THE ASSOCIATIVE GROWTH OF SOME THERMODURIC BACTERIA IN PASTEURIZED MILK

SUMMARY: Three strains of aerobic spore forming bacteria have been studied in association with thermoduric streptococci in pasteurized milk at different temperatures. Generally the development of each component organism of the mixtures was unaffected in the initial stages by the presence of the other, though later the spore formers were frequently adversely affected. Acid production by the mixed cultures was compared with that of the pure cultures under the same conditions of incubation. Generally more acid was formed by the mixtures, particularly at the higher temperatures of incubation. Gas formation was noted by a culture of Streptococcus faecalis in association with the spore formers.     

OBSERVATIONS ON THE EFFECT OF ADDED HYPOCHLORITE ON THE NUMBERS AND KINDS OF BACTERIA IN MILK

SUMMARY: In an investigation of the effect of added hypochlorite on the bacterial flora and bacterial numbers of milk no bactericidal action was apparent within 15 min of adding the disinfectant unless at least 500 p/m available chlorine had been added. After 24 hr storage at atmospheric temperature a reduction in numbers was found with 10 p/m available chlorine and this became more pronounced with increased concentrations. Micrococci seemed more susceptible to the bactericidal effect than streptococci, which formed, after 24 hr storage, the predominant residual flora where 250 and 500 p/m available chlorine had been added. Acid production by lactic streptococci was retarded at concentrations greater than 50 p/m available chlorine in a 5 hr incubation period at 30°, but after a 24 hr incubation the decrease in acid production was only apparent when 500 or 1,000 p/m available chlorine was present. Little effect on acid production by lactic streptococci was found when less than 50 p/m available chlorine had been added. The residual chlorine content of rinses of equipment sterilized in hypochlorite solutions was negligible.     

THE MULTIPLICATION OF COLI-AEROGENES BACTERIA IN MILK STORED AT 3–5°

SUMMARY: When 108 samples of individual farm milk supplies were held at 3—5° for 72 hr, 35·2% showed increases in coli-aerogenes organisms of over one-hundredfold and 10·2% increases of more than one-thousandfold. The coli-aerogenes microflora after refrigeration was dominated by Klebsiella cloacae and K. aerogenes I. While some strains of all the coli-aerogenes types isolated showed growth, though sometimes scanty, on yeast-dextrose agar in 5 days at 3—5°, the majority of the strains of K. cloacae showed luxuriant growth under these conditions and can be considered as typical facultative psychrophiles of milk.     

THE RATES OF GROWTH OF SOME THERMODURIC BACTERIA IN PURE CULTURE AND THEIR EFFECTS ON TESTS FOR THE KEEPING QUALITY OF MILK

SUMMARY: The growth rates of eleven representative thermoduric bacteria, comprising 3 aerobic spore formers, 3 streptococci, 1 Corynebacterium lacticum and 4 micrococci, have been determined in glucose broth and sterile pasteurized milk at 37·5°, 26° and 15°. The spore formers and streptococci were generally not affected by the presence of inhibitory factors in pasteurized milk. When multiplication of micrococci and C. lacticum occurred in milk this was only after a lag period. One micrococcus showed an unusual series of growth phases in glucose broth at 37·5°, possibly due to the appearance of mutants or to adaptation of the organism to growth at that temperature. This was not observed in pasteurized milk. C. lacticum died off when incubated in glucose broth at 37·5°.
None of the keeping quality tests was more effective than any other in detecting these organisms in milk. The micrococci and C. lacticum had little effect on the keeping quality of pasteurized milk within the period of 'commercial life'. Some of the spore formers and streptococci showed marked differences in the end-points with the clot-on-boiling and the alcohol precipitation tests.