=263. Occurrence.=—Many bodies containing starch are presented for
the consideration of the agricultural analyst. First in importance
are the cereals, closely followed by the starchy root crops. Many
spices and other condiments also contain starchy matters. In the
sap of some plants, for instance sorghum, at certain seasons,
considerable quantities of starch occur. In the analysis of cereals
and other feeding stuffs, it has been the usual custom to make no
separate determination of starch, but to put together all soluble
carbohydrates and estimate their percentage by subtracting from 100
the sum of the percentages of the other constituents of the sample.
This aggregated mass has been known as nitrogen-free extract. Recent
advances in methods of investigation render it advisable to determine
the starch and pentosan carbohydrates separately and to leave among
the undetermined bodies the other unclassed substances, chiefly of a
carbohydrate nature, soluble in boiling dilute acid and alkali.
=264. Separation of Starch.=—Starch being insoluble in its natural
state, it is impossible to separate it from the other insoluble matters
of plants by any known process. In bringing it into solution it
undergoes certain changes of an unknown nature, but tending to produce
a dextrinoid body. Nevertheless, in order to procure the starch in a
state of purity suited to analytical processes, it becomes necessary
to dissolve the starch from the other insoluble bodies that naturally
accompany it. As has been shown in preceding paragraphs, there are only
two methods of securing the solution of starch which fully meet the
conditions of accurate analysis. These are the methods depending on the
use of diastatic ferments and on the employment of heat and pressure
in the presence of water. These two processes have been described
in considerable detail in paragraphs =179-181=. It is important, in
starch determinations, to remove from the sample the sugar and other
substances soluble in water and also the oils, when present in large
quantities, before subjecting it to the processes for rendering the
starch soluble.
=265. Desiccation of Amyliferous Bodies.=—The removal of sugars and
oils is best secured in amyliferous substances after they are deprived
of their moisture. As has already been suggested, the desiccation
should be commenced at a low temperature, not above 60°, and continued
at that point until the chief part of the water has escaped. The
operation may be conducted in one of the ways already described
(pp. 12-27). There is great difference of opinion among analysts in
respect of the degree of temperature to which the sample should be
finally subjected, but for the purposes here in view, it will not be
found necessary to go above 105°. Before beginning the operation the
sample should be as finely divided as possible, and at its end the
dried residue should be ground and passed through a sieve of half a
millimeter mesh.
=266. Indirect Method of Determining: Water in Starch.=—It is claimed
by Block[218] that it is necessary to dry starch at 160° in order to get
complete dehydration. Wet starch as deposited with its maximum content
of water has nine molecules thereof, _viz._, C₆H₁₀O₅ + 9H₂O. Ordinary
commercial starch has about eighteen per cent of water with a formula
of C₆H₁₀O₅ + 2H₂O.
The percentage of water may be determined by Block’s feculometer or
Block’s dose-fécule. The first apparatus determines the percentage of
anhydrous starch by volume, and the second by weight.
Block’s assumption that starch can absorb only fifty per cent of its
weight of water is the basis of the determination.
A noted weight of starch is rubbed up with water until saturated, the
water poured off, the starch weighed, dried on blotting paper until it
gives off no more moisture and again weighed. Half of the lost weight
is water, from which the original per cent of water can be calculated.
This at best seems to be a rough approximation and not suited to
rigorous scientific determination.
=267. Removal of Oil and Sugar.=—The dried, finely powdered sample,
obtained as described above, is placed in any convenient extractor
(=33-43=) and the oil or fat it contains removed by the usual solvents.
For ordinary purposes, even with cereals, this preliminary extraction
of the oil is not necessary, but it becomes so with oily seeds
containing starch. The sugar is subsequently removed by extraction
with eighty per cent alcohol and the residue is then ready for the
extraction of the starch. In most cases the extraction with alcohol
will be found sufficient. In some bodies, for instance the sweet potato
(batata), the quantity of sugar present is quite large, and generally
some of it is found. If not present in appreciable amount, the alcohol
extraction may also be omitted. The sample having been prepared as
indicated, the starch may be brought into solution by one of the
methods described in paragraphs =179-181=, preference being given to
the aqueous digestion in an autoclave. The dissolved starch is washed
out of the insoluble residue and determined by optical or chemical
methods =186-194=.
=268. Preparation of Diastase for Starch Solution.=—The methods of
preparing malt extract for use in starch analysis have been described
in paragraph =179=. If a purer form of diastase is desired it may be
prepared by following the directions given by Long and Baker.[219]
Digest 200 grams of ground malt for twenty-four hours with three parts
of twenty per cent alcohol. Separate the extract by filtration and
to the filtrate add about one and a half liters of ninety-three per
cent alcohol and stir vigorously. After the precipitate has subsided
the supernatant alcohol is removed by a syphon, the precipitate is
brought onto a filter and washed with alcohol of a strength gradually
increasing to anhydrous, and finally with anhydrous ether. The diastase
is dried in a vacuum over sulfuric acid and finally reduced to a fine
powder before using. Thus prepared, it varies in appearance from a
white to a slightly brownish powder. Made at different times and from
separate portions of malt, it may show great differences in hydrolytic
power.
=269. Estimation of Starch in Potatoes by Specific Gravity.=—A roughly
approximate determination of the quantity of starch in potatoes can be
made by determining their specific gravity. Since the specific gravity
of pure starch is 1.65, it follows that the richer a potato is in
starch the higher will be its specific gravity. The specific weight
of substances like potatoes is conveniently determined by suspending
them in water by a fine thread attached to the upper hook of a balance
pan. There may be a variation of the percentage of other constituents
in potatoes as well as of starch, and therefore the data obtained from
the following table can only be correct on the assumption that the
starch is the only variable. In practice, errors amounting to as much
as two per cent may be easily made, and therefore the method is useful
only for agronomic and commercial and not for scientific purposes. The
method is especially useful in the selection of potatoes of high starch
content for planting. The table is constructed on the weight in grams
in pure water of 10000 grams of potatoes and the corresponding per
cents of dry matter and starch are given. It is not always convenient
to use exactly 10000 grams of potatoes for the determination, but the
calculation for any given weight is easy.[220]
_Example._—Let the weight of a potato in air be 159 grams, and its
weight in water 14.8 grams.
Then the weight of 10000 grams of potatoes of like nature in water
would be found from the equation 159: 10000 = 14.8: _x_.
Whence _x_ = 931 nearly.
In the table the nearest figure to 931 is 930, corresponding to 24.6
per cent of dry matter and 18.8 per cent of starch. When the number
found is half way between the numbers given in the table the mean of
the data above and below can be taken. In other positions a proper
interpolation can be made if desired but for practical purposes the
data corresponding to the nearest number can be used.
TABLE FOR CALCULATING STARCH IN
POTATOES FROM SPECIFIC GRAVITY.
10000 grams
of potatoes
weigh in Per cent Per cent
water. dry matter. starch.
Grams.
750 19.9 14.1
760 20.1 14.3
770 20.3 14.5
780 20.7 14.9
790 20.9 15.1
800 21.2 15.4
810 21.4 15.6
820 21.6 15.8
830 22.0 16.2
840 22.2 16.4
850 22.4 16.6
860 22.7 16.9
870 22.9 17.1
880 23.1 17.3
890 23.5 17.7
900 23.7 17.9
910 24.0 18.2
920 24.2 18.4
930 24.6 18.8
940 24.8 19.0
950 25.0 19.2
960 25.2 19.4
970 25.5 19.7
980 25.9 20.1
990 26.1 20.3
1000 26.3 20.5
1010 26.5 20.7
1020 26.9 21.1
1030 27.2 21.4
1040 27.4 21.6
1050 27.6 21.8
1060 28.0 22.2
1070 28.3 22.5
1080 28.5 22.7
1090 28.7 22.9
1100 29.1 23.3
1110 29.3 23.5
1120 29.5 23.7
1130 29.8 24.0
1140 30.2 24.4
1150 30.4 24.6
1160 30.6 24.8
1170 31.0 25.0
1180 31.3 25.5
1190 31.5 25.7
1200 31.7 25.9
1210 32.1 26.3
1220 32.3 26.5
1230 32.5 26.7
1240 33.0 27.2
1250 33.2 27.4
1260 33.4 27.6
1270 33.6 27.8
1280 34.1 28.3
1290 34.3 28.5
1300 34.5 28.7
1310 34.9 29.1
1320 35.1 29.3
1330 35.4 29.6
1340 35.8 30.0
1350 36.0 30.2
1360 36.2 30.4
1370 36.6 30.8
=270. Constitution of Cellulose.=—The group of bodies known as
cellulose comprises many members of essentially the same chemical
constitution but of varying properties. The centesimal composition of
pure cellulose is shown by the following numbers:
Carbon, 44.2 per cent
Hydrogen, 6.3 ” ”
Oxygen, 49.5 ” ”
corresponding to the formula C₆H₁₀H₅.
According to the view of Cross and Bevan, cellulose conforms in respect
of its ultimate constitutional groups to the general features of the
simple carbohydrates, but differs from them by reason of a special
molecular configuration resulting in a suppression of the activity of
constituent groups in certain respects, and an increase in activity of
others.[221]
=271. Fiber and Cellulose.=—The carbohydrates of a plant insoluble in
water are not composed exclusively of starch. There are, in addition
to starch, pentosan fibers yielding pentose sugars on hydrolysis and
furfuraldehyd on distillation with a strong acid. The quantitive
methods for estimating the pentosan bodies are given in paragraphs
=150-157=. The method to be preferred is that of Krug (=155=).
In the estimation of cattlefoods and of plant substances in general the
residue insoluble in dilute boiling acid and alkali is called crude or
indigestible fiber.
The principle on which the determination depends rests on the
assumption that all the protein, starch and other digestible
carbohydrates will be removed from the sample by successive digestion
at a boiling temperature with acid and alkali solutions of a given
strength. It is evident that the complex body obtained by the treatment
outlined above is not in any sense a definite chemical compound, but it
may be considered as being composed partly of cellulose.
=272. Official Method of Determining Crude Fiber.=—The method of
estimating crude fiber, adopted by the Association of Official
Agricultural Chemists, is as follows:[222]
Extract two grams of the substance with ordinary ether, at least
almost completely, or use the residue from the determination of the
ether extract. To this residue, in a half liter flask, add 200 cubic
centimeters of boiling 1.25 per cent sulfuric acid; connect the flask
with an inverted condenser, the tube of which passes only a short
distance beyond the rubber stopper into the flask. Boil at once, and
continue the boiling for thirty minutes. A blast of air conducted into
the flask may serve to reduce the frothing of the liquid. Filter, wash
thoroughly with boiling water until the washings are no longer acid,
rinse the substance back into the same flask with 200 cubic centimeters
of a boiling 1.25 per cent solution of sodium hydroxid, free or nearly
free of sodium carbonate, boil at once and continue the boiling for
thirty minutes in the same manner as directed above for the treatment
with acid. Filter into a gooch, and wash with boiling water until the
washings are neutral, dry at 110°, weigh and incinerate completely. The
loss of weight is crude fiber.
The filter used for the first filtration may be linen, one of the forms
of glass wool or asbestos filters, or any other form that secures clear
and reasonably rapid filtration. The solutions of sulfuric acid and
sodium hydroxid are to be made up of the specified strength, determined
accurately by titration and not merely from specific gravity.
The experience of this laboratory has shown that results practically
identical with those got as above, are obtained by conducting the
digestions in hard glass beakers covered with watch glasses. The ease
of manipulation in the modification of the process just mentioned is a
sufficient justification for its use.
=273. Separation of Cellulose.=—Hoppe-Seyler observed that cellulose,
when melted with the alkalies at a temperature as high as 200°, was not
sensibly attacked.[223]
Lange has based a process for determining cellulose on this
observation.[224]
The process, as improved by him, is carried out as follows:
From five to ten grams of the substance are moistened with water and
placed in a porcelain dish with about three times their weight of
caustic alkali free of nitrates and about twenty cubic centimeters
of water. The porcelain dish should be deep and crucible shaped and
should be placed in an oil-bath, the temperature of which is easily
controlled. The contents of the dish are stirred with the thermometer
bulb until all foaming ceases and the temperature of the mixture is
then kept at from 175° to 180° for an hour. After the melt has cooled
to 80° about seventy-five cubic centimeters of hot water are added to
bring it into solution and it is then allowed to cool. The solution is
acidified with sulfuric and placed in large centrifugal tubes. After
being made slightly alkaline with soda lye, the tubes are subjected
to continued energetic centrifugal action until the cellulose is
separated. The supernatant liquid can be nearly all poured off and the
separated cellulose is broken up, treated with hot water and again
separated by centrifugal action. The cellulose is finally collected
upon the asbestos felt, washed with hot water, alcohol and ether, dried
and weighed. With a little practice it is possible to complete the
separation of cellulose in two and one-half hours.
=274. Solubility of Cellulose.=—Cellulose resembles starch in its
general insolubility, but, unlike starch, it may be dissolved in some
reagents and afterwards precipitated practically unchanged or in a
state of hydration. One of the simplest solvents of cellulose is zinc
chlorid in concentrated aqueous solution.
The solution is accomplished with the aid of heat, adding one part by
weight of cotton to six parts of zinc chlorid dissolved in ten parts of
water.
A homogeneous sirup is obtained by this process, which is used in the
arts for making the carbon filaments of incandescent electric lamps.
In preparing the thread of cellulose, the solution, obtained as
described above, is allowed to flow, in a fine stream, into alcohol,
whereby a cellulose hydrate is precipitated, which is freed from zinc
hydroxid by digesting in hydrochloric acid.
Hydrochloric acid may be substituted for water in preparing the reagent
above noted, whereby a solvent is secured which acts upon cellulose
readily in the cold.
A solution of ammoniacal cupric oxid is one of the best solvents for
cellulose. The solution should contain from ten to fifteen per cent of
ammonia and from two to two and a half of cupric oxid.
In the preparation of this reagent, ammonium chlorid is added to a
solution of cupric salt and then sodium hydroxid in just sufficient
quantity to precipitate all of the copper as hydroxid. The precipitate
is well washed on a linen filter, squeezed as dry as possible and
dissolved in ammonia of 0.92 specific gravity. The cellulose is readily
precipitated from the solution in cuprammonium by the addition of
alcohol, sodium chlorid, sugar, or other dehydrating agents. Solutions
of cellulose are used in the arts for many purposes.[225]
=275. Qualitive Reactions for Detecting Cellulose.=—Cellulose may be
identified by its resistance to the action of oxidizing agents, to the
halogens and to alkaline solutions. It is further recognized by the
sirupy or gelatinous solutions it forms with the solvents mentioned
above. The cellulose hydrates precipitated from solutions have in some
instances the property of forming a blue color with iodin.
A characteristic reaction of cellulose is secured as follows: To a
saturated solution of zinc hydrochlorate, of 2.00 specific gravity, are
added six parts by weight of potassium iodid dissolved in ten parts
of water and this solution is saturated with iodin. Cellulose treated
with this reagent is at once stained a deep blue violet color.[226] For
the characteristics of cellulose occurring in wood the researches of
Lindsey may be consulted.[227]
=276. More Rarely Occurring Carbohydrates.=—It is not possible here
to give more space to the rarer forms of carbohydrates, to which the
attention of the agricultural analyst may be called. Nearly a hundred
kinds of sugars alone have been detected in the plant world. For
descriptions of the properties of these bodies and the methods of their
detection and determination, the standard works on carbohydrates may be
consulted.[228]