=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]
Name. Total Rind Seed. Pulp. Juice. Total
weight. skin. sugars Sucrose
in in
juice. juice.
per per per cubic per per
grams. cent. cent. cent. centimeters. cent. 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.
--------------------------------------------------------------------
In whole fruit.
/-----------------------------\
Name.
Acid. Nitrogenous Water. Dry Ash.
bodies. organic
matter.
per per per per per
cent. cent. cent. cent. 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