Starch is the most important storage polysaccharide in plants. It is composed entirely of glucose, so it is a homogeneous polysaccharide. Like other polysaccharides, starch has no sweetness, no reducing properties, and no mutarotation.

Starch is a chain structure composed of maltose units, including amylose and amylopectin. The straight chain part is connected by α-1,4 glycosidic bonds, and the branches are connected by α-1,6 glycosidic bonds. Therefore, incomplete hydrolysis of amylopectin can yield a small amount of isomaltose connected with α-1,6 glycosidic bonds.

Amylopectin structure

The molecular weight of amylose ranges from tens of thousands to hundreds of thousands, with an average of about 60,000, which is equivalent to the condensation of 300-400 glucoses. A linear molecule with no branches. Amylopectin has a molecular weight of more than 200,000 and contains 1,300 glucose or more. Approximately one in every 24-30 glucose units in its molecule has a branched chain structure.

Amylopectin

Although there is no branched structure in the amylose molecule, its conformation is not straight, but curled into a spiral, with six glucose molecules in each circle. . The blue color produced by amylose when exposed to iodine is due to the complexation of multiple triiodide ions (I3-) in the helix. The absorption peak is at 620-680nm. The linear part of amylopectin is shorter, so the complex produced is purple, with light absorption at 530-555nm.

Wheat starch granules stained with iodine

Starch exists in plants in the form of spherical or egg-shaped starch granules. The granules have densely structured crystalline areas and relatively loose amorphous ones. area (amorphous area). This structure makes starch insoluble in cold water. On a microscopic level, starch granules have different refractive indexes for polarized light in different polarization directions due to the optical anisotropy of the crystallization region, which will produce an interesting birefringence (extinction cross) phenomenon.

Birefringence phenomenon of starch granules

When starch granules are heated in water (usually 60-80°C), they will gradually swell and crack, and finally form a uniform paste, which is called gelatinization (gelatinization). During the gelatinization process, starch granules absorb water and swell, reaching 50-100 times their original volume. Taking advantage of this property, starch can be used as a disintegrant and added when making pharmaceutical tablets. When starch swells in gastrointestinal fluids, it breaks the tablet into small particles, allowing the drug inside to be quickly absorbed.

Starch is a polysaccharide and cannot form a true solution in water, but can only form a colloid. Amylose is soluble in hot water, while amylopectin is insoluble in hot water, but it is easier to gelatinize, and the colloidal viscosity formed by gelatinization is higher. Starch from different sources has different proportions. Corn starch and potato starch contain 27% and 20% amylose respectively, while mung bean starch contains 60% amylose. Some starches (such as glutinous rice) are all amylopectin, so they are more sticky; while some legume starches are all amylose.

If the gelatinized starch colloidal solution gradually cools down, the starch molecules will rearrange into a more compact crystal structure and precipitate, which is called aging or retrogradation. Amylose has a long linear region and is easier to arrange neatly, so it ages easily and is difficult to dissolve again after aging (crystal melting temperature is about 150°C). Amylopectin is not easy to age. Therefore, when using starch paste to thicken rice noodles in cooking, starch with a high amylopectin content, such as potato starch, is generally selected; while when making vermicelli and vermicelli, bean starch with a high amylose content is selected.

Amylose is difficult to dissolve again after aging, so it is difficult to digest in the intestine and is a kind of resistant starch. For example, rice and bread will become hard and difficult to digest after being cooled. Therefore, in cooking, we generally try to avoid the phenomenon of aging. However, it has recently been discovered that resistant starch can be fermented by colon flora to produce short-chain fatty acids (SCFA). SCFA can inhibit the growth of harmful bacteria and benefit intestinal health. Being difficult to digest is beneficial to weight loss and can reduce blood sugar fluctuations, so it is now popular again, and some people even eat it cold to lose weight.