Short-range structure in (partially) crystalline potato starch determined with attenuated total reflectance Fourier-transform IR spectroscopy

A fast and direct method, based on infrared spectroscopy, for quantitative determination of starch short-range structure has been developed. The IR spectrum of starch is sensitive to changes in short-range structure in the C—C and C—O stretching region at 1300-800 cm−1. The IR absorbance band at 1047 cm−1 is sensitive to the amount of ordered or crystalline starch and the band at 1022 cm−1 is characteristic of amorphous starch. The ratios (R) of the heights of the bands at 1047 and 1022 cm−1, which expresses the amount of ordered starch to amorphous starch, and 1047 and 1035 cm−1, which is a measure of the amount of ordered starch, showed a linear regression with the amount of potato starch having B-type crystallinity as determined with wide-angle X-ray diffractometry. The IR spectrum and thus the short-range order is also sensitive to water content. In particular, the band at 994 cm−1, which is related to intramolecular hydrogen bonding of the hydroxyl group at C-6, is water sensitive. It is possible to quantify the IR data in terms of short-range order (SIR) over a range of 10–50% water. The method has been applied to quantify the changes in short-range structure during the melting of potato starch with 18 or 26% (w/w) water. The amount of short-range structure and the changes during melting in the (partially) destructured starch samples concur with differential scanning calorimetry and wide-angle X-ray diffractometry measurements.