Читать книгу The Story of Milk - Johan Ditlev Frederiksen - Страница 11

The principal means at our disposal to prevent or control and regulate bacterial growth are:

Cleanliness, Heat, Cold and Disinfectants.

Cleanliness.—Only the most scrupulous cleanliness will prevent contamination. Hence the necessity of thorough scouring and sterilizing of all utensils, and the need for fresh air and pure water.

Heat.—The prevention of fermentation in milk by heating is called “sterilization” or “pasteurization” according to the intensity of the heat and the length of time the milk is subjected to it. Boiling destroys almost all bacterial life. Some germs require, however, exposure to a much higher temperature, up to 250° F. (boiling under pressure), to be entirely eradicated, but for all practical purposes a thorough boiling is considered sufficient to eliminate all danger of contamination. Such boiling is usually termed Sterilization.

Effect of heating milk to various temperatures.

Published by the Toronto Board of Health

Cooling checks bacterial growth and but few germs thrive at a temperature below 50° F. The following table and diagram^[4] show how quickly bacteria multiply in milk at ordinary room temperature, 68° to 70°, which emphasizes the importance of keeping milk cool.

Relative growth of bacteria when held at different temperatures

Tem. of Milk	No. of bacteria per c.c. at beginning	At end of 6 hrs.	At end of 12 hrs.	At end of 24 hrs.	At end of 40 hrs.
°F.
50	10	12	15	41	62
68	10	17	242	61,280	3,574,990

Milk cooler

If the milk had contained 1,000 bacteria per cubic centimeter at the beginning, the part held at 50° F. would have contained 4,100 bacteria at the end of 24 hours, while that held at 68° F. would have contained 6,128,000. The effect of temperature upon the growth of bacteria is shown graphically in the cut.

But even frost does not kill the bacteria. If milk which has been kept sweet or at the desired degree of sourness by cooling is allowed to get warm again, the bacteria which have been kept dormant will get a fresh start. For this reason milk and cream for city supply should not only be cooled strongly, but must be kept thoroughly chilled up to the time they are used, which means that they should be delivered cool to the consumer, and kept on ice in the house, never being allowed to warm up until used.

The diagram (after Conn) shows the rapidity with which bacteria multiply in milk not properly cooled. A single bacterium (a) in 24 hours multiplied to 5 (b) in milk kept at 50° F.; (c) represents the number that develop from a single bacterium in milk kept 24 hours at 70° F.

Disinfectants.—Antiseptics, such as benzoate of soda, formalin, boracic acid, etc., are not permissible in milk, but disinfectants such as soda, lime, washing powders, etc., should be freely used in cleaning utensils, bottles, floors and walls where milk is handled, so as to prevent as far as possible any infection.