426. Some bacteriological aspects of the deterioration of pasteurized milk

Abstract

1. Examinations have been made of milk pasteurized by the L.T. ‘Holder’ and H.T.S.T. processes using the same original bulk raw milk. In the preparation of samples care was taken to eliminate the possibility of post-pasteurization contamination. It was hoped that any differences found would be due solely to the effects of the pasteurizing processes above.2. Samples were examined by (a) colony counts and coliform tests when fresh and after ageing at 18°C. for 24 and 48 hr., (b) methylene-blue and resazurin reduction tests when fresh and after ageing at 18°C. for 45½hr., (c) keeping quality at 18°C. to end-point of 68% alcohol and clot-on-boiling tests. All bacteriological and dye reduction tests were carried out at incubation temperatures of 37, 30 and 22°C.3. In seven trials the experiments were repeated after storage of the original raw milk for 24 hr. at 40–50°F. (4·4–10·0°C.).4. Some detailed examinations were made of the bacterial floras of the raw and corresponding pasteurized milks, both when fresh and after ageing.

Comparative Studies on the Detection of Pollution of Water, Milk, and Ice Cream

“…Also, the relatively higher resistance of certain groups of excretal streptococci towards pasteurisation (63 C for 30 min) than most of the coliform bacteria would seem to explain in a great part the results obtained with pasteurised milk and ice cream. Although heat-resistant coliforms are Correlation between coliformii results and excretal streptococci index in raw and pasteurized milk knoown to exist, their incidence in efficiently pasteurised milk was found to be low (Crossley, 1946;Ashton, 1950). Not all of the milk or ice cream samples which produced acid and gas in MacConkey's broth showed the presence of fecal streptococci.…”

“…This may be explained oni the basis of various possibilities which are known to play a part in coliform results. It is possible that such samples contained coliforms of nonfecal origin (Wilson et al, 1935) or that false positive acid and gas results were produced by organisms other than coliforms (Ashton, 1950). Recently, Williams (1956) showed that a combination of S. faecalis and aerobic sporeformers, for example, Bacillus subtilis, can produce acid and gas in milk cultures.…”

A Processor's View

“…Much of this has recently been summarized in some published work carried out at Shinfield by Franklin (1965), but nevertheless, I must-even if briefly-refer to this earlier work. Ashton (1950) showed that dye reduction tests on raw milk were of little value in predicting the keeping quality of pasteurized milk. Many years before this we had found little relationship between plate counts and reduction time when examining raw milk.…”

The Effect of Cold on Micro‐organisms in Relation to Dairying

“…Possibly, if the refrigeration resources of the dairy are not adequate, an infection with sweet curdling organisms may also occur. Ageing of raw milk a t 4-10" before pasteurization has been shown to lead to increases in the thermoduric peptonizing flora and to reduce the keeping quality of the pasteurized milk (Ashton, 1950).…”