SPECTRA ANALYSIS ON RICE STARCHES FROM DIFFERENT VARIETIES DURING PASTING

Abstract

The pasting behavior of rice starches from different varieties was investigated using both attenuated total reflectance Fourier-transform IR spectroscopy and X-ray diffraction.For IR spectroscopy analysis,absorbance spectra were recorded on a spectrometer equipped with a deuterated triglycine sulphate detector using the Digilab attenuated total reflectance accessory.Starch dispersions(6%)heated to different temperatures were loaded on the OMNI sampler.Each spectrum,obtained at a resolution of 4 cm-1,was an average of 64 scans,recorded against an empty cell as background and was subtracted from the spectrum of water.Spectra were baseline-corrected at 1200 cm-1 and 800 cm-1 by drawing a straight line.All spectra were deconvoluted.The results indicated that the crystalline areas of the granule were mainly ascribed to double helices formed by amylopectin branches.The crystallinity of the granules was disrupted during pasting.The short-range order of waxy(Calmochi101)and medium(Koshi and M202)grain rice starches decreased rapidly with the temperature increasing from 60℃ to 65℃,which indicated that the loss of crystallinity was significant as soon as the temperature reached the pasting onset temperature of each rice starch.At temperatures above 70℃ a relative small decrease of short-range order was observed.In the case of long grain rice starch(Cocodrie and L205),significant decrease of short-range order was observed when the temperature was over 70℃.This was due to the higher pasting onset temperature of long grain rice starch.The rice starches with high amylose content were found to lose the crystallinity more slowly as compared to those starches with low amylose content.It suggested that the amylose could restrain the swelling and disruption of starch granules and inhibit the loss of the crystalline areas of rice starch granules during pasting.Relative crystallinity of pasted starches at different temperatures was determined using X-ray diffraction.The results verified the loss of crystallinity of starch granules during pasting and were consistent with the results obtained by IR spectroscopy analysis.Although the definitions of "crystallinity" were different for the two methods,the results were correlative and comparable and could provide useful information for the study of starch pasting.