Starches rice, coloration with iodine

This results from the transformation of starch by means of heat or by the action of dilute acid or diastase. It is prepared principally from potato, wheat or maize starch and rarely from rice or other exotic starches. Many varieties of dextrin, made in diverse ways, are sold under different names. It occurs as a fine powder, either wliite, dirty white, yellowish or light brown as granules, similar in appearance to gum arabic and as a thick syrup, more or less highly coloured and opaque. In general dextrin has a special odour and taste, which are particularly marked in the pulverulent varieties. It is soluble in water, but insoluble in alcohol. Its solution is strongly dextro-rotatory the value of [a]D varies from 173° to 2250,but is mostly about 200°. With iodine different dextrins give bluish violet to brownish red colorations (the colour is observed by adding the iodine solution drop by drop if the mass is mixed after the first drops are added, the colour disappears). 

As with iodine, starch forms an inclusion complex with bromine vapor.205 Depending on the starch variety, different colors are developed by the complex. Maize and wheat produce an ochre color, rice produces a light-buff color, potato and sago develop a pale-yellow color, and cassava forms a cream color.69 Iodine cyanide (and bromine)-amylose complexes are brown-black and dark brown, respectively.206 The adsorption of chlorine and iodine proceeds according to the Freundlich isotherm. A discontinuity on the Freundlich isotherm plot is reported, which possibly results from the swelling of starch granules454... 

For the tests described below use for acid extraction the residue in the alundum crucible from the water (or water-alcohol) extraction, as described in step 6 of "Standard Method No 32" Before proceeding with the extraction, test for the presence of starch in the above residue by placing a small portion in a 50-ml beaker, adding 10 ml of distd w and bringing it to a boil. Then cool and add a drop of iodine (1 in methanol) so In. Appearance of a blue color indicates the presence of starch Note A Rice hulls may give a faint starch test from small rice particles, and corn meal will give a positive test also. Determine the presence of these materials by examination of the sample to be extracted... 

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