570. Examination of the Fresh Matter.—To avoid the changes which take place in drying fruits and vegetables, it is necessary to examine them in the fresh state. The samples may be first separated into meat and waste, as suggested above, or shredded as a whole. The moisture in the pulp is determined as indicated above, and in a separate portion the soluble matters are extracted by repeated treatment with cold water. The seeds, skins, cellulose, pectose and other insoluble bodies are thus separated from the sugars, pectins, pectic and other acids, and other soluble matters. The insoluble residue is rapidly dried and the relative proportions of soluble and insoluble matters determined. The estimation of these bodies is accomplished in the usual way.
571. Examination of Fruit and Vegetable Juices.—The fruits and vegetables are pulped, placed in a press and the juices extracted. The pressure should be as strong as possible and the press described in paragraph [280] is well suited to this purpose. The specific gravity of the expressed juice is obtained and the sucrose therein determined by polarization before and after inversion. The reducing sugars and the relative proportions of dextrose and levulose are determined in the usual manner. In grape juice dextrose is the predominant sugar while in many other fruits left hand or optically inactive sugars predominate. Soluble gums, dextrin, pectin etc., may be separated from the sugars by careful precipitation with alcohol, or the total solids, ash, nitrogen, ether extract and acids be determined and the carbohydrates estimated by difference. From the carbohydrates the total percentage of sugars is deducted and the remainder represents the quantity of pectin, gum and other carbohydrates present.
572. Separation of Pectin.—Pectin exists in considerable quantities in the juice of ripe fruits (pears) and may be obtained in an approximately pure state from the juices by first removing proteids by the careful addition of tannin, throwing out the soluble lime salts with oxalic acid and precipitating the pectin with alcohol. On boiling with water, pectin is converted into parapectin, which gives a precipitate with lead acetate. Boiling with dilute acids converts pectin into metapectin, which is precipitated by a barium salt.
Pectic acid may be obtained by boiling an aqueous extract (carrots) with sodium carbonate and precipitating the pectic with hydrochloric acid. It is a jelly-like body and dries to a horny mass.
573. Determination of Free Acid.—The free acid, or rather total acidity of fruits, is determined by the titration of their aqueous extracts or expressed juices with a set alkali. In common fruits and vegetables the acidity is calculated to malic C₄H₆O₅, in grapes to tartaric C₄H₆O₆, and in citrous fruits to citric acid C₆H₈O₇. Many other acids are found in fruits and vegetables, but those mentioned are predominant in the classes given.
574. Composition of Common Fruits.—The composition of common fruits in this country has been extensively investigated at the California Station and the following data are derived chiefly from its bulletins.[584]
- (A) = Total sugars in juice.
- (B) = Sucrose in juice.
- (C) = Dry organic matter.
| Name. | Total weight. | Rind skin. | Seed. | Pulp. | Juice. | (A) | (B) |
|---|---|---|---|---|---|---|---|
| grams. | per cent. | per cent. | per cent. | cubic centimeters. | Per cent. | Per cent. | |
| Naval orange | 300 | 28.4 | 27.7 | 107 | 9.92 | 4.80 | |
| Mediterranean sweet orange | 202 | 27.0 | 0.8 | 24.0 | 86 | 9.70 | 4.35 |
| St. Michael’s orange | 138 | 19.2 | 1.6 | 25.9 | 65.4 | 8.71 | 3.48 |
| Malta Blood orange | 177 | 31.0 | 24.0 | 71.0 | 10.30 | 5.85 | |
| Eureka lemon | 104 | 32 | 0.12 | 24.5 | 38 | 2.08 | 0.57 |
| Flesh | Per cent | ||||||
| Apricot | 62.4 | 93.85 | 6.15 | 10.0 | 90.0 | 13.31 | |
| Prune | 25.6 | 94.2 | 5.8 | 21.2 | 78.8 | 20.0 | |
| Plum | 60.4 | 95.2 | 4.8 | 24.7 | 75.3 | 17.97 | |
| Peach | 185 | 93.8 | 6.2 | 22.5 | 77.5 | 17.0 | |
| Skin | Cores | ||||||
| Apple | 183 | 17.0 | 7.0 | 10.26‡ | 1.53‡ | ||
| ‡ In whole fresh fruit. | |||||||
| Name. | Acid | In whole fruit. | |||
|---|---|---|---|---|---|
| Nitrogenous bodies. | Water. | (C) | Ash. | ||
| per cent. | per cent. | per cent. | per cent. | per cent. | |
| Naval orange | 1.02 | 1.31 | 86.56 | 13.04 | 0.40 |
| Mediterranean sweet orange | 1.38 | 0.96 | 85.83 | 13.06 | 0.41 |
| St. Michael’s orange | 1.35 | 1.43 | 84.10 | 15.42 | 0.48 |
| Malta Blood orange | 1.61 | 1.05 | 84.50 | 15.05 | 0.45 |
| Eureka lemon | 7.66 | 0.94 | 85.99 | 13.50 | 0.51 |
| Apricot | 0.68 | 1.25 | 85.16 | 14.35 | 0.49 |
| Prune | 0.40 | 1.01 | 77.38 | 22.18 | 0.44 |
| Plum | 0.48 | 1.33 | 77.43 | 22.04 | 0.53 |
| Peach | 0.25 | 82.50 | 16.95 | 0.55 | |
| Apple[585] | 0.11 | 86.43 | 13.28 | 0.29 | |
575. Composition of Ash of Fruits.—Two or three kilograms of the dried sample are incinerated at a low temperature and burned to a white ash in accordance with the directions given in paragraphs [28-32].