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Monosaccharides are carbohydrate molecules that contain only one sugar moiety in their structure. Fructose is one such carbohydrate. It is a simple ketonic monosaccharide, which is an isomer of the glucose molecule. The hemiacetal functional group of glucose is swapped by the hemiacetal functional group in the fructose molecule. The structure of fructose can be represented in various forms. It is also called fruit sugar. Its name originated from the Latin word ‘Fructus’, meaning ‘fruit’ and ‘ose’, referring, generally, to sugars. The structure of fructose notes is given in the context, including the structure of fructose importance.
Properties of fructose
Fructose, the ketohexose monosaccharide, has its own physical and chemical properties, as per the atoms present in it and its associated functional group. It is a common monosaccharide that is taken up by humans with their daily diet. The properties of fructose are as follows.
Chemical properties of fructose
The chemical properties of the fructose molecules are given below:
It is, generally, a polyhydroxy ketone since it contains many hydroxyl groups in its chain linked to the six parent carbons with one keto functional group.
As a crystalline form, the fructose linear chain forms a ring of furanose.
It is levorotatory as it turns the plane-polarised light to the left side direction.
On losing water molecules, fructose forms hydroxymethylfurfural. When the product is processed into liquid, it forms dimethylfuran, which can be used as alternatives for petrol and diesel.
It undergoes a chemical reaction called the Maillard reaction, which causes browning of the exteriors of the bread while baking it. Fructose easily undergoes this reaction as it is more often present in an open chain than the other dietary monosaccharides.
It acts as a reducing sugar due to the presence of the free ketone group in its open-chain.
Physical properties of fructose
The physical properties of the fructose molecules are given below.
It is the sweetest monosaccharide that we commercially use as sweeteners.
The fructose sugar is at its utmost sweetness at a temperature range of 5-50°C, above which, the sweetness reduces due to the conversion of the six-carbon ring to a five-carbon ring of fructose. The structure of fructose importance is shown in this process.
Fructose molecules decrease the freezing point of a molecule. Thus, it helps in reducing the crystallisation of water in the fruit cells.
It absorbs quickly and retains the moisture content for a longer period.
The melting point of fructose is about 103°C.
Its molecular weight is about 180.16 g/mol.
Structure of fructose
Fructose is a six-carbon-containing monosaccharide, thus, referred to as hexose.
It has a ketone(-CO) functional group at its second carbon of the parent chain, coming under the classification of ketoses.
Linear structure
The linear structure of fructose is shown by the Fischer projection.
It shows the exact alignment of the atoms without disturbing their symmetry and positions.
When the fructose molecule rotates the polarised light rightwards, it is dextrorotatory, which is not predominantly found.
When the fructose molecule rotates the polarised light leftwards, it is levorotatory, which is predominantly found in natural and commercial eatables.
Ring structure
The ring structure of fructose is shown by Haworth projections.
The six-membered carbon chain forms a ring, giving a crystalline structure to the molecule by the resultant pyranose ring.
Being a six-carbon compound, fructose also forms a five-membered ring called furanose, which is the most predominant form of fructose in solutions.
Steps involved in a fructose ring formation
The ring is formed by the intramolecular reaction between the ketone functional group at the second carbon atom and the hydroxyl group at the fifth carbon.
The oxygen atom in the hydroxyl group has the affinity to bond with the second carbon, containing the ketone group.
The hydrogen atom from the hydroxyl of the fifth carbon shifts to bind with the oxygen atom of the ketone group-containing second carbon, forming a hydroxyl group there.
It, thus, forms an ether linkage in between the second and the fifth carbon.
The presence of the ether and the hydroxyl group-containing carbon atom is called anomeric carbon. It is called the hemiacetal group.
Alpha and beta rings of fructose
The rings are further classified as alpha and beta compounds due to the position shift of the hydroxyl group at the second carbon.
When the hydroxyl is downwards, it is termed alpha.
When it is facing upwards, it is beta.
The commercial significance of fructose
The fructose molecules are commercialised for various uses and possess great significance. It has notable employment in the food and beverages industries.
It adds up sweetness, enhances flavour, is a good humectant, decreases the freezing point of compounds, and provides osmotic stability.
It mainly involves beverages, baking processes, canned sweet products like fruits and jams, and dairy products.
It is fruit sugar that is also extracted from the nectars of the flowers.
The bees collect the nectar from the flowers in the form of sucrose and hydrolyze it to fructose.
Conclusion
Fructose is a ketohexose monosaccharide that commercially takes its place as a sweetener. It is a six-carbon compound, acting as a reducing sugar due to the free carbonyl group at its second carbon. It is majorly present as Levo-fructose. Fructose is an isomer of glucose and the structure of fructose is more similar to glucose. Except for that fructose has a keto functional group at its second carbon. It also differs by sometimes forming a five-membered furanose ring, unlike the aldohexose, glucose. Fructose is naturally found in honey, vine fruits and berries.
