The effects of various processing parameters on the rennet coagulation properties of milk were assessed. Using low amplitude oscillation rheometry, the coagulation properties were monitored by measurement of the elastic shear modulus, G', as a function of time, t, from rennet addition; Gapos; was taken as a measure of curd firmness. The Scott‐Blair time dependency model was fitted to the experimental G' It curves for the determination of the following coagulation parameters: gel time, maximum curd firming rate, the set‐to‐cut time at 20 Pa (ie, time to reach 20 Pa) and the curd firmness after a renneting time of 2400 s. The renneting properties were enhanced by increasing the levels of milk protein and fat in the ranges 0.3–7.0% (w/w) and 0.1–10% (w/w) respectively and by two stage homogenization pressure where the first stage pressure, P1, was varied from 0 to 25 MPa and the second stage pressure, P2, was held constant at 5 MP a. The influence of these parameters, within the range investigated, in complementing the gel forming properties decreased in the following order: protein > fat > homogenization pressure. In contrast, the coagulation properties of milk were impaired by high heat treatment, the addition of a commercial microparticulated whey protein based fat substitute and by partial replacement of protein with fat.
Heating impaired the rennet coagulation properties of milk which deteriorated further during storage, i.e. rennet hysteresis occurred. Acidification to pH values ≤ 6.2 or addition of low concentrations of CaCl2 greatly improved the rennet coagulation properties of heated milk. Acidification of heated milk to pH values < 5.5 followed by neutralization to pH 6.6 to produce reformed micelles, resulted in greatly improved rennet coagulation properties except for severely heated milks (120°C for 10 min) which were not coagulable even after acidification/neutralization. Acidification of heated milk to pH values < 5.5 and storage at the low pH for 24 h before neutralization resulted in a further improvement in the rennet coagulation properties. Dialysis of heated milk that had been acidified and reneutralized against an excess of normal milk resulted in a dramatic deterioration of its rennet coagulability. Reheating milk that had been heated, acidified and reneutralized resulted in little change in RCT or gel firmness. Addition of heated milk to raw milk resulted in an increase in RCT of the latter and a reduction in gel firmness.
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