Complex carbohydrates
For more stable and efficient storage of potential energy, plants and animals pack carbohydra te energy in to units much larger than the sugars-dextrin, starch, cel~ulose, and glycogen. All of these are polysaccharides, the molecules of which may contain several hundred times as many glucose units as those of the sugars. Consequently -:hey are much less soluble and more stable but differ markedly among themselves in digestibility and resis~ce to spoilage. Because there is so much moisture in all
owing plants, one essential characteristic of a storage material is insolubility. To be suitable for human food, . owever, a carbohydrate must be subject to digestion by -:he enzymes of the digestive tract. Starches and dextrins :all into this category, but celluloses and hemicelluloses, which also occur in food, cannot be digested by humans.
Dextrins occur mostly as intermediate products in dIe partial hydrolysis of starch by enzymatic action or in
MALTOSE [glucose + glucose)
SUCROSE (glucose + fructose)
LACTOSE (galactose + glucose)
cooking. They are made up of many glucose units joined together with the same linkages as starch. The individual molecules are smaller than those of starch, and they do not have the thickening property of starch. They are water soluble and, depending on their color reaction with iodine, are classified as soluble starch (blue), amylodextrin (purple), erythrodextrin (red), and achrodextrin (colorless). Dextrins are formed when bread or cereals are toasted or flour is browned. They are also used in some infant formula preparations and in products used in soda fountain beverages.
alpha-Limit dextrins are formed in the digestion of the amylopectin form of starch because beta-amylase cannot split the branched chain linkage.
Starch, the chief form of carbohydrate in the diet, occurs in two forms : (1) amylose, a straight chain polysaccharide of glucose units linked together the same as maltose (1,4 glucosidic bonds) and (2) amylopectin, a branched structure of glucose units with a linkage different from maltose at the branchings (1,6 glucosidic bonds) but similar throughout the rest of the chain. Starch is found in cereal grains, vegetables, and other plants. The starch of the grain is mostly in the endosperm, encased in a protective covering of cellulose (the bran or the husk). The starch granule in the endosperm consists of tiny particles of starch usually arranged in concentric layers in a pattern of characteristic shape and appearance. The starch granules in turn may be enclosed in cells of larger size. This storage of starch in plants may be compared to warehouse storing of small packages of prepared cereal in cartons, with the cartons in turn packed in larger containers for ease in handling.
Before starch can be used readily by the body, the outer membrane must be broken, either by grinding or cooking. By the application of heat and moisture the outer cellulose envelope is ruptured, and the moisture permeates the starch granules themselves. Starch granules have an affinity for water, absorbing it like a sponge and increasing greatly in volume. The thickening property of a starch varies according to the source: for example, that of potato starch is greater than those of corn or wheat. After rupture of the cellulose wall by cooking, the starch is in a form that can be acted on more easily by digestive enzymes.
Cooking may carry the breakdown of starch to the dextrin stage if continued long enough. Starch granules break down most rapidly when moisture is present. Long application of dry heat, such as in baking or toasting, ,will also change starch to soluble dextrins. The palatable flavor of the brovvn crust of rolls or bread or the brown part of toast or of toasted cereals is due partly to forms of dextrin.
Modified food starches are natural starches, which have been chemically or physically modified to provide special properties desirable in food processing. The types of modification include crossbonding with phosphates, derlvahzahon such. as esterification, conversion by acid or enzyme activity, and pregelatinization. Depending on their treatment, modified starches may have greater viscosity, more resistance to syneresis (weeping or leaking), improved clarity, or the ability to be dispersed in cold water. They are used in the processing of sauces for canned and frozen foods, frozen fruit pies, infant foods, canned and instant puddings, and gumdrops.
Cellulose, found in the framework of plants, is also a polysaccharide of glucose units but with linkages different from those of maltose and starch. It is the chief constituent of wood, stalks and leaves of all plants, and the outer coverings of seeds and cereals. Cellulose forms the more or less porous walls of cells in which water, starch, minerals, and other substances are stored in the plant much as honev is held in the comb.
No cellulose-splitting enzyme is secreted by the mucosa of the human gastrointestinal tract, but bacterial fermentation or disintegration may play a role in dissolving the substances that bind the cellulose fibers or particles together.
The indigestibility of cellulose is its major asset, as the undigested fiber furnishes the bulk necessary for efficient and normal peristaltic action (muscular contraction) of the intestines. Research has demonstrated that the normal colon performs better when a reasonable amount (4 g-7 g) of bulk or residue is present.
Methylcellulose and other cellulose derivatives that absorb large quantities of water may be used in manufacturing low calorie foods, such as imitation syrups and salad dressings.
Glycogen, or "animal starch," is the form in which the animal stores carbohydrate. It is a polysaccharide similar to amylopectin but ,'lith more branched chains and higher molecular weight. When more glucose than can be immediately metabolized enters the bloodstream, the healthy person combines many glucose molecules (up to 30,000) to form glycogen. By the same token, when glucose is needed, glycogen is broken down and glucose becomes quickly available for energy. In the living mammal both the liver and muscle store glycogen. Approximately three-quarters of a pound of glycogen can be stored by the adult male, one-quarter pound in the liver and one-half pound in muscles. The liver glycogen is the more quickly available for replenishing blood sugar. The muscle glycogen is used primarily as fuel for the muscles. In muscle meat when purchased little glycogen is present because it disappears during rigor mortis. Scallops, oysters, and other shellfish contain significant amounts of glycogen.
Other polysaccharides, such as the pectic substances, alginates, carrageenans (Irish moss), and vegetable s. cannot be digested but are used in various foods use of their colloidal property, that is, the ability to absorb water and form a gel. Commercial pectin, prefrom cull apple peels and cores or the albedo of and available as liquid or powder, is used primarily for making fruit jellies. Agar is used as a thickening agent in candy manufacturing and in processing meats; ginates and carrageenans are used in the manufacture of ice cream to give body and smooth consistency and as stabilizing agents in other food processing. Carageenans, which have the unique ability to stabilize milk , are used in evaporated and flavored milks. Vegetable gums, including arabic, tragacanth, guar, and xanthan, are also used in a variety of products as water, thickeners, and stabilizers.
In the process of human metabolism mucopolysacirles, such as hyaluronic acid and chondroitin sulfate, are synthesized and found in the ground substance of connective tissue where they have structural importance.
Find more: Role of carbohydrates in health
