Human Foods and Their Nutritive Value - Harry Snyder (e book reader pc TXT) 📗
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26. Raw and Cooked Foods Compared.—Raw and cooked foods differ in chemical composition mainly in the content of water. The amount of nutrients on a dry matter basis is practically the same, but the structural composition is affected by cooking, and hence it is that a food prepared for the table often differs appreciably from the raw material. Cooked meat, for example, has not the same percentage and structural composition as raw meat, although the difference in nutritive value between a given weight of each is not large. During cooking, foods are acted upon chemically, physically, and bacteriologically, and it is usually the joint action of these three agencies that brings about the desirable changes incident to their preparation for the table.
27. Chemical Changes during Cooking.—Each of the chemical compounds of which foods are composed is influenced to a greater or less extent by heat and modified in composition. The chemistry of cooking is mainly a study of the chemical changes that take place when compounds, as cellulose, starch, sugar, pectin, fat, and the various proteids, are subjected to the joint action of heat, moisture, air, and ferments. The changes which affect the cellulose are physical rather than chemical. A slight hydration of the cellular tissue, however, does take place. In human foods cellulose is not found to any appreciable extent. Many vegetables, as potatoes, which are apparently composed of cellular substances, contain but little true cellulose. Starch, as previously stated, undergoes hydration in the presence of water, and, at a temperature of 120° C., is converted into dextrine. At a higher temperature disintegration of the starch molecule takes place, with the formation of carbon monoxid, carbon dioxid, and water, and the production of a residue richer in carbon than is starch. On account of the moisture, the temperature in many cooking operations is not sufficiently high for changes other than hydration and preliminary dextrinizing. In Chapter XI is given a more extended account of the changes affecting starch which occur in bread making.
During the cooking process sugars undergo inversion to a slight extent. That is, sucrose is converted into levulose and dextrose sugars. At a higher temperature, sugar is broken up into its constituents—water and carbon dioxide. The organic acids which many fruits and vegetables contain hasten the process of inversion. When sugar is subjected to dry heat, it becomes a brown, caramel-like material sometimes called barley sugar. During cooking, sugars are not altered in solubility or digestibility; starches, however, are changed to a more soluble form, and pectin—a jelly-like substance—is converted from a less to a more soluble condition, as stated in Chapter I. Changes incident to the cooking of fruits and vegetables rich in pectin, as in the making of jellies, are similar to those which take place in the last stages of ripening.
The fats are acted upon to a considerable extent by heat. Some of the vegetable oils undergo slight oxidation, resulting in decreased solubility in ether, but since there is no volatilization of the fatty matter, it is a change that does not materially affect the total fuel value of the food.[11]
There is a general tendency for the proteids to become less soluble by the action of heat, particularly the albumins and globulins. The protein molecule dissociates at a high temperature, with formation of volatile products, and therefore foods rich in protein should not be subjected to extreme heat, as losses of food value may result. During cooking, proteids undergo hydration, which is necessary and preliminary to digestion, and the heating need be carried only to this point, and not to the splitting up of the molecule. Prolonged high temperature in the cooking of proteids and starches is unnecessary in order to induce the desired chemical changes. When these nutrients are hydrated, they are in a condition to undergo digestion, without the body being compelled to expend unnecessary energy in bringing about this preliminary change. Hence it is that, while proper cooking does not materially affect the total digestibility of proteids or starches, it influences ease of digestion, as well as conserves available energy, thereby making more economical use of these nutrients.
Fig. 6.—Cells of
a Partially Cooked
Potato. (After König.)
28. Physical Changes.—The mechanical structure of foods is influenced by cooking to a greater extent than is the chemical composition. One of the chief objects of cooking is to bring the food into better mechanical condition for digestion.[12] Heat and water cause partial disintegration of both animal and vegetable tissues. The cell-cementing materials are weakened, and a softening of the tissues results. Often the action extends still further in vegetable foods, resulting in disintegration of the individual starch granules. When foods are subjected to dry heat, the moisture they contain is converted into steam, which causes bursting of the tissues. A good example of this is the popping of corn. Heat may result, too, in mechanical removal of some of the nutrients, as the fats, which are liquefied at temperatures ranging from 100° to 200° F. Many foods which in the raw state contain quite large amounts of fat, lose a portion mechanically during cooking, as is the case with bacon when it is cut in thin slices and fried or baked until crisp. When foods are boiled, the natural juices being of somewhat different density from the water in which they are cooked, slight osmotic changes occur. There is a tendency toward equalization of the composition of the juices of the food and the water in which they are cooked. In order to achieve the best mechanical effects in cooking, high temperatures are not necessary, except at first for rupturing the tissues; softening of the tissues is best effected by prolonged and slow heat. At a higher temperature many of the volatile and essential oils are lost, while at lower temperatures these are retained and in some instances slightly developed. The cooking should be sufficiently prolonged and the temperature high enough to effectually disintegrate and soften all of the tissues, but not to cause extended chemical changes.
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.
31. Soluble Ferments.—Many plant and animal cells have the power of secreting substances soluble in water and capable of producing fermentation changes; to these the term "soluble ferments," or "enzymes," is applied. These ferments have not a cell structure like the organized ferments. When germinated seed, as malted barley, is extracted, a soluble and highly nitrogenous substance, called the diastase ferment, is secured that changes starch into soluble forms. The soluble ferments induce chemical change by causing molecular disturbance or splitting up of the organic compounds, resulting in the production of derivative products. They take an important part in animal and plant nutrition, as by their action insoluble compounds are brought into a soluble condition so they can be utilized for nutritive purposes. In many instances ferment changes are due to the joint action of soluble and insoluble ferments. The insoluble ferment secretes an enzyme which induces a chemical change, modified by the further action of the soluble ferment. Many of
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