PROCESSES FOR DETECTING AND DETERMINING SUGARS AND STARCHES AND OTHER
CARBOHYDRATES IN CRUDE OR MANUFACTURED AGRICULTURAL PRODUCTS.
=203. Introduction.=—In the preceding part directions have been given
for the estimation of sugars and starches in approximately pure forms.
In the present part will be described the most approved methods of
separating these bodies and other carbohydrates from crude agricultural
products and for their chemical examination. In many respects the
processes which in a small way are used for preparing samples for
analysis are employed on a large scale for technical and manufacturing
purposes. It is evident, however, that the following paragraphs must be
confined strictly to the analytical side of the question inasmuch as
anything more than mere references to technical processes would lead
into wide digressions.
In the case of sugars the analyst is for the most part quite as much in
need of reliable methods of extraction and preparation as of processes
for analysis. With starches the matter is more simple and the chief
methods of separating them for examination were necessarily described
in the previous part.
Sugars in fresh plants exist almost entirely in solution. This is true
of all the great sources of the sugar of commerce, _viz._, the palm,
the maple, the sugar beet and sugar cane. This statement is also true
of fruits and the natural nectar of flowers. By natural or artificial
drying the sugar may be reduced to the solid or semisolid state as in
the cases of raisins and honey. In certain seeds, deficient in water,
sugars may possibly exist in a solid state naturally, as may be the
case with sucrose in the peanut and raffinose in cotton seed.
Starches on the other hand when soluble, are probably not true
starches, but they partake more or less of a dextrinoid nature. Fine
starch particles occur abundantly in the juices of some plants,
as for instance sorghum, where they are associated with sugar and
can be obtained from the expressed juice by subsidence. But even
in such a case it is not certain that the starch enters into the
general circulation. It is more likely formed locally by biochemical
condensation of its constituents. Starches in a soluble or semisoluble
state are transported, as a rule, to the tubers or seeds of plants
where they are accumulated in large quantities as a reserve food for
future growth. For a study of the plant metabolism whereby starch is
produced and for its histological and physiological properties the
reader may consult the standard authorities on vegetable physiology.[170]
=204. Sugar in the Sap of Trees.=—Many trees at certain seasons of the
year, carry large quantities of sugar in their sap. Among these the
maple and sugar palm are preeminent. The sap is secured by cutting a
pocket into the side of the tree or by boring into it and allowing the
sap to run into an appropriate receptacle through a spile. The content
of sugar in the sap of the maple and palm varies greatly. In some cases
it falls as low as one and a half and in others rises to as much as
six or seven per cent.[171] In most cases the sugar in the maple sap is
pure sucrose, but towards the end of the flowing season it may undergo
changes of a viscous nature due to fermentation, or inversion, forming
traces of invert sugar. In this country the sap of the maple may flow
freely on any warm day in winter, but the sugar season proper begins
about February 15th in Southern Ohio and Indiana, and about March 25th
in Vermont. It lasts from six weeks to two months. The sap flows best
during moderately warm, still days, after a light freeze.
In addition to sugar the maple sap contains a trace of albuminoid
matters and some malic acid combined with lime. As a rule it can be
subjected to polarization without preliminary clarification.
=205. Determination of Sugar in Saps.=—In most cases the sap may be
directly polarized in a 200 millimeter tube. Its specific gravity is
obtained by a spindle or pyknometer, and the percentage of sugars taken
directly from the table on page 73, the degree brix corresponding to
the sugar percentage.
On polarizing, the sugar percentage is calculated as follows:
Multiply the specific gravity of the sap by 100 and divide the product
by 26.048. Divide the direct reading of the sap on the sugar scale by
the quotient obtained above, and the quotient thus obtained will be the
correct percentage of sugar in the original solution.
The formula is applicable for those instruments in which 26.048 grams
represent the normal quantity of sugar which in 100 cubic centimeters
reads 100 divisions on the scale. When other factors are used they
should be substituted for 26.048 in the above formula.
The principle of the calculation is based on the weight of the sap
which is contained in 100 cubic centimeters, and this is evidently
obtained by multiplying 100 by the specific gravity of the sap. Since
26.048 is the normal quantity of sugar in that volume of the solution
the quotient of the actual weight divided by that factor shows how many
times too great the observed polarization is. The simple division of
the polariscope reading by this factor gives the correct reading.
_Example_: Let the specific gravity of the sap be 1.015 and the
observed polarization be 15.0. Then the true percentage of sugar in the
sap is found by the equation:
101.5 : 26.048 = 15.0 : _x_.
Whence _x_ = 3.85 = percentage of sugar in the sap.
The process outlined above is not applicable when a clarifying reagent
such as lead subacetate or alumina cream must be used. But even in
these cases it will not be found necessary to weigh the sap. A sugar
flask graduated at 100 and 110 cubic centimeters is used and filled to
the first mark with the sap, the specific gravity of which is known.
The clarifying reagent is added, the volume completed to the second
mark with water, and the contents of the flask well shaken and thrown
on a dry filter. The observation tube, which should be 220 millimeters
in length, is then filled with the clear filtrate and the rest of the
process is as described above. A 200 millimeter tube may also be used
in this case and the observed reading increased by one-tenth.
[Illustration: FIG. 62. LABORATORY CANE MILL.]
[Illustration: FIG. 63. WEIGHING PIPETTE.]
=206. Estimation of Sugar in the Sap of Sugar Cane and Sorghum.=—In
bodies like sugar cane and sorghum the sap containing the sugar will
not flow as in the cases of the maple and sugar palm. The simplest way
of securing the sap of the bodies named is to subject them to pressure
between rolls. A convenient method of obtaining the sap or juice is
by passing the cane through a small three-roll mill indicated in the
figure. Small mills of this kind have been used in this division for
many years and with entire satisfaction. Small canes, such as sorghum,
may be milled one at a time, or even two or three when they are very
small. In the case of large canes, it is necessary that they be split
and only half of them used at once. The mill should not be crowded by
the feed in such a way as to endanger it or make it too difficult for
the laborer to turn. From fifty to sixty per cent of the weight of a
cane in juice may be obtained by passing it through one of these small
mills. Experience has shown that there is a little difference between
the juice as first expressed and the residual sap remaining in the
bagasse, but the juice first expressed may be used for analysis for
control purposes as a fair representative of all that the cane contains.
To determine the percentage of juice expressed, the canes may be
weighed before passing through the mill and the juice collected. Its
weight divided by the weight of the original cane will give the per
cent of the juice expressed, calculated on the whole cane. Instead of
weighing the juice the bagasse may also be collected and weighed; but
on account of the rapidity with which it dries the operation should be
accomplished without delay. The expressed juice is clarified with lead
subacetate, filtered and polarized in the manner described in former
paragraphs. Instead of weighing the juice, its specific gravity may
be taken by an accurate spindle and the volume of it, equivalent to a
given weight, measured from a sucrose pipette.[172]
A sucrose pipette for cane juice has a graduation on the upper part
of the stem which enables the operator to deliver double the normal
weight for the polariscope used, after having determined the density of
the juice by means of a spindle. A graduation of from 5° to 25° of the
brix spindle will be sufficient for all variations in the density of
the juice, or one covering a range of from 10° to 20° will suffice for
most instances. The greater the density of the juice the less volume
of it will be required for the weight mentioned. For general use, the
sucrose pipette is graduated on the stem to deliver from forty-eight
to 50.5 cubic centimeters, the graduations being in terms of the brix
spindle. The graduation of the stem of this instrument is shown in the
accompanying figure. In the use of the pipette it is only necessary
to fill it to the degree on the stem corresponding to the degree brix
found in the preliminary trial.
The quantities of juice corresponding to each degree and fractional
degree of the brix spindle are given in the following table; calculated
for the normal weight 26.048 grams for the ventzke and for 16.19 grams
for the laurent scale. The measured quantities of juice are placed in
a 100 cubic centimeter sugar flask, treated with the proper quantity
of lead subacetate, the volume completed to the mark, and the juice
filtered and polarized in a 200 millimeter tube. The reading of the
polariscope is divided by two for the factor 26.048 and by three for
the factor 16.19.