Computerized rheometric method for studying rennet coagulation of milk

A dynamic viscometer coupled with a microcomputer was used to follow the changes of rheological properties of rennet gelifying milk during period of time up to 2000 s with automatic recording of time, stress and strain every 0.5 s.SCOTT-BLAIR and BURNETT(1958) and DOUILLARD (1973) mathematical models were used to calculate the gelification time (t gel), final gel rigidity (Goa) and the constant Tl representing the time between the gelification point and point when the relative rigidity Gt/Goa reaches 0.1.From analyzed experiments, it is shown that the relative reciprocal coagulation time (ratio : t gel at pH 6.6/t gel at pH x) depended linearely on pH of milk (for the pH range between 6.36 and 6.75) and on amount of added calcium.Coagulation time and constant (Tl) evaluated from the results of phase angle changes were respectively 7 % shorter and 4 times smaller than the ones evaluated from the apparent viscosity evolution.The real part (u') of complex viscosity was changing during coagulation process from 100 to 104 mPas, while storage (G') modulus was increasing from 10-2 to 104 mPa.Relaxation time for coagulated milk was 10-1 against 10 2 s for original milk.Final apparent viscosity of gels (ua) was between 1 and 2 X 105 mPas depending on pH and calcium concentration. Keys words : MHk -Coagulation -Rennet -