We recognize another kind of deterioration in meat and fish that have become tainted, even when no mold is visible, and there is no opportunity for ordinary fermentation. The microscope has given us eyes to see, and as a result of the patient work of the scientist with this instrument we now know that the difficulties in keeping food are caused by the presence of minute vegetable organisms known as molds, yeasts, and bacteria. It is impossible in some cases to draw a sharp line between these different forms of lower life, yet we are able to distinguish them sufficiently for practical purposes. [Illustration: FIG. 28.——Three species of mold. _Buchanan’s Household Bacteriology._] Masses of _mold_ that can be seen with the naked eye are distinguished by a feathery appearance and bright color. Figure 28 shows three species of the green mold that affects jam and jellies. Other species are found in Roquefort and Camembert cheese, and give the flavors characteristic in these cheeses. The presence of _yeast_ can be detected by its action, but it cannot itself be seen without the microscope. When canned fruit or homemade fruit juice “works,” yeasts cells are present in great number. Figure 29 shows one form of yeast, highly magnified, and Fig. 30 shows a single yeast cell. The yeast cake is a mixture of thousands of such cells with some flour or flour and meal, and the cells lie dormant in the cake, until we are ready to use them in bread. (See Chapter XII.) The actual yeast, however, is what Fig. 30 shows it to be, a tiny, one-celled plant, increasing in number by the division of the single cell, or by the budding out of one cell from another. When conditions are favorable the yeast cells increase in number with great rapidity, and some of the sugar that is present is broken down into carbon dioxide gas and alcohol. It is this gas that causes the familiar bubbling when fermentation is taking place. We put yeast cells into bread and cultivate it for this gas. But how does it occur in canned fruits, when its presence is not desired? _Wild yeast_ floats in the air, and lies upon the surface of fruit. All cultivated yeast has been derived from wild yeast. In old-fashioned ways of bread-making no yeast was introduced, a soft dough being left in a warm place to ferment naturally, the yeast cells probably being present in the flour. The yeast that spoils the canned fruit is present in the fruit, in the utensils, or can, and has not been killed as it should be in the canning process. [Illustration: FIG. 29.——One form of yeast. _Buchanan’s Household Bacteriology_] [Illustration: FIG. 30.——A yeast cell. _a_, cell wall. _b_, vacuole. _c_, granules. _d_, nucleus. _e_ and _e_, buds. _Buchanan’s Household Bacteriology._] [Illustration: FIG. 31.——The four types of bacterial cells. _A_, cocci. _B_, bacilli. _C_, spirilía. _D_, branched filamentous organism. _Buchanan’s Household Bacteriology._]