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 kinetics of aggregation of para-casein micelles (pCM) and of heated para-casein micelles (HpCM) were studied and compared by estimating the initial rate of aggregation from the initial rate of turbidity increase. Aggregation of both pCM and HpCM suspensions appeared as cases of slow Brownian flocculation, with a steric contribution to the suspension's stability which could explain their aggregation behavior, including the presence of the Berridge effect at low temperatures. Glycerol addition to the medium, besides the reduction in aggregation rate by increase of medium viscosity, produced an initial aggregation rate increase, probably by interaction with the proteins of the micelle external layer. The differences observed between pCM and HpCM aggregation rate could be related to the changes introduced by high heating in the surface structure of the micelles. Keywords: Heated casein micelles; steric stabilization; rennet coagulation

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