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 interactions of 1,8-anilinonaphthalenesulfonate (ANS) and Nile Red (NR) with bovine casein micelles (CM) were studied by fluorescence spectroscopy. Both fluorescent hydrophobic markers showed blue shifts of their fluorescence emission picks and fluorescence intensity enhancement, indicating their location in low-polarity regions of CM. Studies at two temperatures showed a weak interaction of high binding capacity (K = 0.031 pM-l at 25 "C) for ANS (marker of anionic nature), probably involving hydrophobic and electrostatic components, and a strong one (K = 36.0 pM-l at 25 "C) for NR (a noncharged molecule), clearly hydrophobic and of lower binding capacity.Comparison with the binding observed on CM disassembled in media without added Ca2+ pointed to the possibility that the markers permeate the porous CM structure acceding to casein molecules located into the micelles. Both markers inhibited CM enzymic coagulation, possibly by occupation of hydrophobic regions on renneted CM, thereby lowering the effectiveness of their collisions. The action of rennet in the first step of coagulation produced a decrease of about 10% of the fluorescence intensity of both bound markers, showing that a fraction of them could be located into the outer hydrophilic stabilizing layer of CM.

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The interaction of alkyl sulfonate ligands (AL) with bovine beta-lactoglobulin AB was measured using Trp fluorescence enhancement. One binding site per protein molecule was observed. The location of this site was related with the dimer formation and could be coincident with the fatty acids and SDS binding site. The apparent binding constants for AL were in the range of 10(-)(6) M, at pH 6.8. At pH 3.0 no binding was observed by this fluorescence method. The strength of the interaction was decreasing in the following way: AL16 > AL12 > AL14 >> AL10. Other sites on the monomer were evidenced by the protective action of the AL toward the urea unfolding of the protein.

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