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When cross-linked by heating or by gamma-irradiation and entrapped in cellulose, whey proteins can generate insoluble biofilms with good mechanical properties and high resistance to attack by proteolytic enzymes. Interchain cross-linking of proteins generated an increase in the puncture strength, and a decrease in water vapor permeability. Gelatin was added in film formulation as a stabilizer to improve the puncture strength and film appearance. The structure of the biofilms was also analyzed. SDS-PAGE revealed that heating and gamma-irradiation produce an increase of the molecular weight of the cross-linked protein. Size exclusion chromatography showed a molecular mass of 40 kDa for un-cross-linked whey proteins, whereas for the soluble fractions of the cross-linked proteins, molecular distributions were between 600 and 3800 kDa for the heated proteins and between 1000 and 2000 kDa for gamma-irradiated proteins. No major alteration of the structural conformation of the proteins was observed by FTIR for biofilms obtained after heat treatment, whereas gamma-irradiation induced some modifications in the protein structure. X-ray diffraction analysis suggests that cross-linking by gamma-irradiation seems to modify the conformation of proteins, which became more ordered and more stable.

Color analysis on apple and potato slices coated with calcium caseinate or whey protein solutions showed that the 2 coatings efficiently delayed browning by acting as oxygen barriers. The antioxidant properties of the films were realized using a model allowing the release of oxidative species by electrolysis of saline buffer. Whey proteins were a better antioxidant capacity than calcium caseinate. Furthermore, addition of carboxymethyl cellulose (CMC) to the formulations significantly improved their antioxidative power. Best scavenging of oxygen free radicals and reactive oxygen species was found for films based on whey proteins and CMC which inhibited by 75% the formation of colored compounds produced by the reaction of the oxidative species with N,N‐diethyl‐p‐phenylenediamine.

Sterilized biofilms based on soy protein isolate (SPI, S system) and a 1:1 mixture of SPI and whey protein isolate (WPI, SW system) were achieved through the formation of cross-links by means of gamma-irradiation combined with thermal treatments. The effect of the incorporation of carboxymethylcellulose (CMC) and poly(vinyl alcohol) was also examined. gamma-Irradiation combined with thermal treatment improved significantly the mechanical properties, namely, puncture strength and puncture deformation, for all types of films. Irradiated formulations that contain CMC behave more similarly as elastomers. CMC showed also significant improvements of the barrier properties, namely, water vapor permeability, for irradiated films of the S system and for non-irradiated films of the SW system.

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Biopolymeric coatings were prepared and applied onto paperboard to improve its water barrier property. To prepare whey protein isolate (WPI)/cellulose-based films, WPI and glycerol were dissolved in water with glutaraldehyde (cross-linking agent) and cellulose xanthate. The solution was cast, dried, and insolubilized by entrapment of WPI in regenerated cellulose. Films were combined with beeswax (BW) into a bilayer coating system and then applied onto paperboard by heating compression. Another coating solution consisting of poly(vinyl butyral) (PVB)/zein was prepared by dissolving poly(vinyl alcohol) (PVA) and zein in 70% ethanol with glutaraldehyde and butyraldehyde (functionalization agent). The PVB/zein solution was applied onto paperboard after BW was sprayed. The structure of the PVB/zein-based coatings was analyzed by Fourier transform infrared spectroscopy (FTIR). The water vapor barrier property of coated paperboards was evaluated by water vapor transmission rate (WVTR) measurements. From the FTIR spectra, PVA functionalization after cross-linking and efficient acetalization into PVB were confirmed. WPI/cellulose and PVB/zein coating treatments improved the water barrier properties of paperboard by decreasing the WVTR by 77-78%. Although the BW coating was more efficient (decrease of WVTR by 89%), bilayer coatings composed of BW and polymer coatings had a stronger barrier effect with a decrease of WVTR to 92-95%, hence approaching commercial attributes required to ensure water vapor barrier in paperboard-based food containers (10 g/m(2).day). These results suggest that surface coating by biodegradable polymers may be utilized for the manufacture of paperboard containers in industrial applications.