This series of four papers has outlined a method for predicting the bactericidal efficiency of an ultra-high-temperature sterilizing plant. The method has been illustrated with reference to the performance of an A.P.V. 200 gal./hr. plate-type sterilizing plant, treating at different temperatures milk heavily inoculated with suspensions of B. subtilis and B. stearothermophilus spores.

The method involves the determination of the temperature and flow time distributions in the plant, and of the thermal death characteristics of the organism to be considered. The investigations of plant performance are considered in Part I. The determination of the sporicidal effect of the plant from this information is described in Part II. The effect is expressed in terms of the slope of the thermal death line for the organism considered, at the operation temperature of the plant. By expressing the result in this form it becomes general and applicable to any type of organism, at any operating temperature close to that for which the plant is designed.

The thermal death time of any organism at the operating temperature may be obtained by laboratory experiment, by extrapolation if necessary. In Parts III and IV, laboratory data for B. subtilis spores and B. Stearothermophilus spores are used to calculate the performance of the plant, and the calculated results are compared with the results of direct plant experiment. The agreement is satisfactory.

The interpretation of the results with spores of B. stearothermophilus is complicated by their very marked inhibition by u.h.t.-treated milk. Only about one spore per million germinates and grows in such milk. A false impression of the number of truly surviving organisms after the heat treatment may therefore be obtained. It is not known to what extent this effect occurs with spores of other strains of B. stearothermophilus or with other organisms.

It is not suggested that the theoretical method of estimation of performance will give reliable information on the spoilage level to be expected from a plant under practical dairy conditions. The uncertainties as to the number and types of incident organisms are too great. Comparisons can be made between different plants, however, by comparing the results of the theoretical analyses for the plants. No bacteriological data are then required. This may be a less difficult procedure for manufacturers than a direct bacteriological experiment using heavily inoculated milk. The analysis also enables the contribution to the overall lethal effect of the different part of a sterilizing plant to be assessed.