Elements of the theory and practice of cookery

445. Microscopic examination. — A. Examine under a micro- scope a speck from one of the cloudy spots on the soup-stock, or any other spoiled food. What is it made up of ? 280 THE PRESERVATION OF FOOD 281 B. Examine from time to time a drop from a glass of water allowed to stand undisturbed. The drop is crowded with little living things. Can you pick out some shaped like lead-pencils, others like eggs or billiard-balls, and still others like corkscrews ? Make sketches of them. 446. Bacteria compared with yeast. — Bacteria are single-celled forms of plant-life, like yeast; and, like it, ^^,_^^_,^^ they multiply when they have warmth and food and mois- ture. They exist in QD %: o oo cPOo Fig. 15. — Shapes and groupings of differ- ent kinds of bacteria (much enlarged). Fig. 16. — Bacteria caus- ing typhoid fever, as seen under the micro- scope. far greater numbers than yeasts do, however, swarming in the air, in water, in the ground under our feet. One kind is so small that fifteen hundred in a row would hardly reach across the head of a pin. 447. The life-history of bacteria. — Bacteria multiply by dividing themselves in two, repeating this process 282 THEORY AND PRACTICE OF COOKERY so frequently that were there food and room enough for them all, the world would soon be crowded with bacteria. But there is no danger of this coming to pass ; for they die by millions constantly, one kind preying upon another, while all kinds are destroyed by the occurrence of conditions unfavorable to life. Cold checks their activity and growtli ; the temperature of boiling water, sometimes a lower temperature, kills them. Dry heat is not so fatal to them as moist heat. They have a way, when food and water are scarce, of entering upon a '' resting-stage." This corresponds, in a way, to the "going to seed" of bigger plants when winter comes on. The lively, growing bacteria dis- appear, but not till after they have produced spores, as yeast does when it gets old (§ 225). These little spores are hard to kill ; they seem able to last forever, ready to spring into life when right conditions return. 448. Fermentation and putrefaction. — The life-history of bacteria closely resembles that of yeast. Let us compare the action of yeast upon sugar with the action of bacteria upon soup-stock. The first is fermentation, the second putrefaction. In both cases organic matter is split into something simpler, but fermentation im- proves the flavor of the dough, and produces carbon dioxide to make it porous, while putrefaction spoils good food, and generates unpleasant-smelling gases. Some bacteria cause undesirable fermentation ; for example, that which takes place in fruit improperly canned. THE PRESERVATION OF FOOD 283