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SCIENCE 



[Vol. XVII. No. 432 



learning to recommend that all milk should be sterilized. Indeed, 

 doctors have for a long time been accustomed to recommend 

 boiled milk to patients, but formerly from a mistaken idea. It was 

 always supposed that boiling the milk rendered it more digestible, 

 just as cooking other food makes it more easy to digest. Within 

 recent times, however, we have learned that boiled milk is not 

 more easily digested than fresh milk, but, on tlie contrary, that 

 it is far less easily digested. If an animal is fed with a certain 

 quantity of boiled milk, and subsequently with an equal quantity 

 of fresh mOk, he will digest and absorb only about two-thirds as 

 much of the boiled milk as of the fresh milk. The reason that 

 boiled milk is better than unboiled milk for invalids is because of 

 the presence of bacteria in the latter. In our cities, as we have 

 seen, these are extremely abundant in all milk; and although to 

 the ordinary healthy person they are harmless, they may be a source 

 of irritation to one whose digestive organs are out of order, and 

 therefore in an irritable condition. It is believed that nearly all 

 of the cases of cholera infantum in our cities are due to the bac- 

 teria present in the milk drunk by infants. Nursing children are 

 much less liable to have the disease, since they obtain their milk 

 fresh and free from bacteria. It is not surprising that the doctors 

 in our cities are learning that one of the first things to do in the 

 case of intestinal diseases is to prevent the patient from taking in 

 the large quantities of bacteria which he would swallow with un- 

 sterilized milk. I know of one doctor who goes further, and 

 furnishes his patients with sterilized milk in order that he may be 

 sure they obtain it. 



There are two disadvantages in sterilizing milk by boiling. The 

 first is that the milk is not thereby completely sterilized, and is 

 likely to undergo some fermentation after a time. This is not a 

 very serious matter, however, for the milk thus sterilized is pretty 

 sure to be used before any ot these fermentations occur. Milk 

 that is sterilized is not usually intended for long preservation, but 

 for using immediately, or, at least, within a few days. This be- 

 ing the case, it is not a matter of much importance if some of 

 the spores of the resisting bacteria should be left in it in condition 

 to set up a fermentation after a week or more. 



The other disadvantage is a more serious one. The milk thus 

 sterilized has not the taste of fresh milk. Every one is acquainted 

 ■with the taste of boiled milk, and we all know that it is not so 

 pleasant as that of fresh milk. To some it is quite disagreeable, 

 and children frequently will not touch it. Now, any sort of ster- 

 ilization by boiUng is sure to cause the milk to acquire this taste 

 of boiled milk. This taste appears at about the temperature of 

 160° F., and, since all methods of sterilization by heat raise the 

 temperature much above that point, the taste of boiled milk is 

 always found accompanying such sterilization. 



Now, there is a method of sterilizing milk which avoids the 

 production of this taste, but it is long and tedious. If the milk 

 be heated to a temperature of 155" F. for twenty minutes upon 

 six successive days it is commonly found to be sterilized, and, 

 since it has not been heated to 160", its original taste will be pre- 

 served. Such a process is, of course, too long to be of any practi- 

 cal value, except for scientific experiment. 



The fact is, that with our present knowledge, there has been 

 devised no way of sterilizing milk without either producing the 

 disagreeable taste of boiled milk, or being so long about the pro- 

 cess as to render it of no value in practice. 



It is, however, possible to produce, with ease, a partial sterili- 

 sation. It is frequently of great value to one dealing with milk 

 to delay the soirring as long as possible, and if this fermentation 

 can be put off for a few hours even, it may prove of great use. 

 There has been invented in Paris a method of treating milk which 

 accomplishes just this. It is known by the name of pasteuriza- 

 tion. It consists simply in heating the milk for a few minutes to 

 a temperature of about 155°, or a little higher, and then rapidly 

 cooling it. The short heating does not indeed kill all the bacteria 

 that are in the milk, but it does very much diminish their num- 

 I)ers. So much does this heating check the bacteria growth that 

 it is found to delay the fermentation of milk from twenty-four to 

 forty hours. Of course such a delay as this is of the greatest 

 value in our cities. For accomplishing this pasteurization several 

 machines have been invented, all of which enable a large amount 



of milk to be heated in a short time. In some the milk is caused 

 to run over metal plates that are kept hot by steam ; in others the 

 milk is in a large vessel and the steam conducted into the vessel 

 in a coil of pipes. All of them accomplish the same purpose, but 

 not with equal facility. 



There is one advantage arising from pasteurization which ren- 

 ders its practice even more valuable. It is found that nearly all, 

 if not quite all, of the pathogenic disease germs which are likely 

 to occur in milk, are killed by the pasteurization. It is well rec- 

 ognized to-day that some of our dangerous epidemics are trans- 

 mitted from house to house by means of milk. Milk furnishes a 

 good medium for their growth, and has every chance of becoming 

 contaminated. In cities epidemics of typhoid have been repeat- 

 edly traced to the milk supjoly. Now, if pasteurization is sufiB- 

 cient to kill these disease germs, and if at the same time it delays 

 the souring from twenty to forty hours, and if the milk thus 

 treated retains the taste of fresh milk, and permits the cream to 

 rise on it in the natural way, it is plain that pasteurization is a 

 process which is highly to be recommended. It is not surprising 

 that in Paris, and in some of the large cities of France and Ger- 

 many, pasteurization of milk is becoming more and more com- 

 mon. In Paris it is a regular business, and pasteurized milk is 

 sold at a trifle advance over the price of ordinary milk. People 

 are beginning to prefer it, since it keeps so much better, and is so 

 much safer, and withal has all of the good qualities of fresh milk. 

 It has been suggested that pasteurization of milk in cities should 

 be required by law. So far as I am aware the pasteurization of 

 milk has not yet been introduced into America. 



Lastly, a word in regard to the value of cold in delaying fer- 

 mentation. Every one knows that milk will keep longer if it is 

 kept cool, and it can be preserved almost indefinitely when frozen. 

 But every one is not aware of the great value of a temporary 

 cooling of milk. When milk is drawn from the cow it is at a high 

 temperature, and is, indeed, at just the temperature at which the 

 bacteria will grow the best. The bacteria which get into the milk 

 during the milking, therefore, begin immediately to multiply with 

 great rapidity. If, however, the milk be cooled to as low a tem- 

 perattire as possible, it will take several hours' exposure to the 

 ordinary temperature of the air to bring it back again to the con- 

 dition where the bacteria will grow so rapidly. Indeed, except in 

 the very hottest summer weather, it will not again become so 

 warm as when it left the cow, and hence will not again offer such 

 a good chance for bacteria growth. It follows, then, that a cool- 

 ing of the milk immediately after milking is of the greatest possi- 

 ble value in enhancing its keeping properties. Milkmen should 

 remember that half an hour's cooling of the mUk, or even less 

 than that, immediately after milking, will save several hours in 

 the souring time, and in hot 'summer weather this fact should be 

 remembered as one of the best methods of assisting in supplying 

 customers with good milk. 



Allow me now to summarize the important points which have 

 attracted our attention this afternoon: 



1. The fermentations of milk are varied, although only a few 

 are commonly recognized because the souring of milk usually ob- 

 scures all other fermentations. 



3. All of the fermentations except the fermentations of rennet 

 are caused by micro-organisms getting into the milk after milking 

 and growing there. 



3. The micro-organisms are so abundant around the barn and 

 dairy that they- cannot be kept out of the milk by any amount of 

 care. 



4. The bacteria which produce the abnormal or unusual fer- 

 mentations, like slimy milk, bitter milk, etc., are, however, not so 

 common but that they may be prevented from entering the milk 

 in sufficient quantities to produce serious trouble. 



5. Filth is ordinarily their source, and cleanliness the means of 

 avoiding them. 



6. The souring of milk cannot be prevented even by the great- 

 est cleanliness. 



7. Salicylic acid in proportions of 1-1000 may be of some little 

 value in delaying the souring, but its use is not to be recommended 

 except in special cases. 



8. Milk can be entirely deprived of bacteria by the exposure to 



