Effect of UV-Radiation on the Packaging-Related Properties of Whey Protein Isolate Based Films and Coatings

Abstract

The high oxygen barrier properties of whey protein based films and coatings means these materials are of great interest to the food and packaging industry. However, these materials have poor mechanical properties such as the tensile strength, Young's modulus and elongation at break. Up until now, the influence of ultraviolet (UV) radiation on whey protein films has not been reported in the literature. This study thus investigates the influence of UV-radiation on the properties of whey protein based films. UV-irradiated films showed increased tensile strength and a yellowing that was dependent on the radiation time. After irradiation, the films showed no significant change in the barrier properties, Young's modulus or elongation at break. In addition, a protein solubility study was undertaken to characterize and quantify changes in structure-property relationships. The significant decrease in protein solubility in buffer systems which break disulfide and non-covalent bonds indicates that additional molecular interactions arise with increasing radiation dose. This study provides new data for researchers and material developers to tailor the characteristics of whey protein based films according to their intended application and processing. and scientists are working hard to enhance the physical and barrier properties of protein films. According to Wihodo and Moraru, 1 the influence of film formulation and preparation conditions, plasticization, pH alteration, molecular weight adaption and lipid addition on the functional properties of protein films have been extensively investigated and discussed. To modify the properties of proteins, cross-linking techniques induced by heat, chemicals, enzymes and irradiation and the use of nanocomposites have been reported. 1,[9][10][11][12] The formation of covalent interactions such as intermolecular disulfide bonds (heat treatment), ε-(γ-glutamyl)-lysyl bonds (transglutaminase cross-linking) and covalent cross-links between aromatic acids (Ultraviolet (UV) radiation) lead to stabilization of the polymer network and are therefore essential for the mechanical and barrier properties. 13 Recently completed research projects showed that existing whey protein formulations are suitable as barrier layers. [8][9][10]14,15 The influence of UV-radiation on the functional properties and the structureproperty relationships now need to be investigated.According to literature, some research groups 1,7,16-20 have already worked on UV-irradiated protein-based films. Ustunol and Mert 19 irradiated whey protein solution for 3 h with UV light and described increased tensile strength of the cast films but there was no significant difference in the oxygen and water vapour barriers. However, no publications can be found on the influence of UV-irradiation on cast whey protein films.The objective of this study was to investigate if and to what extent the whey protein based barrier layers are influenced by UV irradiation, namely the irradiation time [min] and radiation dose [J/m 2 ]. Of particular inter...

UV Radiation Induced Cross-Linking of Whey Protein Isolate-Based Films

“…Ustunol and Mert [24] stated that UV radiation of the film-forming solution has no significant effect on Young's modulus. Young's modulus of the UV-treated cast films has been studied by Schmid et al [12], who can partly confirm this effect. This means that a significant increase could be observed when measuring in the cross direction, but not in machine direction.…”

“…The distance between the sample and the UV source was set to 70 mm. The experimental setup of the ultraviolet radiation was installed in a metal housing to protect the operator from the radiation following the configuration by Schmid et al [12]. Films were radiated on both sides.…”

“…It is stated that UV radiation significantly increases the crosslinking between the protein fractions at a radiation time of 10 min. Schmid et al [12] concluded that a large proportion of the cross-linkages are already linked during the first 10 min of radiation. It is assumed that the required energy for the formation of radicals is already present at a dose of about 3 J/cm 2 , resulting in protein cross-linking by a chain-growing process, which was qualitatively confirmed in this study with the methods described by Schmid et al [12].…”

“…Then, the proteins are not able to form cross-links anymore and the free volume of the films increases. In addition, the films are about twice as thick as the coatings described by Schmid et al [12]. The films were radiated on both sides to compensate for the higher layer thickness.…”

“…Double bonds, including aromatic rings of amino acids like tyrosine or phenylalanine, can absorb UV radiation, followed by the formation of free radicals and intermolecular covalent bonds [5,6]. Films derived from soy protein, corn zein, wheat gluten, peanut protein, fish gelatin, egg albumin, sodium caseinate, and whey protein have been treated with UV radiation [5][6][7][8][9][10][11][12], mainly stating various changes in final film properties. However, there is very little information on the structural changes such as UV-light-induced cross-linking mechanisms in protein films.…”