The considerable falls in viscosity of sodium caseinate solutions under the action of rennet are shown to be due to changes in volume and shape (or either alone) of loosely coiled long chain molecules, resulting from the discharge of ionization (‘Third-type electro-viscous effects’). This reduction may be as well effected by small amounts of NaCl or by the separated decomposition products of the rennin-caseinate reaction. Even at the highest practicable caseinate concentrations (9–10% by weight) the solutions are truly fluid over the range of shear-rate of the Ostwald viscometers used

The initial rate of fall of viscosity is found to be proportional to the maximum (initial) specific viscosity, leading to an equation Xk1 = 1/τ where X the proportional viscosity loss, varies with the source of caseinate but is constant for all normal rennets; k1, the first-order reaction constant, varies with the rennet but is independent of the source of casein over a wide range; and τ, which has the dimensions of time, varies with both caseinate and rennet.

The parameter k1 may be found either from the exponential viscosity fall or calculated from the asymptotic initial slope and has been shown to serve as a useful index of rennet activity. This method of rennet testing does not depend on the keeping qualities of rennins, rennets, milks or caseinates. Details of the preparation of caseinates and of the methods for practical testing of commercial rennets are given.