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Starch Structures

Physically, after extraction and drying, normal starch is a white powder consisting of a mixture of amylose and amylopectin in semi-crystalline granules. Starch granules are microscopic structures approximately 0.5-100 μm in diameter. In shape, they are spherical, elliptical, or polyhedral. The size and morphology of starch granules is characteristic of the organ and species in which they are produced (Jane et al. 1994). Starch granules appear rather similar in size and morphology with and without amylose. Under most environmental conditions, starch granules can be considered moderately inert with little capacity to hold water. These characteristics of starch granules make them ideal vessels for storage and shipping, whether in grain or tubers or from processed isolated starch.

AMYLOPECTIN:

α-1,4 glucan lightly-branched with α-1,6-glucosidic linkages

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Chemically, starch is classified as a complex carbohydrate and is a mixture of two polymers of glucose: amylose and amylopectin. Amylose is a generally linear α-1,4 glucan which is sometimes lightly-branched with α-1,6-glucosidic linkages. Amylopectin is normally in higher molecular weight than amylose. It is also an α-1,4 glucan, but is highly-branched with α-1,6-glucosidic linkages. The proportions on a dry weight basis of amylose and amylopectin in starches isolated from storage tissues like potato tubers or cereal grain is normally between 20 to 30 percent amylose and 70 to 80 percent amylopectin. In addition to amylose and amylopectin, granules contain small quantities of protein and lipid. Between species there is variation in the structure of amylopectin (Jane et al. 1999), the size and structure of amylose (Hizukuri et al. 1981; Takeda et al. 1984; Takeda et al. 1986; Takeda et al. 1987; Takeda et al. 1989), the nature and amounts of proteins (Mu-Forster & Wasserman 1996; Han et al. 2002a; Han et al. 2002b) and lipids (Morrison 1988; Tester & Morrison 1992).

 

Models of amylopectin clustering describe our best understanding of how these molecules come together to form the 3-dimensional structure of starch granules.

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