Comparative study of the action of anionic and non-ionic hydrophobic fluorescent markers on the enzymic coagulation of heated bovine casein micelles

The action of anionic (ANS, TNS) and non-ionic (NR) hydrophobic fluorescent markers on the enzymic coagulation of heated bovine casein micelles (HCM) was studied by spectrophotometric and spectrofluorimetric techniques. In the presence of the markers a conformational change following the κ-casein proteolysis was observed. Both ANS and TNS were partially released during this process, showing that a fraction of the markers was bound in the neighborhood of the site of action of chymosin. These anionic markers produced a decrease of the aggregation rate, probably by increase of the negative charge of the aggregating particles. Turbidity measurements suggested that the presence of ANS and TNS introduced changes in the aggregation mechanism, giving place to less compact aggregates. On the other hand, the binding of NR is not affected by the κ-casein proteolysis and tended to produce an increment of the aggregation rate, especially at high marker concentrations. Although turbidity measurements showed differences in the aggregation mechanism, the fractal dimension of the aggregates did not change by NR action. The effect of this non-ionic marker could be produced through an increment of the surface hydrophobicity and the number of regions able to give interparticle links, thereby increasing the number of effective collisions.