Starch

Structure of the amilous molecule.

Structure of the amylopectin molecule.

Starch, or starch, is a macromolecule that is made up of two different glucose polymers: amylose (in a proportion of 25%) and amylopectin (75 %)). It is the reserve carbohydrate of most vegetables. Much of the properties of flour and bakery and pastry products can be explained by knowing the characteristics of starch.

Etymology

The Spanish term starch, although it may seem of Arabic origin, comes directly from Latin, like many other Spanish terms that also begin with al- without this being the usual Arabic particle. Starch comes, then, after some dissimilation, from the Latin word amy̆lum, a vulgar adaptation of the medieval Latin amidum, which derives from the Greek ámylon: that is not ground, which is associated with the Indo-European mel: to grind or refine.

Components

Starch is made up of two compounds with different structures:

• Amilosa: It is formed by α-D-glucopiranosas united by hundreds or thousands (normally 300 to 3000 units of glucose) by α-(1 → 4) in a chain without branching, or very sparsely branched by α-(1 → 6 links). This chain adopts a helical arrangement and has six monomers for every helix return. It usually constitutes 25 to 30% of starch.

• Amilopectin: Represents the remaining 70-75 %. It is also formed by α-D-glucopiranosas, although in this case it forms a highly branched chain in which there are α-(1 → 4) unions, as indicated in the previous case, and many α-(1 → 6 links that originate places of branching every twelve monomers. Its molecular weight is very high, since each molecule usually gathers from 2000 to 200 000 units of glucose.

However, the ratio between these two components varies depending on the organism in which it is found.

The starches in cereals contain small amounts of fat. Starch-associated lipids are generally polar lipids, which require polar solvents such as methanol-water for extraction. Generally the lipid level in cereal starch is between 0.5 and 1%. Non-grain starches are essentially lipid-free.

From a chemical point of view, it is a mixture of two very similar polysaccharides, amylose and amylopectin; they contain crystalline and non-crystalline regions in alternating layers. Since crystallinity is produced by the arrangement of amylopectin chains, waxy starch granules have a similar degree of crystallinity as normal starches. The ordered, radial arrangement of starch molecules in a granule is evident by viewing the polarizing cross (white cross on a black background) in a polarizing microscope when the polarizers are placed at 90° to each other. The center of the cross corresponds with the hilum, the center of granule growth.

Amylose is the product of the condensation of D-glucopyranoses by means of a(1,4) glycosidic bonds, which establishes long linear chains with 200-2500 units and molecular weights up to one million; that is, amylose is an a-D-(1,4)-glucan whose repeating unit is a-maltose. It has the facility to acquire a three-dimensional helical conformation, in which each turn of the helix consists of six glucose molecules. The interior of the helix contains only hydrogen atoms, and is therefore lipophilic, while the hydroxyl groups are located on the exterior of the helix. Most starches contain around 25% amylose. The two commercially available corn starches commonly known as high amylose have apparent mass contents of around 52% and 70-75%.

Amylopectin differs from amylose in that it contains branches that give it a tree-like molecular shape; branches are attached to the central stem (similar to amylose) by a-D-(1,6) bonds, located every 15-25 linear glucose units. Its molecular weight is very high as some fractions reach up to 200 million daltons. Amylopectin constitutes about 75% of the most common starches. Some starches are made exclusively of amylopectin and are known as waxy. Potato amylopectin is the only one that has phosphate ester groups in its molecule, most frequently attached in an O-6 position, while the remaining third do so in an O-3 position.

Shape of starch grains

Starch grains in potato cells seen with an electronic sweeping microscope.

The sizes and shapes of the starch grains of the endosperm cells vary from one cereal to another; in wheat, rye, barley, maize, sorghum and millet, the grains are simple, while those of rice are compound. Oats have simple and compound grains, with the latter predominating.

Most of the starch grains of the prismatic and central endosperm cells of wheat have two sizes: large, 30-40 microns in diameter, and small, 1-5 microns, while those of the endosperm cells sub-aleurone, are mainly intermediate in size 6-15 µm in diameter. In the sub-aleurone endosperm cells there is relatively more protein and the starch grains are less packed than in the rest of the endosperm.

Hydration

Starch granules are insoluble in cold water, but they can contain water when the temperature increases, that is, the starch granules undergo the process called gelatinization. During gelatinization, leaching of amylose occurs, total gelatinization normally occurs within a more or less wide temperature range, with the largest granules being the first to gelatinize.

The various states of gelatinization can be determined. These states are: the initiation temperature (first observation of the loss of birefringence), the average temperature, the final temperature of the loss of birefringence (TFPB, is the temperature at which the last granule in the field of observation loses its birefringence) and the gelatinization temperature range.

At the end of this phenomenon, a paste is generated in which there are chains of highly hydrated low molecular weight amylose that surround the aggregates, also hydrated, of the remains of the granules.

Retrogradation

It is defined as the spontaneous insolubilization and precipitation, mainly of amylose molecules, due to the fact that their linear chains are oriented parallel and react with each other by hydrogen bonds through their multiple hydroxyls; It can be carried out by various routes that depend on the concentration and temperature of the system. If a concentrated solution of amylose is heated and cooled rapidly to room temperature, a rigid, reversible gel forms, but if solutions are dilute, they become opaque and precipitate when allowed to stand and cool slowly.

Retrogradation is directly related to the aging of bread, the amylose fractions or the linear sections of amylopectin that retrograde, form zones with a very rigid crystalline organization, which requires high energy for them to break and the starch to gelatinize..

The amylose and amylopectin molecules are dispersed in the aqueous (gelatinized) starch solution. After cooling, the linear portions of various molecules are placed parallel due to the formation of H-bonds. This forces the water molecules apart and allows the molecules to crystallize together.

When starch is dissolved in water, the crystalline structure of the amylose and amylopectin molecules is lost and they hydrate, forming a gel, that is, they become gelatinized. If this gel is cooled, and even if it is left at room temperature long enough, the molecules rearrange, placing the linear chains in parallel and forming hydrogen bonds. When this rearrangement occurs, the retained water is expelled out of the network (a process known as syneresis), that is, the solid phase (amylose and amylopectin crystals) and the aqueous phase (liquid water) are separated.

The phenomenon of syneresis can be observed in everyday life in pastry creams, yogurts, sauces and purées.

Gelling

Starch and archeology

Marunguey starch grain (Zamia amblyphyllidia) recovered in liptical tool.
Utu-27, Puerto Rico (Utu-27)ca. 1100 d.C.).

Due to the differentiated morphological qualities that starch granules have depending on the plant to which they belong, a paleoethnobotanical research technique (starch grains in archaeology) has been designed to be of great help for the archeology of tropical regions of the world.

Many plants, especially tuberous and seed plants, had not been identified in the archaeological contexts of the tropics, a situation that limited the knowledge that could be had about the importance that plants had for the ancient peoples of these areas.

Starch granules, being enduring structures in archaeological tools related to food production and other derivatives, can be recovered and identified. The process of extracting starch from archaeological tools begins with the collection of sediment samples in the pores, cracks and fissures of said tools to then subject them to a chemical separation process (by means of centrifugation with cesium chloride).

Thanks to the application of the study of starch grains in archaeology, there are currently several investigations on the origin and evolution of plants in the American neotropics that have served to begin to trace, in an effective way, many of the dynamics biocultural issues around the development of economic plants (wild and domestic) and the sociocultural complexity of indigenous peoples.

Starch and digestion

The digestion process, in all living organisms, involves the breakdown of complex molecules of high molecular mass into simpler ones so that nutrients can be absorbed. It involves a series of reaction mechanisms, among which we find the addition of water, known as hydrolysis.
In the case of starch, the amylases secreted by the salivary glands and the pancreas, are responsible for breaking down carbohydrates. In this way, polysaccharides such as starch found in food are degraded to disaccharides such as maltose, with the ability to cross the mucosa and be absorbed in the intestine.

Starch as an antidote

Starch is used as an antidote in iodine poisoning, and it should be noted that it acts as an antidote only for this substance.

Iodine is a highly toxic substance due to its oxidizing power and can cause digestive causticization and respiratory tract irritation, the latter due to the fact that some of its commercial solutions are volatile. Iodine poisoning, and its derivatives, clinically manifests mainly with digestive irritation.

The action protocol when poisoning occurs in adults consists of emptying the stomach and adding a 10% starch suspension to perform gastric lavage. If lavage cannot be performed, administer starchy oral route, that is, 100 ml every 5-10 minutes and repeat 3-4 times. The mechanism on which the action as an antidote of starch is based is its ability to neutralize the iodine.

The bluish color of the return water from the gastric lavage indicates that it is being effective, so you should continue lavage. This bluish color of the return water from gastric lavage is due to a chemical reaction between iodine and starch. The iodine dissolves and the I₃^- ions enter the starch molecule (specifically amylose) producing a blue colored optical effect. This chemical reaction is the basis on which the iodine test is developed.

The mechanism of action of starch as an antidote consists in the inactivation of the non-absorbed fraction of iodine. Therefore, the antidote is effective during the first hours after intoxication since as time passes, the iodine is will absorb and the starch will have no therapeutic use. Starch is capable of absorbing iodine in a wide range of concentrations. In certain cases, when the concentration of free iodine is high enough, in addition to binding to amylose, it will be able to bind to another starch component, amylopectin.

Iodine poisoning is rare, but can occur in some situations, such as individuals with thyroid disease, the elderly, fetuses and neonates, or patients with other risk factors. In these populations, the likelihood of developing iodine-induced thyroid dysfunction (hypo- or hyperthyroidism) is increased.

In normal situations, the iodine ingested from the diet would not be enough to cause intoxication, so it is more common for these situations to occur after the intake of a large number of iodine supplements. Some medications such as amiodarone can also cause iodine poisoning. Therefore, they are generally voluntary poisonings whose starch treatment can be carried out quickly. Starch treatment is performed as a complement to gastric lavage and activated charcoal, which will prevent further iodine absorption.