THE PASTEURISATION OF MILK.

Otago Witness, Issue 2604, 10 February 1904, Page 6

THE PAS T EURISATION OF MILK .

This is the text of a very insliuelive contribution to an anmnl report of the Wisconsin Agricultural Experiment Station. The article in Question :s: s the

joint product of Professors E. H. Farrington and Hon. L. Russell, hoi.li of whom are well known in dairying circles in the United States. They deal with the subject at very gre c at length, and exhaustively discuss the several features of the different systems of pasteurisation at present in vogue. In explaining the objects of pasteuration, and showing the effect which it produces on milk, these American experts point out that -when milk or cream is maintained at a temperature of 155deg for 15 or 20 minutes it is rendered much thinner, although the amount of fat present is not at all affected. The cream on such milk does not rise to the Furface, and this gives the impression that the product is deficient in butter fat, even though the Babcock tester may show a normal condition. The cause of this diminished consistency was ascertained by Babcock and Russell in 1896 to be due to the condition of the fat globules in the milk. In normal milk of cream these globules r-re aggregated together for the most part in tiny clots or clusters, but where milk is heated above 140deg these miscroscopic masses are broken down, and the fat globules- are uniformly diffused throughout the milk serum.

To obviate this difficulty with pasteurised products, milk or cream may be treated for a longer time at a temperature at which the fat aggregations will not be broken down — namely, about 140deg. If, however, milk was treated at this temperature, it would, according to the limits set in the investigations of Forster and de Man, require one hour's exposure to destroy the germs of tubercle. But recently Dr Smith has reopened the question as to the temperature and time limit at which the bacillu. of tuberculosis loses its vitality.

He finds tit at in ordinary circumstances tho tubc-rel© orgr-ik-m derived fom L .vinesources is generally killed in 15 cr 20 minutes at an exposure to 140dcg. Even after an exposure lasting but 10 minutes the bacilli were killed in most cases. In milk the conditions were subject to pome variation*. Where this fluid was heated in tub-* and a iwlhcl? foimed on the surface — called " scalded skin " — this layer of fai globules find coagulated pioteids protected the tubercle bacilli to such tin extent that not infrequently an exposure of 140deg Falir. for even 60 minutes did not entirely destroy the geims. Tliis condition, it is pointed out. y\ oulJ apply where milk is pdsixu;;&=d iq botles, but not where the mecLtP-nical agitation of tthe milk is continuous, and the "scalded skin"' layer is thereby prevented from forming. To te«L the effect of a temperature of 140dej (n the creaming properties of millc, a sample jar was taken from some raw milk and placed in cold water to .^erve as a standard for comparison. The remainder of the milk was heated in a pas-tc-un-tr to a temperature of 140da:r. and samples were withdrawn at varying intervals of time between 15 and 60 minutes, the rest of the m:lk being then raised L o 155deg for 15 to 30 minutes, and sampled in the s>ame way. All the samples were kept in cold running water at a temperature approximating 50deg. Daily observations tvere made as to the tihickness of the cream layer on each jar and as i.o the keeping qualities of the milk. The results are tubulated, and from che figures in the table, which it is not necessary to reproduce, the follow ing inferences are drawn : —

(1) That pasteurising at 156deg for 15 minutes ard then cooling to SOdeg changes the character of the milk, so that tite creaming of the milk ;s; s not only greatly retarded but actually lessened in quantity. '2) Comparing the milk pasteurised ■it- 14 Odes: for either the 15 or oO minutes period, the table shows that the cream rises quickly and as completely as on normal milk. The samples treated at at 140deg were taken in triplicate, and the close uniformity of results shows that the creaming of such milk was practically uninjured. Tn samples pasteurised at D4odeg for 60 minutes the creaming property was also normil. This indicates thafc in pasteurisation the factor of duration of ex posure has no material effect in lessening the creaming property ; this depends more or the temperature employed.

As to the keeping quality oi milk pasteurised at 140deg, it is pointed out that this, quality turns mainly on the destruction of the lactic acid bacteria. Numerous experiments have shoArn that the thermal death point in !.!>is case is in the neighbourhood of 135deg to 140deg where the

exposure lasts for 10 minutes. It was therefore considered that so far as keeping qualicy was concerned milk' or cream treated at 140deg for at least 15 minutes would keep practically as well as if heated to a highei temperature. This was dtmonstrated by tests, the average of which showed tint the row milk soured in about two day, v hcrcas all the pasteurised samples kept for at least six days ; also, fi'aat there was practically no difference in the keeping quality between the milk lie.it ed to L4od?f, for either 15 or 30 minutes, and that heated to 155deg for a period of 15 minutes.

The temperature recommended for the pasteurisation of milk and cream in tho pa«t lias- been 155deg for a period of 15 to 20 minutes. Thif limit mas chosen because it had hci;i? regarded as the point at which the iuh ivlc bacillus is do*tioyed in a moist medium. When, however, cream or milk is treated to a temperature of 140deg, or above, the physical condition of the fat globules is changed, sc that cream appears much thinner and milk loses its property of rapid creaming. This objection can be overcome, as is shown by the preceding experiments, if milk or cream is not treated above tins temperature. Not only is ttae creaming property of the milk r.nd the "body" or consistency of cream unaffected, out the keeping quality i« practically as good as it is where the product is heated to a tempera tine of 155 do S .

All that is necessary to secuie good keeping quality is to destroy the vegetative bacteria,, and as this is accomplished at a temperature of 140d-eg, if the exposure is made for a sufficient period of time, no advantage in this respect is o be gained by heating to a higher temperature. This being true, it only remains to determine with certainty how long an exposure must be made to destroy the tubercle bacillus. The limit that has been heretofore considered necessary where the exposure was ma eta at 140deg was one hour, but recent extensive experiments, in •which all conditions have been mopt carefully controlled, show that time miy be materially shortened where milk' is agitated during pasteurising. The use of a temperature of l&Odeg will So away with the only objection "that has ever been urged against pasteurised products.