Constance Higginbottom

501. The effect of storage at different relative humidities on the survival of micro-organisms in milk powder and in pure cultures dried in milk

The survival of the microflora during the storage of dried milks has been shown to vary considerably from sample to sample even when the bacteria appeared to be of similar types and the milks were dried in the same plant. The variation may have been due to differences in the moisture content of the dried milk, but no data were available with which to test this hypothesis. Preliminary experiments with three different types of milk powder showed that over the limited range of humidity used, the lower the relative humidity in which the milk powders were stored, the greater the survival of the bacteria contained in the powder. The scope of the investigation was then extended to cover a wider range of relative humidity and longer periods of storage. It was found that although in general the higher the humidity (or moisture content) the lower the survival rate of the microflora in the milk powder, survival was greater at 5-20per thousand relative humidity than at humidities above and below this level. A brief account of part of this work has been published elsewhere. The fact that survival was greatest at 15-25per thousand R.H. had been observed by WATTS for dried cultures of Streptococcus agalactiae . To determine whether this was a characteristic common to several types of bacteria the investigation was continued to include bacteria typical of the microflora of milk powders. Pure cultures of these bacteria in skim milk were dried and stored at varying relative humidities. Maximum survival was found at 5-20per thousand R.H. Dried suspensions of bacterial spores have been shown to be markedly sensitive to the complete absence of humidity, although their viability was unaffected by exposure to 5per thousand R.H. for two years.

365. The bacteriological quality of british spray-dried milk powder

The increased use of dried-milk products in human feeding during recent years stimulated investigations into and more rigid control of the bacteriological quality. Spray-dried milk powder from the same eight British plants has been examined for bacteriological quality at this Institute in several different years and the results show the marked improvements which have been made.

367. Bacterial growth in reconstituted spray-dried milk

1. Over 200 samples of spray-dried milks from eight British plants have been examined for total and spore counts, numbers of β-haemolytic colonies and for moulds, yeasts and conliform bacteria. 2. The mean count and standard deviation range for total and spore count are reported. No β-haemolytic colonies other than those due to spore-forming bacilli were detected. 3. After ageing the reconstituted milk for 24 hr. at 15–5° C. the mean count (37° C.) was unchanged; at 22° C. the increase was 2000-fold and at 37° C. the milk had clotted within this time. 4. The predominating organisms in a random selection of the reconstituted milks are described, both for the fresh milk and after ageing at 15·5 and 22° C. A marked increase i n spore-forming bacilli was noted at 22° C. 5. The relation of the flora of reconstituted milk to its food-poisoning potentialities is briefly discussed.

The alcohol test applied to sterilized milk.

A positive 80% alcohol test was not obtained in sterilized milks containing fewer than 10 5 bacteria/ml with Bacillus subtilis , B. licheniformis and B. cereus , and usually did not occur until the stationary phase of growth had been maintained for some days. A positive alcohol test with less than 10 5 bacteria/ml was associated with the lower maximum population density of B. brevis and B. circulans 152. The alcohol test after 24 h at 37°C was found unreliable as an indicator of the keeping quality of sterilized milk at 22°C. The possibility of using the alcohol test after 3 days at 37°C or a bacterial count after the same incubation period is discussed briefly.

571. The effect of menadione (2-methyl-l: 4-naphthoquinone) on the keeping quality of milk

1. The addition of 1 µ g. menadione/ml. to milk had no effect on the souring of the milk at 37° C. With higher concentrations of menadione the effect on the keeping quality at 37° C. varied with different milks. In some milks the clot-on-boiling stage was delayed by 1 hr. with 10 µ g. menadione and by 2 to over 5 hr. with different milks containing 100 µ g. menadione per ml. Some milks, however, showed no alteration in the keeping quality at 37° C. with as much as 1000 µ g. menadione/ml. 2. The response of individual milks to the presence of menadione appeared to be related largely to the bacterial flora of the milk, The relative insensitivity of Gram negative rods, including coliform bacteria and of enterococci, to menadione was confirmed. Milks containing large numbers of coliform bacteria showed the least and those free of such bacteria the greatest response to menadione. 3. Menadione fed to milking cows at the rate of 100 mg·/day for a period of 3 weeks had no effect on the keeping quality of the milk when it was held at 37, 22 or 15·5° C. The time of incubation required for the milk to attain a titratable acidity equivalent to 2 ml. N/9-NaOH, to become unstable to 68 % alcohol or to boiling showed no difference between the milk from the cows given menadione and that from the control cows. 4. The menadione had no demonstrable effect on the appetite or the well-being of the cows nor on their milk yield. 5. When as much as 100 μg. menadione was given per cow per day, no menadione was detected in the milk under conditions suitable for the detection of 1 μg. menadione/ml.milk.

366. Dye-Reduction tests in the bacteriological examination of dried milk

1. Over 100 samples of roller-dried full-cream milk and rather less than 100 samples of spray-dried milk reconstituted and incubated for 20 hr. at 55, 37, 39, 22 and 15·5° C. have been examined by the methylene-blue or resazurin-reduction tests at 37° C. The results have been compared with the plate count at 37 or 30° C. of the freshly reconstituted milk and with the keeping quality at 15·5° C. 2. A range of reduction times suitable for routine use was obtained after incubation at 22° C., but the results showed a very poor correlation with plate count at 37 or 30° C. and with keeping quality. 3. After incubation at 55, 37 and 30° C. the reduction times were too short, often showed delayed end-points, and the milk frequently clotted during examination. 4. Results with resazurin were similar to those using methylene blue and no significant differences resulted from the two different methods of reconstitution.