333. The coliform flora of milk and dairy products

471. The role of micro-organisms in dye-reduction and keeping-quality tests: I. The change in dominant bacterial flora during incubation at 37·5 and 22° C. and the part played by staphylococci and thecoli-aerogenesgroup

1. The present work was planned to provide an explanation of the lack of correlation for individual samples between the results of dye-reduction tests at 37·5° C. and keeping-quality tests at 22° C. A special study was made of the particular part played by the staphylococci (in thirty-eight samples) and thecoli-aerogenesgroup (in thirty-seven samples) in dye reduction at 37·5° C. and in keeping quality at 22° C. A detailed study was also made of the changes in bacterial flora of twelve samples during incubation at these temperatures.

2. In milk, after overnight storage at 14° C., staphylococci were found to constitute only a small proportion of the bacterial flora, but their numbers increased during incubation at 37·5° C. (for the dye-reduction test) and at 22° C. (for the clot-on-boiling test). At the time of dye reduction staphylococci were found to constitute a major part of the bacterial flora, particularly in samples taking 5 hr. or more to reduce the dye. Although the staphylococci increased in numbers during incubation at 22° C. they constituted only a relatively small proportion of the flora at the time of the end-point of the clot-on-boiling test.

3. Initially thecoli-aerogenesgroup were found to constitute only a small proportion of the total bacterial flora. Some growth usually occurred during incubation at 37·5 and 22° C.

426. Some bacteriological aspects of the deterioration of pasteurized milk

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.

412. Coliform bacteria in New Zealand butter

1. The limitations have been explored of a simplified form of Grimes's litmus milk method, as used for a presumptive test for coliform organisms in butter.

2. To improve on this test, the merits and demerits of renneted litmus milk and litmus milk agar have been investigated, and the latter found preferable.

3. Of 719 samples of export salted butter tested for coliform contamination, only 19·1% gave confirmed positive results using MacConkey broth as the presumptive medium, 21·9% were positive using a litmus milk agar medium, and 30·2% (on 434 samples only) were positive using renneted milk.

4. The coliform organisms isolated were classified into the usual types and a seasonal variation in the coli to aerogenes ratio noted.

5. An observation is made on what appears to be the inhibitory effect of bile salt on the gas production of several strains of the aerogenes-cloacae group isolated in this investigation.

Survival of Escherichia coli, strain W, during the manufacture of cottage cheese

Cottage cheese was manufactured in 10-liter experimental vats by the direct-acid-set method from milk that was inoculated with a heat resistant strain of Escherichia coli. Growth or survival of Strain W (ATCC 9637) E. coli was determined at various stages of the cheese making operation after the cheese-skim milk was inoculated to give counts of 2.5 X 10(4) or 4.0 X 10(5) cells/ml. Numbers of coliform organisms remained constant at the inoculated concentration in the cheese milk up to a cooking temperature of 43 C. At 43 C, when curd was separated from the whey, the curd (not washed) had coliform counts that were two log cycles greater than the whey. These trends were in milks with both cell counts. Washing of the curd with acid and 10 ppm chlorine reduced the number of coliform cells in the curd at all cooking temperatures as compared with unwashed curd. Acid wash of the curd at pH 5 did not reduce the coliform counts below those of unwashed curd. Cooking temperatures of 54 C were necessary to destroy (less than 1 cell/ml) E. coli Strain W, in either the unwashed or acid-chlorine washed curd. Holding curd with an initial average log count of 6.26 coliform cells/ml at constant temperatures of 38, 43, 49, and 54 C confirmed that 54 C for 50 min was necessary to reduce the average count to less than 1 cell/ml in isolated curd. Coliforms in whey were reduced to that concentration after 10 min at 54 C.