By the use of the coaxial cylinder rheometer, the viscoelastic properties of reconstituted skim milk and renneted milk gel have been measured under various temperatures, concentration of skim milk and rennet.
The aqueous solutions of skim milk in the concentration range from 35 to 50 weight percent show a remarkable non-Newtonian behavior over a frequency range between 10-4 and 10-1cps. The viscosity at the zero frequency, therefore, could not be measured. According to Ferry's reduction method, however, the master curves for the reduced viscosity and rigidity have been obtained for both the temperature and concentration relationships. The activation energy calculated from the linear relation between log aT and 1/T is 93kcal/mol.
As skim milk coagulates by adding commercial rennet, the viscoelasticity of renneted milk gel increases at the lapse of setting time. The velocity of gelation varies with the concentration of skim milk, the temperature of setting and the concentration of added rennet. The velocity constant K of gelation was defined as the reciprocal of the setting time when the value of viscosity and rigidity reaches the half value of maximum respectively. It has been found that log K increases linearly with temperature above about 25°C, below which the enzyme action of rennet is inactive. The activation energy for gelation is about 59kcal/mol.
The relation between K and the concentration of rennet c, is represented by an equation K=K∞(1-e-kc), where K∞ is the maximum velocity constant and k is a constant. The gelation of milk seems to be caused primarily by the adsorption of rennet to casein molecules which is followed by the denaturation.