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Spectrofluorometric study on surface hydrophobicity of bovine casein micelles in suspension and during enzymic coagulation

Gatti, C.A., Risso, P.H., Pires, M.S.
Journal of agricultural and food chemistry 1995 v.43 no.9 pp. 2339-2344
cattle, casein, hydrophobicity, fluorescent dyes, coagulation, rennet, fluorescence emission spectroscopy, inorganic ions, calcium, micelles
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 micromolar-1 at 25 degrees C) for ANS (marker of anionic nature), probably involving hydrophobic and electrostatic components, and a strong one (K = 36.0 micromolar-1 at 25 degrees 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 Ca(2+) 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.