The effects of composition and some processing treatments on the rennet coagulation properties of milk

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 rime, t, from rennet addition; G' was taken as a measure of curd firmness. The Scott-Blair rime dependency model was fitted to the experimental G'/t curves for the determination of the following coagulation parameters: gel time, maximum curd firming rate, the set-to-cut time at 20 Pa tie, 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, P-1, was varied from 0 to 25 MPa and the second stage pressure, P-2, was held constant at 5 MPa. 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.