Structure and Properties

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

Galactose unit

H OH

H OH

HOCH

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