Native starches

Native starch naturally exists in the form of starch granules with different shapes, sizes, and surface properties. X-ray diffraction can be used to classify starches into A-, B-, and C- (combination of A and B) types based on the packing of the semicrystalline structures of starch granules. Most cereal

Fig. 9.1 Factors leading to the SDS state.

starches (maize, wheat, rice) are of the A-type, tuber starches (potato, yam) are B-type, and legume starches (kidney bean, soybean) normally belong to C-type. Some A-type starches (maize, sorghum, millets, and large granules of wheat, rye, and barley at the equatorial groove) have surface pores connected to interior cavities through channels (Fannon et al., 1992). There are no such surface pores in B-type starch granules. This macrostructural difference between native starch granules has importance during digestion as starch-digesting enzymes enter the pores and channels and digest the granules in an inside-out manner. As far as the nutritional quality of starch is concerned, native A-type starches inherently have a high amount of SDS while native B-type starches are essentially resistant (high RS) (Ferguson et al., 2000), based on the in vitro Englyst assay (Englyst et al., 1992). In an in vivo study, Seal et al. (2003) clearly showed the changes in blood glucose levels after consumption of native maize starch (Fig. 9.2), producing a typical profile of SDS. There have been two patents (Axelsen and Smith, 2001; Qi and Tester, 2005) on SDS in which native A-type starches were used as the sole ingredient to prepare medical foods for treatment of glycogen storage disease (GSD1) (Chen et al., 1984) and other diabetic conditions.

Our own investigation of native starch digestion properties shows that A-type native maize starch is a near-ideal SDS source (Zhang et al., 2006a,b). This is because native granules, at any given time point of digestion in the Englyst assay, provide similar RDS, SDS, and RS amounts, and thus continue to provide slow release of glucose over an extended period of time. It is well known that the semicrystalline structure of starch granules is composed of alternating concentric layers of ordered and dense crystalline s

g 0 30 60 90 120 150 180 210 240 270 300 330 360

Time (min)

Fig. 9.2 Change from baseline in plasma glucose concentrations with time after consumption of 50 g rapidly hydrolysed (A) and 50 g slowly hydrolysed (■) native maize starch for healthy subjects (from Seal et al., 2003).

g 0 30 60 90 120 150 180 210 240 270 300 330 360

Time (min)

Fig. 9.2 Change from baseline in plasma glucose concentrations with time after consumption of 50 g rapidly hydrolysed (A) and 50 g slowly hydrolysed (■) native maize starch for healthy subjects (from Seal et al., 2003).

regions and less ordered amorphous regions (also called amorphous background) from the hilum to the surface of the granules. This is considered a lamellar structure. Based on our investigation (Zhang et al., 2006a), the surface pores and interior channels are the starting points for enzymic digestion, and gradually the amorphous background and dense crystalline regions are evenly digested by enlarging the interior channels through a side-by-side digestion mechanism from the inside of the granule to the outside. This is the mechanism for the slow digestion property of A-type maize starch and could conceivably be replicated in other food materials to create a similar digestion effect. B-type starches have a somewhat similar granular structure with different crystalline and amorphous arrangements, but there are no surface pores or interior cavities. Thus, digestion occurs through pitting from the outside of the starch granules. Additionally, B-type crystallites are somewhat resistant to digestion compared with A-type crystallites (Gérard et al., 2000). Therefore B-type native starch is largely RS (~70%) due to the harder surface and the nature of B-type crystallites.

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