CHAPTER II
CHANGES IN COMPOSITION OF FOODS DURING COOKING AND PREPARATION
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.