Outlines of dairy bacteriology, 10th edition / A concise manual for the use of students in dairying

by H. L. Russell; E. G. Hastings

Types of acid-forming bacteria. When substances undergo decomposition, it is a common belief that compounds offensive to the odor and taste are formed; but such is not necessarily the case. The products of the decomposition may be as agreeable and as harmless as the compounds decomposed. Whether the decomposition products of any substance are offensive or not is dependent on the kinds of micro-organisms acting on it. There are forms of acid-producing bacteria that change milk in odor, taste, and appearance, yet the sour milk is not offensive in any sense of the word. Other bacteria also sour the milk, but produce offensive odors and a disagreeable taste. Thus, the acid-forming bacteria may be divided into two main groups, which may be designated as desirable and undesirable. This division is of importance to the butter and cheese maker and to the consumer of milk.

Desirable acid-forming bacteria. If milk is produced under clean conditions, it is not likely to have a disagreeable odor or taste at any time, even when it is sour; rather the taste is agreeable like that of good butter milk. The curd is perfectly homogeneous, showing no holes or rents, due to the development of gas, and there is but little tendency for the whey to be expressed from the curd. This type of fermentation is largely produced by the group of bacteria to which has been given the name, Bacillus lactis acidi.

The main by-product of this group of bacteria is lactic acid; small amounts of acetic acid and alcohol, with traces of other compounds, are also formed. The agreeable odor and to some extent the flavor of milk fermented by these bacteria is due to other by-products than lactic acid, for this has no odor and only a sour taste. The acid fermentation of milk is often called the lactic acid fermentation. In reality only the fermentation produced by the desirable group in which lactic acid is the most evident by-product should be thus called.

Fig. 20.—Different Types of Curds.
On the left a solid, homogeneous curd produced by desirable bacteria; on the right, the curd produced by harmful bacteria. Note the gas holes and free whey.

The bacteria of this group may enter the milk from the dust coming from the coat of the cow. They are also found in the barn dust and on cultivated plants. Under ordinary farm conditions, the larger part of those found in milk come directly from the utensils. If the milk is drawn under extremely clean conditions and care is taken to sterilize the utensils, but few acid-forming bacteria of any kind will enter the milk; under such conditions most of the acid-forming bacteria will belong to the group in question. They find, however, such favorable conditions for growth in milk that they develop more rapidly than most other types with which milk becomes seeded; consequently under normal conditions, they gain the ascendency and so control the type of fermentation.

The desirable type of acid-forming bacteria do not form spores; hence, are easily killed by heating the milk. They can grow in the presence or in the absence of free oxygen. In the bottom of a can of milk or in the middle of a cheese, there is no air, yet these bacteria grow as well under these conditions, as in milk exposed to the air. The range of temperature for growth varies from 50° to 100° F. but development is most rapid at 90° to 95° F. and about 1 per cent of acid is formed.

Another group of bacteria which may be classed among the desirable acid-forming organisms is constantly found in milk. They have little to do with the ordinary acid fermentation as they grow very slowly at ordinary temperatures. If a sample of raw milk is placed at the temperature of the animal body, the acidity will reach 1 per cent in a few hours. Thereafter the acidity will increase slowly and may reach three per cent or above. The continued increase in acid is due to the growth of long rods of the Bacillus Bulgaricus type, which apparently enter the milk with the fecal matter. The nature of the change produced by them in milk is very similar to that caused by Bact. lactis acidi in that lactic acid is the chief product; no gas is produced and hence the curd is uniform in appearance. Temperatures from 100° to 110° F. favor their development. Organisms belonging to this group are used in the preparation of the fermented milks now so widely sold in the cities.

These desirable, acid-forming bacteria are of the greatest service in every branch of the dairy industry, whether in butter or in cheese making, or in the sale of milk in the city. The dairy industry is dependent upon fermentative activity, as much as the manufacture of beer or wine, and the main basis of this is the acid fermentation of the milk by these desirable types of bacteria.

Although milk contains a large amount of nitrogenous substances (casein and albumen), it does not undergo putrid decomposition, as do meat and eggs, not because it is not fitted for the growth of the bacteria causing that type of change, but because the acid formed in it stops the growth of the putrefactive bacteria. If a sample of milk is placed in a stoppered bottle, it will have much the same taste and odor at the end of several months as at the end of a few days. The acid acts as a preservative, like the vinegar in pickles, or the acid in silage and in sauerkraut. Meat placed in a stoppered bottle which is then filled with milk will be preserved.