Monosaccharides are classified by the number of carbons in the saccharide unit—triose (three carbons), tetrose (four carbons), pentose (five carbons), and hexose (six carbons). The chemical names of many of the carbohydrates end in “-ose”, which means “sugar.” Pentose and hexose sugars are more common in foods, the main pentoses being ribose and arabinose, and the three most predominant hexoses being glucose, fructose, and galactose.
Ribose is an extremely important component of nucleosides, part of the genetic material deoxyribonucleic acid (DNA), ribonucleic acid (RNA), and the energy-yielding adenosine triphosphate (ATP). Ribose also plays an important role as part of vitamin B2 (riboflavin).
This pentose contributes to the structure of many vegetable gums and fibers.
Glucose is the most common hexose found in foods, and the major sugar in the blood. It is present in its free form in fruits, honey, corn syrup, and some vegetables. It also exists as the repeating saccharide unit in starch and glycogen, and is incorporated into many fibers. Refined glucose, called dextrose in the food industry, is used in the production of candies, beverages, baked goods, canned fruits, and alcoholic beverages. Glucose is also the major ingredient of corn syrup, which is made commercially by hydrolyzing cornstarch.
Also called fruit sugar or laevulose, fructose is found primarily in fruits and honey. Fructose is the sweetest of all sugars, yet it is seldom used in its pure form in food preparation because it can cause excessive stickiness in candies, over browning in baked products, and lower freezing temperatures in ice cream. High fructose corn syrup. however, is the preferred and predominant sweetening agent used in soft drinks.
Seldom found free in nature, galactose is part of lactose, the sugar found in milk. A derivative of galactose, galacturonic acid, is a component of pectin, which is very important in the ripening of fruits and the gelling of jams.
Sugar alcohols are not carbohydrates, but the alcohol counterparts of specific carbohydrates. Their ability to contribute sweetness, combined with their tendency to be slowly absorbed, make them useful ingredients in various dietetic foods if used in small amounts. Some of these infrequently discussed sugar alcohols are glycerol, inositol, mannitol, sorbitol, and xylitol. Glycerol serves as the three-carbon backbone of triglyceride molecules (fat molecules). Inositol is a sugar alcohol found commonly in cereal bran. Nlannitol, sorbitol, and xylitol are often used by food manufacturers as sweetening agents, especially in dietetic candies or gums. These sugar alcohol corn-pounds differ from monosaccharides by a slight rearrangement of their atoms; the carbohydrate’s hydroxyl group (-OH) is replaced with an aldehyde or ketone group (C=0).
As free water decreases, so does the water activity. Water activity is measured by dividing the vapor pressure exerted by the water in food (in solution)—by the vapor pressure of pure water (Pw), which is equal to 1.00.
Enzyme: A protein that catalyzes (causes) a chemical reaction without itself being altered in the process.