THE PROBLEM OF MILK SUPPLY

Otago Daily Times, Issue 25514, 19 April 1944, Page 4

THE PROBLEM OF MILK SUPPLY

PASTEURISATION

This is the second part of an article contributed last year to the British Medical Journal by G. S. WILSON, M.D., F.R.C.S., Professor of Bacteriology as applied to Hygiene, London School of Hygiene and Tropical Medicine. The first part was printed yesterday.

ii Question 7: What is Pasteurisation? Answer.—Pasteurisation is a loose terms applied to several forms of heat treatment of milk. In this country, however, only two methods of pasteurisation are officially allowed—namely, pasteurisation by the nolder method and pasteurisation by the hignlemperature short-time (H.T.S.T.) method. In the holder method the milk must be retained at a temperature of not less than 145 degrees F. and not more than 150 degrees F. for at least 30 minutes, and then cooled to a temperature of not higher than 55 degrees I'. The milk must not be heated more than once. The processing plant must be provided with indicating and recording thermometers, and temperature records j'Ast be preserved for a period of not less than one month. In the H.T.S.T. method the milk must be retained at a temperature of not less than 162 degrees F. for at least 15 seconds in an apparatus that is thermostatically controlled and provided with an automatic device to divert the flow of any milk which has not been so treated. The milk must then be cooled as under the holder process. The efficiency of either method may be tested by means of the phosphatase reaction. The enzyme phesphatase is destroyed by a time-temperature combination slightly higher than that necessary to destroy tubercle bacilli. Milk, therefore, in which the phosphatase test is negative can be regarded for all practical purposes as free from pathogenic organisms.

Question 8: What Effect Has Pasteurisation on the Properties and Nutritive Value of Milk?

Answer.—About 5 per cent, of the lactalbumin is coagulated; the coagulation time by rennin is lengthened, but the casein clot formed consists of finer particles and is of a more open texture; there is a diminution of about 5 per cent, in the soluble calcium and phosphorus; there may be some loss of iodine through volatilisation; about 10 per cent, of vitamin B 1 is destroyed, and about 20 per cent, of vitamin C; the flavour of the milk is somewhat altered owing to the removal of volatile products from the raw milk, but in properly pasteurised milk the acquirement of a cooked flavour is practically negligible and cannot be distinguished except by a very few persons; the keeping quality of the milk is increased considerably. Against these positive changes may be set a number of negative findings. Observations on rats, mice, calves, and children have failed to reveal any change in the biological value and digestibility of the protein, in the availability of the calcium and phosphorus, in the amount of vitamin A or D, or in the total energy or nutritive value of the milk for the growing animal. Question 9: Does Pasteurisation Destroy Pathogenic Organisms in the Milk? Answer.—The tubercle bacillus is destroyed by a temperature of 138 degrees F. in 30 minutes, of 145 degrees F. in five to ten minutes, of 150 degrees F. in two to five minutes, and of 160 degrees F. in 12 seconds. It will be seen that there is an ample margin of safety in the time-temperature combination laid down for *the holder method, and a fair margin of safety in that laid down for the H.T.S.T. method of pasteurisation. Other pathogenic organisms—with the exception of Staphylococcus aureus, which is not pathogenic by the mouth-—are more susceptible to heat than the tubercle bacillus, so that their survival in pasteurised milk is improbable. Question 10: Is Pasteurisation Antagonistic to Clean-milk Production? Answer.—lt is not sufficiently appreciated that the quality of the pasteurised product depends to, a considerable extent on the cleanliness of the raw milk. It is therefore to the advantage of the large dairy companies practising pasteurisation to obtain the highest quality of raw milk. To most farmers it is immaterial whether their milk is pasteurised or not. Their real motive for improving the cleanliness of their supply is financial. The operation of the accredited-milk scheme in this country showed that a bonus payment of Id a gallon for accredited milk led

to an enormous increase in the num- ■ ber of registered suppliers, in spite of the fact that a large part ,of the milk so produced was being taken to London and the large towns to be pasteurised. It is sometimes objected that a farmer who has taken considerable trouble to produce clean milk will bs disheartened if he finds that his milk is to be mixed with that of a lower standard of cleanliness from other farmers. Perhaps this is true, but it is an argument not against pasteurisation, but against the bulking of milk. Question II: Is Pasteurisation Antagonistic to the Eradication of Dis- ‘ eased Animals from Milking Herds? Answer.—As with clean-milk production, the stimulus to eradicate disease is almost purely financial, and is unaffected by the subsequent history of the milk. This has been amply shown by the success of the Tuberculosis (Attested Herds) Scheme, in which financial support was given to the producer who was anxious to eradicate tuberculosis from his herd. It is also borne out by the experience of the United States. In 1936, for example, no less than 99.8 per cent, of raw milk and 99.2 per cent, of pasteurised milk distributed in towns of 1000 inhabitants and over was derived from T.T. herds. The two figures are so nearly identical as to lend no colour to the view that pasteurisation retards the process of disease eradication. Question 12: How Often is Pasteurisation Inefficient? *

Answer.—The term “ pasteurised ” is often applied erroneously to any heattreated milk, not solely to milk that has been heat-treated in a licensed plant by cne or other of the two methods laid down by the Ministry of Health. Many of the statements that pasteurised milk has been found to contain tubercle bacilli are incorrect if the word “ pasteurised ” is limited, as it should be, to officially pasteurised milk. Stringent requirements are exacted under the Milk (Special Designations) Orders of 1936 and 1938. In addition we have now a reliable test—the phosphatase test—for determining whether or not the milk has been adequately heat-treated. The proportion, of inefficiently pasteurised milk is therefore much less thana 10 years ago. Examination of 1493 samples of pasteurised milk taken during 1934 to 1936 in London, Manchester, Sheffield, and Liverpool failed to reveal living tubercle bacilli in a single sample, in spite of the fact that the raw supply was often infected. It is not denied that the record of small dairies may be less satisfactory than that of the larger dairies supplying our great cities, but if compulsory pasteurisation was introduced more thorough supervision would be exercised over these smaller plants. Admittedly, owing tq some undetected fault in the plant or to some human error, the milk may at times be inefficiently heated; but there is no more to condemn pasteurisation on this account than there is to condemn railway transport because of the occasional failure of the signal system. Question 13: Does the Consumption of Raw Infected Milk in Childhood Protect against Pulmonary Tuberculosis in Adult. Life? •

Answer. —Though some doctors believe this to be true, there is no reliable evidence in its support. Those who do advocate the consumption of raw infected fail to realise that what they are recommending is a method of indiscriminate vaccination with living tubercle bacilli in the hope that, by a process of natural selection, the survivors will be more resistant in later life to invasion by the human tubercle bacillus. Against what other disease, it may be asked, would a method of vaccination be tolerated that is known to demand at present the sacrifice of 1500 to 2000 lives a year? Conclusion . Cleanliness. of production is no safeguard against tuberculosis or . conta- ‘ gious abortion in the cows; and T.T. milk is just as exposed as ungraded milk to infection from human sources. Though the ultimate ideal may be clean milk produced from disease-free herds and protected:from human contamination, there is no other immediate practical solution to the problem of supplying safe milk to the public but pasteurisation. Whether we like it or not, the logic of the case is inexorable.