape. The kEM,kTS and kE value order was reversed when the basis was changed to mole plasticizer/mole β-Lg. The latter results clearly reflect the effect of plasticizer number of O atoms.