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41 
8.—Tue Microscope anp Foop ADULTERATION. 
By Lester Reep, F.1.C., F.C.S. 
(Read December 16th, 1903.) © 
Wuite the analyst depends chiefly upon chemical methods for 
his qualitative, and almost entirely so for his quantitative work, 
it would be impossible to avoid falling back upon the invaluable 
help of the microscope in many cases. I propose this evening to 
pass briefly in review a few of these. 
When a sample of flour is submitted for analysis, the question 
of admixture with foreign starches is almost wholly a micro- 
scopical one, and it is fortunate for our purpose that the shapes 
and other characters of starch granules are as distinctive as they 
are. 
With a microscope of very moderate power it is possible to 
become intimate with some of the more striking peculiarities of 
some of the more common starches, and with the use of a 
polarizer further distinctions may be brought to light. We 
cannot assign a fixed or invariable diameter to any particular 
starch. We have only the average diameter to go by, if we 
. attempt to judge by size alone. 
Potato-starch granules are some of the largest, and may be 
roughly compared to oyster-shells, which they resemble not only 
in outline, but in their concentric, distinct, ring-like markings ; 
with polarized light they give a well-marked cross, and show 
colours well with selenite; their size is from 60 to 100 microns 
(the micron being ‘001 millimetre). 
In wheat-starch the markings are comparatively faint, and a 
near approach to the circular form is often attained. 
Maize-starch is smaller than that of wheat, and somewhat 
angular in form. 
' Rice-starch is still smaller, and very angular. 
The natural starches of cocoa and pepper are very small, the 
latter from 0.5 to 5 microns. 
If we are at any time in doubt whether the objects we are 
looking at under the microscope are starch granules or not, there 
is a very simple test by which the point may be decided. We 
introduce a droplet of iodine solution under the cover-glass. 
This solution is pale straw-coloured, but whenever it reaches and 
passes a starch granule, the latter becomes deep blue, approaching 
to black, if the iodine solution be strong. This is due to the fact 
that a blue compound, commonly called iodide of starch, is formed 
by the action of the iodine upon the starch, of which the granule 
is mainly composed. 
