Packaging-related properties of protein- and chitosan-coated paper

Abstract

The mechanical and gas-barrier properties of paper and paperboard coated with chitosan-acetic acid salt (chitosan), whey protein isolate, whey protein concentrate and wheat gluten protein were studied. Paper sheets were solution-coated using a hand applicator. In addition, bi-layer composites of wheat gluten and paper or paperboard were produced by compression moulding, and the chitosan solution was also applied on paperboard using curtain coating. Young's modulus, fracture stress, fracture strain, tearing strength, air permeance and oxygen permeability were assessed. The mechanical and air permeance measurements of solution-coated paper showed that chitosan was the most effective coating on a coat weight basis. This was due to its high viscosity, which limited the degree of penetration into the paper. The proteins, however, also enhanced the strength and toughness of the paper. Compression-moulded wheat gluten/paper or paperboard, as well as curtaincoated chitosan paperboard laminates, showed oxygen barrier properties comparable to those of paper and paperboard coated with commercial barrier materials. None of the composites could be delaminated without fibre rupture, indicating good adhesion between the coatings and the substrates. INTRODUCTIONEnvironmental concerns, including the limitation of greenhouse gases and the presumably higher cost of petrochemicals in the future, have made renewable materials more interesting as a substitute for synthetic oxygen-barrier packaging materials, including ethylene-vinyl alcohol copolymers, poly(vinylidene chloride), polyesters and polyamides. Interesting renewable barrier materials include various proteins and polysaccharides, mainly because of their high content of hydrogen bonds.Even though oils are the group which has been most studied among renewable paints and coatings in non-food applications, 1,2 some studies have considered proteins as coatings on paper. Han and Krochta 3 studied the increase in gloss and the increase in oil resistance of paper coated with whey protein isolate (WPI). Chan and Krochta 4 reported a significant reduction in oxygen permeability (OP) for paperboard coated with denatured and undenatured WPI. Compared to their study, we have used a lower content of plasticizer and we have measured the mechanical properties. Trezza et al. 5 reported a reduction in the OP of paper coated with corn zein (a corn protein). The OP was, however, still significantly higher than that of most conventional polymers. Other studies/patents of protein coatings on paper have focused mostly on grease resistance and/or water permeability, 6-10 or on improving the strength of paper.11 Studies of chitosan coatings on paper, paperboard and cellophane have been reported. [12][13][14][15][16][17] These reports, however, focus mainly on antimicrobial or medical applications, although barrier properties are also occasionally reported. Several, mostly Japanese, patents on chitosan-cellulose blends exist.Many studies have also dealt with whey and other proteins as oxygen-barrier...

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