What are Disaccharides?

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Chemical structure of disaccharides

Disaccharides, meaning “two sugars”, are the carbohydrates formed when two monosaccharides are joined, basically when the 2 monosaccharide molecules undergo a condensation reaction which involves the elimination of a small molecule, such as water, from the functional groups only. Like monosaccharides, disaccharides dissolve in water, taste sweet, and are called “sugars”. Disaccharide is one of the four chemical groupings of carbohydrates which are monosaccharide, disaccharide, oligosaccharide and polysaccharide. One of the best known disaccharide is ‘sucrose’ and others commonly found in nature are ‘lactose’ and ‘maltose’. Sucrose is formed as a result of “photosynthesis”, a process of producing food in plants and is found in table sugar. Sucrose is made up of glucose and fructose. Lactose, present in milk, is made up of glucose and galactose. Lactose has a complex molecular structure, and due this some people are unable to digest it properly.


How are Disaccharides formed?

Disaccharides are formed when two monosaccharides are joined together and a molecule of water is removed. This is known as “dehydration synthesis or condensation reaction”. The left over dry substance in the synthesis of disaccharide can be stored with relative ease, and used as an ingredient in the creation of many different types of foods.


How are Disaccharides classified?

There are two types of Disaccharides. They are as depicted below

  • Reducing Disaccharide: In this type of disaccharide, the reducing sugar is a free ‘hemiacetal’ unit. Hemiacetal are compounds that are derived from aldehydes and ketones respsecitvely. Aldehydes are organic compounds. This functional group, with the structure R-CHO, consists of carbonyl center bonded to hydrogen and an R group. The -CHO is called the aldehyde or formyl group. Many fragrances are aldehydes. Ketones are also organic compound with the structure RC (=O) R’ in which the C=O is the ketone group. Examples of reducing disaccharide are maltose and Cellobiose.
  • Non-reducing Disaccharides: In this type, monosaccharides have no free hemiacetal unit. Examples of non-reducing disaccharides are Sucrose and Trehalose.


What are the physical properties of Disaccharide?

Depending on the monosaccharide constituents, disaccharides some variation of the following properties

  • Crystalline,
  • Sometimes water-soluble
  • Sometimes sweet-tasting and sticky-feeling.


What are some common Disaccharides?

Some of the common Disaccharides are as follows.

  • Sucrose (table sugar, cane sugar, beet sugar): It is a white, odorless crystalline powder with a sweet taste. It is best known for its role in human nutrition. Sucrose is made up of glucose (monosaccharide) and fructose (monosaccharide).
  • Lactulose: It is a synthetic sugar used in the treatment of constipation and in the liver diseases. It is formed from fructose and galactose (monosaccharide).
  • Lactose (milk sugar): It is found in milk. Lactose is formed from galactose and glucose. Lactose has a complex molecular structure, and so some people are unable to digest it properly.
  • Maltose: It is also known as malt sugar and is formed from bonding between two units of glucose. It is used in the making of soft candies such as chocolates and fruit based-treats.
  • Trehalose: It is also known as mycose or tremalose. It is formed from bonding within glucose units. It serves as an antioxidant.
  • Cellobiose: This disaccharide is derived from the condensation of two glucose molecules.


What is the role of Disaccharides in human health?

Too much of disaccharides causes spike in blood sugar and leads to a disease called “Diabetes”. But, some types of disaccharides are used because they cause fewer spikes in blood sugar and are preferred by ‘type 2 diabetes’ patients. e.g., Maltose. While glucose molecules are still present, they tend to create less of spike in blood sugar levels and are absorbed into the body easier than regular table sugar. But, too much of these safer disaccharides can cause diarrhea.

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