INFECTION OF MILK.

New Zealand Herald, Volume LXVIII, Issue 21030, 14 November 1931, Page 17

INFECTION OF MILK.

HARMFUL BACTERIA. CARE AND CLEANLINESS. ENSURING SAFE SUPPLY. BY Q.U.M. There is an idea prevalent in tlie minds of many milk producers that it is quite essential that the milk he pasteurised, and that, no matter how bad the milk may have been originally, the operation of pasteurisation is sufficient to render it quite pure and fit for human consumption. This is most unfortunate and is quite wrong. Tlvo only way to be sure that the milk is safe is to take all precautions to ensure that it does not become contaminated from tho time that it is first obtained from tho cow until it is delivered to the householders' door. Pasteurisation is most certainly not a sure method of rendering a dirty milk innocuous. Certainly, when properly carried out, pasteurisation is very effective and serves to kill most of tho bacteria in milk. No one has ever claimed that pasteurisation can render a milk completely sterile. Should any readers ever visit a bacteriological laboratory they would bo amazed at the elaborate precautions' which have to be taken to ensure complete sterilisation. For example, one common process is to boil for twenty minutes, and then repeat the process on two successive days. Most people are under tho impression that boiling is quite adequate to kill completely all bacteria, but bacteriologists have found that many bacteria can survive even after twenty minutes boiling, and that to kill them it is necessary that the operation bo repeated on two successive days. Bacteria in Milk. Compare this with the operation of pasteurisation, which, as commonly carried out, consists of heating the milk to 145 degrees or thereabouts, and holding at this temperature for about thirty minutes. Pasteurisation, as commonly carried out, does not destroy all the bacteria iti milk, and it, does not render an infected milk safe for human consumption. Bacteria in milk can be divided into two classes, The normal milk bacteria, such as streptococcus lactis, which is responsible for the natural souring of milk, are representative of one class, and apart from tho fact that they cause milk to go sour, are otherwise quite harmless. The other types are the pathogenic bacteria, which cause illness and epidemics, and most of this type find in milk an ideal culture medium and thrive and grow with great rapidity. It is almost an impossibility to obtain a milk which does not contain a few of the normal milk bacteria, and what happens when this milk becomes infected with pathogenic bacteria is that a battle in miniature takes place between tho lactic and the pathogenic bacteria, and, there usually being a big preponderance of tho lactic type, the harmful bacteria are killed off, or, at least, considerably restricted in growth. One can readily see that, far from being harmful, tho presence of streptococcus lactis is an advantage—so long as there is a possibility of infection with disease-producing organisms. Result of Pasteurisation.

The lactic bacteria arc easily destroyed by the usual process of pasteurisation — J more so than the pathogenic bacteria —and j the result, of pasteurising an infected milk is that, after the process has been completed, ifc is possible that undesirable germs are free t.o breed unhindered by the lactic bacteria which have been killed off. These pathogenic bacteria increase at a terrific rate, and the state may soon be reached when the total number of bacteria present is greater than if tho milk had been left unpasteurised. It is unfortunate that this fact is not realised more than it is. What must be understood is that tho only way to be sure that milk is free from infection is to prevent infection, and not to attempt to cure tho infection. The terrific speed at which bacteria propagate is not realised. Reproduction takes place by fission, tho bacterial cell swelling and dividing into two. Ihis picccss can take place 48 times in 24 hours. This means that a single bacteriacan swell up, divide into two, and. within a period of 24 hours, be responsible for 231,474,976,710,656 descendants. Sources of Contamination. Bacteria find entrance to the milk from many sources. It should riot be necessary here to stress the need for absolute cleanliness in the buckets, machines and other equipment with which the, milk comes in contact. Instead we will mention just a few of the lesserknown, but nevertheless, dangerous sources of infection. hxcreta contains over 30,000,000,000 baHeria, per mime, and earth and straw up to MX),000.000 per ounce. The. leaves of grass and plants arc liberally coaled with micro organisms, and for that matter, practically everything associated with a dairy farm is alive, v. it h teeming millions. l.'jvctt the. ait of a clean cowshed contains on an average about 300.000 organisms per cubic yard, which number rises to over 1,000,000 while milking is in progress. It. has been shown that up to 20.000 bacteria, fall into an open bucket every minute. These figures alone should servo to indicate sonic precautions that should at uavs be taken. Tho cloth used for washing" the udders is usually in a, tilt by condition, and when like Ibis its main function is to transfer m a very effective manner pathogenic bacteria from a diseased cow to every other cow in the herd. The.-water supply of many farms is another source of infection, and whenever it is derived from rain water tanks or shallow wells should always be sterilised before being used for any purpose. Strainer Should Hot Be Needed.

However t 10.-ui I lie cow and shed, il. is impossible to obtain perfectly slcnlo milk, as the. udder, even when healthy, always contains sonic bacteria wliicli find their way in from the openings in the I Pa ts MTerc is always a largo number of bacteria in the milk duel and tlm first portion of tho fore milk from each teat should not bo used. Tests have shown lhat fore milk contains 16.000 hactrr..j per cubic centimetre, middle milk 4.JU and shippings .560. Next to dirty containers (ho strainer h probably the worst, sourco of infection as, duo to its construction, if. is very difficult to sterilise properly. '.lho mero fact that a strainer is considered necessary should bo sufficient to condemn the milking methods of that particular farm. Th or( , is a mistaken idea prevalent that, straining removes the, bulk of the. bacteria from milk. The fallacy of this can be better understood by considering the respective sizes <">f baetena and the openings in sieve cloth. Since it takes 25.000 bacteria to cover one inch in length, 400 of them could lie placed in a. row in the space between two wires of tho finest strainer, and through one of the square openings in the gauze a regiment of 160,000 bacteria, could march abreast. Bacteria are much smaller than the. drops n { f.it, in suspension in tho milk and therefore, a strainer fine enough to remove bacteria, would also remove all of the butter-fat, thus allowing only skim milk to pass through the strainer.