MILK AND BACTERIA

New Zealand Mail, Issue 1746, 23 August 1905, Page 57

MILK AND BACTERIA

THE EFFECT OF DIFFERENT TEMPERATURES IN DETERMINING THE SPECIE'S OF BACTERIA WHICH GROW IN MILK.

(H. W. Conn and W. M. Esten, 16th Annual Report, Storrs Agricultural Experiment Station.)

For the last three years work has been done in the Storrs laboratory upon the relation of the growth of bacteria in milk to a variety of conditions. The most important conclusions reached in the work hitherto reportetd have been : 1. At a temperature of 68deg. F. the common Bacterium aeidi lactis grows with special rapidity, and seems to he more favoured by this temperature than .is any other species. 2. At 55deg. F the ordinary organisms do not grow so readily as at 68deg., hut other species of bacteria do develop readily. 3. The percentage of acid bacteria increases with the total number •of bacteria in ordinary market milk, provided that milk is kept at moderately warm temperatures. The purpose of these experiments, reported in this paper, has been : 1. To extend the observations of the effect of temperature upon the species of bacteria which grow in milk, so as to include a wider range of temperatures. 2. To determine whether there is any regularity in the results. “The significance of the ouertions thus studied is considerable. Their bearing upon the hygienic problems of milk will be recognised when it is remembered that normal lactic bacteria are quite harmless to health, and that the troubles attributed to milk bacteria must he associated with other species. The development of lactic bacteria has been shown to check the growth of other species. Hence it follows that conditions favouring the growth of Tactic bacteria may actually make the milk more rather than less, wholesome; while conditions that check their growth may favour the development of other more harmful bacteria, and, while

PRTSSERVING THE MILK from souring, actually render it less wholesc me.

“The bearing of these facts upon dairy problems is no less close. It is known that cgrtain dairy species are usefnl to the butter maker and necessary to the cheese maker, while others are deleterious. Some species rain the taste of butter, and other fill cheese with gas, producing the gassy (or spongy) curd, and swelled cheese, which the cheese maker so much dreads. To check the growth of these species, and to favour the growth of the desired types, is the object of controlling dairy conditions. The close relation of these problems to temperature makes it especially desirable to know what effect variation in temperature has upon the growth of different types of bacteria. It is for the purpose of obtaining light upon the two problems, the one of the dairy and the other of public hygiene, that the long series of experiments outlined in this paper were undertaken, and the importance of the problems gives value to the results, even though, for reasons to be explained, the results can as yet be regarded as only approximately accurate.” SUMMARY OP RESULTS. Prom the experiments recorded, together with those published in the Annual Report of 1902-3, the authors give the following general summary of the effect of different temperatures upon the development of the types of bacteria in ordinary markef milk:— 1. The effect of variations in temperature upon the development of the different species of bacteria in milk is not always the same under apparently identical conditions. Such variations are to be expected when we bear in mind that different samples of milk vary so widely in their bacterial content at the outset. The presence of an especially persistent species in any lot of milk may, at any time, be expected to interfere with the normal course of events, and to produce unusual results. But in

spite of these variations, there appears to be clearly discernible a normal development of bacteria associated with different temperatures. 2. There is, in all cases, a certain period at the beginning when there is no increase in total numbers of bacteria. During this period some species are multiplying, while others are apparently dying. The length of this period depends upon temperature. At 98deg. F. it is very short, while at 32deg. it may last from 6 to 8 days, since, at this temperature, milk may, in six days, actually contain fewer bacteria than when fresh.

3. After this preliminary period there also follows a multiplication of bacteria ; but the types that develop differ so markedly that samples of the same milk kept at different temperatures are, at later periods, very different in their bacterial content, even though they contain the same number of bacteria.

4. The development of the ordinary lactic species, Bacterium lactis acidi. in practically all cases, checks the growth of other species of bacteria, and finally kills them, since the bacteria regularly decrease in actual numbers after the lactic bacteria have become very abundant. The development of lactic bacteria thus serves as a protection both to the milk and the person drinking it, since it prevents the growth of other bacteria. This effect is probably due to the production of lactic acid, and is not noticeable in milk only a few hours old. The presence of large numbers of lactic bacteria in milk is not an indication that the MILK IS UNWHOLESOME. 5. In practically all the samples of milk kept at 68deg. F. the multiplication of the Bact. lactis acidi begins ouiekiy, and progresses with great rapidity. They grow so rapidly that they produce acid enough to curdle the milk in about 40 hours, the growth of other specie® being held in check. Milk, when curdled at this temperature, shows a smooth acid curd, with no gas hubbies, such as a butter maker or a cheese maker likes to see, and contains commonly over 90 per cent, cf Bact. lactis acidi. This is the temperature favourable to ordinary dairy processes. 6. A totally different result appears in milk kept at 98deg. F. The results are somewhat more variable than at 68deg. F. Occasionally the Bact. lactis acidi grows vigorously at this temperature, but the common effect is, that a different type of lactic bacteria develops. ’The type favoured by this temperature is the species B. lactis aerogenes. It is a gasproducer. and produces a different type of acid from that produced by B. acidi lactici. It forms a curd full of gas bubbles, and is an enemy of the butter and cheese maker. If B. coli communis (which causes spongy curd) is in the milk, this also grows luxuriantly at 98deg. F. The souring of milk occurs very rapidly at this temperature. 7. In milk kept at 50deg. F., neither of the types of lactic bacteria seems to be favoured. The delay in growth lasts two or three days after which all types of bacteria appear to develop somewhat uniformly. Sometimes the lactic bactera develop abundantly, sometimes only slightly. The neutral bacteria almost always grow rapidly, and the liquefiers, in many cases, become abundant. In time the milk is apt to curdle, commonly with an acid reaction, hut it never shows the predominance of B. lactis acidi found at 68deg. F. At 50 deg. F., therefore, the lactic bacteria are not so influenced as at 68deg., and they exert ne check upon the growth of other bacteria. The milk, therefore, in time becomes more decidedly affected by bacteria than at 68deg., and its wholesomeness more under suspicion. 8. From our experiments there seems to be no difference between the effect of 50deg. and 33deg. F. upon the bacteria, except upon the rapidity of growth; 33deg. very markedly checks the growth of bacteria, but later they grow to great r umbers, as at 50deg. the lactic bacteria fails to outweigh the other species, so that all types develop abundantly. A few species appeal* to he particularly well adapted to this low temperature, and are especially abundant at the end of the experiments. 9. The curdling point, appears to he quite independent of the number of bacteria present. In one sample, at 98deg. F., the milk curdled with only 8,000,000 per cubic centimetre, while in others there have been found 4,000,000,OCX) per c. c., without any curdling. These differences are due partly to the development of enzymes, and partly to the products of some species neutralising the actions of others. The amount of acid present at the time of ordinary acid curdling does not widely vary. 10. Milk is not necessarily wholesome because it is sweet, especially if it has been kept at low temperatures. At the temperature of an ice chest milk may remain sweet for a long time and yet contain enormous numbers of bacteria, among which are species more likely to be unwholesome than those that develop at 68deg. F. From this standpoint tin suggestion arises that instances of ict -cream poisoning are perhaps due to the preservation of cream for several days at a low temperature, feuch Treatment keeping the milk sweet, but favouring the development of specie-; of bacteria' that are, at higher tempera - tures, checked by the lactic organisms.