Starch: Plant polysaccharide

Starch is a polysaccharide produced by most of higher plants and by some mosses, ferns, protozoa, algae and bacteria as energy and carbohydrate storage material; it is also the main source of carbohydrates in human diet.

The diversity in starch granule structure and properties is appalling. Even cereal starches widely vary in properties. Starch occurs in intracellular granules and does not have any function as structure-building molecule in plant cells, but rather functions as a storage polysaccharide.

The word of starch comes from Middle English word, strechen meaning ‘to stiffen’. Starch is produced by plant in the form of granules mainly containing two biopolymers: amylose and amylopectin.

The amylose molecule is a linear, unbranched structure in which the glucose residues are made up of α-(1-4)-d linked polyglucan in a helix conformation.

Amylopectin, on the other hand, is a branched-chain polymer, the branch points occurring through a-1,6 glycosidic bonds. In amylopectin about 5% of linkage consists of α(1-6) bonds resulting in a large multi-branched macromolecule with branch-on-branch structure and a molecular size ranging between 50 and 500x106, one of the largest polymers present in nature.

Plants normally store starch with a small amount of water, lipids and proteins in semi-crystalline granules inside amyloplasts and chloroplasts.

Starch granules are insoluble in water and alcohol but can be dissolved in hot water during a process known as gelatinization. During gelatinization, the starch granules are irreversibly destroyed releasing the amylose and amylopectin biopolymers.

The temperature at which gelatinization process takes place is around 70 ◦C for most starch granules, undergoing an irreversible transition in which the molecular structure of amylose and amylopectin is disrupted.

Starch provides humans with energy 4 kcal per gram and is hydrolyzed to glucose, supplying the glucoses that is necessary for brain and central nervous system functioning.

To digest starches, humans have specialized digestive enzymes called amylases. These enzymes breakdown starches into glucose, which is either used immediately as an energy source or stored in the liver and to be released when needed.
Starch: Plant polysaccharide

Amylopectin

Starch is composed of two polymers of different structure; amylose is essentially linear, whereas amylopectin is a branched component.

The amount of amylopectin varies among different starches. Waxy varieties contain almost 100% amylopectin. Starch normally contains about 20–30% amylose and 70–80% amylopectin.

Amylopectin is a branched-chain polymer, the branch points occurring through a-1,6 glycosidic bonds.

The estimation of the chain length distribution is of primary importance for characterizing the molecular structure of amylopectin.

The branched amylopectin molecule contains regions with low and high levels of branches. In highly branched regions, side-chains of amylopectin are grouped, forming crystalline zones (clusters).

Crystalline region is less susceptible to enzymatic hydrolysis, water penetration, and other chemical reactions than amorphous region.

Side chains of the amylopectin molecule can be divided in A, B, and C chains. C chains constitute the backbones of the amylopectin molecules, to which B-chains are linked that at the same time carry one or more branches. The B chains are classified into B1, B2, B3 and B4 depending on their length and the number of clusters that they span.
Amylopectin

Texture of alkaline noodles

Inclusion of alkaline salts in noodle formulas imparts a characteristics aroma and flavor, a yellow color and a firm, elastic texture.

Alkaline agents improve the texture of the cooked product, making it more chewy with less of a tendency to soften and paste after cooking.

The desired textural characteristics are a bright, even light yellow appearance; free of any darkening or discoloration; a firm clean bite; a chewy and elastic texture with some degree of springiness; and a satisfactory al dente reaction on biting.

Alkaline noodles prepared with NaOH are sticker and less firm than alkaline noodles made with blends of sodium and potassium carbonate, possibly NaOH adversely affects gluten development.

In addition, the starch characteristics (amylose/amylopectin ratio) seemed to play an important role in altering the texture of noodles.
Texture of alkaline noodles

Starch content in food

Starch is the most important, abundant, digestible food polysaccharide and is therefore a major source of energy in human diets.

Starches are the major storage polysaccharides in foods of plant origin. Common food starches are derived from seed (such as wheat, maize, rice barley) and root (such as potato, cassava/tapioca) sources.

Starch has a negligible osmotic pressure, which allows plants to store large reserves of carbohydrate without disturbing the cell’s water relations.

Starch is composed by two glucose polymers – amylose and amylopectin. The amylose to amylopectin ratio is normally 1:3 for most cereal, tube and root starches.

Moist heat causes starch grains to swell and rupture, thus converting starch to a form that is readily digested.

In the body, much of the glucose may be utilized directly as a source of energy, but some of it is converted into fat, the muscles utilizing fatty acids indirectly as fuel for energy. Excess carbohydrates not required for energy, when ingested (eaten) will be stored in the body as fat.

The starch plays an important role in formation of texture and quality of the starch-based food products.

Starches are used mainly in the food and paper industries, with 57% of produced starch consumed in the food industries and 43% in the nonfood sector.

Food starches are commercially manufactured and available for use in products such as baked food, beverages canned, frozen and glassed foods, confections, dairy products, dry goods, meat products and canned food.
Starch content in food

Starch

Starch is by far the most important source of carbohydrates in the human diet amounting to approximately 50% of total carbohydrate in the United States, but often as much as 75% total carbohydrates in some of the developing countries.

Starch is a polysaccharide made up of many glucose units bonded together – 3000 or so in each molecule of starch. Shorter carbohydrate chains composed of 3 to 1o glucose molecules are called oligosaccharides.

Starch is partly digested in the mouth and changed in the small intestine to glucose.

All the major economic sources of starch are plants. Starch and starch like molecules can also be found in other kingdoms, of life, including bacteria, algae and animals.

Starchy foods such as potatoes, bread and pasta provide other nutrients as well as starch. For example, bread is a good source of protein, B-group vitamins and minerals such as calcium and iron.

Other major sources of starch include cereal grains, such as wheat, rice, rye, millet, sorghum and corn. These grains contain 76 percent starch. Tubers, such as potatoes and cassavas and root vegetables such as parsnips, also supply starch.

Starch

Sources of starch

Starch sources are numerous, with common ones derived from cereal grains such as a wheat, corn or rice.

Wheat yields a cloudy, thick mixture, while cornstarch produces more clear mixtures such as gravies or sources.

Food such as bread potatoes and rice are valuable sources of dietary starch and provide significant proportion of the carbohydrate requirement of a well balance diet.

Plant cells store glucose as starches – long, branched or unbranched chains of hundreds or thousands of glucose molecules linked together.

These giant starch molecules are packed side by side in groups such as wheat or rice, in root crops and tubers such as yams and potatoes, and in legumes such as peas and beans.

Some foods have been modified to increase their starch content. For instant, ‘high starch’ potatoes have been produced by modifying an enzyme in the growing potato tuber that is involved in making starch.

Starch in the diet is also important because the foods in which it occurs tend to be high in dietary fiber. The presence of this dietary fiber slows down the rate at which glucose from starch is absorbed into the blood stream.

In most human societies peopled depend on as staple grain for much of their food energy: rice in Asia, wheat in Canada, the United States and Europe corn in much of Central and South America: millet, rye, barley and oats elsewhere.

Sources of starch