May 15, 1891.] 



SCIENCE. 



273 



rendering the milk sour to the taste and precipitating the caseine 

 in the form of the curd. 



Hardly less familiar to you all is the fermentation produced by 

 the action of rennet. You will allrecall this action produced by 

 the addition to the milk of a little rennet which you have ob- 

 tained from a calf's stomach. The milk curdles quickly, and after 

 a little a whey separates from the curd. In this case the action is 

 quite different from that of the souring. It is entirely indepen- 

 dent of the milk sugar, and is connected with the caseine of the 

 milk. The caseine undergoes a chemical change under the influ- 

 ence of the rennet. In common sweet milk the caseine is in a 

 condition of partial solution, and while it is in solution the milk 

 is of course a liquid. But under the influence of the rennet a 

 chemical change takes place, the nature of which we do not yet 

 fully understand. So far as we can determine to-day, the change 

 consists of a separation of the caseine into two parts, oneof which is 

 soluble, and therefore remains in solution in the whey, while the 

 other is insoluble, and as soon as it is formed it is immediately 

 precipitated as the curd. While, then, the souring of milk con- 

 cerns the milk sugar alone, the fermentation by rennet is con- 

 nected only with the caseine. 



A third form of milk fermentation is the alcoholic fermentation. 

 Milk does not readily undergo the alcoholic fermentation. When 

 yeast is added to a solution of ordinary cane sugar it causes the 

 sugar to be decomposed into alcohol and carbonic acid. If yeast 

 is put into milk, however, instead of undergoing an alcoholic fer- 

 mentation, it wiil under ordinary conditions undergo a change 

 into lactic acid, and will consequently sour. Nevertheless, an al- 

 coholic fermentation of milk does sometimes occur. The Arabs, 

 wandering around the deserts, have been for a long time accus- 

 tomed to prepare from the milk of theur mares an intoxicating 

 drink which contains considerable alcohol. This drink they call 

 " koumiss." It is prepared by simply putting the milk into ilasks, 

 and adding to it a little already fermented milk, which starts the 

 process anew, and soon gives rise to a considerable amount of 

 alcohol. In the Caucasus Itfountains it has somewhat recently 

 been noticed that the common people have a method of preparing 

 an alcoholic drink from ordinary cows' milk. The milk is placed 

 in leather-flasks, and there is added to it some small lumps called 

 " kephir grains." These kephir grains contain various yeasts and 

 bacteria, and they are possessions of the common people, who 

 hand them down from generation to generation. Where they 

 originally came from is unknown. They have the power of set- 

 ting up fermentation in the milk, at first the ordinary lactic fer- 

 mentation, but this is soon superseded by the formation of alcohol, 

 a,nd on the second day the milk is in condition to drink. Since it 

 has been found that milk can be made to undergo an alcoholic 

 fermentation, k simple method has been discovered of producing 

 it at will from cows' milk. All that is necessary to do is to add 

 to the milk a little ordinary cane sugar and then a little yeast, 

 and the fermentation that takes place will produce alcohol, and 

 give us a beverage to which the Arab name " koumiss " is applied. 

 This condition of milk is frequently prescribed as a food in hospi- 

 tals, since it seems to be more easily digested than ordinary milk, 

 the caseine being coagulated into small flakes that are readily 

 acted on by the digestive juices. 



The next fermentation that we will notice is that producing 

 bitter milk. All of you must be familiar with this peculiar trouble. 

 At certain seasons of the year, especially in the fall, milk seems 

 to iiave a tendency to become extremely bitter without becoming 

 sour. Quite naturally, this has been ascribed to some special food 

 which the animals get hold of at this season. It is, however, a 

 troublesome matter, for it spoils the milk and injures it for all 

 dairy purposes. 



A fermentation, not quite so common, but far more trouble- 

 some when it occurs, is that known as slimy milk. Perhaps 

 some of you hive had experience with this milk, that can be more 

 readily sold by the yard than by the quart or gallon. The milk, 

 after milking, rapidly becomes viscous, thickening to such an ex- 

 tent that the vessel in which it is placed may be inverted without 

 spilling the milk. So slimy does it become that it can sometimes 

 be pulled out into long threads, like molasses candy. Such milk 

 is of course worthless. It cannot be churned, the cream will not 



rise on it, and it is useless for cheese-making. Of course no one 

 wants to drink it. Up in Norway, however, the people are said 

 to be fond of drinking, or rather eating, this slimy milk, and have 

 learned to prepare it artificially by putting a small plant into the 

 milk. With us, however, it is nothing but a troublesome nuisance, 

 and the farmer who finds it in his milk usually tries every im- 

 aginable remedy to check it. 



Milk not infrequently undergoes a change by which it becomes 

 rancid. It has the smell of rancid butter, and chemical study has 

 shown that the trouble is due to the formation of the same mate- 

 rial which gives the taste to the rancid butter, viz., butyric acid. 

 Such a fermentation, though very common, is not ordinarily seen 

 in the dairy, since it is concealed by other more prominent 

 changes, and thus escapes notice. 



One of the commonest fermentations of milk is what we may 

 call that of alkaline curdling. Under its influence the milk cur- 

 dles without becoming acid. I am sometimes asked why milk 

 sometimes becomes " loppered " without losing its sweet taste. It 

 is due to the effect of the fermentation that we are now consider- 

 ing. Such a curdling seems to be similar to that produced by the 

 action of rennet. Indeed, careful study seems to indicate that 

 the two are almost, if not precisely, identical, and that these 

 alkaline fermentations are produced by the formation of a ferment 

 similar to rennet. This form of fermentation represents a class 

 of which there are many varieties. They are accompanied by 

 various odors and smells, and the milk seems to be undergoing 

 decomposition. The various forms of tainted milk may be usu- 

 ally ascribed to the class of fermentations now considered. They 

 are certainly very common, almost always occurring in milk 

 which has stood for a short time, but commonly they escape no- 

 tice, since the souring of the milk is so much more prominent that 

 it entirely conceals the alkaline curdling. Experiment, however, 

 easily isolates this fermentation. 



Once in a while dairymen are troubled by a blue milk, not blue 

 milk like that of the city restaurant, which is blue simply because 

 the cream has been removed from it and water added, but milk 

 which is blue from a special fermentation. Such milk appears 

 like other milk when it is drawn, but just about the time it begins 

 to sour, small blue patches may be seen in it. These patches in- 

 crease in size, and finally, by the time the milk is quite sour, it 

 has assumed a brilliant blue color. No one wants to drink such 

 milk, though it is probable that it would not do any injury if it 

 were drunk. There is no poison in it that chemists can discover, 

 and it has been fed to small animals like rats without doing any 

 injury. But still no one with his eyes open will drink it, and if 

 it is known that the mUk from a certain farm is subject to this 

 fermentation it wiU be thoroughly avoided. Sometimes this blue 

 milk becomes so common that it may almost be regarded as an 

 epidemic. 



Blue milk is not the only colored milk that arises as the result 

 of fermentation. Yellow milk sometimes occurs. I have had in 

 my laboratory mUk that is just the color of a lemon ; other speci- 

 mens with an amber color. Red milk is occasionally found, 

 sometimes occurring spontaneously as a troublesome infection, 

 and easily produced artificially in the bacteriologisfs laboratory. 

 Sometimes milk assumes a green color, though never quite so 

 brilliant as a grass green. Such milks usually have a vile odor, 

 and are plainly undergoing a putrefactive decomposition. A violet 

 milk is also occasionally seen. 



Lastly, I may mention a series of fermentations uuder the head 

 of miscellaneous. Various foi-ms of decomposition changes occur 

 which do not really belong under any of the above classes, and 

 which have not been sufficiently studied to enable us to say much 

 about them. They simply indicate that in the above list we have 

 by no means exhausted the fermentations which are likely to oc- 

 cur in milk, and that future study will reveal much more in this 

 line. 



It is only within a comparatively few years that this long list 

 of fermentations has been known. Little by little, as milk has 

 been studied by modern scientific methods, has the number of 

 these known fermentations increased, and nearly every year adds 

 one or more to the list of the fermentations to which milk is 

 known to be subject. 



