Carbohydrates in foods

Carbohydrates are an important fraction in most foods and are a major sources of energy. In this section you will refresh your knowledge on the structure and function of carbohydrates and fibre.

Dietary carbohydrates are a diverse group of substances with various chemical, physical and physiological properties. They are made of carbon (C), oxygen (O) and hydrogen (H) in the proportions given by the formula C n(H 2 O)n. That’s why they are called carbohydrates (C = carbon; H 2 O = water or hydrate). The classification of carbohydrates is based on molecular size, degree of polymerization and type of linkage and character of individual monomers.

Monosaccharides

Monosaccharides are carbohydrates which typically form a single ring. As can be derived from the name, monosaccharides are single sugars (mono = one, saccharide = sugar). The most important monosaccharides in nutrition each contain 6 carbon atoms, 12 hydrogens, and 6 oxygens (C 6 H 12 O 6). By solving this exercise you will find out that three monosaccharides are most important in nutrition. Those three form the building blocks of naturally occurring di-, oligo- and polysaccharides. They all have the same number and kind of atoms, but in different arrangements. These chemical differences result in differences in sweetness.

Disaccharides

Disaccharides (di = two, saccharide = sugar) are pairs of monosaccharides linked together. A dissacharide is formed by a chemical reaction known as condensation (see glossary). During a condensation reaction, a hydroxyl (OH) group from one monosaccharide and a hydrogen atom (H) from the other combine to create a water molecule (H 2 O). To split a dissacharide in two monosaccharides, a chemical reaction known as hydrolysis (see glossary). is needed. During this reaction a water molecule splits to provide the H and OH needed to complete the resulting monosaccharides. Hydrolysis reactions occur during digestion.

Sugar alcohols (or polyols)

Sugar alcohols (or polyols) are found in nature (e.g. in fruits) and are prepared commercially. They are a chemically reduced form of mono- or disaccharides. Sugar alcohols are used in large quantities by the food industry as sweeteners. One of the advantages of sugar alcohols over sucrose is that they are not digested by bacteria in the mouth and therefore do not cause caries. Furthermore, sugar alcohols are not digested or absorbed in the small intestine, but are fermented by the large gut micro flora. As a consequence, sugar alcohols contribute less energy than sucrose. A disadvantage is that a high intake of sugar alcohols (50 grams or more at one time) can lead to laxative effects.

Oligosaccharides

Oligosaccharides (oligo = few, saccharide = sugar) are polymers consisting of 3-9 monosaccharide units. They are not widely occurring or of great importance in foods and food products, except for several galactosylsucroses and fructo-oligosaccharides. The galactosylsucroses occur in vegetables, such as peas, beans and lentils. The fructo-oligosaccharides occur in cereals (e.g. wheat and rye) and in various vegetables (e.g. onions, garlic, asparagus, and artichokes). Fructo-oligosaccharides are an example of non- digestible oligosaccharides.

Polysaccharides (or complex carbohydrates)

Polysaccharides are relatively complex carbohydrates consisting of more than 9 monosaccharide units. They can be subdivided into starches and non-starch polysaccharides.

Starch

Starch is a polymer of only glucose units. Starch is the main food reserve in plants and consequently the major carbohydrate of the human diet. Common food starches are derived from seeds (e.g. wheat, maize, rice, and barley) and roots (e.g. potato and cassava).

Two kinds of covalent linkages occur between the glucose molecules in starch, giving rise to two kinds of glucose chains. Amylose is composed of straight chains, with carbon 1 of one glucose linked to carbon 4 of the next glucose. This is called alfa-1,4 linkage*. Amylopectine is made up of straight chains like amylose but has occasional branches where the carbon 6 of a glucose is linked to the carbon 1 of another glucose (alfa-1,6 linkage**).

*see glossary: linkage

**see glossary: glucosidic bound

It was long thought that starch is completely digestible. However some starch (in raw potato's and unripe bananas for example) occurs in a partially crystalline form in granules. This crystalline starch is resistant to digestion by pancreatic enzymes. The crystalline structure is lost when starch is heated in water, thus permitting digestion to take place. Recrystallization takes place to a variable extent after cooking and may result in higher resistant starch values than originally present in the raw food. As a result of food processing (cooking, cooling, freezing), resistant starch values in a food may be elevated up to 20%.

Non-starch polysaccharides (NSP): Non-starch polysaccharides (NSP) consist of the polysaccharides other than starch that are insoluble in 80% ethanol. With regard to human nutrition, it is important to consider that most of the nutritional important NSP are present in plant cell walls (and thus constituents of dietary fibre). Other NSP occur in the form of gums, waxes and mucilages.