Surface hydrophobicity of whey protein concentrate (WPC) under heated (85 degrees C for 5, 10, 20, 30, 40, and 60 min) and unheated conditions was measured using cis-parinaric acid (CPA), 1-anilino-8-naphthalenesulfonate (ANS), and a fluorescence quenching method using acrylamide as a quencher. This last method evaluates the degree of exposure of tryptophanyl residues in proteins to the solvent. The initial slope of Stern-Volmer plots, K(app), was used as an index of protein hydrophobicity. Surface hydrophobicity of WPC exhibited good relation with surface functional properties such as emulsifying and foaming. Analysis of the data obtained in this work showed that the fluorescence quenching method gave results similar to those obtained using CPA and ANS. Therefore, this simple technique is satisfactory in effectively obtaining information about the hydrophobicity of whey proteins.

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The aim of the present study was to develop and characterize edible films produced from whey protein concentrate (WPC) and plasticized with different contents of glycerol (Gly) and/or trehalose (Tre) in order to evaluate new edible film formulations for their potential use in food packaging applications. Additionally, potential changes in the film mechanical properties during storage at ambient and freezing conditions were considered. Moisture content, solubility, thickness, transparency, microstructure, colour parameters, and mechanical properties were assessed. The films incorporated with Tre were more insoluble in water than WPC/Gly films, being more suitable for food applications. WPC/Gly and WPC/Tre films were clear enough to be used as see-through packaging. However, when Tre was included into WPC/Gly film formulations, film opacity increased. Scanning electron microscope (SEM) images suggested that this phenomenon may be related to the growth of Tre crystals in the film matrix. Moreover, when Tre concentration increased in the WPC/Gly matrix, film surface was more heterogeneous. Interestingly, the presence of Tre in WPC-based films was effective in preventing Maillard reaction after heating. WPC/Tre films were the most rigid but the least stable for storage, resulting more susceptible to rupture and cracking. Only WPC/Gly and WPC/Gly-Tre 8% films were rather flexible, manageable, and stable up to 90 days of storage under ambient and freezing conditions. These findings can be used to better design applications of edible films containing plasticizers that may crystallize over time in order to optimize film formulation in a rational manner towards their eventual application as food packaging.

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Functional properties of whey protein concentrates (WPC) are primarily dependent on the degree of denaturation of beta-lactoglobulin (beta-LG), the major globular whey protein. Irreversible modifications in the tertiary structure and association state of beta-LG after heat treatment were studied by partition in aqueous two-phase systems and fluorescence quenching. Partitioning of preheated beta-LG in two-phase systems containing 5% (w/w) poly(ethylene glycol) and 7% (w/w) dextran, between pH 6.0 and7.0, are appropriately related with the intensity of heat treatment. An increase in the partition coefficient of beta-LG was observed with increasing temperature of heat treatment. On the other hand, fluorescence quenching of beta-LG by acrylamide was used to study the conformational flexibility of the protein at pH values between 4. 0 and 9.0. The values of bimolecular quenching rate constant (k(q)) obtained showed that beta-LG appears to be more flexible at high pH values, while at low pH the protein assumes a more compact form. The efficiency of acrylamide quenching on preheated beta-LG was substantially more pronounced than for the untreated protein. This difference can be ascribed to the presence of unfolded monomers and aggregates of denatured molecules formed after heat treatment, whose tryptophanyl residues are more exposed to the solvent. In conclusion, the results suggest that partition studies in aqueous two-phase systems and fluorescence quenching are very useful tools to detect changes in conformation and aggregation of beta-LG induced by heat treatment.