Iodine affinity

To determine the amylose content of starch, the iodine reaction has been most commonly used because amylose and amylopectin have different abilities to bind iodine. The methods such as blue value (absorbance at 680 nm for starch-iodine complex using amylose and amylopectin standards), and potentiometric and amperometric titration have been used for more than 50 years. These procedures are based on the capacity of amylose to form helical inclusion complexes with iodine, which display a blue color characterized by a maximum absorption wavelength (kmax) above 620 nm. During the titration of starch with iodine solution, the amount (mg) of iodine bound to 100 mg of starch is determined. The value is defined as iodine-binding capacity or iodine affinity (lA). The amylose content is based on the iodine affinity of starch vs. purified linear fraction from the standard 100 mg pure linear amylose fraction has an iodine affinity of 19.5-21.0mg depending on amylose source. Amylopectin binds 0-1.2mg iodine per 100mg (Banks and Greenwood, 1975). The amylose content determined by potentiometric titration is considered an absolute amylose content if the sample is defatted before analysis. 

Irradiation Acidity Gelation Iodine affinity Amylose... 

Table 6.1 Iodine affinities and amylose contents of starches79...

Using results of these kinds of studies, the characteristic structure of amylose can be differentiated from that of amylopectin. Amylose has a small number of branches and crystallizes and precipitates when complexed with 1 -butanol. The iodine affinity of amylose is much greater (i.a. 18.5 to 21.1) than that of amylopectin (i.a. 0.0 to 6.6),79,152-158,163,169-174 and the iodine affinity of amylose (3-limit dextrin is similar to that of the parent amylose. The average chain length of amylose (3-limit dextrins is much larger than that of the amylopectin (3-limit dextrin.160... 

The iodine affinities, blue values, max, beta-amylolysis limits and branch chain length distributions are similar for the starch samples harvested at different stages (7 to 30 days DAF) of development of waxy rice grains,331 japonica rice328 and indica rice.332 The gelatinization temperature of maize starch increases from 12 to 24 DAF, but is reduced at 36 DAF.333... 

Amyloses of five commercially important starches (wheat, corn, rice, tapioca and potato) were purified by fractional crystallization of their amylose-alcohol complexes and shown to be free of amylopectin.111,205 All five amyloses showed comparable iodine affinity, blue value and max of their polyiodide complexes (Table 10.5), which is consistent with their being (1 —4)-linked a-glucans with an average DPn of —500 or above.206 The average size of amylose molecules in cereal starches is smaller than... 

White et al.8,30,31 have, in several papers, studied the chemical composition of oat starch amylose and amylopectin (Table 15.2). Gel permeation chromatography (GPC) of the total starch gave rise to two peaks, but some of the material had an unusual iodine affinity and was, therefore, described as an intermediate fraction.8... 

Botanical source B value Iodine affinity max /3-Amylolysis limit (%) Chain length hi P0 (ppm) P6 (ppm)... 

The presence and content of phosphoric acid moieties in starch also does not affect the course of reaction.66 78 For several types of amylose, the iodine affinity varies between 18.5 and 20% in a 0.05 M solution of iodine.79... 

Materials Emulsifier Added, % Iodine Affinity of Filtrate Amylose-Complexing Index... 

The nomenclature of starch components has become highly confused through the indiscriminate use of such terms as a and /3-amylose, amylose and amylopectin, amyloamylose and erythroamylose, etc. It does not seem advisable to apply any of these older terms to the new and unique fractions obtained by selective precipitation methods. It is therefore proposed that the term A-fraction be applied to that portion of the starch which is preferentially precipitated by various alcohols or by higher fatty acids, and which exhibits a high affinity for iodine. Correspondingly, the residual non-precipitated portion of the starch, which possesses low iodine affinity, is termed the B-fraction. These designations apply to laboratory-prepared samples, and do not carry any a priori connotations of purity, homogeneity, or structure. 

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,... 

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