Souring of milk is produced by lactic-acid bacteria. During it some of the sugar of milk (lactose) is converted into lactic acid. (What is the effect of acid upon milk ?) Bacteria in living take what they need for food by breaking up the substances used. The products resulting from this, their life-activity, finally make their own growth impossible, though not essentially the growth of other bacteria. Lactic-acid bacteria ultimately cease to grow as milk sours. Souring then goes no further; the sugar that is then unchanged remains as sugar.

Milk sours easily; that is, lactic-acid fermentation occurs readily when milk is open to the air at or above the usual house-temperature (70° F.). Milk that does not sour under such conditions within a few hours has generally had some chemical added to counteract the acidity or prevent the fermentation.

Temperature below 70° F. checks souring, as it is unfavorable to bacterial growth. Milk should be kept on ice or in a cooled atmosphere; it should be cooled immediately after a milking, to avoid souring. Sudden change of temperature will often sour milk that has stood, as will mixing milk of different ages. Sour milk is of value in cooking; is advised as a drink by some diet-specialists, but only as the kinds of bacteria that it will contain are scientifically controlled.

In the house, as in the dairy, milk must be kept in clean utensils, covered but not air-tight (as some bacteria grow in it only in the absence of air), and at low temperature in an atmosphere free from odors, as milk readily absorbs these. It should be kept in an isolated compartment in a refrigerator.

Some Bacteria that may be in Milk.

a, lactic acid, produce souring; b, produce slimy milk. (Conn and Marshall).