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Sugars belong to the carbohydrate family. They may be simple, or form pairs or even long chains of thousands of units. Complex carbohydrates are also known as polysaccharides and, unlike simple sugars, their size means they do not taste sweet. Carbohydrates form the structural elements and natural energy reserves of plants, and also provide a major source of energy for animals and humans.
© Shutterstock / chromatos - Molecular structure of different kinds of sugar

How sugars are classified

As the building blocks of life, sugars occur naturally in our basic DNA, as well as in fruit, vegetables, honey, sugar beet and sugar cane. Animals provide us with sugars too, in dairy products for example. Sugars are part of the carbohydrate family, alongside starch and cellulose, and are central to the energy metabolism of living beings. However, sugars do not necessarily make food taste sweet.

Sugars consist of carbon, hydrogen and oxygen atoms, with the hydrogen and oxygen usually found in the same ratio as in water. Sugars are classified according to the number of combined simple molecules. The sugars we are most familiar with are composed of six carbon atoms and are called monosaccharides or simple sugars. These molecules can be combined together to form pairs (disaccharides), small groups (oligosaccharides), or enormous structures containing thousands of units (polysaccharides).

‘Mono-‘, ‘di-’, ‘oligo-’, ‘poly-’: simple sugars in complex groups

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.

Calorific value and digestion

Starch and sugars have the same calorific value of 4 kcal per gram.

During digestion, complex sugars are hydrolysed, meaning they are broken down and reduced to monosaccharide molecules, which pass through the wall of the small intestine. Once these monosaccharides have reached the bloodstream, they are carried to the liver, where they are metabolised. Glucose can either be released into the blood or stored in the form of glycogen or fatty acids.

Derivatives of sugar

Polyols are alcohols derived from sugars. Although only slightly less calorific than sugars (3 kcal per gram instead of 4), they are far less likely to cause tooth decay. Sorbitol, a polyol with a naturally sweet taste, can be found in prunes and is used as a sweetener in ‘sugar-free’ chewing gum and sweets. Xylitol, initially produced from birch bark, is also used to sweeten food, but also for the refreshing sensation it creates in the mouth, owing to its negative heat of solution.