Carbohydrates
Authors: BeMiller James N. 
Published: August 2004

Carbohydrates are found in all plant and animal cells. They are the most abundant of the organic compounds, so abundant that it is estimated that well over half of the organic carbon on earth exists in the form of carbohydrates. Most carbohydrates are produced and found in plants. Carbohydrate molecules make up about three‐fourths of the dry weight of plants; most of this is found in cell walls as structural components. Carbohydrates also constitute important energy reserves in plants; one carbohydrate, starch, provides about three‐fourths of the calories in the average human diet on a worldwide basis. But the nutritional aspects are only a part of the story of carbohydrates. They have many important industrial uses in such diverse areas as the adhesive, agricultural chemical, fermentation, food, paper and related products, petroleum production, pharmaceutical, and textile industries. The variety of carbohydrates in nature is large, and the number of theoretical possibilities is essentially limitless. The basic carbohydrate molecule possesses an aldehyde or ketone group and a hydroxyl group on every carbon atom except the one involved in the carbonyl group. As a result, carbohydrates are defined as aldehyde or ketone derivatives of polyhydroxy alcohols and their reaction products. The name carbohydrate (hydrate of carbon) is derived from the fact that the basic carbohydrate molecule has the formula Cn(H2O)n. In common practice, all low molecular weight carbohydrates are called sugars. Numerical prefixes designated the number of carbon atoms are tri‐, tetra‐, penta‐, hexa‐, hepta‐, etc. In systemic nomenclature, the suffix for the names of aldehyde sugars is ‐ose and for ketose sugars‐ ulose. However, common names are routinely used, creating exceptions in both cases. Most carbohydrates exist in the form of polysaccharides. Polysaccharides are the principal components of cell walls of land plants (cellulose), seaweeds, and some microorganisms and store energy (starch in plants and glycogen in animals). They are important in the human diet and in many commercial applications. Most carbohydrates have two kinds of reactive groups: the carbonyl group and primary and secondary hydroxyl groups. Aldose and ketose molecules can form hemiacetals intramolecularly, as well as by reacting with another molecule. Such an intramolecular reaction forms a ring. The most common rings are the six‐membered pyranose ring, and the five‐membered furanose ring. Most monosaccharides occur in combinations, most often with either more of the same sugar or different sugars in the form of polymers (polysaccharides). Less frequently, except in the case of sucrose, they are joined together in oligosaccharide chains. Mono‐ and oligosaccharides may also be linked to nonsugar organic compounds. These combined forms of sugars are known as glycosides. By far the most abundant of the naturally occurring oligosaccharides is the disaccharide sucrose, ordinary table sugar from sugar cane or sugar beets. The two monosaccharide units in sucrose are α‐D‐glucopyranosyl and β‐D‐fructofuranosyl units. The two units are joined head‐to‐head, making sucrose a nonreducing disaccharide, an unusual structure in nature. Polysaccharides are naturally occurring polymers of monosaccharide (sugar) units. In precise chemical nomenclature, polysaccharides are glycans and are described as being composed of glycosyl units. Oligosaccharides and other compounds with glycosidic bonds can be made synthetically. The classic and still widely used reaction is the Koenigs‐Knorr reaction. Hydroxyl groups can be both oxidized to carbonyl groups and removed by reduction. Sugars that have the hydroxyl group missing from one or more of the carbon atoms are called deoxy sugars. As polyhydroxy compounds, carbohydrates react with aldehydes and ketones to form cyclic acetals. Replacement of a hydroxyl group with an amino group at any position produces an aminodeoxysugar. Unsaturated sugars are useful synthetic intermediates. The most commonly used are the so‐called glycals (1,5‐ or 1,4‐anhydroalditol‐1‐enes). Carbohydrates have widespread utilization, both as low cost, high volume commodities and as low volume specialty chemicals. Significant uses in terms of volume are surveyed here. Not covered are the lower volume uses involving carbohydrates; these are mainly pharmaceutical applications involving antibiotics, antigens, and synthetic drugs. Crystalline α‐D‐glucopyranose is generally sold as dextrose. Glucose is also isomerized toD‐fructose to produce high fructose corn syrup (HFCS). CrystallineD‐fructose also finds use in the food industry. Sucrose is widely used in the food industry to sweeten, control water activity, add body or bulk, provide crispness, give surface glaze or frost, form a glass, provide viscosity, and impart desirable texture. It is used in a wide variety of products from bread to medicinal syrups. Oligo‐ and higher saccharides are produced extensively by acid‐ or enzyme‐catalyzed hydrolysis of starch, generally in the form of syrups of mixtures. Polysaccharides are the most abundant of the carbohydrates, and they comprise the greatest part of industrial utilization. Most of the low molecular‐weight carbohydrates of commerce are produced by depolymerization of starch. Polysaccharide materials of commerce fall into three classes: cellulose, a water‐insoluble material; starches, which are not water‐soluble until cooked; and water‐soluble gums. Cellulose is the principal cell‐wall component of higher plants and the most abundant polysaccharide. Approximately one‐half the mass of perennial plants and one‐third the mass of annual plants is cellulose. The greatest amount of cellulose used is the purified wood pulp used in the manufacture of paper, etc. Hemicelluloses are a large group of polysaccharides that are associated with cellulose in the primary and secondary cell walls of all higher plants, but otherwise have no relationship to cellulose. Hemicelluloses are heteroglycans. They do not comprise a distinct class of chemical structures. Both woody and nonwoody tissues contain 20–35% hemicelluloses. Starch granules must be cooked before they will release their water‐soluble molecules. Starch use permeates the entire economy because it (corn starch in particular) is abundantly available and inexpensive. All green plants package and store carbohydrate (D‐glucose) in the form of starch granules. In granule form, starch is quasicrystalline (displays spherocrystalline patterns), dense, insoluble in cold water, and only partially hydrated. Normal corn starch is composed of 20–30% of the linear polysaccharide amylose and 70–80% of the branched polysaccharide amylopectin. Alkaline hypochlorite treatment produces whiter (bleached) products that produce softer, clearer gels. Most hypochlorite‐oxidized starch and all ammonium persulfate‐oxidized starch is used in the paper industry. Dextrins (so‐called converted starches), acid‐modified starches, starch ethers, starch esters, and cross‐linked starches have also been prepared. Food starches, especially those made from waxy maize, potato, and tapioca starch are usually both cross‐linked and phosphorylated, acetylated, or hydroxypropylated to provide appropriate gelatinization, viscosity, and textural properties. Examples of their application are their use in canned foods and in the preparation of spoonable salad dressings. Other starches of interest include cationic starches, pregelatinized starches, starch graft copolymers (a product made by polymerizing acrylonitrile onto gelatinized starch and used as an additive to absorbent soft goods such as disposable diapers), and cold‐water swelling starches. Heating a starch in water causes the granules to swell. At sufficient solids concentration the swollen granules occupy most of the space, and a viscous mass called a paste results. Application of shear to these fragile, swollen granules results in formation of a molecular dispersion. The process of granule swelling with concurrent hydration and solubilization of starch molecules is called gelatinization. The viscosity obtained by cooking a suspension of starch is determined by the starch type, derivatization or modification, solids concentration, pH, amount of agitation during heating, maximum temperature reached, time held at that temperature, agitation during holding, and the presence of other ingredients. Gums are polymeric substances which, in an appropriate solvent or swelling agent, form highly viscous dispersions or gels at low dry substance content. Commonly, the term industrial gums refers to water‐soluble polysaccharides, glycans in official carbohydrate nomenclature, or polysaccharide derivatives used industrially. Particularly in the food industry, the term hydrocolloid is often used interchangeably with gum. The usefulness of such industrial gums is based on their physical properties, in particular their capacity to thicken or gel aqueous solutions and otherwise to control water. Secondary characteristics are responsible for their utilization as adhesives, binders, bodying agents, bulking agents, crystallization inhibitors, clarifying agents, etc. Gums are tasteless, odorless, colorless, and nontoxic. None, except the starches and starch derivatives, are broken down by human digestive enzymes.