Plasticizer effect on the glass transition temperature of beta-lactoglobulin films

The effect of plasticizer on beta-lactoglobulin (beta-Lg) film glass transition temperature (T-g) was investigated to elucidate the plasticization mechanism. The T-g of film containing polyethylene glycol 400 (PEG 400) as a plasticizer could not be observed because an ice-melting peak occurred during heating. Films plasticized by propylene glycol (PG), glycerol (Gly), and PEG 200 possessed similar film T-g values, related to the plasticizer water-binding properties. However, sorbitol (Sor) and sucrose (Suc) produced higher film T-g values due to their larger M-W and solid-state plasticizer behavior. Lubrication of beta-Lg molecules by plasticizer domains external to the beta-Lg is suggested as a film plasticization mechanism because no shift in film T-g was observed with increasing plasticizer content. The size, shape, and physical state of plasticizer were the key factors affecting film properties, consistent with previous results on mechanical and oxygen permeability (OP) properties.