Abstract
Influence of water sorption and phase transitions requires clarification to understand plasticizer mechanisms on the physico-chemical properties of biodegradable and edible films. The effects of plasticizers as glycerol, xylitol, sorbitol, polyethylene glycol 400 (PEG 400) and oleic acid were investigated on water sorption, phase transitions and properties in whey protein concentrate and carboxy methyl cellulose blend films (WPC-CMC). Stability after aging at different humidity (50% RH and 75% RH for 30 days) was determined. Sorption isotherms of glycerol-plasticized films showed the highest water affinity and binding sites led to the lowest glass transition temperature (Tg). Xylitol showed similar water sorption behavior to sorbitol but higher plasticization effect. An increased C-atom of glycerol, xylitol and sorbitol gave identical infrared spectra. Conversely, oleic acid modified protein secondary structures contributed to the lowest water sorption and highest Tg as indicated by amide I and amide II bands in the infrared spectra. Highly hydrophilic plasticizers as glycerol, xylitol and sorbitol showed less sensitivity to water-induced Tg reduction. Water sorption greatly improved plasticization effects in PEG 400 and oleic acid which contained non-polar components. Aging accelerated the Maillard reaction in WPC-CMC and led to physico-chemical changes and destabilized films. Light transmission decreased concurrently with increased brown pigments that improved the water barrier and stiffness. Higher humidity accelerated molecular mobility and facilitated physico-chemical changes of the polymer blends. Oleic acid showed the least stability during aging, while sorbitol greatly enhanced water barrier properties and stability of the blend films. Molecular structures of plasticizer greatly influenced water sorption, modified molecular orientation of biopolymers which subsequently impacted properties and stability of the films.
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