Analysis of iodine absorption curve of starch by chemometrics to evaluate the eating quality of rice [Oryza sativa]

The amylose content of milled rice is usually determined colorimetrically by the iodine test. An automatic analyzer was developed to obtain the iodine absorption spectrum ranging from 400 to 900 nm. The influence of the molecular weight of amylose on the maximum absorption wavelength (lambdamax), and that of the temperature during the grain filling period on the iodine absorption spectrum and on the starch pasting properties were examined. The lambdamax of the amylose with above about 70,000 molecular weight was 598 nm, and a positive correlation was observed between the absorbance at lambdamax and the amylose content. The lambdamax was observed at a shorter wavelength in the rice grown at a high temperature during grain filling, and the shift of lambdamax by the temperature during grain filling was larger in the low-amylose varieties (dull) and breeding lines than in the non-glutinous varieties. The difference between the minimum and final viscosities in RVA (Y) was shown by an equation, Y=(29.7X-27.1)/(0.73X-0.72), where, X is the ratio of absorbance at 400-600 nm (Fr. I) to that at 600-900 nm (Fr. II). Thus, the lambdamax and the Fr. I/Fr. II ratio are expected to be useful as efficient eating-quality selection indexes in rice breeding.