Food Structure


Wheat starch:water (1:2) dispersions were studied by small angle x-ray scatter diffractometry (SAXS), and by light, polarizing microscopy, and scanning electron microscopy (SEM). X-ray scatter data were collected in the 64-661 A range. Radii of gyration (Rg) of spherically shaped regions and d-spacings were calculated for samples that were treated as follows: 25° C and run at 25° C; heated to 88° C and run at 82° C; heated to 88° C, stored at 25° C for 2 h or 3 days and run at 25° C; heated to 88° C, stored at 2° C for 3 days and run at 6° C. Samples comparable to those used for SAXS experiments were stored without x-ray irradiation to evaluate the effects of x-ray exposure on starch structure. All samples were evaluated for differences in granule morphology and loss of birefringence using light, SEM and polarizing microscopy.

SAXS results indicated that a well-defined d-spacing that was seen between 260-296 A in unheated granules is related to birefringence of the starch granule and was not found again after heating, cooling, and storage. These observations indicate that a more random order was present after heating. The 260-296 A spacing fits the 3 cluster, ordered model for amylopectin in which an extended amylopectin molecule has a diameter of 500 A. The Rg data further support this model because an Rg value for unheated starch was found at 249 A. A maximal Rg change occurred for starch heated to 88° c and stored for 3 days at 2° C (Rg 175 A). These data suggest that heating, cooling and storage result in a molecular reorganization extending over a relatively large distance and involving both a loss of order (shown by loss of the 260-296 A spacing) and a tightening of the structure (shown by the decrease in the Rg values).

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