Carbohydrates (saccharides) are classified as follows:

 1. Monosaccharides are those sugars that cannot be hydrolyzed into simpler saccharides. They may be classified as trioses, tetroses, pentoses, hexoses, or heptoses, depending on the number of carbon atoms (3-7), and the location of the carbonyl group C=O. If it is a terminal aldehyde, it is an aldose. If it is not terminal, then there is a ketone and is designated as ketose. For trioses there are only two possibilities— glyceraldehyde and dihydroxyacetone. Examples are listed in Table 1. In addition to aldehydes and ketones, the polyhydric alcohols (sugar alcohols or polyols), in which the aldehyde or ketone group has been reduced to an alcohol group, can occur naturally in foods. They are also synthesized by reduction of monosaccharides for use in the manufacture of foods for weight reduction and for diabetics. They are poorly absorbed, and have about half the energy yield of sugars. The biggest side effect is flatulence; bacteria in the intestine ferment the sugar alcohol that was not absorbed.

2. Disaccharides are condensation products of two mono saccharide units, for example, lactose, maltose, isomaltose, sucrose, and trehalose.

 3. Oligosaccharides are condensation products of 3 to 10 mono saccharides. Most are not digested by human enzymes.

 4. Polysaccharides are condensation products of more than 10 monosaccharide units; examples are the starches and dextrans, which may be linear or branched polymers.

Polysaccharides are sometimes classified as hexosans or pentosans, depending on the constituent monosaccharides (hexoses or pentoses, respectively). In addition to starches and dextrans (which are hexosans), foods contain a wide variety of other polysaccharides that are collectively known as nonstarch polysaccharides; they are not digested by human enzymes, and are the major component of dietary fiber. Examples are cellulose from plant cell walls (a glucose polymer) and inulin, the storage carbohydrate in some plants (a fructose polymer).

Table1. Classification of Important Monosaccharides

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