A viscometric study of the breakdown of casein in milk by rennin and rennet

Abstract

SummaryIn the later stages of the reaction between rennin and casein drastic viscometric methods are undesirable, but in the first stages there is a fall in viscosity which may be satisfactorily measured in an Ostwald viscometer. Using fat-free milk, the viscosity at first falls linearly with time. At low rennet concentrations (Ce) this may be said to constitute a zero-order reaction (constantk0). At higher rennet concentrations and after longer times, the reaction passes to first order (constantk1). After very long times it doubtless becomes more complex.The values ofk0are proportional, over a considerable range, to the milk concentration (Cm), those ofk1being independent ofCm.For pure rennink0andk1are proportional toCe, but for commercial rennets they vary as a power (N) ofCeand the value ofNappears to measure the rennin purity. The potentialities of the method for assessing rennet activities are also discussed.If the reduced viscosity (specific viscosity÷Cm) of fat-free milks is plotted againstCm, good straight lines are obtained which may be extrapolated to zero concentration to give a reliable value of intrinsic viscosity. The intrinsic viscosity falls progressively during the protein breakdown process but the slope of the curves (‘second order term’) remains unchanged.