Dr. Brukner has contributed to the _Proceedings_ of the Vienna Academy of
Sciences a paper on the "Chemical Nature of the Different Varieties of
Starch," especially in reference to the question whether the granulose of
Nageli, the soluble starch of Jessen, the amylodextrin of W. Nageli, and
the amidulin of Nasse are the same or different substances. A single
experiment will serve to show that under certain conditions a soluble
substance maybe obtained from starch grains.
If dried starch grains are rubbed between two glass plates, the grains
will be seen under the microscope to be fissured, and if then wetted and
filtered, the filtrate will be a perfectly clear liquid showing a strong
starch reaction with iodine. Since no solution is obtained from uninjured
grains, even after soaking for weeks in water, Brukner concludes that the
outer layers of the starch grains form a membrane protecting the interior
soluble layers from the action of the water.
The soluble filtrate from starch paste also contains a substance identical
with granulose. Between the two kinds of starch, the granular and that
contained in paste, there is no chemical but only a physical difference,
depending on the condition of aggregation of their micellæ.
W. Nageli maintains that granulose, or soluble starch, differs from
amylodextrin in the former being precipitated by tannic acid and acetate
of lead, while the latter is not. Brukner fails to confirm this
difference, obtaining a voluminous precipitate with tannic acid and
acetate of lead in the case of both substances. Another difference
maintained by Nageli, that freshly precipitated starch is insoluble,
amylodextrin soluble in water, is also contested; the author finding that
granulose is soluble to a considerable extent in water, not only
immediately after precipitation, but when it has remained for twenty-four
hours under absolute alcohol. Other differences pointed out by W. Nageli,
Brukner also maintains to be non-existent, and he regards amidulin and
amylodextrin as identical. Brucke gave the name erythrogranulose to a
substance nearly related to granulose, but with a stronger affinity for
iodine, and receiving from it not a blue but a red color. Brukner regards
the red color as resulting from a mixture of erythrodextrin, and the
greater solubility of this substance in water.
If a mixture of filtered potato starch paste and erythrodextrin is dried
in a watch glass covered with a thin pellicle of collodion, and a drop of
iodine solution placed on the latter, it penetrates very slowly through
the pellicle, the dextrin becoming first tinctured with red, and the
granulose afterward with blue. If, on the other hand, no erythrodextrin is
used, the diffusion of the iodine causes at once simply a blue coloring.
With regard to the iodine reaction of starch, Brukner contests Sachsse's
view as to the loss of color of iodide of starch at a high temperature. He
shows that the iodide may resist heat, and that the loss of color depends
on the greater attraction of water for iodine as compared with starch, and
the greater solubility of iodine in water at high temperatures.
The different kinds of starch do not take the same tint with the same
quantity of (solid) iodine. That from the potato _arum_ gives a blue, and
that from wheat and rice a violet tint; while the filtrate from starch
paste, from whatever source, always gives a blue color.
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