MILK PURITY.

Auckland Star, Volume LXIX, Issue 177, 29 July 1938, Page 8

MILK PURITY.

BACTERIA CONTROL.

PLATE COUNT CRITICISED

DOES NOT MEASURE NUMBERS

One of the regulations laid down by the Auckland Metropolitan Milk Council to govern milk and cream standards demands that no milk shall be sold which has a bacterial plate count exceeding 100,000 per cubic centimetre. The value of this plate count test to determine purity is severely criticised by Professor G. S. Wilson and his assistants at the London School of Hygiene and Tropical Medicine. This information is contained in the report of the Agricultural Research Council of Great Britain, which was issued last month.

J Professor Wilson's investigations, which occupied three years, covered an examination of the different testa, including the plate count. In his findings, he states that ostensibly the plate test measures the numbers of bacteria in milk, though, in fact, it does not. Bacterial Variations. It is pointed out that, on account of the difference between various species of bacteria in their nutritional, respiratory and temperature requirements; on account" of the fact that many organisms may be dead; and, most important of all, on account of the gross irregularity in the distribution and clumping of the -organisms in the milk, the plate count merely registem the number of bacterial units capable of multiplying under the particular conditions selected.

j Since the average number of bacteria per clnmp is variable from one milk to another, and from time to time in the same milk, and since these clumps may disintegrate to an uncontrollable extent oaring the process of dilution, it follows that the figures yielded by the plate count are arbitrary and not strictly comparable from milk to miUc. They •re merely approximate and have no absolute significance. The technique is complex, difficult to standardise, and requires highly skilled workers. Besides demanding costly apparatus and a delay of at least two days in the result, the plate count seems to afford no better index of the sanitary conditions of production or of the keeping quality of the milk than the Breed test or the modified methylene blue test.' Methylene Test Favoured. Professor Wilson favours the modified methylene blue reduction test, which, he claims, appears to fulfil most of the requirements demanded of a test for the routine grading of raw milk. It consists in observing the time required for a solution of methylene blue to lose its colour when mixed with the milk to be tested. Bacteria hasten the reduction, and thus shorten the time. The test is simple and inexpensive. It has a very small experimental error, and can be carried out by relatively unskilled workers on large numbers of samples. It demands a" minimum of equipment. It is capable of classifying milk on the basis of cleanliness into the maximum number of grades desirable, and it affords on the whole a very good index of the keeping quality of the milk. Besides these advantages, it gives more information about the milk than does the plate count.