FRUITS, MELONS AND VEGETABLES.

568. Preparation of Sample.—Fresh fruits and vegetables are most easily prepared for analysis by passing them through the pulping machine described on [page 9]. Preliminary to the pulping they should be separated into skins, cores, seeds and edible portions, and the respective weights of these bodies noted. Each part is separately reduced to a pulp and, at once, a small quantity of the well mixed substance placed in a flat bottom dish and dried, first at a low temperature, and finally at 100°, or somewhat higher, and the percentage of water contained in the sample determined. The bulk of the sample, three or four kilograms, is dried on a tray of tinned or aluminum wire, first at a low and then at a high temperature, until all or nearly all the moisture is driven off. The dried pulp is then ground to as fine a powder as possible and subjected to the ordinary processes of analysis; viz., the determination of the moisture, ash, nitrogen, fiber, fat and carbohydrates.

In this method of analysis it is customary to determine the carbohydrates, exclusive of fiber, by subtracting the sum of the per cents of the other constituents and the nitrogen multiplied by 6.25 from 100.

569. Separation of the Carbohydrates.—It is often desirable to determine the relative proportions of the more important carbohydrates which are found in fruits and vegetables. The pentoses and pentosans are estimated by the method described in paragraph [150]. The cane sugar, dextrose and levulose are determined by extracting a portion of the substance with eighty per cent alcohol and estimating the reducing sugars in the extract before and after inversion by the processes described in paragraphs [238-251]. The percentages of sugars deducted from the percentage of carbohydrates, exclusive of fiber, give the quantity of gums, pentosans, cellulose and pectose bodies present.

Pectose exists chiefly in unripe fruits. By the action of the fruit acids and of a ferment, pectose, in the process of ripening, is changed into pectin and similar hydrolyzed bodies soluble in water. The gelatinous properties of boiled fruits and fruit juices are due to these bodies, boiling accelerating their formation. In very ripe fruits the pectin is completely transformed into pectic acids. The galactan is estimated as described in [585].

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.
Name.	Acid	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].

The composition of the ash is determined by the methods already described.[586]

The pure ash of some common whole fruits has the following composition:[587]

(A) = Per cent pure ash in fruit.
(B) = Per cent manganomanganic oxid.
(C) = Per cent phosphorus pentoxid.
(D) = Per cent sulfur trioxid.

Name.	(A)	Per cent potash.	Per cent soda.	Per cent lime.	Per cent magnesia.	Per cent ferric oxid.	(B)
Prune	0.47	63.83	2.65	4.66	5.47	2.72	0.39
Apricot	0.51	59.36	10.26	3.17	3.68	1.68	0.37
Orange	0.43	48.94	2.50	22.71	5.34	0.97	0.37
Lemon	0.53	48.26	1.76	29.87	4.40	0.43	0.28
Apple	1.44	35.68	26.09	4.08	8.75	1.40
Pear	1.97	54.69	8.52	7.98	5.22	1.04
Peach	4.90	27.95	0.23	8.81	17.66	0.55

Name.	(C)	(D)	Per cent silica.	Per cent chlorin.
Prune	14.08	2.68	3.07	0.34
Apricot	13.09	2.63	5.23	0.45
Orange	12.37	5.25	0.65	0.92
Lemon	11.09	2.84	0.66	0.39
Apple	13.59	6.09	4.32
Pear	15.20	5.69	1.49
Peach	43.63	0.37

576. Dried Fruits.—A method of preserving fruits largely practiced consists in subjecting them, in thin slices or whole, to the action of hot air until the greater part of the moisture is driven off. The technique of the process is fully described in recent publications.[588] It has been shown by Richards that fruit subjected to rapid evaporation undergoes but little change aside from the loss of water.[589]

In the analyses of dried fruits the methods already described are used. The presence of pectin renders the filtration of the aqueous extract somewhat difficult, and in many cases it is advisable to determine the sugars present in the extract without previous filtration.

577. Zinc in Evaporated Fruits.—Fruits are commonly evaporated on trays made of galvanized iron. In these instances a portion of the zinc is dissolved by the fruit acids, and will be found as zinc malate etc., in the finished product. The presence of zinc salts is objectionable for hygienic reasons, and therefore the employment of galvanized trays should be discontinued. The presence of zinc in evaporated fruits may be detected by the following process.[590] The sample is placed in a large platinum dish and heated slowly until dry and in incipient combustion. The flame is removed and the combustion allowed to proceed, the lamp being applied from time to time in case the burning ceases. When the mass is burned out it will be found to consist of ash and char, which are ground to a fine powder and extracted with hydrochloric or nitric acid. The residual char is burned to a white ash at a low temperature, the ash extracted with acid, the soluble portion added to the first extract and the whole filtered. The iron in the filtrate is oxidized by boiling with bromin water and the boiling continued until the excess of bromin is removed. A drop of methyl orange is placed in the liquid and ammonia added until it is only faintly acid. The iron is precipitated by adding fifty cubic centimeters of a solution containing 250 grams of ammonium acetate in a liter and raising the temperature to about 80°. The precipitate is separated by filtration and washed with water at 80° until free of chlorids. The filtrate is saturated with hydrogen sulfid, allowed to stand until the zinc sulfid settles and poured on a close filter. It is often necessary to return the filtrate several times before it becomes limpid. The collected precipitate is washed with a saturated solution of hydrogen sulfid containing a little acetic acid. The precipitate and filter are transferred to a crucible, dried, ignited and the zinc weighed as oxid. Small quantities of zinc salts added to fresh apples which were dried and treated as above described, were recovered by this method without loss. Other methods of estimating zinc in dried fruits are given in the bulletin cited.

Evaporated apples contain a mean content of 23.85 per cent of water and 0.931 per cent of ash.

The mean quantity of zinc oxid found in samples of apples dried in the United States is ten milligrams for each 100 grams of the fruit, an amount entirely too small to produce any toxic effects. When zinc exists in the soil it will be found as a natural constituent in the crop.[591]

578. Composition of Watermelons and Muskmelons.—In the examination of melons a separation of the rind, seeds and meat is somewhat difficult of accomplishment, since the line of demarcation is not distinct. In watermelons the separation of rind and meat is made at the point where the red color of the meat disappears. In muskmelons no such definite point is found and in the examination of these they are taken as a whole. The total moisture, ash and nitrogen may be determined in the whole mass or in the separate portions. The sugars are most conveniently determined in the expressed juices. The mean composition of the melons given below is that obtained from analyses made in the Department of Agriculture.[592]

Composition of Melons.
	Total weight, grams.	Juice, per cent.	Total proteids, per cent.	Ash, per cent.
Watermelons	10330	meat 83.99	6.12	0.37
Watermelons	10330	rind 81.02	6.12	0.37
Muskmelons	3407	80.23	6.45	0.57
Composition of Juice.
	Sucrose in juice, per cent.	Reducing sugars in juice, per cent.		Ash per cent.
Watermelons	meat 1.92	meat 4.33		meat 0.31
Watermelons	rind 0.34	rind 2.47		rind 0.38
Muskmelons	1.02	3.04		0.53