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

Abstract

Summary: In 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 (C_e) 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 (C_m), those ofk₁being independent ofC_m.For pure rennink₀andk₁are proportional toC_e, but for commercial rennets they vary as a power (N) ofC_eand 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÷C_m) of fat-free milks is plotted againstC_m, 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.