Page:Popular Science Monthly Volume 58.djvu/254

246 it in virtue of a rennet-like ferment, whilst a third class precipitate and dissolve the casein of the milk, along with the development of butyric acid. The process whereby milk is submitted to a heat of 65° to 70° C. for twenty minutes is known as pasteurization, and the milk so treated is familiar to us all as pasteurized milk. Whilst the pasteurizing process weeds out the lactic acid bacteria from the milk, a temperature of 100° C. for one hour is necessary to destroy the butyric acid organisms: and even when this has been accomplished there still remain in the milk the spores of organisms which are only killed after a temperature of 100° C. for three to six hours. It will, therefore, be seen that pasteurization produces a partial, not a complete sterilization of the milk as regards its usual bacterial inhabitants. The sterilization to be absolute would require six hours at boiling point. But for all ordinary practical purposes pasteurization is an adequate procedure. All practical hygienic requirements are likewise adequately met by pasteurization, if it is properly carried out, and the milk is subsequently cooled. Milk may carry the infection of diphtheria, cholera, typhoid and scarlet fevers, as well as the tubercle bacillus from a diseased animal to the human subject. For the purpose of rendering the milk innocuous, freezing and the addition of preservatives are inadequate methods of procedure. The one efficient and trustworthy agent we possess is heat. Heat and cold are the agents to be jointly employed in the process, viz., a temperature sufficiently high to be fatal to organisms producing a rapid decomposition of milk, as well as to those which produce disease in man; this is to be followed by a rapid cooling to preserve the fresh flavor and to prevent an increase of the bacteria that still remain alive. The pasteurized process fulfils these requirements.

In conjunction with Dr. Hewlett, I had occasion to investigate in how far the best pasteurizing results might be obtained. We found that 60° to 68° C. applied for twenty minutes weeded out about 90 per cent, of the organisms present in the milk, leaving a 10 per cent, residue of resistant forms. It was found advisable to fix the pasteurizing temperature at 68° C, in order to make certain of killing any pathogenic organisms that may happen to be present. We passed milk in a thin stream through a coil of metal piping, which was heated on its outer surface by water. By regulating the length of the coil, or the size of the tubing, or the rate of flow of the milk, almost any desired temperature could be obtained. The temperature we ultimately fixed at 70° C. The cooling was carried out in similar coils placed in iced water. The thin stream of milk was quickly heated and quickly cooled as it passed through the heated and cooled tubing, and whilst it retained its natural flavor, the apparatus