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Structure and Properties

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Carbohydrates are hydroxylized aldehydes or ketones and are the most abundant of organic compounds. They serve as fuel, energy storage, basic building blocks in DNA and RNA, and as structural elements of bacterial and plant cell walls. Carbohydrates are bonded to other macronutrients in glycoproteins and glycolipids, which play important roles in cell membranes, as for instance in cell-cell recognition. Food carbohydrates are built almost exclusively from the monosaccharides, glucose, fructose, and galactose. Depending on the number of monosac-charides bonded together, one can distinguish di-, oligo-, and polysaccharides (A).

Among the disaccharides, sucrose (or saccharose) plays a predominant role. It is commonly called "sugar," and can be produced from sugar cane and sugar beets. Lactose (milk sugar) is the predominant carbohydrate in milk. It represents a major energy source for infants. Maltose (malt sugar) is of lesser significance in foods, but is produced in large amounts when carbohydrate polymers are digested.

Most food carbohydrates consist of starch, composed of amylose and amylo-pectin. These macromolecules (polysaccharides) are made exclusively from glucose building blocks—a single amylopectin molecule may consist of hundreds of thousands of glucose molecules. Starch is the energy storage polysaccharide of plants. Cereal grains contain ~75 %, potatoes ~65 % starch (in dry matter). The animal body can store only limited amounts of carbohydrates in the liver and muscle as glycogen. Stored carbohydrates bind water, using more space than lipids while containing less energy. This is why the human body uses primarily fats for energy storage. Even though the structure of glycogen is similar to that of starch, it has no role in human nutrition since any stored glyco-gen is almost completely broken down in foods ready for consumption. A multitude of other polysaccharides occurs in many plant species. These and others are nowadays added to foods in large amounts during processing. Usually, these additives are derived from starch: on hydrolysis, it is converted to glucose. Through physical, chemical, or enzymatic modification it becomes more water-soluble, and thereby more suitable for food processing.

All carbohydrate polymers are linked by glycosidic bonds (B). Depending on the OH groups involved in the bonds, different linkage types exist, affecting the digestibility of the various food polysac-charides. The gastrointestinal tract contains specific enzymes able to split a-glycosidic bonds (e.g., maltose, sucrose). The human body also makes an enzyme to split the p-glycosidic bonds in lactose but lacks enzymes specific to any other p-glycosidic linkages (see p. 78).

Structure and Properties 57

- A. Carbohydrate Classification

Carbohydrate	Occurrence	Structure and Characteristics
Monosaccharides
D-Glucose	Fruit, honey - traces in most	Water-soluble hexose
(dextrose)	plants
D-Fructose	Fruit, honey - traces in most	Water-soluble hexose
(fruit sugar)	plants
D-Galactose	Component of lactose -	Water-soluble hexose
released during digestion
Disaccharides
Sucrose or saccharose	Sugar beets, sugar cane,	Water-soluble disaccharide, consisting of
(table or cane sugar)	fruit, maple syrup	glucose and fructose linked by a-1,2 bonds
Lactose
(milk sugar)	Milk, milk products	Water soluble disaccharide, consisting of
Maltose		galactose and glucose linked by |-1,4 bonds
(malt sugar)	Sprouts, forms during starch	Water soluble disaccharide, consisting of
	digestion	glucose and glucose linked by a-1,4 bonds
Polysaccharides
Amylose	Starch, cereals, potatoes	Linear glucose polymer with a-1,4 bonds,
		water soluble
Amylopectin	Starch, cereals, potatoes,	Branched glucose polymer with a-1,4 and
	thickeners	a-1,6 bonds, non-water soluble
Glycogen	Liver, muscle	Branched glucose polymer with a-1,4 and
(animal starch)		a-1,6 bonds, water soluble
Inulin	Artichokes	Fructose polymer, water soluble
Technical saccharides
Dextrin	Food additive	Short fragments of an a-1,4 glucose polymer
Invert sugar	Food additive	Hydrolyzed sucrose, equal parts glucose and
		fructose
Glucose syrup	Food additive	Hydrolyzed starch (glucose)
High fructose corn	Food additive	Hydrolyzed starch, partially isomerized
syrup (HFCS)

I- B. Glycosidic Bonds -

hoch2

Hz hoch2 H y

Maltose

(glucose-a-1, 4-glucose)

Sucrose

(glucose-a-1, 2-fructose)

Lactose

HOCH

Sucrose

(glucose-a-1, 2-fructose)

HOCH^ OH H

HOCH^ OH H

H OH

H OH

Galactose unit

H OH

H OH

HOCH

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