Amylose binding capacity

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. 

Amylose, although water soluble, gives an unstable solution which irreversibly precipitates. It is mainly responsible for the deep blue coloration given by starch and iodine. Solutions of amylopectin are relatively stable. The iodine-binding capacity, on the other hand, is very low. A small amount of covalently bound phosphate normally appears with starch but its exact location within the molecule is not known. 

Generally, the phosphorus content in starches is associated with different pasting properties, and it confers a larger ion binding capacity. In wheat and com starch, phosphorus is present largely or wholly as adsorbed phos-phatides (extractable with boiling 85% methanol) associated preferentially with the amylose fraction. 

The binding capacity of iodine by starch was studied by spectrophotomet-ric, potentiometric, amperometric, and rheological methods. A sharp increase in the viscosity of starch solutions occurred after the amylose helix... 

Fig. 13.—Iodine-binding capacity (IBC) of amylose as the function of the concentration of I ions (in mol of KI). (Reprinted with permission from J. Hollo and J. Szejtli, Brauwissenschaft, 13 (1960) 348-352.)...

Properties of Complexed Amylose and Amylopectin 7 Iodine-Binding Capacity... 

Different starches show varying flavour binding capacity. Starches with a low amy-lose content (e.g. tapioca at 17%) and waxy starches consisting only of amylopectin have only a weak binding capability those with a high amylose content (like potato or maize) have a greater one. Gelatinized potato starch has been researched best. 

The iodine-binding capacity of starch is dependent on the degree of polymerization (DP). Amylose forms with iodine a helical inclusion complex with an intense blue colour, which possesses an absorption maximum at wavelengths between 620 and 680 nm. Amylopectin has much less iodine-binding capacity because of its branched character, leading to a red-violet colour with absorption maximum of 540 nm 31. 

Kim and Robinson have shown that the helical conformation of amylose was the most important factor in the binding of surfactant molecules in the helical cavity of amylose chain, resulting in a slight decrease in the intrinsic viscosity, fall in the p-amylolysis limit and the iodine binding capacity(29)... 

Some physical properties (e.g., water-binding capacity, gelatinization temperature, swelling power) of starches from pearl millets have been studied. Some starches were shown to retrograde more readily than others. Only small variations were found in the amylose content of the starches, suggesting that other physicochemical factors may be more important in determining the properties of the starches. 

The capacity of starch to stain blue-black with iodine suggests that some of the amylose is present in the starch in the V-form. The lipids present in cereal starch would bind to amylose if it were in the V-form, and yet X-ray analysis does not show the presence of the V-polymorph in cereal starches (i.e., most of the amylose would be in the amorphous form). The conclusion is that although a significant part of the amylose is probably in the helical form, the three-dimensional order necessary to give a crystalline diffraction pattern is absent. Indeed, the crystalline nature of starch is now attributed to the presence of amylopectin and not to amylose. Starch from waxy mutants contains only amylopectin (and no amylose), but this starch has the same degree of crystallinity and the same X-ray pattern as the regular starches that contain both components. 

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