Human Foods and Their Nutritive Value | Page 8

hundredths of a per cent. The essential oils have no direct food value, but indirectly, like the organic acids, they assist in promoting favorable digestive action, and are also valuable because they impart a pleasant taste. Through poor methods of cooking and preparation, the essential oils are readily lost from some foods.
16. Mixed Compounds.--Food materials frequently contain compounds which do not naturally fall into the five groups mentioned,--carbohydrates, pectose substances, fats, organic acids, and essential oils. The amount of such compounds is small, and they are classed as miscellaneous or mixed non-nitrogenous compounds. Some of them may impart a negative value to the food, and there are others which have all the characteristics, as far as general composition is concerned, of the non-nitrogenous compounds, but contain nitrogen, although as a secondary rather than an essential constituent.
17. Nutritive Value of Non-nitrogenous Compounds.--The non-nitrogenous compounds, taken as a class, are incapable alone of sustaining life, because they do not contain any nitrogen, and this is necessary for producing proteid material in the animal body. They are valuable for the production of heat and energy, and when associated with the nitrogenous compounds, are capable of forming non-nitrogenous reserve tissue. It is equally impossible to sustain life for any prolonged period with the nitrogenous compounds alone. It is when these two classes are properly blended and naturally united in food materials that their main value is secured. For nutrition purposes they are mutually related and dependent. Some food materials contain the nitrogenous and non-nitrogenous compounds blended in such proportion as to enable one food alone to practically sustain life, while in other cases it is necessary, in order to secure the best results in the feeding of animals and men, to combine different foods varying in their content of these two classes of compounds.[7]
NITROGENOUS COMPOUNDS
18. General Composition.--The nitrogenous compounds are more complex in composition than the non-nitrogenous. They are composed of a larger number of elements, united in different ways so as to form a much more complex molecular structure. Foods contain numerous nitrogenous organic compounds, which, for purposes of study, are divided into four divisions,--proteids, albuminoids, amids, and alkaloids. In addition to these, there are other nitrogenous compounds which do not naturally fall into any one of the four divisions.
[Illustration: FIG. 4.--APPARATUS USED FOR DETERMINING TOTAL NITROGEN AND CRUDE PROTEIN IN FOODS.
The material is digested in the flask (3) with sulphuric acid and the organic nitrogen converted into ammonium sulphate, which is later liberated and distilled at 1, and the ammonia neutralized with standard acid (2).]
Also in some foods there are small amounts of nitrogen in mineral forms, as nitrates and nitrites.
19. Protein.--The term "protein" is applied to a large class of nitrogenous compounds resembling each other in general composition, but differing widely in structural composition. As a class, the proteins contain about 16 per cent of nitrogen, 52 per cent of carbon, from 6 to 7 per cent of hydrogen, 22 per cent of oxygen, and less than 2 per cent of sulphur. These elements are combined in a great variety of ways, forming various groups or radicals. In studying the protein molecule a large number of derivative products have been observed, as amid radicals, various hydrocarbons, fatty acids, and carbohydrate-like bodies.[8] It would appear that in the chemical composition of the proteins there are all the constituents, or simpler products, of the non-nitrogenous compounds, and these are in chemical combination with amid radicals and nitrogen in various forms. The nitrogen of many proteids appears to be present in more than one form or radical. The proteids take an important part in life processes. They are found more extensively in animal than in plant bodies. The protoplasm of both the plant and animal cell is composed mainly of protein.
Proteids are divided into various subdivisions, as albumins, globulins, albuminates, proteoses and peptones, and insoluble proteids. In plant and animal foods a large amount of the protein is present as insoluble proteids; that is, they are not dissolved by solvents, as water and dilute salt solution. The albumins are soluble in water and coagulated by heat at a temperature of 157° to 161° F. Whenever a food material is soaked in water, the albumin is removed and can then be coagulated by the action of heat, or of chemicals, as tannic acid, lead acetate, and salts of mercury. The globulins are proteids extracted from food materials by dilute salt solution after the removal of the albumins. Globulins also are coagulated by heat and precipitated by chemicals. The amount of globulins in vegetable foods is small. In animal foods myosin in meat and vitellin, found in the yolk of the egg, and some of the proteids of the blood, are examples of globulins. Albuminates are casein-like proteids found in both animal and vegetable foods. They