It has been shown that the rate of shear of butter when cut by a wire increases as a power of the load applied to the wire. The values of this power are high (index n = 8 to 15, average 11 at 12.5° C.) and show a high structural viscosity in the butter. There is no indication that butter exhibits a true yield value, but the (rate of shear)/(load) curve is such that a practical yield value is found. The practical yield value (denned as the minimum load required to produce a measurable rate of flow) provides a convenient measure of the hardness of butter. The viscosity of butter decreases rapidly as rate of shear is increased, but the rate of decrease varies from one sample to another. Measurements of spreading capacity should be made at rates of shear comparable with those which occur when butter is spread with a knife.

The effect of temperature over the range 6–18° C. on the hardness and structural viscosity of a number of butter samples has been investigated. The effect of reworking has also been studied.