CONICET | Buscador de Institutos y Recursos Humanos

The aim of this study was to determine the effect of Ca2+, Na+, Zn2+ and Mg2+ on the milk coagulation process induced by the enzymatic pool P7 (EPP7), secreted by Bacillus sp. P7. The significance of the concentration of each cation on the time of coagulation (tc) and fractal dimension of clots obtained (Df) was analized. The turbidity (t) of casein micelle (CM) suspensions in the range of 450-650 nm were recorded. b parameter, which is related to the particle size, was obtained from the slopes of the linear graphs of log t vs. log time (t). Df, which estimate the degree of compactness of the aggregates formed, is the maximum b value achieved and tc was determined as the time at which b stabilizes after the sharply rising. The effect of these four cations on the EPP7 enzymatic activity and on the initial size (initial b) of the particles were also studied. The presence of Ca2+, Na+ or Zn2+ increased initial b values, which corresponds to the formation of larger CM. This makes sense since the shielding of the negative casein charges facilitate their aggregation. The variables tc and Df proved to be also dependent of Ca2+, Na+ and Zn2+. On the other hand, Mg2+ has not a significant effect on these variables (p>0.05). As the concentration of Ca2+ increased (up to 15 mM) the Df decreased; however, the increase in the concentration of Zn2+ (up to 0.25 mM) the Df increased. The concentration of Na+ (up to 100 mM) changes the effect of Zn2+ on the Df: at higher NaCl concentration, less is the effect of Zn2+ (the ionic strength is probably the responsible of this change, not the Na+ specifically). These three cations caused a decrease of tc. This finding confirms that the tc is not directly related to the enzymatic activity of EPP7, since Ca2+ and Mg2+ had a stimulatory effect on EPP7 activity, while Na+ and Zn2+ caused a decrease on its activity. In conclusion, the cations evaluated have a differential effect on the CM aggregation process.