Nature of Crystallinity in Native and Acid Modified Starches

Crystalline characteristics of natural and acid modified starches (cereals, millets, pulses, root, and tuber) were examined by X-ray diffraction studies. The (hkl) planes were identified using A-, B- and C- type starch cell parameters. The size of crystallite along the direction [hkl] was calculated for all the reflections in the starches. Smallest crystallites (35 to 56 Å) are observed for the reflections at (2θ) 11.2 to 11.5° corresponding to (111) reflection plane, waxy rice showing the lowest value of 35 Å. Largest crystallite size of the order 200 Å is observed at 2θ 20.0 to 20.4° corresponding to (032) reflection plane with finger millet giving the highest value in the order of 289 Å. Acid modification leads to minor changes in 'd' spacing but crystallite size changes quite considerably. It increases in some, decreases in others, and remains unchanged in few cases. There was no uniform pattern observed either with respect to acid used, source of starch or the peak at which the changes occur. In finger millet starch, crystallite size becomes homogeneous upon acid treatment.