Classification of Carbohydrates
Sources of Carbohydrates
The three major groups of carbohydrate food are monosaccharide, disaccharides, and polysaccharides.
The simplest form of carbohydrate is the monosaccharide, often called simple sugar. Following are the three main monosaccharide’s important in human nutrition.
1 Glucose. A moderate sweet sugar, glucose is found naturally preformed in a few foods, but mainly it is created in the body from starch digestion. In human metabolism, all other types of sugar are converted by the body into glucose. It is the form in which sugar circulates in the bloodstream and is oxidized to give energy. It is found in some foods such as corn syrup.
2 Fructose. The sweetest of the simple sugar, fructose is found in fruits and honey. In human metabolism, it is converted to glucose for energy.
3 Galactose. The simple sugar galactose is not found free in foods but is produced in human digestion from lactose (milk, sugar) and is then changed to glucose for energy. This reaction is reversible and, during lactation, glucose may be reconverted to galactose, since the lactose component in breast milk is produced from gelactose.
Disaccharides are double sugars made of two monosaccharide. The three main disaccharides with their two components are
1 sucrose = glucose + fructose
2 lactose = glucose + gelactose
3 maltose = glucose + glucose
In each of these disaccharides, glucose is one of the two components.
Sucrose is common table sugar. It is the most prevalent disaccharide in the diet, contributing about 25% of the total carbohydrate calories. It is found in many food sources, including cane and beet sugar, brown sugar sorghum cane and molasses, maple syrup, pineapple, and carrot toots.
Lactose is the sugar in milk. During lactation it is formed in the body from glucose to supply the carbohydrate component of milk. It is the least sweet of the disaccharide, about one sixth as sweet as sucrose. Thus it is often used in high-carbohydrate, high-calories liquid feedings when the necessary quantity of sucrose would be too sweet to be tolerated. When milk sours, as in the initial stages of cheese making the lactose is changed to lactic acid and separates in the liquid whey from the remaining solid curd. The curd is then processed for cheese. Therefore, although milk has relatively high carbohydrate content (lactose), one of its main products – cheese – has none.
Maltose occurs in malt products and in germinating cereals. As such, it is a negligible dietary carbohydrate. However, it is important as an intermediate product of starch digestion.
Polysaccharides are even more complex carbohydrates made up of many units of one monosaccharide. The most important polysaccharides in human nutrition include starch dextrin, cellulose, pectin, and glycogen.
Starch is the most significant polysaccharide in human nutrition. It is a compound made up of glucose chains; hence it yields only glucose on digestion. Starch granules vary in size and shape according to the source. Potato granules, for example, are relatively large, whereas rice granules are small. Grinding or cooking helps make the starch available for use by breaking down the cell walls.
Starch is by far the most important source of carbohydrate and account for about 50% of the total carbohydrate intake in diet in the world. In most countries where it is the staple food substance, it makes up an even higher proportion of the total diet. Major food source include cereal grains, potatoes and other root vegetables, and legumes.
Dextrins are polysaccharide compounds that are intermediate products of starch breakdown and the formation of maltose during digestion. Dextrin- Maltose, an infant formula preparation, is a combination of dextrin and maltose.
Cellulose is a polysaccharide that human beings cannot digest because they lack the necessary digestive enzymes. Therefore it remains in the digestive tract and contributes important bulk to the diet. This bulk helps move the digestive food mass along and stimulates peristalsis. Cellulose forms the supporting framework of plants. The main sources are stems and leaves of vegetables, seed grains converting, skins, and hulls.
Pectin’s are also nondigestable polysaccharides. They are found mostly in fruits and possess a gel-like thickening quality. They are often used as a base for jellies. This ability to solidify to a gel also makes them useful in cosmetic and drugs.
Glycogen, often called animal starch, is formed in the body from glucose and is stored in relatively small amounts in the liver and muscle tissues.