330. The technique of the bacteriological examination of dried milks

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Abstract
1. A technique (B) for the reconstitution of both roller- and spray-dried milks is described which yields a liquid which is more satisfactory to handle, and gives clearer plates, higher counts and better replicates than the technique (A) at present recognized. The most important improvement in the new technique involves reconstitution, with a short holding period, at 50° C.2. For spray-dried full-cream milks the mean increases in the counts obtained by technique B as compared with technique A were: for 3 days at 37°C, slightly more than a three-fold increase; for 5 days at 30°C, slightly more than a two-fold increase; and for 3 days at 55° C. an 86% increase. The corresponding increases for spray-dried separated milk were for 3 days at 37°C, a two-fold increase, for 5 days at 30°C. a three- fold increase, and for 3 days at 55°C, a 23% increase. For roller-dried full-cream milk the increases were markedly smaller, i.e. 44, 41 and 29 %.3. After 6 months' storage spray-dried full-cream milks showed a slightly greater difference between counts by technique B than by technique A.4. A mean increase in plate count of about 10% was obtained by incubating plates for 5 instead of 3 days at 37° C. Although this increase was statistically significant, it is not considered that 5 days' incubation offers any practical advantage over 3 days' incubation.5. At 30°C. incubation for 5 days is essential. Thus the mean count after 5 days at 30° C. showed, for spray-dried powders, at least a two-fold increase over that at 3 days, though for roller-dried powders the increase was only 20 %. The percentage increase for spray-dried milks varied for individual samples and for samples from different plants.