Fig. 7.—Cells of
Raw Potato, Showing Starch
Grains. (After König.)
There is often an unnecessarily large amount of heat lost through faulty construction of stoves and lack of judicious use of fuels, which greatly enhances the cost of preparing foods. Ovens are frequently coated with deposits of soot; this causes the heat to be thrown out into the room or lost through the chimney, rather than utilized for heating the oven. In an ordinary cook stove it is estimated that less than 7 per cent of the heat and energy of the fuel is actually employed in bringing about physical and chemical changes incident to cooking.[[13]]
29. Bacteriological Changes.—The bacterial organisms of foods are destroyed in the cooking, provided a temperature of 150° F. is reached and maintained for several minutes. The interior of foods rarely reaches a temperature above 200° F., because of the water they contain which is not completely removed below 212°. One of the chief objects in cooking food is to render it sterile. Not only do bacteria become innocuous through cooking, but various parasites, as trichina and tapeworm, are destroyed, although some organisms can live at a comparatively high temperature. Cooked foods are easily re-inoculated, in some cases more readily than fresh foods, because they are in a more disintegrated condition.
In many instances bacteria are of material assistance in the preparation of foods, as in bread making, butter making, curing of cheese, and ripening of meat. All the chemical compounds of which foods are composed are subject to fermentation, each compound being acted upon by its special ferment body. Those which convert the proteids into soluble form, as the peptonizing ferments, have no action upon the carbohydrates. A cycle of bacteriological changes often takes place in a food material, one class of ferments working until their products accumulate to such an extent as to prevent their further activity, and then the process is taken up by others, as they find the conditions favorable for development. This change of bacterial flora in food materials is akin to the changes in the vegetation occupying soils. In each case, there is a constant struggle for possession. Bacteria take a much more important part in the preparation of foods than is generally considered. As a result of their workings, various chemical products, as organic acids and aromatic compounds, are produced. The organic acids chemically unite with the nutrients of foods, changing their composition and physical properties. Man is, to a great extent, dependent upon bacterial action. Plant life also is dependent upon the bacterial changes which take place in the soil and in the plant tissues. The stirring of seeds into activity is apparently due to enzymes or soluble ferments which are inherent in the seed. A study of the bacteriological changes which foods undergo in their preparation and digestion more properly belongs to the subject of bacteriology, and in this work only brief mention is made of some of the more important parts which microörganisms take in the preparation of foods.
30. Insoluble Ferments.—Insoluble ferments are minute, plant-like bodies of definite form and structure, and can be studied only with the microscope.[[1]] They are developed from spores or seeds, or from the splitting or budding of the parent cells. Under suitable conditions they multiply rapidly, deriving the energy for their life processes from the chemical changes which they induce. For example, in the souring of milk the milk sugar is changed by the lactic acid ferments into lactic acid. In causing chemical changes, the ferment gives none of its own material to the reacting substance. These ferment bodies undergo life processes similar to plants of a higher order.
Fig. 8.—Lactic Acid
Bacteria, Much Enlarged.
(After Russell.)
All foods contain bacteria or ferments. In fact, it is impossible for a food stored and prepared under ordinary conditions, unless it has been specially treated, to be free from them. Some of them are useful, some are injurious, while others are capable of producing disease. The objectionable bacteria are usually destroyed by the joint action of sunlight, pure air, and water.