Page:Popular Science Monthly Volume 55.djvu/657

 Under these conditions the bacteria do not commonly have an opportunity of growing sufficiently to produce their poisonous products before the milk is consumed. Practically, then, these bacteria that resist the moderate heat of Pasteurization are of no serious importance in connection with the healthfulness of milk. Pasteurized milk has been deprived of all its strictly pathogenic bacteria, and the germs still left will commonly have no opportunity to grow very much before the milk is consumed. It is therefore the confident belief of many that Pasteurization is actually a safer method of treating milk than sterilization. Moreover, the results appear to be equally favorable, for Pasteurization is claimed to produce an effect upon diarrhœal diseases equal to that of sterilization.

But the most important argument for Pasteurization seems to be that it is really practical, and can be introduced upon a scale vastly more extended than can sterilized milk. The practice of Pasteurizing milk has doubtless been followed not a little by private families, but from the very outset it has appeared that the proper method of dealing with the matter is to treat the milk at a general distributing center, rather than to depend upon the consumer to do it. Not a few devices have been suggested for accomplishing the purpose satisfactorily and rapidly. The machines invented are planned upon two different principles. In one plan the milk is placed in some large vessel holding many gallons and is here heated, commonly by steam coils. It is allowed to remain here at the desired temperature for twenty minutes to half an hour, and is then cooled. This method is necessarily slow—so slow, indeed, that it is impractical for use where large amounts of milk must be treated rapidly for general distribution. It probably could not be used for the milk supply of a city. The other method is called that of continuous flow. Here the milk is allowed to flow continuously over a heated surface, which brings it quickly to the desired temperature. It is kept hot for only a short time, however, and it then flows over a cooled surface, where the temperature is brought down again and the milk is finally delivered from the machine in a continuous stream of cooled milk. Great objections have been urged against this process, from the fact that it is not thorough. The milk is retained at the high temperature for such a short time that many of the bacteria are not killed. The Pasteurization is decidedly less thorough than by the other method. But here, again, before condemning the process it is necessary to consider its purpose. If it is to destroy all the bacteria, or as large a number of them as is possible, it is of course unsatisfactory. If, however, the purpose is to treat the milk cheaply and rapidly in such a manner as to remove the danger of disease