The hygiene and marketing of fresh cream as assessed by the methylene blue test

Summary

The hygiene and marketing of fresh cream in England and Wales was investigated by a working party of the Public Health Laboratory Service (PHLS) between 1 October 1968 and 31 July 1969. Thirty-one cream-producing dairies were visited and observations made in the light of the Code of Practice published by the Milk and Milk Products Technical Advisory Committee of the Ministry of Agriculture, Fisheries and Food, and the Scottish Home and Health Department. Suggestions are made in this report to strengthen the code.

A total of 5184 samples of fresh cream comprising 4385 heat-treated, 282 clotted and 517 untreated were examined. Details of production, age of the cream, distance and mode of distribution along the retail chain were available for most samples, and this history was, as far as possible, related to the laboratory findings. In the laboratory the samples were examined by the methylene blue test, colony count, conform and Escherichia coli I test and for pathogens. Staphylococcus aureus was grown from 59 out of 3417 samples of cream; 54 of these were from untreated cream. Phage typing indicated that a proportion of these strains were of animal, presumably bovine, origin. Other human pathogens isolated included one each of Salmonella typhimurium, Brucella abortus, E. coli type O126 and Clostridium welchii.

It appeared that heat-treated cream was much better, bacteriologically, than untreated; and that large dairies, in general, had better premises, more hygienic methods of preparation and the advantages of mechanical filling and capping of cartons; thus they offered the consumer a better product, bacteriologically, than some of the smaller dairies.

A statistical analysis of the results of the tests used showed the methylene blue test, in spite of some anomalies, to be of more use as a screening test than any of the others. As this test is cheap and easy to carry out, the working party thought that it should be the test of choice. In view of the known anomalies, however, the working party recommends that the test should remain a screening or advisory test and have no penal function. Results should be reported as follows: cream not decolorizing the dye in 4 hr. at 37° C. (± 0·5°) after overnight incubation at 20° C. (± 0·5°) would be accepted as satisfactory; between ½ and 4 hr. at 37° C. (± 0·5°) as fairly satisfactory; but if the dye were decolorized at the end of the overnight incubation the sample would be provisionally classified as ‘unsatisfactory; requires further investigation’. A repeat sample would then be examined. If samples were repeatedly unsatisfactory, consultations would be arranged between the local authority, the laboratory and the dairy, with perhaps inspection of the dairy premises and retail storage conditions and further, more searching tests on samples made in an effort to eradicate production faults.