August i6, 1889.] 



SCIENCE. 



117 



vessels in which the mill: is placed. These, unless recently washed 

 in boiling water, contain bacteria clinging to their walls. These 

 bacteria begin to grow as soon as the milk gets into the vessels, 

 and in a few hours will multiply so as to be extremely abundant. 



Number of Bacteria in Milk. — Different Species. 



The number of bacteria in milk will depend chiefly on three 

 things: i. The cleanliness of the vessels ; 2. The temperature of 

 the milk, warmth being favorable to their growth ; 3. The length 

 of time that the milk has been standing. Ordinarily the number 

 of bacteria in the air is of comparatively little importance, unclean 

 vessels being the great source of contamination. If, however, the 

 vessels are perfectly clean, the number of organisms in the air be- 

 comes the important factor. In cream which has been allowed to 

 " ripen " for a few days, the number is extremely great. In the 

 specimens of ripened cream which we have examined, from 10,000 

 to 100,000 individuals have been found in a single drop, the latter 

 number being usually nearer the truth than the former. Even 

 under conditions most unfavorable for their growth, in a cool 

 cellar during the winter, 12,000 have been found in a single drop. 

 These are capable of multiplying with the greatest rapidity, pro- 

 ducing hundreds of thousands in a few days. 



Not only is the number of individuals very great, but the number 

 of different species is considerable. Some thirty or more different 

 species of bacteria have been found during the winter in speci- 

 mens examined in the laboratory. No single specimen of cream 

 contained them all, but each contained several species. 

 ' The number of bacteria present has, however, no significance 

 until we know something of their effect. Some are harmless, some 

 are hurtful ; some affect cream, milk, and butter injuriously, and 

 others do not. The effect produced by most of these organisms 

 upon milk is striking. 



Of the large number of organisms found in milk, two or three 

 seem to be characteristic. The first is the one that produces the 

 ordinary souring of milk {Bacillus acidi lactici). This organism, 

 upon being introduced into sterilized milk, grows rapidly, and soon 

 breaks up the milk-sugar that is present into either lactic or acetic 

 acid and carbonic acid. The acid thus formed causes the milk 

 both to curdle, by hardening or coagulating its albuminous matter, 

 and to acquire its well-known sour taste and odor. This organism 

 is very abundant in the air in warm weather, but in the winter 

 seems to be much less abundant : indeed, it can at times almost be 

 said to be absent. Milk has been kept in an open dish in the 

 laboratory, during cold weather, for two weeks without its going 

 through the characteristic changes of souring. It finally curdled, 

 but with a peculiar odor of decay, and did not sour in the typical 

 manner at all. The vessel in this case was absolutely clean, so 

 that the air was the only source of contamination. The changes 

 which did take place were produced by bacteria other than the 

 common sour-milk bacterium, this one not seeming to be present 

 at all. The fact that the typical souring was thus prevented shows 

 that the common sour-milk bacterium was not present in the air at 

 the time, at least in any great quantity. Such an experiment 

 would not succsed in the summer. 



A second species almost always found in milk is Oidiuni lactis. 

 This produces no important change in milk. It grows rapidly, but 

 does not cause the milk to sour or curdle. Besides the two men- 

 tioned, a large number of other species have marked effects upon 

 milk. 



Action of Different Kinds of Bacteria in Milk. 



As concerns their action, we may divide them into four classes : 

 I. Some, like the bacteria of sour milk, cause the milk to sour by 

 breaking up the milk-sugar into lactic or acetic acid and carbonic 

 acid : curdling of the milk results. 2. Many produce the same 

 result, but only at somewhat higher temperatures. At ordinary 

 temperatures, they grow, but do not curdle the milk ; in a warm 

 oven, however, the milk will soon curdle. Accordingly, these 

 would sour and curdle the milk in summer, but would not do so, 

 or would do so less readily, in winter. The temperature and time 

 required to produce the curdling differ with different species of 

 bacteria. 3. Some do not have the power of breaking up milk- 

 sugar, do not produce any acid, and do not coagulate the milk. 

 The milk remains liquid, and sometimes becomes decidedly alka- 



line. 4. A few species curdle the milk, but produce no acid, the 

 milk becoming alkaline instead. The majo/ity of bacteria of milk 

 and cream which have been experimented upon produce a souring 

 and curdling of milk at some temperature. Experiments have also 

 indicated that the action in all these species is somewhat similar ; 

 i.e., the breaking-up of the milk-sugar into an acid and some other 

 product. But, although the action is thus fundamentally the 

 same, the details of the action vary with each different species of 

 bacteria. 



The curdling is very different in character with different species. 

 In some cases a hard curd and a clear liquid are formed ; in others 

 a curd is formed, but no liquid is separated from it ; in still other 

 cases the whole milk is turned into a semi-gelatinous mass. Some- 

 times the curd is easily broken or cracked, like the curd of common 

 milk ; in other cases it is very tenacious, sticky, and slimy. Some- 

 times the curd is dissolved in a few days, and the milk is left as a 

 clear and almost transparent liquid. Here the caseine seems to 

 undergo a change similar to digestion ; i.e., conversion into pep- 

 tones. 



In connection with the curdling, there also arises in all cases a 

 characteristic odor, which differs with different species of bacteria. 

 There is a sour smell, a smell like sour bread, a smell like soft-soap, 

 like salt mackerel, like a pig-pen, like the barnyard, and in many cases 

 a smell of putrefaction. Besides these, there are others that cannot 

 be described because of the lack of words in our language to distin- 

 guish odors. As far as the studies have gone, the effect of each 

 species of bacteria upon the milk seems to be different from all 

 others. The dairyman or the housewife would in most cases say 

 that the milk had soured, but careful study shows that in reality 

 the different bacteria do produce effects differing to a greater or 

 less extent. The results of the experiments seem to indicate that 

 what is commonly known as the souring of milk is not always- 

 caused by the common sour-milk bacterium, as has been usually, 

 supposed, but is frequently produced by others, and that the prod- 

 ucts formed are different. Particularly is this true in winter. 



Bacteria in Cream. 



Experiments were undertaken in the expectation that the so- 

 called " ripening" of cream would prove to be a definite change 

 due to the growth of bacteria. Having found that the souring of 

 milk is less simple than had been supposed, one is prepared to find 

 that the "ripening" of cream is also a complex process. It is not 

 easy to say just what is meant by " ripened " cream. In ordinary- 

 farm practice, cream is usually allowed to stand for a few days be- 

 fore churning, when it becomes somewhat thickened, and acquires 

 a pleasantly sour odor. In the creameries the cream is also 

 ripened, though for a shorter time, and it does not become so thick 

 or so sour. That the ripening is due to the growth of bacteria 

 there can be no doubt. Ripened cream always contains these or- 

 ganisms in almost inconceivable abundance. In some places the 

 ripening is hastened by adding a little sour cream as a "starter."^ 

 This simply means the addition of a large number of bacteria, 

 which of course hastens the process. Sometimes an aitificial 

 starter in the form of an acid is added. This practice proceeds 

 upon the supposition that the ripening is due to the formation of an 

 acid, which is probably a secondary matter. It is doubtful if this 

 kind of a staiter has any definite value. 



By successive heatings, specimens of cream have been deprived 

 of all bacteria, and it is then found that the cream remains un- 

 changed indefinitely. In these specimens of sterilized cream have 

 been planted the various species of bacteria that have been experi- 

 mented upon. All of them grow well in the cream, 'and each has 

 its characteristic effect ; but no one of them has yet been found to 

 produce exactly what would be called ripened cream. Some curdle 

 it ; some cause it to putrefy. From all of the experiments it may 

 be concluded that the ripening of cream is a complex matter. The 

 souring is apparently due to a process similar to the souring of 

 milk ; the thickening, in part to the curdling of the small amount 

 of rnilk left with the cream, and in part to immense numbers of 

 bacteria that develop. Another important factor in the ripening 

 of cream is the decomposition of the albuminous matter present. 

 In general we infer that different kinds of bacteria assist in the 

 ripening of cream, but doubt whether any one has such a definite 



