The purpose of any pasteurizing or sterilizing plant is to bring about the heat inactivation of bacteria or bacterial spores. It is generally considered for convenience that the bactericidal effect takes place almost entirely in a holding period, during which the milk can be considered to be held at a fixed temperature for a minimum time. In practice, since the milk cannot be instantaneously heated and cooled, there is a progressively increasing lethal effect throughout the period when the milk is approaching its holding temperature, and a decreasing effect in the cooling period. There is, therefore, a considerable lethal effect above that due to the nominal holding time. This has been shown in a paper by Ball(1) in which methods of calculation developed for the canning industry are applied to the problem of high-temperature, short-time pasteurization of milk. The higher the final milk temperature, the greater will be the proportion of lethal effect due to the come-up time, until, at temperatures of the order of 135° C., the lethal effect during holding may be relatively slight.