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

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

Fig. 23.—Curd Test.
A good curd obtained from milk containing no harmful bacteria but many desirable acid-forming organisms.

The Wisconsin curd test is made as follows: Samples of the milk to be tested are placed in sterile pint fruit jars. The milk is warmed to 90° F., ten drops of rennet are added to each sample, and as soon as the curd is solid, it is cut into small pieces with a case knife so as to facilitate the expulsion of the whey. As the curd settles to the bottom of the vessel, the whey is poured off at intervals so that a pat of firm curd is left. As the milk curdles the bacteria are enmeshed and are carried with the curd. The jars are kept at a temperature of 100° to 105° F., since this temperature is favorable to growth of the bacteria that are sought, the gas-forming organisms. At the end of ten to twelve hours, the jars are examined; if the curd is solid, the texture firm, not mushy or slimy on the surface, if the odor is agreeable, it indicates that the milk contains few or none of the undesirable forms of bacteria. If the curd is full of gas holes, it is apparent that undesirable bacteria are present and under such circumstances the curd will not have an agreeable odor. If the gas-forming bacteria are numerous, the curd may even be spongy from the abundance of gas holes, and the undesirable odor more pronounced. Such curds are tough and rubbery. In some cases a bad flavor or odor is apparent even though the texture of the curd is not open and full of holes. The curd, the surface of which is slimy indicates undesirable organisms. A solid curd of agreeable odor is indicative of the presence of the desirable acid-forming bacteria. Such a milk is excellent from the standpoint of the butter or cheese maker, but may not be so desirable from the standpoint of the milk dealer on account of its poor keeping qualities. On the other hand a milk suitable from the standpoint of the milk dealer, on account of its low germ content, and hence good keeping quality, may give a poor curd test. It is certain to contain some bacteria, especially those from the interior of the udder while it may contain none of the desirable acid-forming organisms without which a curd of good texture and flavor can not be obtained. The bacteria in the clean milk will grow rapidly at the high temperatures at which the curds are kept and the changes they will produce as to flavor and odor may be undesirable. The milk might be judged as poor when in reality it might be a most excellent sample, and if kept at the ordinary storage temperatures, it might keep for days. The test when used for market milk should be interpreted with this in mind.

Fig. 24.—Curd Test.
The curd obtained from milk containing many gas-forming bacteria. The irregular, angular holes are mechanical, due to the imperfect fusion of the pieces of curd.

If the results are to be of any value, the test must be made with care to avoid all sources of error; the tester must know that the bacteria causing the gas and bad flavors in the sample were originally present in the milk at the time the sample was taken, and that they have not come from the containers used or from other sources. To insure these conditions the jars must be thoroughly cleaned and then sterilized just before use by placing them in cold water and bringing them to the boiling point, or sterilized by a thorough steaming. The sample of milk of a patron must be taken so as to avoid contamination from the milk of the other patrons. This can best be done by filling the jars as the milk is poured from the patron's can into the weigh can. In cutting the curds, the knife used must be dipped in hot water between each test to cleanse the same. In short, the test should be carried out with great care so that the tester is certain of the results obtained.

Other tests for the bacteriological condition of milk will be described in Chapter IX.

Overcoming abnormal fermentations. The lactic acid bacteria are often looked upon as normal to milk, and it is certain that they are to be classed as harmful, only as they injure the keeping qualities of milk. In milk designed for butter and cheese their presence is necessary. At times these desirable forms of bacteria may disappear, and be replaced by less desirable types. In one case it was observed that the usual lactic bacteria had been replaced in a cheese factory supply by an acid-forming organism that produced an intensely bitter taste in the milk, thus rendering the cheese of no value. When such harmful forms appear, they must be overcome, and the normal types of bacteria replaced. A thorough cleaning of the milk utensils, attention to the cattle and all places from which such bacteria may find their way into the milk is often sufficient to cause a disappearance of the trouble. If the acid-forming bacteria have disappeared, the inoculation of the milk with cultures in ways later to be discussed is often of advantage. At times more stringent measures must be employed in order to destroy the harmful bacteria, such as the use of strong disinfectants.

Disinfection and disinfectants. If any building or room becomes infected with disease-producing bacteria, or if organisms causing abnormal fermentations become established in a factory, the use of a disinfectant that will destroy with great rapidity the life of bacteria is necessary. The disinfection of all types of dairy apparatus and utensils can be accomplished by thorough cleansing, and by the use of steam or boiling water. The disinfection of rooms and stables cannot be so readily accomplished.

Consideration must always be given to the resistance of the organism it is desired to destroy. Those that form spores are very resistant toward all chemical agents, while those that do not produce these resistant bodies are easily killed. In the dairy and factory, it is often necessary to destroy the organisms that develop in decomposing organic matter. Here, as in all disinfection, a thorough cleaning should precede the application of any disinfectant. Some chemicals act as deodorants, i.e., destroy the offensive odor, without removing the cause. It is impossible effectually to destroy bacteria embedded in a mass of organic matter, and through the removal of the material itself, the larger part of the bacteria will be removed. The disinfectant then comes in direct contact with the surface to be disinfected, consequently destroys the bacteria not removed in the cleaning.