Physical changes in Milk caused by the action of rennet: V. Effects of varying concentrations of milk powder and of low-temperature pretreatment

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Abstract
1. When the concentration of fat-free milk-powder is increased, other factors being kept constant, the elastic and viscous setting-parameters of the curd also progressively increase, being related to the concentration by power-equations. The intercepts, which correspond closely to ‘coagulation times’, likewise increase, but nearly linearly with concentration 2. These effects depend mainly on protein concentration. Quite large quantities of added lactose make little difference to the parameters. 3. The slopes of the logarithmic setting curves vary quite independently of their intercepts, so that ‘coagulation times’ provide a very incomplete indication of the rate of setting of curd. 4. A comparison of setting data obtained in the ordinary way at a series of temperatures with those from experiments in which the rennet is first allowed to act on the milk at near freezing-point, show that, in the latter case, there is considerable delay before the normal equation of setting is established in the curd. It is also clear that high temperatures are detrimental to this process of setting, just as they are when the usual procedure is followed. 5. The general law that the rate of setting is, at any time, proportional to the amount of still unset casein, is established much sooner if the time zero is defined in such a way as to take all the processes involved together. If an ‘induction period’ is subtracted, either by letting the rennet first act in the cold, or by taking the time zero in the normal process at the coagulation, and not the renneting, time, a much longer time elapses before the curves become linear. We are indebted to many colleagues at the N.I.R.D. for helpful discussions, especially Dr S. J. Rowland and Dr R. Aschaffenburg.