293. Bacteriological aspects of the manufacture of spray-dried milk and whey powders, including some observations concerning moisture content and solubility

1. A bacteriological study was made of the commercial operation over a period of 4½ years of two Kestner evaporating and spray-drying plants, handling a fairly good and a very poor milk supply respectively. Processes investigated comprised handling of raw milk, pasteurization, climbing film evaporation, handling of evaporated milk, and spray drying. Data were also obtained showing the influence of variations during processing on the moisture content and solubility of powders. Separated milk, full-cream milk, and whey were handled. 2. The general bacterial flora of spray-dried powders and the influence of individual manufacturing processes, packing and storage were studied and possible control methods suggested. Data showing the extent of both hourly and seasonal variations were also obtained. 3. A method of bacteriological analysis is described which gave reasonably good agreement between different workers. 4. Examination of 671 powder samples showed a wide variation in individual plate counts from 200 to 19,500,000 per g. The flora on standard milk agar at 37° C. was of a specialized type and comprised comparatively few species. Thermoduric streptococci of the ‘enterococcus’ and ‘viridans’ groups predominated, Str. durans and Str. thermophilus being the commonest species; in addition, five species of micrococci, probably non-thermoduric, also occurred commonly. Aerobic spore-forming bacilli and three Achromobacter species were found regularly, but their numbers were not of practical importance. Spore-forming anaerobic bacilli were present in 14% of powder samples, probably in very small numbers, and were most frequent during the winter months. 5. Coliform organisms were rarely found in 1 ml. of reconstituted milk, but were isolated from 25% of powder samples when 20 ml. of reconstituted milk was examined; coliforms occurred most frequently during the winter months. False positives due to anaerobes were common, especially in stored powders, and confirmatory tests of presumptive positives were essential. Examination of 198 coliform strains from powders and 164 strains from raw milk showed that coli types occurred less frequently in the powders, whilst aerogenes-cloacae types, particularly cloacae, were more frequent. 48·5% of the powder strains were heat resistant, as compared with 2·2% of the raw-milk strains. It was shown that the coliform flora of powder was partly due to plant contamination by heat-resistant strains, although some non-heat-resistant strains could survive spray drying. It seemed possible that coli types were less resistant to drying than aerogenes-cloacae.