The fruit-acids are composed of carbon, hydrogen, and oxygen, and are burned in the body the same as sugar, or fats. The actual energy-producing content of fruit is not large, and depends almost entirely upon the sugar content. The nutrient elements of fruit consist of fruit-sugar, combinations of salts, organic acids, and various flavoring or aromatic substances. These same salts, acids, etc., purchased at the drug store, and administered separately, would be of no particular value, and might produce harmful results, but in the various combinations of fruits they have very important places in the diet.
Fruit as an aid to digestion
One of the most important functions that fruit performs in the body is that of an artificial solvent, or an aid to digestion. To make food serve this purpose well would require some knowledge in regard to chemical harmonies, quantity, etc. To illustrate: If the stomach does not secrete a sufficient quantity of hydrochloric acid, fruit-acid should be absolutely omitted, as any acid, except hydrochloric acid itself, tends to inhibit the formation of the normal stomach acid. And this in turn tends to fermentation of the sugars and starches—causing acid fermentation and all the symptoms that accompany this condition.
Effect of acid fruits
So it is very important to prevent all the causes and sources of fermentation if we would prevent the development of all the various diseases that arise from acid conditions of the stomach, and autointoxication. This explains why people of rheumatic tendency cannot take acid fruit. Citrus fruits, however, and limes, lemons, oranges, grape-fruit, etc., are beneficial in rheumatism and conditions of lowered blood alkalinity, because they are changed to alkalis in the system, and reinforce the blood alkalinity. People of rheumatic tendency, therefore, should confine the diet as nearly as possible to starchless foods, omitting all but the citrus fruits.
Classification of fruit according to acidity
In the lesson on "Vieno System of Food Measurement" I give the energy value of various fruits, and also the nitrogen factor. These tables consider fruits in the same light with other foods; that is, as sources of energy and nitrogen. In the table which follows, the more important fruits are grouped according to their total acidity. The figures represent the volume of acidity, not strength:
| ACID FRUITS | SUBACID FRUITS | SWEET AND | |||
| NON-ACID FRUITS | |||||
| Limes | 95 | Raspberries | 16 | Grapes | 8 |
| Lemons | 78 | Plums | 14 | Prunes | 7 |
| Grapefruit | 39 | Cherries | 13 | Raisins | 6 |
| Cranberries | 37 | Peaches | 12 | Bananas | 6 |
| Pineapples | 22 | Blackberries | 12 | Persimmons | 4 |
| Oranges | 20 | Apples | 11 | Figs | 4 |
| Apricots | 18 | Pears | 3 | ||
| Strawberries | 18 | Dates | 3 | ||
The fruits in the above table are all reasonably wholesome, and the particular fruits to be used depend as much upon convenience as upon the nature of the food substances. The above groups, however, will be given much attention in dietetic prescriptions, and the food scientist should become thoroughly familiar with this classification.