Amylose iodine adsorption

When the A-fraction from corn starch is potentiometrically titrated with iodine, its affinity for the latter is sharply reduced in the presence of small amounts of fatty acid. Thus the iodine adsorption of recrystallized A-fraction (originally 18.7%) is reduced to 12.4%, 3.5% and 0% by the addition respectively of 2%, 5% and 10% of palmitic acid. Raw corn starch contains approximately 0.66% of fatty acids, corresponding to approximately 2% on the basis of the linear A-fraction. Thus, a third of the linear component in raw com starch is inactivated. In a sense, Taylor and coworkers were correct in assuming an association between a-amylose and fatty acid, but they erred in presuming the combination to be an ester. 

Kerr and Severson have superimposed selective precipitation on aqueous extraction technique, by treating the soluble extract with butyl alcohol. This gives an A-fraction (designated by Kerr as crystalline amylose ) of exceptionally high iodine adsorption, but in yields amounting to only 5-6% of the starch. The products from corn and tapioca starches analyze 20.5% and 20.7% iodine adsorption respectively, as compared with a maximum value of 19.0% after four recrystallizations of the Pentasol-precipitated A-fraction from corn starch. This suggests that the A-fraction may be somewhat diversified in molecular size, and that aqueous extraction preferentially dissolves the shorter linear chains. It is not to be expected that all the molecules of a natural high polymer should be of uniform size. 

We have reported a novel reversible interaction of amylose-iodine-complex with proteins, which is closely associated with the coil— helix transition of amylose chain, and the high adsorptive capacity of this complex appears to be due to hydrophobic Interactions (1 6). 

Starches can be separated into two major components, amylose and amylopectin, which exist in different proportions in various plants. Amylose, which is a straight-chain compound and is abundant in potato starch, gives a blue colour with iodine and the chain assumes a spiral form. Amylopectin, which has a branched-chain structure, forms a red-purple product, probably by adsorption. 

Although the dipolar and resonating nature of the interaction of amylose and iodine is well established, Schlamowitz173 regards the iodine in a starch complex as being in a predominantly non-polar form, and Meyer and Bern-feld174 refute the helix theory and consider that adsorption of iodine occurs on colloidal micelles in amylose solutions. Most of the experimental facts which Meyer presents can, however, be satisfactorily explained on the helical model. 

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... 

The potentiometric, iodine titration clearly shows the adsorptive powers of both starch fractions, namely, their relative affinities for iodine in a quantitative way. As is well known, the iodine affinity of amylose depends on its chain length, the shorter chains having the lower affinity. Evidently,... 

The adsorption of iodine by amylose chains to give a deep blue complex is universally used to qualify starch, and to quantify amylose. For some industrial uses, it is imperative that starch hydrolysates contain no starch, and this can be confirmed by cooling the matrix to 5-10°C, adding 0.02moll iodine drop-wise, and observing whether a blue color appears the detection limit is 50 mg per kg. 

---