J. Burnett

716. Physical changes in milk caused by the action of rennet: I. Description of apparatus for measuring rigidity moduli and internal viscosities, tests of reliability and some observations on syneresis

A method for measuring the increasing rigidity moduli and internal viscosities of milk during rennet setting is described. Curves are given showing the degree of reproducibility on replicate samples of dried and liquid milks, and of the effects of changes in calcium levels in the case of the former. A tentative interpretation of the shape of the main part of the setting curve is proposed. The upper part of the curve generally becomes erratic with the onset of syneresis, but some dried milks do not show this behaviour. Some curious phenomena concerning the production of syneretic liquid (whey) are described and methods for obviating the disturbing effects of syneresis are discussed.

Physical changes in milk caused by the action of rennet: IV. Effects of varying rennet concentration and temperature.

1. Under carefully standardized conditions, the increase in the over-all rigidity modulus of curd when the time after rennetting is doubled, varies inversely as the cube root of the rennet concentration. The intercepts of the extrapolated setting curves on the time axis and the observed coagulation times vary inversely as the concentration. 2. With the same (normal) rennet concentration, some rheological parameters increase or decrease progressively as temperature is increased from 21° to 41° C.; others pass through maxima or minima, all between 29° and 35° C. 3. Long-term ‘creep’ experiments have been done over the above temperature range, creep and slow recovery increasing progressively with temperature. Relaxation experiments, though showing the simplest distribution of relaxation times at about 32° C., are too greatly influenced by unavoidable differences in firmness at the different temperatures for it to be possible, at this stage, to draw conclusions about their temperature dependence. 4. A comparison of some of these results with data published by a few other workers is attempted.

Preliminary experiments on the rheology of bovine blood-clot

SummaryThe bahaviour of clotted cows blood, subjected at 32° C to stresses in a U-tube gelometer designed to follow the coagulation of milk, may be closely represented by a simple (Maxwell) model consisting of a dashpot and spring in series. The dashpot contains aNewtonian fluid and the spring is very nearlyHookean. The elasticity shows no appreciable after-effect or static fatigue. This simple form of behaviour is confirmed in creep and in all but the latest stages of constant strain relaxation, the relaxation time being about 70 sec. In all these respects, the clotted cows blood proves to be a much simpler rheological system than is the milk gel.ZusammenfassungDas Verhalten von geronnenem Rinderblut bei 32° C in einem U-Röhren-Gelometer, das gebaut wurde, um die Koagulation der Milch zu verfolgen, mechanischer Beanspruchung unterworfen, kann ziemlich genau durch ein einfaches (Maxwell-) Modell dargestellt werden, das aus einem Dämpfungskolben und einer Feder in Reihe besteht. Der Kolben verhält sich newtonisch und die Feder entspricht nahezu der einer demHooke- schen Gesetz folgenden.Die Elastizität zeigt keinen wesentlichen Nacheffekt, keine statische Ermüdung. Diese einfache Art des Verhaltens bestätigt sich auch aus dem Kriechverhalten und für alle außer den Endstadien, aus der Spannungsrelaxation, wobei die Relaxationszeit ungefähr 70 sec beträgt. In allen Hinsichten erweist sich das geronnene Rinderblut als ein viel einfacheres rheologisches System als koagulierte Milch.

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

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.