Carbohydrates
Authors: BeMiller James N. 
Published: July 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 one‐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 small 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, and very important biological functions. The variety of carbohydrates in Nature is large, and the number of theoretical possibilities is essentially limitless. The simplest carbohydrate molecules possess 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 (the aldoses) or ketone (the ketoses) derivatives of polyhydroxy alcohols and their reaction products. Low molecular weight carbohydrates are often called sugars. Simple sugars have two kinds of reactive groups: the carbonyl group and primary and secondary hydroxyl groups. Aldose and ketose molecules can form hemiacetals intramolecularly. Such an intramolecular reaction forms a ring. Most monosaccharides occur in combinations, most often with either more of the same sugar or different sugars in the form of polymers (polysaccharides) in which the sugar units (in a ring form) are joined by glycosidic (acetal) linkages. Less frequently, they are joined together in short chains (oligosaccharides). 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 joined head‐to‐head, making sucrose a nonreducing disaccharide, an unusual structure in nature. Mono‐ and oligosaccharides may also be linked to nonsugar organic compounds. These combined forms of sugars are known as glycosides. Most carbohydrate (both in quanity and kinds of structures) exists in the form of polymers of sugars termed polysaccharides. Polysaccharides are the principal components of cell walls of land plants, seaweeds, and some microorganisms and store energy. 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. 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 can be esterified and etherified and 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. Carbohydrates have widespread utilization, both as low cost, high volume commodities and as low volume specialty chemicals, mainly used in pharmaceutical applications as antibiotics, antigens, and synthetic drugs.D‐Glucose, known commercially as dextrose, is isomerized toD‐fructose to produce high fructose syrups (HFS). Sucrose is widely used in the food industry to sweeten, control water activity, provide crispness, give surface glaze or frost, form a glass, and impart desirable texture. 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. 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 products. 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. 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 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 dense, insoluble in cold water, and only partially hydrated. Normal starches are 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 hydroxypropylated, phosphorylated, or acetylated to provide appropriate processing 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, 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 loss of crystalline order is called gelatinization. The viscosity obtained by cooking a suspension of starch is determined by the starch type, types(s) of modification(s), 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 that, 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 or polysaccharide derivatives used industrially (other than native and modified starches, which are also included in the definition, but that are always considered as a separate and distinct category). 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 systems 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.