The large and varied class of foodstuffs derived from plants have a strikingly common feature as far as their chemical composition goes. They are all very rich in carbohydrates, and as a rule are poor in fats. They contain vegetable proteins in different proportions.
Vegetable proteins exist as vegetable albumin and vegetable casein. Vegetable albumin is not unlike egg albumin, and is coagulated by heat. Legumin or vegetable casein is coagulated by rennet and by acids, and is not coagulated by heat. There is a striking difference in the protein from animal and vegetable sources, the former yielding 11 per cent, gluta-minic acid on decomposition, the latter 37 per cent. Little is known as to the significance of this. Vegetable proteins do not contain many of the substances known as "nucleo-proteins," and are therefore considered by some authorities as a more suitable source of protein in gout. Extractives are present in vegetables in abundance, and belong to the group of chemical substances called amines. Asparagin, found in the potato, is one of the best known; these are of no value as foods.
Though there is no true gelatine substance in carbohydrates, there is a peculiar group of bodies known as "pectoses." These substances give to fruits their power of forming jellies when boiled. Little is known of their exact chemical nature.
The salts of plants exist largely as salts of potash and phosphorus; in many of the cereals (e.g. wheat) iron is also present in combination with phosphoric acid. This differs from animal food, where the salts are mainly chlorides; thus in the eating of vegetables it is important to add sodium chloride (common salt).
On the whole, plant life is not rich in fats. The fat resembles in chemical composition the animal fats, but contains as a rule more of the oily constituent (olein) and less of the solid components. The solid vegetable fats are cocoa-nut oil, palm oil, and cocoa butter, none of which are in great use as foods. The fluid oils are classed as drying and non-drying oils, owing to the fact that on exposure to air some absorb oxygen and become dry and solid, e.g., linseed, hemp, walnut, and poppy; others are non-drying oils, e.g., olive oil, cotton-seed oil, and almond oil.
Vegetable butters are now made from mixtures of these fats, and closely resemble cooking and table butter. In addition to the oils obtained by pressure, most plants contain volatile or essential oils, to which they owe their fragrance. These oils are diffused through every part of a plant, but are more concentrated in certain parts. Thus in the orange and lemon they are found in the rind, in the rose tribe they are found in the petals, in the umbelliferae chiefly in the seeds. They are mainly used in perfumery, but they enter also largely into flavouring agents, and act as condiments to excite the appetite and stimulate the digestive juices.
The carbohydrates form the great bulk of vegetable food, and are represented by starch, sugar, and cellulose. There is a framework of cells and binding tissue known as cellulose; starch is the substance stored in the cells; and sugar is the partially digested starch which circulates through the plant. These will be shortly considered.
Cellulose belongs to the carbohydrate group, but is characterised by its extraordinary insolubility: neither cold or hot water has any effect upon it, and in order seriously to influence it, prolonged boiling with a dilute acid is necessary. Cotton wool may be taken as almost pure cellulose, and paper is just felted vegetable fibre. When a plant is young its cellulose can be digested readily by man, but as the plant grows older the cellulose becomes more woody in structure and gets enveloped in resinous material, so that it is indigestible like paper or sawdust. It is this peculiarly insoluble envelope that makes the whole vegetable group difficult to digest unless very thoroughly cooked.
The nutritive parts of the plant are the starch, which is stored up in the cells, and the sugar, which circulates through the framework. The starches and sugars are convertible: the sugar is the digested nutriment of the plant, while the starch is the form in which the nourishment of the plant is stored for further use. It is also interesting to note that all starch eaten by animals has first to be converted into sugar before it can be used for the needs of the body.
Starch is really made up of a number of minute cells; microscopically they are of various shapes, depending on the plant from which they are taken, e.g., round, oval, polyhedral. The starch granules show concentric markings, showing that the cells have been built up layer by layer. Starch grains are insoluble in cold water; when boiled, the starch grains swell up and burst the cellulose envelopes, and forms with water the sticky substance used in laundry work. Cooking is essential for vegetable foods in order to rupture the starch granules so that the broken-up grains may be brought more readily into contact with the digestive juices. Dry heat applied to starch, as in baking of bread, pastry, tart-making, partially converts the carbohydrate into another form known as dextrin (British gum). A temperature of about 300° F. is necessary to produce this. Dextrin differs from starch in being very soluble in water, and gives a reddish-brown instead of blue reaction with iodine.