Optical Interference Studies on Single Starch Granules* A. Chandrashekar, K. Somashekar and A. Savitri, Mysore (India) zyx Studies on the optical interference by potato, sorghum, wheat, pigeon pea and cowpea starch granules indicated that the potato granule was ordered mostly on the centre of the granule and that the molecular arrangement of the birefringent portion was perpendicular to the radial. This was more so with the legume starches where the entire granule was ordered. Ccrcal starches had birefringent material on the periphery and non-bircfringent material in the centre and in this case, the birefringent material was radially arranged. Optische Interferenz-Untersuchungen an einzelnen Starkekor- nern. Untersuchungen der optischen Interferenz der Starkekorner von Kartoffeln, Sorghum, Weizen, Taubenerbsen und Kuherbsen zeigten. daB das Kartoffelstarkekorn in seinem Zentrum am starkstcn geordnet war und dab die molekulare Anordnung des doppelbrechenden Anteils senkrecht zu dem radialen Antcil verlief. Dies war bei den Legumino- scnstarken, bei denen das ganze Korn geordnet war, in hoherem MaBe der Fall. Getreidestarken hatten doppelbrechendes Material an der Peripherie und nicht doppelbrechendes Material im Zentrum: in diesem Falle war das doppelbrechende Material radial angeordnet. 1 Introduction Birefringence of starch granule is a very well known phenome- non and is strangely one of the least studied of the various granule properties. Birefringence is defined as the difference in refractive index (R. zyxwvutsrqp I.) between two mutually perpendicular directions (nl and n2) and occurs in material with a two- dimensional difference in order and is represented by An (nl -n2). The relationship between three-dimensional order crystallinity and birefringence has not been established in starch. Starch granules appear with a typical dark and nature cross when observed under polarized microscope. The intensity of birefringence is dependent on the thickness of the granules and on the orientation of the molecules within the granule. The dark arms of the polarization cross occur along the local average axes of the optic refractive index ellipsoides and intersect at the hilum in most starches. The optic axis as highest refractive index direction coincides with the chain direction of starch and most other biopolymers so that the appearance of starch granules under the polarizing microscope gives a strong clue as to the orientation of the molecular chains within the various regions of the granule. Early in the 1940's zyxwvutsrq Speich [l] and Frey-Wyssling [2] worked on the measurement of birefringence of starch granu- les. French [3] developed optical maps of granules by viewing granules at different angles under polarized light. An optical interference technique has been used in this study to under- stand molecular orientation within the granule and the internal distribution of birefringent material. Borch et al. [4] have recorded anisotropic scattering by single starch granules. 2 Materials and Methods Potato (Solanum tuherosum), tiir (Cujunus zyxwvuts cujan), cowpea (Vignu catjung). wheat (Triticum aestivum) and sorghum (Sor- ghum hicolor) were all procured from the local market for the isolation of starch. Starches were isolated by the alkali soaking method zyxwvutsrq [S]. The moisture contents of all the starches were brought to 10% to measure birefringence. The optical interference experiments were carried out as described by Krishnamurthy and Somashekar [6] and are shown zyxwvu * Presented at the Cereal Carbohydrate Conference, Aug. 5th-7th. 1986. Purdue University, West Lafayette. Indiana (U.S.A.). in Fig. 1. Starch granules were placed on a slide wetted with water, covered with a cover slip and the birefringence was immediately measured. D - - P- zyxwvut 52 zyxwvut Q zyxwvut r n lM ii A - D-- Figure 1. Set up for measuring optical in- terference of single starch granules. S,-Source (sun), L-Lens, D-Slit, P-Polarizer, Q-Quartz, &Starch (sample), TM-Travel- ling microscope, A-Analyzer, Sp-Spectro- sp r 1 scope. The principle is described here briefly. When a bircfringent material held along the slow direction between crossed polars is observed through a spectroscope, the spectrum of white light transmitted through the system consists of a series of dark horizontal bands of different wave-lengths. On placing a second birefringent material in line with the first, the bands are shifted, according to the extent of interference of light by the second material. The direction and extent of shift and the region at which the shift occurs maximally and direction of dispersion (i.e., towards the red or blue region) of the spectrum are determined. The interpretations of these observations to obtain some ideas on the molecular orientation of the interfering material as starch is described by Krishnarnurthy and starch/starke 39 (1987) Nr. 6, zyxwvutsrqponm S. 195- 197 0 VCH Verlagsgesellschaft mbH. D-6940 Weinheim. 1987 01138-905h/87/060h-Ol')5$02.50/0 195