Notes on Starch
by Eben van Tonder
1 August 2020
Below are instructive quotes and information on Starches.
“While cellulose is the major structural polysaccharide, plant energy storage and regulation utilizes a combination of similar polysaccharides that combined are referred to as starch. The two major components of starch are called amylopectin (the major constituent) and amylose (the minor constituent). Starches are usually present in the form of intramolecularly hydrogen-bonded polymer aggregates or granules.
Starch is the second most abundant polysaccharide weightwise. It is widely distributed in plants where it is stored as reserve carbohydrate in seeds, fruits, tubers, roots, and stems.
Commercially starch is prepared from corn, white potatoes, wheat, rice, barely, millet, cassava, tapioca, arrowroot, and sorghum. Amylopectin, which is sometimes called the B fraction, is usually the major type of starch present in grains. However, amylose, which is sometimes called the A fraction, is present exclusively in a recessive strain of wrinkled pea. Thus, the fraction of amylopectin and amylose varies with respect to the particular plant and the usual weather, age, and soil conditions. Amylose serves as a protective colloid. Mixtures of amylose and amylopectin, present in native starch, form suspensions when placed in cold water. An opalescent starch paste is produced when this suspension is poured into hot water.
While cellulose can be considered a highly regular polymer of D-glucose with the units linked through a β-1,4 linkage, amylose is a linear polysaccharide with glucose units linked in an α-1,4 fashion while amylopectin contains glucose units with chains of α-1,4 glucopyranosyl units but with branching occurring on every 25–30 units, with the chainbranch occurring from the 6 position. While this difference in orientation in how the glucose units are connected appears small, it causes great differences in the physical and biological properties of cellulose and starch. For instance, humans contain enzymes that degrade the α-glucose units of starch allowing it to be metabolized as a major food source but we are not able to convert the β unit, found in cellulose, into glucose so that wood and other cellulose-intensive materials are not food sources for us. Also, the individual units of cellulose can exist in the chair conformation with all of the substituents equatorial, yet amylose must either have the glucosyl substituent at the 1 position in an axial orientation or exist in a nonchair conformation.
As noted before, starch can be divided into two general structures, branched amylopectin and largely linear amylose. Most starches contain about 10–20% amylose and 80–90% amylopectin, though the ratio can vary greatly.
Amylose typically consists of over 1000 D-glucopyranoside units. Amylopectin is a larger molecule containing about 6000 to 1,000,000 hexose rings essentially connected with branching occurring at intervals of 20–30 glucose units. Branches also occur on these branches giving amylopectin a fan or treelike structure similar to that of glycogen. Thus, amylopectin is a highly structurally complex material. Unlike nucleic acids and proteins where specificity and being identical are trademarks, most complex polysaccharides can boast of having the “mold broken” once a particular chain was made so that the chances of finding two exact molecules is very small.
An important characteristic of amylose is its ability to form a blue-colored solution in the presence of iodine. The formation of this complex has been widely employed for the detection of starch in general and amylose in particular. The iodine atoms are believed to lie along the hollow core of the amylose. While amylopectin also interacts with iodine, it does so much more weakly giving a reddish-purple complex.
Starch granules are insoluble in cold water but swell in hot water, first reversible until gelatinization occurs, at which point the swelling is irreversible. At this point the starch loses its birefringence, the granules burst, and some starch material is leached into solution. As the water temperature continues to increase to near 100 deg C, a starch dispersion is obtained. Oxygen must be avoided during heating or oxidative degradation occurs. Both amylose and amylopectin are then water-soluble at elevated temperatures. Amylose chains tend to assume a helical arrangement giving it a compact structure. Each turn contains six glucose units.
The flexibility of amylose and its ability to take on different conformations are responsible for the “retrogradation” and gelation of dispersions of starch. Slow cooling allows the chains to align to take advantage of inter- and intrachain hydrogen bonding, squeezing out the water molecules, leading to precipitation of the starch. This process gives retrograded starch, either in the presence of amylose alone or combined in native starch, which is generally difficult to redisperse. Rapid cooling of starch allows some inter- and intrachain hydrogen bonding, but also allows water molecules to be captured within the precipitating starch allowing it to be more easily redispersed.
Most uses of starch make use of the high viscosity of its solutions and its gelling characteristics. Modification of starch through reaction with the hydroxyl groups lowers the gelation tendencies decreasing the tendency for retrogradation.
Starch is the major source of corn syrup and corn sugar (dextrose or D-glucose). In addition to its use as a food, starch is used as an adhesive for paper and as a textile-sizing agent.
Oligomeric or small-chained materials called cyclodextrins are formed when starch is treated with a particular enzyme, the amylase of Bacillus macerans. These oligomeric derivatives generally consist of six, seven, eight, and greater numbers of D-glucose units joined through 1,4-α linkages to form rings. These rings are doughnut-like with the hydroxyl groups pointing upward and downward along the rim of the doughnut. Like crown ethers used in phase transfer reactions, the cyclodextrins can act as “host” to “guest” molecules. In contrast to most phase transfer agents, cyclodextrins have a polar exterior and nonpolar interior. The polar exterior allows the cyclodextrins, and often the associated guest, to be water-soluble. The nonpolar interior allows nonpolar molecules to be guest molecules. Cyclodextrins are being used as enzyme models since they can first bind a substrate and through substituent groups, act on the guest molecule—similar to the sequence carried out by enzymes.
A major effort is the free-radical grafting of various styrenic, vinylic, and acrylic monomers onto cellulose, starch, dextran, and chitosan. The grafting has been achieved using a wide variety of approaches including ionizing and ultraviolet/visible radiation, charge-transfer agents, and various redox systems. Much of this effort is aimed at modifying the native properties such as tensile (abrasion resistance and strength) and care (crease resistance and increased soil and stain release) related properties, increased flame resistance, and modified water absorption. One area of emphasis has been the modification of cotton and starch in the production of super-absorbent material through grafting. These materials are competing with all synthetic crosslinked acrylate materials that are finding use in diapers, feminine hygiene products, wound dressings, and sanitary undergarments.
Charles E. Carraher, Jr.. 2003. Seymour-Carraher’s Polymer Chemistry. Sixth Edition. Marcel Dekker Inc.