The decay of foods of all kinds is due to certain minute organisms or germs, which have the power of producing certain chemical changes. They belong to the lowest form of plant life, and are so small that they cannot be seen by the naked eye or without the aid of a microscope. The air we breathe, the water we drink, all fruits and vegetables, are teeming with these minute living plants. There are a great many kinds of these germs, and, although some of them do great harm, the great majority of them are beneficial rather than injurious.
The organisms which concern the housewife can be divided into three classes: - Yeasts, Moulds, and Bacteria.
Every housewife is familiar with the yeast plant and its habits. It thrives in substances containing sugar, which it decomposes into a gas called carbon dioxide or carbonic acid gas and alcohol. It is this gas which makes it possible for us to procure a nice spongy loaf of bread, and when we wish to hasten fermentation in bread-making we generally add a little sugar to the yeast. Yeasts are readily killed at a high temperature, and so they are not a source of great trouble. The fermentation of canned fruits and preserves is usually due to the presence of yeasts.
Moulds, like yeasts, thrive in mixtures containing sugar, as well as in acid substances, and can grow on almost any kind of foodstuff in the presence of warmth and moisture. They are somewhat more resistant to heat than yeasts, but are killed at the temperature of boiling point. Moulds as a rule attack jellies, preserves, and pickles, but do not generally spoil canned fruits and vegetables. Moulds develop from spores, which are always floating about in the air, and if one of these spores settles on suitable food it will germinate if there is sufficient warmth. They thrive particularly well in darkness and dampness. Moulds are, however, not as injurious as bacteria and yeasts, and if discovered before they have penetrated too far, they can be removed a<nd the food used or resterilised.
The spoiling of vegetables is due primarily to bacteria. They are particularly fond of protein foods or foods rich in nitrogenous substances, such as meat of all kinds, fish, eggs, milk, and also peas, beans, lentils, and maize or mealies. Bacteria do not develop in substances containing a large percentage of sugar or acid, and, therefore, fruits and vegetables which are slightly acid are not favoured by bacteria, such as tomatoes, rhubarb, etc., nor do preserves, jams, jellies, and canned fruits support bacterial growth.
Bacteria are much more resistant to heat than either yeasts or moulds, and grow with amazing rapidity. One bacterium under favourable conditions will produce about twenty millions in the course of twenty-four hours. The reproduction of bacteria is brought about by one of two processes. The germ either divides itself in two parts, making two cells instead of one, while each cell in turn divides; or else it reproduces itself by means of spores. These spores can be likened to the seeds of an ordinary plant, and they are the chief difficulty in canning vegetables. The organisms themselves are readily killed at a temperature of boiling point, but the spores or seeds retain their vitality sometimes after several hours' boiling, and upon cooling will germinate. The method which is usually employed in scientific work is known as the three days' process or "intermittent" sterilisation, when the work is done on three successive days. The first day the vegetables are heated to boiling point and kept at that temperature for about one hour, and is repeated on two successive days. The boiling on the first day is said to kill all the organisms, and the second day their seeds or spores, which have had time to develop during the interval. The boiling on the third day is not always necessary, but is advisable in order to be sure that sterilisation is complete. However, the writer has been very successful with the continuous process, provided the articles of food have been sterilised the proper length of time. Instead of sterilising the food on three successive days for one hour, it may be done for two hours on two successive days.