Outlines of Dairy Bacteriology, 8th edition / A Concise Manual for the Use of Students in Dairying

by H. L. Russell

A bacteriological examination of the various cultures which have been placed on the market not infrequently reveals an impure condition. In several cases the writer has found a not inconsiderable number of liquefying bacteria mixed with the selected organism. Molds not infrequently are found in cultures put up in the dry form. Doubtless the effect of these accidental contaminations is considerably less in the case of a starter composed of a distinctively lactic acid-producing organism than with a form which is less capable of thriving vigorously in milk, and it should be said that these impurities can frequently be eliminated by continued propagation.

The virility and vigor of the starter is also a fluctuating factor, dependent in part at least, upon the conditions under which the organism is grown. In some cases the germ is cultivated in solutions in which acid cannot be formed in abundance. Where the conditions permit of the formation of acid, as would be the case if sugar was present with a lactic acid-producing species, the vitality of the culture is often impaired by the action of the gradually accumulating acid. Some manufacturers attempt to minimize this deleterious condition by adding carbonate of lime which unites with the acid that is formed.

Propagation of starters for cream-ripening. The preparation and propagation of a starter for cream-ripening is a process involving considerable bacteriological knowledge, whether the starter is of domestic origin or prepared from a pure-culture ferment. In any event, it is necessary that the starter should be handled in a way so as to prevent the introduction of foreign bacteria as far as possible. It should be remembered at all times that the starter is a live thing and must be handled throughout its entire history in a way so as to retain its vitality and vigor unimpaired. The following points should be taken into consideration in growing the starter and transferring it from day to day:

1. If a commercial starter is used, see that it is fresh and that the seal has not been broken. If the culture is too old, the larger part of the organisms may have died out before it is transferred, in which case the effect of its addition to the sterilized milk would be of little value.

When the commercial ferment is received, it should be stored in the refrigerator pending its use so as to retard as much as possible the changes that naturally go on in the culture liquid. Be careful that the bottle is not exposed to the influence of direct sunlight for in a transparent medium the organisms may be readily killed by the disinfecting action of the sun's rays.

2. If a home-made starter is employed, use the greatest possible care in selecting the milk that is to be used as a basis for the starter.

3. For the propagation and perpetuation of the starter from day to day, it is necessary that the same should be grown in milk that is as germ-free as it is possible to secure it. For this purpose sterilize some fresh skim-milk in a covered can that has previously been well steamed. This can be done easily by setting cans containing skim-milk in a vat filled with water and heating the same to 180° F. or above for one-half hour or more. Steam should not be introduced directly. This process destroys all but a few of the most resistant spore-bearing organisms. This will give a cooked flavor to the milk, but will not affect the cream to which the starter is added. Dairy supply houses are now introducing the use of starter cans that are specially made for this purpose.

4. After the heated milk is cooled down to about 70° or 80° F., it can be inoculated with the desired culture. Sometimes it is desirable to "build up" the starter by propagating it first in a smaller volume of milk, and then after this has developed, adding it to a larger amount.

This method is of particular value where a large amount of starter is needed for the cream-ripening.

5. After the milk has been inoculated, it should be kept at a temperature that is suitable for the rapid development of the contained bacteria, 65°-75° F., which temperature should be kept as uniform as possible.