The basic physical structural unit of starch is the granule, which has a distinctive microscopic appearance for each botanical source. The common starches are readily identifiable by using a polarizing light microscope to determine their size, shape, and form and the position of the hilum (botanical center of the granule). Jane et al. (1994) have demonstrated that starch granules from different biological sources (roots and tubers, grains, maize, peas and beans, fruits, and nuts) could present a wide variety of fascinating morphologies. Scanning electron micrographs of grain starches from their work are presented in Figure 4.
There is also an observed set of common characteristics and those specific to each biological source, indicating substantial genetic control. Among possible genetically controlled factors are the particular types and amounts of synthetic enzymes that function in the biosynthesis of the starch molecule. Biosynthesis occurs in the amyloplast organelle, whose membranous structure and physical characteristics could impart a particular shape and morphology to the individual starch granules. This in turn may have an effect on the arrangement and association of the amylose and amylopectin molecules in the granule and on the morphology of the granule (Jane et al. 1994).
The modes of biosynthesis of starch and the nature of starch granular organization are still subjects of considerable research, and the outcome of the biosyn-thetic process is different in diverse plant species as well as in different genotypes of the same species. Because starch is synthesized in plastids, those structures must possess all the enzymes necessary for granule formation. The starch grows by apposition (deposition of material on the outside). The new layer deposited on the outside of the granules varies in thickness, depending upon the amount of carbohydrate available at the time (Hoseney 1986). Starch biosynthesis is a complex process that is mediated by several groups of biosynthetic enzymes. It is the balance of biosynthetic enzyme activities that defines the ultimate structure
of a particular starch. Mutations in a specific biosynthetic enzyme activity can skew the structural balance to yield a distinctive starch biopolymer structure. Functional properties of the starches are then related not only to their structure as polymers but also to the packing of polymers within the granules (Banks and Greenwood 1973).
Molecular arrangement within a starch granule is to some extent radial, as evidenced by fibrillar fracture surfaces that can extend radially along growth rings, when starch granules are fractured (French 1984). Starch granules are birefringent, indicating a high degree of internal order.Birefringence, the ability to refract light in two directions, is evidenced by distinctive patterns under a polarizing microscope. The loss of birefringence indicates disruption of the molecular arrangement in the crystalline areas and is used as a major criterion for gelatiniza-tion. X-ray diffraction studies show that the molecular arrangement in a starch granule is such that there are crystalline (micellar) regions imbedded in an amorphous matrix. The molecular nature of the crystallinity within the starch granule is not fully understood but certainly involves amylopectin in both nonwaxy and waxy starches.
There is no good understanding of the state of amylose in a normal starch granule. More is understood about the ordered nature of amylopectin in a starch granule (Thompson et al. 2000). However, the linear amylose molecules are believed to be interspersed between amylopectin, rather than located in bundles (Jane et al. 1992). Findings by Morrison et al. (1993a, b) indicated that in cereal starches, there are two amorphous forms of amylose: lipid-free amylose and lipid-complexed amylose. For high-amylose maize starches (HAMS), X-ray diffraction patterns provide evidence that single-helical amylose-lipid complexes may be partially crystalline. And HAMS are even less crystalline than normal maize starch, perhaps due to a lower proportion of amylopectin (Thompson et al. 2000). Blanshard (1987) suggested that limited cocrystallinization between amylose and amylopectin may occur.
By combining old and new results provided over the years by a range of microscopic techniques, Gallant et al. (1997) gathered some of the pieces of the puzzle concerning starch granule internal structure and organization (Fig. 5). Considerable evidence now exists from SEM, TEM, and enzyme degradation studies and more recently from AFM, which indicate that the crystalline and amorphous lamellae of amylopectin are organized into larger, more or less spherical structures, termed blocklets. This was an idea first hinted at by Nageli in 1858 and Badenhuizen in 1936. The blocklets range in diameter from around 20 to 500 nm, depending on the botanical source and location in the granule.
The amorphous fraction controls the variation in granule volume due to its ability to absorb and release the free water of a raw starch granule. Figure 5 shows the existence of radial channels within starch granules, believed to be composed predominantly of semicrystalline or amorphous material and through which amylose can exit the granule structure. This also reinforces the real existence of a granule radial structure and clearly defined locales (possibly between the more crystalline blocklets) that are more easily degraded by enzymes. These have implications for determining the resistance of starch to digestion (Gallant et al. 1997).
These support earlier work on the nature of the outer surface and its relationship to the chemical and enzymatic reactivity of granules. Native granules also exhibit resistance to enzyme-catalyzed digestion, and each of the granules is attacked in a characteristic pattern (Leach and Schoch 1961). The pattern of digestion that develops when native maize and wheat starch granules are treated with amylases indicates that some areas of their surface are more susceptible to
attack than are others (e.g., Evers and McDermott 1970; Evers et al. 1971; Fuwa et al. 1977). Fannon et al. (1992) showed that pores are found along the equatorial groove of large granules of wheat, rye, and barley starches but not on other starches (rice, oat, tapioca, arrowroot, canna). They proposed that the pores affect the pattern of attack by amylases and by at least some chemical reagents. Kanen-aga et al. 1990 also reported that maize starch (which has pores) was more susceptible to enzymatic digestion than potato starch (on which no pores were found) and which Leach and Schoch (1961) found was digested in a different pattern.
These pores are characteristic of particular species of starch and not produced by drying (Fannon et al. 1992).
In later work (Fannon et al. 1993), it was shown that pores previously observed on the external surface of sorghum starch granules open to serpentine channels that penetrate into the granule interior. This provides evidence on observations that the enzymatic digestion of maize starch begins at the hilum (Leach and Schoch 1961). It is likely that at least some channels penetrate at the hilum. This suggests that hidden surfaces of channels for those starches with pores on the external surfaces must be considered in order to evaluate susceptibility to enzyme attack. A possible function is the regulation of the rate of starch granule conversion into D-glucose during germination (Gallant et al. 1997; Fannon et al. 1993).
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