The second suggestion, methods of preparing and preserving food so that it shall be free from poisonous and harmful substances, indicates the necessity for some knowledge of bacteriology. The various fermentative and putrefactive changes which take place in food substances are caused principally by the growth in them of microscopic forms of plant life known by the general name of micro-organisms.
When micro-organisms grow in masses, as may be seen in the green and yellow molds of bread and cake, they are plainly visible to the naked eye, but to distinguish individuals a microscope of high magnifying power is necessary. The common mildew, the decay of apples, melons, and other fruits, the rot of vegetables, and the decomposition of eggs and meat are due to the transforming power of these invisible agents. One of the most common and best known is yeast, which has been more studied and is probably better understood than any of the ferments. It is frequently mentioned to illustrate the transforming power of these infinitely tiny forms of life. A bit of yeast is like a little mass of seeds, each a single cell; these, when they are placed in a proper medium—in other words, find the surroundings of food, moisture, and warmth necessary for their life—multiply with extraordinary rapidity, using what they require of the food in which they find themselves, decomposing sugar and starch and establishing changes which result in carbonic acid and alcohol as the chief products. We take advantage of the production of carbonic acid by yeast to make our loaves of light and wholesome bread.
Micro-organisms are everywhere: they exist in the earth and the sea; in plants and animals; on the surface of our bodies and in the digestive canal; in cooked and uncooked food; in refuse, particularly animal waste; on our clothing, books, furniture, and in the dust of the atmosphere. Wherever they find suitable food, warmth, and moisture they increase with wonderful rapidity, and, if undisturbed would in time completely transform the object upon which they fall. However, by removing any one of the factors necessary to their growth, they cease to multiply, and under such conditions some species remain inert, some die.
Like other forms of life, micro-organisms by their growth give rise to various products which may be either harmless or harmful. Of the latter may be mentioned noxious gases which pollute the air and poisonous substances which render our food unwholesome. The souring of milk, and the putrefactive changes which, in the presence of heat, so rapidly set in in eggs, meat, oysters, lobsters, crabs, and other albumen-containing foods, are among the results of their transforming power. Perhaps the most important point for us to consider here is, that it is highly