236. Mastication is an important part of digestion, and when foods are not thoroughly masticated, additional work is required of the stomach, which is usually an overworked organ because of doing the work of the mouth as well. Although much of the mechanical preparation and mixing of foods is of necessity done in the stomach, some of it may advantageously be done in the mouth. The stomach should not be required to perform the function of the gizzard of a fowl.
237. Palatability of Foods.--Many foods naturally contain essential oils and other substances which impart palatability. These have but little in the way of nutritive value, but they assist in rendering the nutrients with which they are associated more digestible. Palatability of a food favorably influences the secretion of the gastric and other digestive fluids, and in this way the natural flavors of well-prepared foods aid in digestion. In the feeding of farm animals it has been found that when foods are consumed with a relish better returns are secured than when unpalatable foods are fed. To secure palatability the excessive use of condiments is unnecessary. It is possible to a great extent during preparation to develop and conserve the natural flavors. Some foods contain bitter principles which are removed during the cooking, while in others pleasant flavors are developed. Palatability is an important factor in the digestibility of foods.
238. Physiological Properties of Food.--Some food materials, particularly fruits and vegetables, contain compounds which have definite physiological properties, as tannin which is an astringent, special oils which exert a cathartic action, and the alkaloids which serve as irritants to nerve centers. Wheat germ oil is laxative, and it is probable that the physiological properties of graham and whole wheat breads are due in some degree to the oil which they contain.[67] The use of fruits, herbs, and vegetables for medicinal purposes is based upon the presence of compounds possessing well-defined medicinal properties. As a rule food plants do not contain appreciable amounts of such substances, and the use of food for medicinal effect should be by the advice of a physician. The physiological properties of some foods are due to bacterial products. See Chapter XX.