3.3.1. Rennet clotting time (RCT)
RCT was highly positively correlated with pH, protein, casein
and ES, but negatively correlated with Ca2+, fat and total phosphorus
content (Table 5). The strong positive correlation with pH
would be expected from earlier studies. Lucey (2002) showed that
the activity of rennet increased with decrease of pH, while other
studies have demonstrated that the aggregation of destabilised micelles
increases at lower pH due to solubilisation of micellar calcium
phosphate, a decrease in net surface charge and
dissociation of casein from the micelles (Fox & McSweeney,
2003). The Ca2+ was negatively correlated with RCT, in agreement
with Tsioulpas (2005) who showed that increasing Ca2+ would
accelerate milk coagulation because Ca2+ in the serum phase
reduces the negative charge on the surface of casein micelles,
weakening repulsion and accelerating their coagulation. Fox and
McSweeney (2003) suggested that initiating coagulation required
a minimum concentration of Ca2+ of about 1.5 mM.
Jõudu, Henno, Kaart, Püssa, and Kärt (2008) showed that an
increase in milk protein, casein, casein fractions, and the casein
number decreased the rennet coagulation time of milk which
was not in agreement with the current study. The weak negative
correlation between RCT and fat content observed in Table 5 is
probably secondary to the correlations of fat with pH and protein
(Table 4).