Measurement of wheat starch granule size distribution using image analysis and laser diffraction technology

Starch was isolated from flour of four wheats representing hard red winter (Karl), hard red spring (Gunner), durum (Belfield 3), and spell (WK 86035-8) wheat classes. Digital image analysis (IA) coupled with light microscopy was used to determine starch size distributions where the volume of granules was calculated as spherical particles or oblate spheroids. Starch granules were classified into three size ranges: A-type granules ( > 15 mu m), B-type granules (5-15 mu m), and C-type granules ( < 5 mu m). An error was noted in using digital image analysis because the perimeter of some granules touch the edge (PTE) of the field being analyzed. To correct for this error, the PTE granules were manually replaced into the field by measuring their diameters and entering them into the database. The results showed differences in the starch size distributions between the classes of wheat evaluated, as well as the method of analysis. Four laser diffraction sizing (LDS) instruments were used to measure granule distributions of the four classes of wheat. LDS compared with IA resulted in a approximate to 40% underestimation of the A-type granule diameter and a approximate to 50% underestimation of the B-type granule diameter. A correction factor (adjustment) was developed from IA data to correct LDS analysis. LDS data correlations before adjustments to IA data were R-2 = 0.02(ns) to 0.55(***). After adjustment, these correlations improved to R-2 = 0.81(***) to 0.93(***) depending on the class of wheat starch evaluated.