Structure and Thermal Properties of Starch Granules

Starch is a complex carbohydrate that is widely used in food products. It is a semicrystalline granule with a tight radial pattern of molecules. Starch has varying molecular structures, depending on the botanical source. Normal starch contains around 70–80% amylopectin and 20–30% amylose. Waxy starch has mostly amylopectin, while high-amylose starch contains more than 40% amylose. The amylose content affects the thermal, pasting, gelling, and digestive properties of starch. Starch granules are disrupted by heating in excess water, a process known as gelatinization, which renders the molecules fully accessible to digestive enzymes. Starch molecules can retrograde when cooled and form tightly packed structures stabilized by hydrogen bonds. Starch is almost completely digestible in the small intestine for typical human diets.

The Structure of Starch Granules

Native starch is a semicrystalline granule, with different shapes and sizes, depending on its botanical source. Within the granule, starch molecules are tightly packed in a radial pattern. Starch has characteristic features that vary in molecular structure, polymorph type, morphological properties of the starch granule, gelatinization and pasting properties, and enzyme digestibility.

Normal starch contains around 70–80% amylopectin and 20–30% amylose. Waxy starch consists mostly of amylopectin with less than 8% amylose, and high-amylose starch contains more than 40% amylose. The average molecular sizes of the amylose molecules vary between an average degree of polymerization (DP) of 324–4920 with around 9–20 branch point equivalents, to 3–11 chains per molecule. The amylose content has a significant impact on the thermal, pasting, gelling, and digestive properties of starch. Amylopectin is a much larger molecule than amylose, with a molecular weight of 1 × 107 –1 × 109 and a highly branched structure with approximately 95% α-(1→4) and 5% α-(1→6) linkages. The DP of amylopectin is usually 9600–15,900. Part of the amylose in lipid-containing granules exists as an amylose inclusion complex, where the lipid chains occupy a hydrophobic core located within a single amylose helix. The amount of lipid complexed with amylose ranges from 15% to 55% of the amylose fraction in cereal starches. Two forms of amylose/lipid complexes, the amorphous and crystalline amylose/ lipid complexes, have been reported. Amorphous amylose/lipid complexes have lower dissociation temperatures (<100 °C) than the crystalline amylose/lipid complexes (>100 °C).

In normal starch granules, branched chains of amylopectin form double helices and contribute to the crystalline structures, which are packed in three different patterns to form the X-ray diffraction patterns of the A-, B-, and C-types. The A-type crystalline structure polymorph has a monoclinic unit cell, the B-type polymorph has a hexagonal unit cell, and the C-type polymorph has a combination of the A- and B-type starches. The amylopectin of A-type starch has a larger proportion of short branched chains (DP, 6–12) but a smaller percentage of longer branched chains (DP > 12) than that of B-type starch. The starch branched chain length of amylopectin has been found to be a key factor in determining the crystalline polymorph types. In addition, amylopectin of A-type starch has some branched linkages located in the crystalline region, but B-type starch has almost all the branched linkages located in the amorphous region, which explains the less porous structure and digestibility of the B-type starch granules when compared to that of A-type starch.

Starch Gelatinization and Retrogradation

Starch granules are disrupted by heating in excess water in a process commonly known as gelatinization, rendering the molecules fully accessible to digestive enzymes. Some types of hydrated cooking operations are typical examples of the preparation of starchy foods for consumption, which facilitate more rapid digestion. Starch gelatinization is an endothermic transition commonly analyzed using differential scanning calorimetry (DSC). The gelatinization properties are affected by the structure of amylopectin, the amylose content, and the minor components present in the starch granules, which depend on the botanical origin of the starch. In general, starch consisting of amylopectin with longer branched chains needs a higher gelatinization temperature and enthalpy change because longer branched chains favor the formation of stable, double-helical crystalline structures.

When the gelatinized starch is cooled, starch molecules reassociate and can form tightly packed structures stabilized by hydrogen bonds. These structures are thermally very stable and can only be rehydrated at 80–150°C, depending upon the extent and nature of the retrogradation. Both amylopectin and amylose molecules can retrograde, but retrogradation of amylopectin requires several days or even longer to form crystallites with a low dissociation temperature (40–60°C), due to its branched structure and short branch chains. The linear amylose molecules retrograde faster and form crystallites with a higher dissociation temperature (130–170°C).

Starch Digestibility

Starch is digested by α-amylase in the mouth, by pancreatic amylase and glucoamylase, and by sucrose and isomaltase in the membranes of microvilli within the small intestine. The total digestive tract digestibility is almost complete for typical human diets. Measurements on human subjects with ileostomy have shown that the percentage of starch being digested in the small intestine is normally higher than 95% in common starch-rich foods, such as white bread, freshly cooked potatoes, breakfast cereals, and rice.

However, there still are some starches that are less easily digested in the small intestine because of specific reasons. The main reasons for indigestibility involve the large and compact particle size of cereal seeds, ungelatinized starch granules, or retrograded starch. In addition, chemical modification of starch, lipid-complexed starch, and some heat or acid treatments all increase the enzyme resistance of starches. When starch-rich foods are not finely ground, such as in breakfast cereals, the digestibility of the starch decreases. The RS fraction is therefore a relevant component when considering the nutritional content.

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