=176. Constitution of Starch.=—The molecule of starch is without doubt
formed by the condensation of a large number of hexose bodies. On
account of its great insolubility its molecular weight has not been
determined with any degree of accuracy. Its formula may be expressed
either as (C₆H₁₀O₅)ₙ or (C₁₂H₂₀O₁₀)ₙ. It is insoluble in cold water and
other common solvents and does not pass into solution in any reagent
without undergoing a change of structure. In hot water it forms a paste
and when heated under pressure with water it undergoes a partial change
and becomes soluble. Heated with acids or subjected to the action of
certain ferments it suffers hydrolysis and is transformed into dextrin,
maltose and dextrose. In analytical work an attempt is usually made to
transform the starch entirely into dextrose, the quantity of which is
then determined by some of the processes already given. All starches
possess the property of giving an intensely blue color with iodin and
this reaction serves to detect the most minute quantity of the material.
Starch grains derived from different sources are distinguished by
differences in size and appearance. In most cases a careful examination
of the starch particles will reveal their origin.[145] The greatest
part of the cereal grains is composed of starch, the percentage ranging
from sixty to eighty. Rice has the greatest percentage of starch in
its composition of any substance. Certain root crops are also rich
in starch, such as the potato, artichoke and cassava. Starch appears
as one of the first products of vegetable metabolism, according to
some authorities, preceding the formation of sugars. By reason of its
greater complexity, however, it is more probable that the production
of simple sugars precedes the formation of the more complex molecule.
Starch granules are probably used as a food by the plant in the
building of more complex structures and the excess of this food is
stored in the seeds and in tubers.
=177. Separation of Starch Particles.=—Advantage is taken of the
insolubility of the starch particles to secure their separation from
the other vegetable structures with which they are associated. The
substances containing starch are reduced to a pulp as fine as possible,
and this pulp being placed in a fine cloth the starch particles are
washed through the cloth with water. The milky filtrate carrying the
starch is collected in an appropriate holder and, after some time, the
particles subside. They may then be collected and dried. While this
process is the one used commercially in the manufacture of starch, it
can only give approximate data respecting the actual quantity of starch
in a given weight of the sample. It is not quite possible by this
method to get all the starch separated from the rest of the vegetable
matter, and particles of foreign substances, such as cellulose and
albuminoid matters, may pass through the filter cloth and be found
with the deposited granules. It follows from this that the quantitive
determination of the starch in a given sample by any direct method is
only approximately exact.
=178. Methods of Separation.=—Hot acids cannot be safely employed
to dissolve starch from its natural concomitants because other
carbohydrate bodies become soluble under similar conditions. In such
cases the natural sugars which are present should be removed by cold
water and the starch dissolved from the residue by a diastatic ferment.
Instead of this the sugars may be determined in a separate portion
of the pulped material and the starch, together with the sugars,
determined, and the quantity of sugar found deducted from the final
result.
In these cases the final determinations are made on the sugars, after
inverting the sucrose, and proceeding as directed for invert sugars
in paragraph =141=. The starch, after separation with diastase, is
converted into dextrose by one of the methods to be given and the
resulting dextrose determined by one of the approved methods.
=179. Separation with Diastase.=—Diastase or malt extract at a
temperature of about 65° rapidly renders starch soluble. Cereals,
potato meal and other starch-holding bodies are dried, first at a low
temperature, and extracted with ether or petroleum to remove fat. The
material is then rubbed up with water, boiled, cooled to 65°, and
treated with malt extract (diastase) prepared as given below. One
kilogram of ground green malt is mixed with one liter of glycerol
and an equal quantity of water, and allowed to stand, with frequent
shaking, for eight days. After that time the mixture is filtered,
first through a small filter press and afterwards through paper. In
case no filter press is at hand the mixture may be pressed in a bag
and the liquor obtained, filtered. Malt extract obtained in this way
will keep its diastatic properties for a long time. In its use, blank
determinations must be made of the dextrose produced by treating equal
portions of it with hydrochloric acid. For three grams of starchy
material twenty-five cubic centimeters of the malt solution should be
used and the mixture kept at 65° for two hours.[146]
=180. Method in Use at the Halle Station.=—The method of separating
starch from cereals, potatoes and other starch-holding materials,
employed at the Halle station, is essentially the same as already
described.[147]
The malt extract used is prepared immediately beforehand, inasmuch
as no preservative is added to it. It can be quickly prepared by
digesting, for a short time at not above 50°, 100 grams of finely
ground dried malt with one liter of water and separating the extract by
filtration. This extract will keep only a few hours.
The material in which the starch is to be determined is dried and
extracted with ether. From two to four grams of the extracted
material, according to the amount of starch which it contains, are
boiled for half an hour with 100 cubic centimeters of water, cooled to
65°, treated with ten cubic centimeters of malt extract and kept at
the temperature named for half an hour. It is then again boiled for
fifteen minutes, cooled to the temperature mentioned and again treated
with malt extract as above. Two treatments with malt extract are
usually sufficient to bring all the starch into solution. Finally it
is again boiled and the volume completed to 250 cubic centimeters and
thrown upon a filter. Two hundred cubic centimeters of the filtrate are
converted into dextrose by boiling with hydrochloric acid, and the rest
of the analysis is conducted in the usual manner. The dextrose value of
the quantity of malt extract used must be determined upon a separate
portion thereof, and the quantity of dextrose found deducted from the
total amount obtained in the analysis.
[Illustration: FIGURE 47. AUTOCLAVE FOR STARCH ANALYSIS.]
=181. Separation by Hydrolysis with Water at High
Temperatures.=—Instead of dissolving the starch with diastase, it may
be brought into solution by heating with water under pressure. The
former method employed of heating in sealed flasks has been entirely
superceded by heating in an autoclave. The materials are best held in
metal beakers furnished with a cover which prevents loss from boiling
if the pressure should be removed too rapidly after the completion
of the operation. The autoclave is a strong metal vessel capable of
resisting the pressure of several atmospheres. It is furnished with a
pressure gauge C and a safety valve D, as shown in the figure. The top
is securely screwed on by means of a wrench, shown at the right hand
side. In the figure a portion of the case is represented cut away to
show the arrangement of the metal beakers inside.
In the method of Reinke, as practiced at the Halle station, and in
this laboratory, about three grams of the starchy substance are placed
in each of the metal beakers with twenty-five cubic centimeters of
a one per cent lactic acid solution and thirty cubic centimeters of
water. The contents of the beaker are thoroughly mixed and they are
then heated for two and a half hours in the autoclave, at a pressure
of three and a half atmospheres. The addition of the lactic acid is
for the purpose of protecting any sugar which may be present from
decomposition at the high pressure and temperature employed. After
the completion of the heating, the autoclave is allowed to cool, the
cover is removed and the beakers taken out and their contents washed
with hot water into quarter liter flasks. After cooling, the volume
is completed with cold water, and after standing for half an hour,
with frequent shaking, the contents of the flasks are filtered and
200 cubic centimeters of the filtrate in each case converted into
dextrose with hydrochloric acid in the usual way. In order to obtain
agreeing results, it is highly necessary that the substance before
treatment should be ground to a fine powder. The addition of the lactic
acid, as practiced in the reinke method, tends to give somewhat high
results, due probably to the hydrolytic action of the acid on the
fiber present. When starchy bodies are heated in the autoclave for the
determination of their starch by polarimetric methods, or for ordinary
determinations, the use of lactic acid should be omitted.
_Example._—The following data indicate the methods of calculation to
be followed in the determination of the percentage of starch in the
material by diastatic hydrolysis: Three grams of a barley were inverted
by diastase, as directed above, the volume of the solution made a
quarter of a liter, filtered, 200 cubic centimeters of the filtrate
converted into dextrose by hydrochloric acid, the volume completed to
half a liter with water and fifty cubic centimeters thereof oxidized
by the alkaline copper solution in the usual way. The amount of copper
obtained was 331 milligrams, corresponding to 174 milligrams of
dextrose. The amount of malt extract used in hydrolyzing the barley
mentioned above, was ten cubic centimeters. The diastatic solution
inverted with hydrochloric acid and treated as indicated above, yielded
191 milligrams of copper, corresponding to ninety-eight milligrams of
dextrose in ten cubic centimeters of the malt extract. The quantity of
malt extract represented in the final determination of copper, however,
was only one and six-tenths cubic centimeters. We then have:
Total dextrose 174 milligrams
Dextrose in one and six-tenths cubic centimeters
malt extract 16 milligrams
Dextrose corresponding to 240 milligrams of
barley 158 milligrams
Calculated on the proportion that dextrose is to starch, as ten is to
nine, this is equivalent to 142 milligrams of starch. The percentage
of starch in the original substance, therefore, was equivalent to 142
multiplied by 100, divided by 240, _viz._, 59.17.
=182. Principles of the Methods of Determination.=—In the approximately
pure state in which starch exists in the trade, it may be determined
by conversion into dextrose and estimating the latter by one of the
methods given. It is probable that there is no known method by which
starch can be entirely converted into dextrose, and all the methods of
hydrolysis, when used for quantitive purposes, must be standardized,
not by the theoretical quantity of dextrose which a given weight of
pure starch should yield, but by the actual quantity obtained. Starch
is not largely converted into dextrose by any of the diastatic ferments
which produce principally maltose and dextrins. Recourse must therefore
be had to strong acids. In practice, hydrochloric is the one usually
employed. By the action of a hot mineral acid, not only is starch
converted into dextrose, but also the dextrose found is subjected to
changes. In such cases an opposing action seems to be exerted by the
hydrolytic agent, a part of the dextrose formed suffering a partial
condensation, and thus assuming a state of higher molecular weight,
approaching the constitution of the dextrins. Another part of the
dextrose may also suffer oxidation and thus disappear entirely in
respect of the further steps in starch analysis.
In such cases, the best the analyst can do is to conduct the hydrolysis
in as nearly as possible constant conditions, and to assume that the
percentage of dextrose present at a given time bears a constant ratio
to the quantity of starch hydrolyzed. In reality almost all the starch
appears finally as dextrose, and by proceeding on the assumption noted
above a fairly satisfactory accounting may be made of the remainder.
Starch being insoluble, it cannot be determined directly by its
rotatory power. When heated for a few hours in contact with water at a
high pressure, starch becomes soluble, and in this state has a fairly
constant gyrodynat, _viz._, [_a_]_{D} = 197°.
Starch is also rendered soluble by rubbing it in a mortar for about ten
minutes with an excess of strong hydrochloric acid, and in this way
a quick approximate idea may be obtained of the percentage present.
Starch prepared in this manner, however, has a strong reducing power on
metallic salts, showing that a part of it has already, even in so short
a time, assumed the state of maltose or dextrose. The gyrodynat of
pure anhydrous starch in such conditions varies from [_a_]_{D} = 197°
to [_a_]_{D} = 194°. Starch is also rendered soluble by boiling with
salicylic acid, whereby a solution is obtained having a gyrodynat of
[_a_]_{D} = 200°(circa). The methods of procedure for the analysis of
starch will be set forth in detail in the following paragraphs.
=183. Estimation of Water.=—In prepared or commercial starches the
water may be determined by heating in a partial vacuum. The temperature
at first should be low, not exceeding 60°. After drying for an hour
at that heat the temperature may be gradually increased. The last
traces of water come off from starch with difficulty, and the final
temperature may be carried a little beyond 100° without danger of
decomposition.
Ost recommends the use of an atmosphere of hydrogen or illuminating
gas.[148] One and a half grams of the finely powdered sample are
placed in the drying tube described in paragraph =23=, and heated in
a stream of dry hydrogen. The temperature at first is kept at about
60° for several hours and is then gradually increased to 120°. Ost
states that even at 150° the sample preserves its pure white color,
but so high a temperature is not necessary. Maercker, at the Halle
station, makes use of the same process, but employs illuminating gas
instead of hydrogen. The importance of beginning the desiccation at
a low temperature arises from the fact that at a higher temperature,
before the greater part of the water is driven off, the starch will
suffer a partial fusion and form a paste which is very difficult to
dry. The dried sample must be kept in a stoppered vessel to prevent the
absorption of hygroscopic moisture.
=184. Estimation of Ash.=—When the drying is accomplished in a flat
platinum dish, the same sample may serve for incineration. Otherwise
the incineration may be accomplished in another portion of the sample
by following directions already given.[149]
=185. Nitrogen.=—Even very pure samples of starch may contain a little
nitrogen which is most conveniently determined by moist combustion.[150]
As a rule, in commercial starches of good quality, the quantity of pure
starch may be considered to be the remainder after subtracting the sum
of the weights of water, ash and nitrogen multiplied by 6.25, from the
original weight of the sample taken.
_Example_:—
Per cent of moisture found 12.85
” ” ” ash found 0.08
” ” ” nitrogen × 6.25 0.27
-----
Sum 13.20
Per cent of pure starch in sample 86.80
Samples of starch usually contain also traces of fat and fiber, and
these when present in weighable quantities, should be determined and
proper deductions made.
=186. Hydrolysis with Acids.=—The acids commonly chosen for hydrolyzing
starch are sulfuric and hydrochloric. The former has the advantage of
being more easily removed from the finished product but the latter
performs the work with less damage to the sugars formed. For commercial
purposes sulfuric and for analytical practice hydrochloric acids are
commonly employed.
[Illustration: FIGURE 47 (BIS). MAERCKER’S HYDROLYZING APPARATUS FOR
STARCH.]
The best process for analytical purposes is the one proposed by
Sachsse.[151] In this method the starch is heated with the hydrolyzing
mixture in the proportion of three grams to 200 cubic centimeters
of water and twenty of hydrochloric acid of 1.125 specific gravity,
containing five and six-tenths grams of the pure gas. The heating is
continued for three hours on a steam-bath. Maercker recommends, instead
of the above procedure, heating for two hours at gentle ebullition in
an oil-bath. In this method three grams of the starch are reduced to
paste with 200 cubic centimeters of water, and then boiled for two
hours with fifteen cubic centimeters of hydrochloric acid of 1.125
specific gravity. The erlenmeyers in which the hydrolysis takes place
are heated in an oil-bath and are provided with reflux condensers made
of long glass tubes on which some bulbs have been blown, as shown in
the accompanying figure. In all cases after hydrolysis the solution
is neutralized, made to a standard volume and an aliquot part, after
filtration, diluted to contain an amount of dextrose suited to the use
of the table by Allihn for calculating the percentage of sugar. In
diluting the solution preparatory to the estimation of dextrose, it is
well to remember that nine parts of starch will furnish theoretically
ten parts of dextrose. Since three grams of the sample are used,
containing approximately eighty-five per cent of starch, the quantity
of dextrose present is a little less than three grams. The solution
should therefore contain not less than 300 cubic centimeters.
=187. Factor for Calculating Starch from the Dextrose Obtained.=—If all
the starch could be converted into dextrose without loss, the quantity
of it could be easily calculated theoretically on the supposition that
the formula of starch is (C₆H₁₀O₅)ₙ. The factor by this assumption is,
starch = dextrose × 0.90. If the starch have the formula assigned to it
by Nägeli, _viz._, C₃₆H₆₂O₃₁ the formula becomes, starch = dextrose ×
0.918.
Ost prefers to work by Sachsse’s method and to use the factor 0.925 to
convert the dextrose into starch.[152]
In view of all the facts in the case it appears that the analyst will
reach nearly correct results by converting the starch into dextrose
by heating for three hours at 100° with hydrochloric acid or for two
hours at gentle ebullition as directed above, determining the resultant
dextrose and multiplying the weight thereof by 0.92.
=188. Polarization of Starch.=—Starch may be prepared for polarization
by dissolving it in cold hydrochloric acid. The process as carried
out by Effront is as follows.[153] Five grams of starch are rubbed
with twenty cubic centimeters of cold concentrated hydrochloric
acid for nearly ten minutes or until the solution is quite clear.
The volume is completed to 200 cubic centimeters with water and the
solution polarized. By this process there is always produced a notable
quantity of reducing sugars, and for this reason it must be admitted
that a portion of the starch has suffered complete hydrolysis. Ost
therefore recommends the use of an acid of 1.17 specific gravity,
and the gyrodynat of the soluble starch thus produced is found to
vary from [_a_]_{D} = 196°.3 to 196°.7. When acid of 1.20 specific
gravity is employed the gyrodynat falls to [_a_]_{D} = 194.2.[154] For
approximately correct work the solution with the weaker hydrochloric
acid and subsequent polarization is to be recommended as the most rapid
method for starch determination.
It will be of interest to add the observation that the gyrodynat of
maltose has lately been redetermined by Ost, who finds it to be
[_a_]_{D}²⁰ ° = 137°.04 ± 0.19.[155]
=189. Solutions of Starch at High Pressure.=—Starch may also be brought
into a condition suited to polarization by dissolving in water at a
high temperature and pressure. The solution is accomplished in an
autoclave as described in =181=.
From two to three grams of starch are used and from eighty to ninety
cubic centimeters of water. The starch is first reduced to a pasty
state by heating with the water and, when evenly distributed throughout
the flask, is rendered soluble by heating from three to five hours in
an autoclave at from two to three atmospheres. The material is entirely
without action on an alkaline copper solution. After heating, the
volume of the solution is completed to 100 cubic centimeters and it is
then polarized. The gyrodynat of starch dissolved in this way varies
from [_a_]_{D} = 196°.5 to 197°.[156]
Starch is prepared by Baudry for polarization by boiling with salicylic
acid.[157] The gyrodynat of starch dissolved in this way is [_a_]_{D} =
200°.25.
=190. Polarization after Solution in Dilute Nitric Acid.=—Guichard
recommends saccharification with ten per cent nitric acid (ten cubic
centimeters strong acid, ninety cubic centimeters water).[158] This
treatment, even after prolonged boiling, gives only a light straw color
to the solution which does not interfere with its polarization with a
laurent instrument.
In working on cereals four grams of the finely ground material, in
which the bran and flour are intimately mixed, are used.
The material is placed in a flask of about 500 cubic centimeters
capacity, with 100 cubic centimeters of the dilute acid. The flask is
closed with a stopper carrying a reflux condenser. After boiling for
an hour the contents of the flask are filtered and examined in the
saccharimeter. The dextrose formed is determined by the polarimetric
data and the quantity of starch transformed calculated from the
dextrose. The following formula is used:
_av_ × 25 × 0.016
_A_ = ------------------.
2 × 52.8
In this formula _a_ = the rotation in angular degrees, _v_ = the volume
of the liquid and _A_ = the starch transformed.
In this method no account is taken of the sucrose and other sugars
which are present in cereals. In the case of sucrose the left-handed
sugar produced by treatment with nitric acid would diminish the
rotation to the right and thus introduce an error. On the other hand
the dextrose formed from the fiber of the bran would be calculated as
starch. If these two errors should be compensating the method might
prove practical.
=191. Rapid Estimation Of Starch.=—For the rapid estimation of starch
in cereals, cattle foods and brewery refuse, Hibbard recommends a
method which is carried out as follows:
The malt extract is prepared by covering ground, dry malt with water
containing from fifteen to twenty per cent of alcohol. The object of
adding alcohol is to preserve the filtered extract. It exercises a
slight retarding effect on the action of the diastase, but prevents the
malt extract from fermenting. After standing for a few hours in contact
with the malt, the liquid is separated by filtration and is then ready
for use. The substance in which the starch is to be determined should
be dry enough to be finely pulverized, but previous extraction with
ether is omitted. Enough of the material to contain at least half a
gram of starch is placed in a flask with fifty cubic centimeters of
water and from one to two cubic centimeters of malt extract added. The
mixture is at once heated to boiling with frequent shaking to prevent
the formation of clots. The addition of the diastase before boiling is
to aid in preventing the formation of lumps. After boiling a minute
the mixture is cooled to 60° and from two to three cubic centimeters
of the malt extract added. It is then slowly heated until it again
boils, consuming about fifteen minutes, when, after cooling, it is
tested with iodin for starch. If a blue color be produced the operation
above described is repeated until it fails to reappear. The mixture
is then made up to a standard volume, thrown on a linen filter and an
aliquot part of the filtrate, representing from 200 to 300 milligrams
of starch, is boiled with five cubic centimeters of hydrochloric acid,
of thirty per cent strength, for half an hour. The total volume of the
liquid before boiling should be completed to sixty cubic centimeters.
By the method above described, it is claimed that the determination of
starch in a cereal or similar substance can be completed within two
hours. The chief amount of time saved is in the heating with the malt
extract, which instead of being continued for two hours, as usually
directed, can be accomplished in thirty minutes.[159]
=192. Precipitation of Starch with Barium Hydroxid.=—The tendency of
carbohydrate bodies to unite with the earthy bases has been utilized by
Asboth as a basis for the quantitive determination of starch.[160]
About three grams of the finely ground sample containing the starch,
or one gram of pure starch, are rubbed up in a mortar with water and
the detached starch remaining suspended in the wash water is poured
off. This operation is repeated until all the starch is removed. In
difficult cases hot water may be used. The starch thus separated is
heated in a quarter liter flask to the boiling point to reduce it to
the condition of paste. When the paste is cold it is treated with fifty
cubic centimeters of the barium hydroxid solution, the flask closed and
well shaken for two minutes. The volume is then completed to the mark
with forty-five per cent alcohol, the flask well shaken and allowed
to stand. In a short time the barium-starch compound separates and
settles. Fifty cubic centimeters of the clear supernatant liquor are
removed with a pipette, or the liquor may be passed through a filter
and the quantity mentioned removed for titration of the residual barium
hydroxid after the addition of a few drops of phenolphthalein solution.
The quantity of barium hydroxid remaining, deducted from the original
quantity, gives the amount which has entered into composition with
the starch; the composition of the molecule being BaOC₂₄H₄₀O₂₀, which
contains 19.10 per cent of barium oxid and 80.90 per cent of starch.
The set solution of barium hydroxid must be preserved from contact with
the carbon dioxid of the air. The burette should be directly attached
to the bottle holding the set solution, by any of the usual appliances,
and the air entering the bottle must be deprived of carbon dioxid. The
water used in the work must be also free of air, and this is secured by
boiling immediately before use.
_Example._—A sample of flour selected for the analysis weighed
3.212 grams. The starch was separated and reduced to paste in the
manner described above. Thirty and four-tenths cubic centimeters of
tenth-normal hydrochloric acid were exactly neutralized by ten cubic
centimeters of the barium hydroxid solution. After treatment as above
described, fifty cubic centimeters of the clear liquor, corresponding
to ten cubic centimeters of the added barium hydroxid, required 19.05
cubic centimeters of tenth-normal hydrochloric acid. Then 30.4 - 19.05
= 11.35, and 11.35 x 5 = 56.75, which number corresponds to the total
titration of the residual barium hydroxid in terms of tenth-normal
hydrochloric acid. This number multiplied by 0.0324, _viz._, starch
corresponding to one equivalent of barium, gave 1.8387 grams of starch
or 57.24 per cent of the weight of flour employed.
The barium hydroxid method has been given a thorough trial in this
laboratory and the results have been unsatisfactory when applied to
cereals. The principle of the process, however, appears to be sound,
and with a proper variation of working details, it may become practical.
=193. Disturbing Bodies in Starch Determinations.=—Stone has made a
comparison of the standard methods of starch determinations, and the
results of his work show that in the case of pure starch all of the
standard methods give approximately correct figures. For instance, in
the case of a pure potato starch, the following data were obtained:
By inversion with hydrochloric acid, 85.75 per cent; by inversion with
oxalic and nitric acids, 85.75 per cent; by solution in salicylic acid,
85.47 per cent; and by precipitation with barium hydroxid, 85.58 per
cent.[161]
When these methods are used, however, for the determination of starch
in its original state, the widest variations are secured. Stone shows
that these variations are due chiefly to the inverting effect of the
reagents employed upon the pentosans present. In experiments made with
pure xylan obtained from wheat straw, the methods employed gave from
44.73 to 67.16 per cent of material, which would be calculated by
the usual methods as starch. Stone also shows that the pentosans are
practically unaffected by the action of diastase or malt extract. Pure
xylan treated with diastase, under the condition in which starch is
converted into maltose and other soluble carbohydrates, fails to give
any subsequent reaction whatever with alkaline copper solution. In
all cases, therefore, where starch occurs in conjunction with pentose
bodies, it is necessary to separate it by diastatic action before
applying any of the methods of conversion of the starch into dextrose
or its precipitation by barium hydroxid.
=194. Colorimetric Estimation of Starch.=—The production of the
intensely blue color which starch gives with iodin has been used not
only as the basis of a qualitive method, but also of many attempts at
quantitive determination. These attempts have, as a rule, been attended
with very unsatisfactory results, due both to the extraordinary
delicacy of the reaction and to the fact that starches of different
origin do not always give exactly the same intensity of tint when
present in the same quantity. At the present it must be admitted that
little should be expected of any quantitive colorimetric test.
In case such a test is desired the procedure described by Dennstedt
and Voigtländer may be followed.[162] A weighed quantity of the
starch-holding material, containing approximately half a gram of
starch, is placed in a two liter flask and boiled with a liter of
water. After cooling, the volume is completed to two liters and
the starch allowed to subside. Five cubic centimeters of the clear
supernatant liquor are placed in a graduated cylinder holding 100, and
marked in half cubic centimeters. One drop of a solution of iodin in
potassium iodid is added and the volume completed to the mark. A half
gram of pure starch is treated in the same way and different measured
portions of the solution treated as above until the color of the first
cylinder is matched. From the quantity of pure starch in the matched
cylinder the quantity in the sample is determined. The test should be
made in duplicate or triplicate. If a violet color be produced instead
of a blue, it may be remedied by treating the sample with alcohol
before the starch granules are dissolved.
=195. Fixation of Iodin.=—In addition to forming a distinctive blue
color with iodin, starches have the power of fixing considerable
quantities of that substance. The starches of the cereals have this
power in a higher degree than those derived from potatoes. In presence
of a large excess of iodin the starches of rice and wheat have a
maximum iodin-fixing power of about nineteen per cent of their weight.
When only enough of iodin is employed to enter into combination
the percentage absorbed varies from nine to fifteen per cent. The
absorption of iodin by starches is a matter of importance from a
general chemical standpoint, but as at present determined has but
little analytical value. It is evident, however, that this absorption
must take place according to definite chemical quantities and the
researches of investigators may in the future discover some definite
quantitive method of measuring it.[163]
=196. Identification of Starches of Different Origin.=—It is often
important, especially in cases of suspected adulteration, to determine
the origin of the starch granules. For this purpose the microscope
is the sole resort. In many cases it is easy to determine the origin
of the starch by the size or the shape and marking of the grains. In
mixtures of more than one kind of starch the distinguishing features of
the several starches can be clearly made out in most instances. There
are, however, many instances where it is impossible to discriminate
by reason of the fact that the characteristics of starch granules
vary even in the same substance and from year to year with varying
conditions of culture.
In many cases the illustrations of the forms and characteristics of
starch granules which are found in books are misleading and no reliance
can be placed on any illustrations which are not either photographs or
drawings made directly from them. In the microscopic study of starches
the analyst will be greatly helped by the following descriptions of the
characteristic appearance of the granules and the classifications based
thereon.[164]
=197. Vogel’s Table of the Different Starches and Arrowroots of
Commerce.=—_A._ Granules simple, bounded by rounded surfaces.
I. Nucleus central, layers concentric.
_a._ Mostly round, or from the side, lens-shaped.
1. Large granules 0.0396-0.0528 mm, _rye starch_:
2. Large granules 0.0352-0.0396 mm, _wheat starch_:
3. Large granules 0.0264 mm, _barley starch_.
_b._ Egg-shaped, oval, kidney-shaped: Hilum often long
and ragged:
1. Large granules 0.032-0.079 mm, _leguminous starches_.
II. Nucleus eccentric, layers plainly eccentric or meniscus-shaped.
_a._ Granules not at all or only slightly flattened:
1. Nucleus mostly at the smaller end; 0.06-0.10 mm,
_potato starch_:
2. Nucleus mostly at the broader end or towards the
middle in simple granules; 0.022-0.060 mm, _maranta
starch_.
_b._ Granules more or less strongly flattened.
1. Many drawn out to a short point at one end.
_a._ At most 0.060 mm long, _curcuma starch_:
_b._ As much as 0.132 mm long, _canna starch_:
2. Many lengthened to bean-shaped, disk-shaped, or
flattened; nucleus near the broader end; 0.044-0.075 mm,
_banana starch_:
3. Many strongly kidney-shaped; nucleus near the edge;
0.048-0.056 mm, _sisyrinchium starch_:
4. Egg-shaped; at one end reduced to a wedge, at the other
enlarged; nucleus at smaller end; 0.05-0.07 mm,
_yam starch_:
_B._ Granules simple or compound, single granules or parts of granules,
either bounded entirely by plain surfaces, many-angled, or by partly
round surfaces.
I. Granules entirely angular.
1. Many with prominent nucleus: At most 0.0066 mm, _rice starch_:
2. Without a nucleus: The largest 0.0088 mm, _millet starch_:
II. Among the many-angled also rounded forms.
_a._ No drum-shaped forms present, angular form predominating.
1. Without nucleus or depression very small; 0.0044 mm,
_oat starch_:
2. With nucleus or depression; 0.0132-0.0220 mm.
_a._ Nucleus or its depression considerably rounded;
here and there the granules united into differently
formed groups; _buckwheat starch_:
_b._ Nucleus mostly radiate or star-shaped; all the
granules free; _maize_ (_corn_) _starch_:
_b._ More or less numerous kettledrum and sugar-loaf like forms.
1. Very numerous eccentric layers; the largest granules
0.022-0.0352 mm, _batata_ (sweet potato) _starch_:
2. Without layers or rings; 0.008-0.022 mm.
_a._ In the kettledrum-shaped granules the nucleal
depression mostly widened on the flattened
side; 0.008-0.022 mm, _cassava starch_:
_b._ Depression wanting or not enlarged.
_aa._ Nucleus small, eccentric; 0.008-0.016 mm,
_pachyrhizus starch_:
_bb._ Nucleus small, central, or wanting.
_aaa._ Many irregular angular forms;
0.008-0.0176 mm, _sechium starch_:
_bbb._ But few angular forms; some with
radiate, nucleal fissure; 0.008-0.0176 mm,
_chestnut starch_.
_C._ Granules simple and compound; predominant forms, oval, with
eccentric nucleus and numerous layers; the compound granule made up
of a large granule and one or more relatively small kettledrum-shaped
ones; 0.025-0.066 mm, _sago starch_.
=198. Muter’s Table for the Detection of Starches when Magnified about
230 Diameters.=
[All measurements are given in decimals of an inch.]
_Group I_: All more or less oval in shape and having both hilum and
rings visible.
-------------+----------------+-------------+-----------------------
Name. | Shape. | Normal | Remarks.
| |measurements.|
-------------+----------------+-------------+-----------------------
Tous les mois|Oval, with flat | 0.00370 |Hilum annular, near one
| ends | to 0.00185 | end and incomplete
| | | rings.
| | |
Potato |Oval | 0.00270 |Hilum annular, rings
| | to 0.00148 | incomplete, shape and
| | | size very variable.
| | |
Bermuda |Sack-shaped | 0.00148 |Hilum distinct annular,
arrowroot | | to 0.00129 | shape variable, rings
| | | faint.
| | |
St. Vincent |Oval-oblong | 0.00148 |Hilum semi-lunar, rings
arrowroot | | to 0.00129 | faint, shape not very
| | | variable.
| | |
Natal |Broadly ovate | 0.00148 |Hilum annular, in
arrowroot | | to 0.00129 | center and well marked
| | | complete rings.
| | |
Galangal |Skittle-shaped |About 0.00135|Hilum elongated, very
| | | faint incomplete
| | | rings.
| | |
Calumba |Broadly | ” 0.00185|Hilum semi-lunar, faint
| pear-shaped | | but complete rings,
| | | shape variable.
| | |
Orris root |Elongated-oblong| ” 0.00092|Hilum faint, shape
| | | characteristic.
| | |
Turmeric |Oval-oblong, | ” 0.00148|Very strongly marked
| conical | | incomplete rings.
| | |
Ginger |Shortly conical,| ” 0.00148|Hilum and rings
| with rounded | | scarcely visible,
| angles. | | shape variable but
| | | characteristic.
-------------+----------------+-------------+-------------------------
_Group II_: With strongly developed hilum more or less stellate.
-------------+--------------+-------------+-------------------------
Name. | Shape. | Normal | Remarks.
| |measurements.|
-------------+--------------+-------------+-------------------------
Bean |Oval-oblong |About 0.00135|Fairly uniform.
| | |
Pea |Like bean | 0.00111 |Very variable in size,
| | to 0.00074 | with granules under
| | | 0.00111 preponderating.
| | |
Lentil |Like bean |About 0.00111|Hilum, a long depression
| | | seldom radiate.
| | |
Nutmeg |Rounded | ” 0.00055|The small size and
| | | rounded form
| | | distinctive.
| | |
Dari |Elongated | ” 0.00074|Irregular appearance and
| hexagon | | great convexity
| | | distinctive.
| | |
Maize |Round and | ” 0.00074|The rounded angles of the
| polygonal | | polygonalgranules
| | | distinctive.
-------------+--------------+-------------+-------------------------
_Group III_: Hilum and rings practically invisible.
-------------+--------------+-------------+-------------------------
Name. | Shape. | Normal | Remarks.
| |measurements.|
-------------+--------------+-------------+-------------------------
Wheat |Circular and | 0.00185 |Very variable in size and
| flat | to 0.00009 | very dull polarization
| | | in water.
| | |
Barley |Slightly |About 0.00073|The majority measuring
| angular | | about 0.00373
| circles | | distinctive, and a few
| | | four times this size.
| | |
Rye |Like barley | 0.00148 |Small granules, quite
| | to 0.00009 | round, and here and
| | | there cracked.
| | |
Jalap |Like wheat | |Polarizes brightly in
| | | water.
| | |
Rhubarb | do. | 0.00055 |Polarizes between jalap
| | to 0.00033 | and wheat, and runs
| | | smaller and more convex.
-------------+--------------+-------------+-------------------------
Senega |Like wheat | 0.00148- |
| | 0.00009 |
Bayberry | do. | 0.00074- | Measurements the
| | 0.00011 | only guide.
Sumbul | do. | 0.00074- |
| | 0.00009 |
-------------+--------------+-------------+-------------------------
Chestnut |Very variable | 0.00090- |Variable form, and small
| | 0.00009 | but regular size,
| | | distinctive.
| | |
Acorn |Round-oval |About 0.00074|Small and uniform size,
| | | distinctive.
| | |
Calabar bean |Oval-oblong | 0.00296 |Large size and shape
| | to 0.00180 | characteristic.
| | |
Licorice |Elongated-oval|About 0.00018|Small size and shape
| | | distinctive.
| | |
Hellebore |Perfectly | 0.00037 |Small, regular size and
(green or | rotund | to 0.00009 | rotundity, distinctive.
black) | | |
| | |
Hellebore |Irregular | 0.00055 |Irregular shape and faint
(white) | | to 0.00009 | central depression,
| | | distinctive.
-------------+--------------+-------------+-------------------------
_Group IV_: More or less truncated at one end.
-------------+--------------+-------------+------------------------
Name. Shape. | Normal | Remarks.
| |measurements.|
-------------+--------------+-------------+------------------------
Cassia |Round | 0.00111 |Round or muller shaped
| | to 0.00018 | granules and faint
| | | circular hilum.
| | |
Cinnamon |Like cassia | 0.00074 |More frequently truncated
| | to 0.00009 | than cassia, and
| | | smaller.
| | |
Sago (raw) |Oval-ovate | 0.00260 |Has circular hilum at
| | to 0.00111 | convex end and rings
| | | faintly visible.
| | |
Sago | ” | 0.00260 |Has a large oval or
(prepared) | | to 0.00111 | circular depression,
| | | covering one-third
| | | nearly of each granule.
| | |
Tapioca |Roundish | 0.00074 |A little over fifty per
| | to 0.00055 | cent truncated by one
| | | facet, and a pearly
| | | hilum.
| | |
Arum |Like tapioca |About 0.00056|Smaller than tapioca and
| | | truncated by two facets.
| | |
Belladonna | do. | |Not distinguishable
| | | from tapioca.
| | |
Colchicum | do. |About 0.00074|Larger than tapioca,
| | | and contains many
| | | more truncated
| | | granules.
| | |
Scammony | do. | ” 0.00045|Smaller than tapioca,
| | | more irregular, and
| | | hilum not visible.
| | |
Cancella |Very variable | 0.00033- |Very variable, form and
| | 0.00022 | small size the only
| | | points.
| | |
Podophyllum |Like tapioca |About 0.00040|Like scammony, but has
| | | visible hilum in most
| | | of the granules.
| | |
Aconite | do. | ” 0.00037|Like tapioca, but half
| | | the size.
-------------+--------------+-------------+------------------------
_Group V_: All granules more or less polygonal.
-------------+--------------+-------------+-------------------------
Name. | Shape. | Normal | Remarks.
| |measurements.|
-------------+--------------+-------------+-------------------------
Tacca |Poly- or | 0.00075 |Distinguished from maize
| hexagonal | to 0.00037 | by its sharp angles.
| | |
Oat |Polygonal |About 0.00037|Larger than rice and
| | | hilum visible in some
| | | granules.
| | |
Rice | do. | 0.00030- |Measurement using
| | 0.00020 | one-eighth or
| | | one-twelfth inch power,
| | | and then hilum visible.
| | |
Pepper | do. | 0.00020- | Do.
| | 0.00002 |
Ipecacuanha | do. |About 0.00018|Some round and truncated
| | | granules, adhering in
| | | groups of three.
-------------+--------------+-------------+-------------------------
=199. Blyth’s Classification.=—Blyth gives the following scheme for the
identification of starch granules by their microscopic appearance.[165]
_Division I.—Starches showing a play of colors with polarized light and
selenite plate_:
The hilum and concentric rings are clearly visible, and all the starch
granules, oval or ovate. Canna arrowroot, potato, arrowroot, calumba,
orris root, ginger, galangal and turmeric belong to this division.
_Division II.—Starches showing no iridescence, or scarcely any, when
examined by polarized light and selenite_:
Class I.—The concentric rings are all but invisible, and the hilum
stellate. The bean, pea, maize, lentil, dari and nutmeg starches are in
this class.
Class II.—Starches which have both the concentric rings and hilum
invisible in the majority of granules: this important class includes
wheat, barley, rye, chestnut, acorn, and many starches in medicinal
plants.
Class III.—All the granules are truncated at one end. This class
includes sago, tapioca and arum, several drugs and cinnamon and cassia.
Class IV.—In this class all the granules are angular in form and it
includes oats, tacca, rice, pepper and ipecacuanha.
=200. Preparation of Starches for Microscopical Examination.=—The
approximately pure starches of commerce may be prepared for microscopic
examination by rubbing them up with water and mounting some of the
suspended particles by one of the methods to be described below.
In grains, seeds and nuts the starch is separated by grinding with
water and working through fine linen. The starch which is worked
through is allowed to subside, again beaten up with water if necessary
and the process continued until the grains are separated sufficiently
for microscopic examination. A little potash or soda lye may be used,
if necessary, to separate the granules from albuminous and other
adhering matter. The analyst should have a collection of samples of all
common starches of known origin for purposes of comparison.
The granules are mounted for examination by plain light in a medium
of glycerol and camphor water. When polarized light is used the
mounting should be in Canada balsam.[166] The reader can find excellent
photomicrographs of the more common starches in Griffith’s book.[167]
=201. Appearance in Balsam with Polarized Light.=—Mounted in balsam
the starches are scarcely visible under any form of illumination
with ordinary light, the index of refraction of the granules and the
balsam being so nearly alike. When, however, polarized light is used
the effect is a striking one. It is very easy to distinguish all the
characteristics, except the rings, the center of the cross being at the
nucleus of the granule.
With the selenite plate a play of colors is produced, which is peculiar
to some of the starches and forms the basis of Blyth’s classification.
=202. Description Of Typical Starches.=—The more commonly occurring
starches are described by Richardson as they appear under the
microscope magnified about 350 diameters.[168]
The illustrations, with the exception of the cassava starch, and the
maize starch accompanying it were drawn by the late Dr. Geo. Marx from
photographs made by Richardson in this laboratory. The two samples
excepted were photographed for the author by Dr. G. L. Spencer.
_Maranta Starch._—Of the same type as the potato starch are the various
arrowroots, the only one of which commonly met with in this country
being the Bermuda, the starch of the rhizome of _Maranta arundinacea_,
and the starch of turmeric.
The granules are usually not so varied in size or shape as those of
the potato, averaging about 0.07 millimeter in length as may be seen
in Fig. 48. They are about the same size as the average of the potato,
but are not often found with the same maximum or minimum magnitude,
which circumstance, together with the fact that the end at which the
nucleus appears is broader in the maranta and more pointed in the
potato, enables one to distinguish the two starches without difficulty.
With polarized light the results are similar to those seen with potato
starch, and this is a ready means of distinguishing the two varieties,
by displaying in a striking way the form of the granule and position of
the hilum.
_Potato Starch._—The starch grains of the potato are very variable in
size, being found from 0.05 to 0.10 millimeter in length, and in shape
from oval and allied forms to irregular and even round in the smallest.
These variations are illustrated in Fig. 49, but the frequency of the
smaller granules is not as evident as in some other cases. The layers
are visible in some granules with great distinctness and in others
hardly at all, being rather more prominent in the starch as obtained
from a freshly cut surface. The rings are more distinct, too, near
the hilum or nucleus, which in this, as in all tuberous starches, is
eccentric, shading off toward the broader or more expanded portion
of the granule. The hilum appears as a shadowy depression, and with
polarized light its position is well marked by the junction of the arms
of the cross. With polarized light and a selenite plate a beautiful
play of colors is obtained. The smaller granules, which are nearly
round, may readily be confused with other starches, but their presence
serves at once to distinguish this from maranta or Bermuda arrowroot
starch. Rarely compound granules are found composed of two or three
single ones each with its own nucleus.
_Ginger Starch._—This starch is of the same class as those from the
potato and maranta and several others which are of underground origin.
In outline the granules are not oval like those named, but more
rectangular, having more obtuse angles in the larger ones and being
cylindrical or circular in outline in the smaller, as indicated in Fig.
50. They average nearly the same size as maranta starch, but are much
more variable, both in size and form. The rings are scarcely visible
even with the most favorable illuminations.
_Sago Starch._—This exists in two modifications in the market; as raw
and as prepared sago. In the prepared condition it is characterized by
a larger circular depression in the center of most of the granules. The
rings are not visible. They are mostly circular in form or approaching
it, and vary from 0.025 to 0.065 millimeter in diameter, as indicated
in Fig. 51.
[Illustration: FIG. 48. MARANTA STARCH × 350.]
[Illustration: FIG. 49. POTATO STARCH × 350.]
[Illustration: FIG. 50. GINGER STARCH × 350.]
[Illustration: FIG. 51. SAGO STARCH × 350.]
[Illustration: FIG. 52. PEA STARCH × 350.]
[Illustration: FIG. 53. BEAN STARCH × 350.