Читать книгу The Story of Milk - Johan Ditlev Frederiksen - Страница 12

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Louis Pasteur, French scientist and originator of the process of pasteurization.

Pasteurization depends upon the fact that almost all bacteria, and especially the disease-producing species, are checked in their growth and made harmless, if not totally destroyed, by instantaneous exposure to a temperature of 175° F., or a more or less prolonged exposure to lower temperature, for instance for 20 minutes to 157° or 30 minutes to 145°. These lower temperatures are recommended in the case of new milk to be consumed as such, in order not to change its digestibility. Pasteurizers with “holding devices” are therefore largely taking the place of the “continuous” sterilizing machines, which allow only momentary exposure to the highest temperature.

Whatever method is used it is essential that after the heating is finished the milk should be cooled as quickly as possible to a temperature sufficiently low to prevent development of the germs that have not been entirely destroyed by the heat and which, if the milk is left for any length of time at a temperature favorable for bacterial life—anywhere between 65° and 110°,—will begin to grow again. Cool the milk to below 60° and if possible to 50° or 40°.

Sometimes a second pasteurization is practiced, the milk being purposely allowed to stand at 70° to 80° for 12 to 24 hours after the first pasteurization for the germs which escaped destruction to develop into full-grown bacteria. They are then killed by the second heating before the milk is finally chilled to stop any further growth.

Pasteurization can be done by placing the milk in any tin or enamelled or glass vessel, set in another vessel containing water (a double boiler). Heat until the milk has reached the desired temperature, hold it at that temperature the necessary time, and then place the vessel in cold running water or in ice water until the milk is thoroughly chilled. It is not advisable to place the hot milk in the ice box as cooling in air is too slow. Not until it is thoroughly cooled in water is it safe to put it in the ice box to keep it cool.

N. J. Fjord, Danish investigator

In the seventies Prof. N. J. Fjord, in Denmark, applied to milk the process which had been developed by Louis Pasteur in France to give keeping quality to wine and beer. A Danish dairy expert, J. Moldenhauer, now connected with the New York State Department of Agriculture, first brought the process to this country and used pasteurization in a city creamery in Kentucky. American experiment stations established the temperatures and the time of exposure necessary for best results, and no one has done more for the introduction of pasteurization than the New York philanthropist, Nathan Straus, who has established pasteurizing plants and milk distributing stations in many localities, thereby contributing so largely to the lowering of the death rate among the children of the poor.

The Straus Pasteurizer

A continuous pasteurizer

The following directions are given by the Straus Pasteurized Milk Laboratories of New York for the pasteurization of milk for babies:

1. Only use fresh, filtered milk, which has been kept cold, and proceed as follows:

2. Set the bottles, after they have been thoroughly cleaned, into the tray (a), fill them to the neck, and put on the corks or patent stoppers.

3. The pot (b) is then placed on a wooden surface (table or floor) and filled to the three supports (in the pot) with boiling water.

4. Place tray (a), with the filled bottles, into the pot (b), so that the bottom of the tray rests on the three supports, and put cover (c) on quickly.

Milk cooler

5. After the bottles have been warmed up by the steam for five minutes, remove the cover quickly, turn the tray so that it drops into the water. The cover is to be put on again immediately. This manipulation is to be made very quickly, so that as little steam as possible can escape. Thus it remains for twenty-five minutes.

6. Now take the tray out of the water and cool the bottles with cold water and ice as quickly as possible, and keep them at this low temperature till used.

7. Before use, warm the milk—in the bottles—to blood heat. Never pour it into another vessel.

8. The milk must not be used for children later than twenty-four hours after pasteurization. Never use remnants.

In a Continuous Pasteurizer a constant stream of milk is fed into the machine, heated by flowing over a metal surface with steam or hot water on the opposite side, and cooled by running over a cooler furnished with a stream of cold water or ice water.