CHAPTER VI
THE PREPARATION OF SOURED MILK IN THE HOUSE
There is no great difficulty in making soured milk at home: the necessary operations are quite simple, but at the same time they must be conducted with precision and care, otherwise the results may be unsatisfactory and disagreeable; there may even sometimes be danger in badly prepared sour milk. It is always an advantage in such matters to understand the reason of things, and a few notes on the surrounding conditions, and what has to be accomplished, may be of assistance to the would-be experimenter.
The majority of intelligent people are now acquainted with the fact that the germs of bacteria are to be found everywhere on the surface of the earth, in air, and in water, and that they are the sole cause of the decay of all manner of perishable articles.
The distribution is unequal—bacteria are much more plentiful where there is decaying matter—in dirty houses, sewage, or other contaminated water, etc. Milk is a splendid food for bacteria, and numerous varieties multiply in it exceedingly, and many of these are injurious, producing putrefactive changes which render the milk unwholesome, even poisonous in some cases. Others are beneficial, and are absolutely necessary for the souring of milk for making butter or cheese and for the ripening of the latter. The soured milk which is the subject of this book is the work of certain lactic-acid-producing bacteria, and the problem we have before us is to encourage the growth of the latter to the uttermost and to exclude the others.
As bacterial germs are present in the air and readily sow themselves into any medium with which they come in contact, the first consideration is to get good fresh milk which has been as little exposed to the air as possible. The second is to conduct the experiment where the germs are fewest, and in cleanly surroundings, far removed from decaying matter and free from taints and smells.
Fig. 38.—Photo-micrograph of smear of one-month culture of Bacillus bulgaricus. In spite of its age, the culture is perfectly free from any foreign organisms, which would otherwise lower its value for the preparation of soured milk, and might, indeed, make it directly injurious.