Heat denaturing of whey proteins exposes free sulfhydryl groups which promote intermolecular disulfide bond formation, thus allowing formation of insoluble films. Whey protein isolate (WPI) and #beta#lactoglobulin films have similar water vapor permeability (WVP) and oxygen permeability (OP). Film properties are affected by protein chain-to-chain interactions, which in turn are affected by presence of low-molecular-weight plasticizers. Plasticizer content and relative humidity (RH) both have an exponential effect on film permeability. At film compositions where glycerol- and sorbitol-plasticized WPI films have equal mechanical properties, sorbitol-plasticized films have lower OP. Inhibition of intermolecular disulfide bond formation with sodium dodecy sulfate (SDS) increases WPI film solubility, extendibility and flexibility, while having little effect on WVP. Inhibition of sulfhydryl-disulfide interchange with N-ethyl maleimide reduces WPI film solubility and elongation, with little effect on other film mechanical properties or WVP. Reduction of disulfide bonds with cysteine has no effect on WVP of WPI films. Heat curing of glycerol-plasticized WPI films increases film strength, while decreasing film extendibility, flexibility and WVP. Overall, WPI film properties are affected both by the amount of intermolecular disulfide bond formation and by plasticizers competing for protein chain-tochain hydrogen bonding.
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