The most common monosaccharides in our food are glucose, fructose and galactose.
The combination of two monosaccharides creates a disaccharide. The best-known disaccharide is sucrose, or table sugar, derived from sugar cane or sugar beet. It is made up of one glucose molecule and one fructose molecule. Maltose consists of two glucose molecules, and can be found in germinated barley. As for lactose, a sugar of animal origin, this results from the union of one glucose molecule and one galactose molecule. Oligosaccharides are composed of between three and nine monosaccharide molecules. Some oligosaccharides cannot be assimilated by our digestive system, for example raffinose (formed of one galactose molecule, one glucose molecule and one fructose molecule), stachynose (two galactose molecules, one glucose and one fructose molecule) and verbascose (three galactose molecules, one glucose and one fructose molecule). Oligosaccharides are found in pulses and in some roots, and ferment when in contact with our intestinal flora. This fermentation releases gases (methane, carbon dioxide and hydrogen) which cause bloating and flatulence.
Polysaccharides consist of over 10 monosaccharide molecules and, most often, they comprise thousands or even tens of thousands of units combined together. The bulkiness of these complex sugars prevents them from interacting with our taste receptors; therefore, they do not confer a sweet taste. These macromolecules are polymers and act as a form of carbohydrate storage. Starch, an energy reserve from the plant world, is a long ramified chain of several thousand glucose units. It can be found in roots, tubers and seeds. Glycogen is the animal kingdom’s equivalent of starch. It is synthesised from glucose in the liver and muscles, and is rapidly available when the organism needs energy. Dietary fibre, derived exclusively from plants, is a complex group of polymers that we cannot digest. Cellulose, for example, is made up of around 10 000 glucose units.
Polysaccharides also include the alginates contained in seaweed, and pectin, which is mainly present in fruit, such as apples. These are structural carbohydrates used as food additives acting as thickening agents, gelling agents, emulsifiers or stabilisers.