Morphology and Ultrastructure of Starch Grain

Dry starch granules observed by optical microscopy appear transparent. The development of confocal laser scanning microscopy (CLSM) permitted to obtain, after staining with rhodamine, three-dimensional images of starch granules (van de Velde et al., 2002). The potato starch granules are ellipsoids of a size generally ranging from 45 to 75 μm, and the corn starch granules are of polyhedral shape having a size of about 15-20 μm. The wheat starch presents two types of spherical grains of relatively similar size 30 μm (grains A) and 5 μm (grain B). Even if potato starch or tapioca starch grains appear smooth by scanning electron microscopy (SEM), Fannon et al. (1992) showed that the cereal starch grains (wheat, barley, rye, etc.) have pores of about 100 nm on the surface, with the exception of oats and rice. These pores are generated at the time of the biosynthesis of the starch granule and lead to a greater sensitivity to enzymes. Moreover, Bule´on et al. (1997) and Waigh et al. (1997) showed by synchrotron radiation and X-ray micro-diffraction that the radial orientation is stronger at the periphery of potato starch grain than at the center. Recently, this organization was confirmed by atomic force microscopy (Baker et al., 2001).

An organization of the starch granule in concentric layers has been evidenced by various techniques. It is possible to observe by optical microscopy, on potato starch granules hydrated, an alternating concentric layer with low and high refractive indices. These layers have been interpreted as “growth rings” (Frey-Wyssling and Buttrose, 1961). These layers, visible in transmission electron microscopy on partially hydrolyzed grain cuts are alternately amorphous and crystalline (Yamaguchi et al., 1979). The hydrolysis method applied influences the results. Acid hydrolysis results in an increase in overall crystallinity of the grain, whereas it remains unchanged after enzymatic hydrolysis (α-amylolysis) (Colonna et al., 1988). Similarly, the layers observed after acid hydrolysis have a thickness of 1-3 μm, whereas the layers observed after enzymatic treatment have a thickness of 0.3-0.5 μm (Gallant and Guilbot, 1969; Gallant et al., 1972). These dimensions are close to those observed by CLSM after labeling the starches of the grains by safranin (van de Velde et al., 2002) and by SEM on partially hydrolyzed granules (Gallant et al., 1972; Smith, 1999) or fractured (Oates, 1997). Indeed, the starch grains observed by SEM show an “onion” organization with several sheets of variable size ranging from 120 to 400 nm. Recently, Li et al. (2006) showed by contrast interferential microscopy that this organization is marked in the potato starch grains, but less obvious to other botanical sources, including corn.

The most resistant layers to hydrolyze (crystalline layers described previously) have a semicrystalline organization (Yamaguchi et al., 1979; Oostergetel and van Bruggen, 1993). Many authors have confirmed this alternation of amorphous and crystalline lamellae by small-angle X-ray diffraction (XRD) and measured a repeat distance of about 9 nm (Oostergetel and van Bruggen, 1989; Jenkins et al., 1993; Jenkins and Donald, 1995; Donald et al., 2001).

Jenkins and Donald (1995) proposed, from the model of fringed micelle of French (1981), a model of organization of the starch granule containing the different structural levels. The thickness of the semicrystalline repetition would correspond to the cluster structure of amylopectin: The crystalline lamellae would be mostly constituted by amylopectin short chains (DP 15), whereas the amorphous strips would contain most of the branch points.

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