FERMENTS

Two classes of ferments are of importance in connection with milk: (1) “unorganized” or chemical ferments, the “enzymes,” and (2) “organized” ferments such as bacteria and yeast.

Enzymes

Rennet.—Among the unorganized ferments, Rennet or Rennin is highly important on account of its power of coagulating or curdling milk by precipitation of the casein. Rennet is extracted from the stomach of the suckling or milk-fed calf, where it serves in digesting the calf’s food. It is in the market in the form of a liquid extract as well as a dry powder compressed into tablets (Rennet Tablets and Junket Tablets). The characteristics and use of rennet are described under “Cheese Making” in Chapter [III].

Pepsin is another enzyme the office of which in the process of digestion is to dissolve albuminoids. It is not considered identical with rennet though in an acid solution it will curdle milk. It occurs in the stomachs of grown animals fed on solid food and is usually produced from hogs’ stomachs.

Bacteria

Everywhere, in the air, in water, in the soil, and clinging to every object in the world, are minute organisms known under the common names of bacteria or microbes. In contrast to rennet and the other “unorganized” ferments, bacteria belong to the “organized” class. Some are harmful, producing putrefaction, dissolution, poisons or disease; others are beneficial, leading to desirable fermentations and changes; others again are indifferent, neither good nor bad, but harmless.

As the milk comes from the cow it is almost free from bacteria, but milk makes an excellent soil for many of these organisms to grow in, and they soon get in, to multiply with enormous rapidity at any temperature from 60° to 100° F.

Lactic Acid Bacilli are bacteria of special importance to the dairyman, for they convert sugar of milk into lactic acid and produce various more or less agreeable flavors. They are also powerful germicides and scavengers, destroying or neutralizing the products of other bacteria which in the absence of these bacteria and the lactic acid produced by them would play havoc with the food and produce putrefaction or disease. Everybody who handles milk knows that pure sour milk or buttermilk in which lactic acid bacteria abound keeps well for a long time, free from other fermentations which have no chance to develop in their presence. It is due to this purifying property that Metchnikoff recommended Bulgarian sour milk as a health food, asserting that it prevents harmful fermentations in the digestive channel.

The Control of Bacteria

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.
501012154162
68101724261,2803,574,990

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.