329. The effect of the method of reconstitution and of the temperature of incubation on the plate count of spray-dried milk powder

Bacteriological standards are used in the assessment of the quality of dried milk powder. From a knowledge of total numbers of bacteria and the relative numbers of the various types deductions are made as to the quality of the raw milk supply, the cleanliness of the plant and the over-all efficiency of the process. Figures have been published by several workers showing plate counts of roller- and spray-dried milk powder, but the technique of reconstituting and of plating varied. Sterile water appears to have been the general reconstituting fluid, but sometimes lithium hydroxide (N/10 solution) was used, especially when the powder was difficult to dissolve. The temperature of the reconstituting fluid is not always mentioned, but it appears to have varied from room temperature to 60–65° C. The usual incubation temperature for the plates was 37° C, though 21–22 and 55–56° C. were also used for special purposes. InStandard Methods for the Examination of Dairy Products, eighth edition (1), the technique laid down mentions water, carefully warmed to 43–49° C, as the reconstituting fluid, and either 37 or 32° C. (for 48 hr.) as the incubation temperature, the choice being left to theenforcement officials having jurisdiction. American workers have for some years strongly advocated the use of 32 rather than 37° C. as the incubation temperature for routine plate counts of samples of raw milk, the advantage being that slight inaccuracies in the temperature of the incubator have less influence on the plate count which is, moreover, closer to that obtained by direct counts. The data offered in this paper show that the advantage of a similar incubation temperature (30° C. was used in these experiments) is equally obvious in the plating of dried milk powder.