Structure and Properties of Starch Granules

Starch granules are formed in the amyloplasts. These granules are simple or compound and consist of concentric or eccentric layers of varying density. They are of varying size (2-150 pm). 

On the basis of results obtained from different physical methods, the model shown in Fig. 4.25 is under discussion for the crystalline regions of the starch granule. It contains double helices of amylopectin (cf. 4.4.4.14.4), mixed amy-lose/amylopectin double helices, V helices of amylose with enclosed lipid (cf. 4.4.4.14.3), free amylose, and free lipid. 

With the aid of X-ray diffraction diagrams, native starches can be divided into t) es A, B, and C. An additional form, called the V-type, occurs in swollen granules (Fig. 4.26). While types A and B are real crystalline modifications, the C-type is a mixed form. The A-type is largely present in cereal starches, and the B-type in potatoes, amylomaize, and in retrograded starches (resistant starch, cf. 4.4.4.16.3). The C-type is not only observed in mixtures of corn and potato starches, but it is also found in various legume starches. When suspended in cold water, air-dried starch granules swell with an increase in diameter of 30 0%. If this suspension is heated, irreversible 

Source Shape Diameter (pm) Crystallinity (%) Gelatinization temperature (°C) Swelling at 95°C Amylose Percentage i%r Polymeri- sation degree Amylopectin Iodine binding constant Polymeri- sation degree  

Weight of swollen starch, based on its dry weight loss of soluble polysaccharides is considered. 

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