Fructose (), or fruit sugar, is a common monosaccharide, i.e. a simple sugar. It is classified as a reducing hexose, more specifically a ketonic simple sugar found in many plants, where it is often bonded to glucose to form the disaccharide sucrose. A white, water-soluble solid, it is one of the three dietary monosaccharides, along with glucose and galactose. Fructose is found in honey, tree and vine fruits, flowers, berries, and most root vegetables.
History Fructose was discovered by French chemist Augustin-Pierre Dubrunfaut in 1847. The name "fructose" was coined in 1857 by the English chemist William Allen Miller. Pure, dry fructose is a sweet, white, odorless, crystalline solid, and is the most water-soluble of all the sugars.
Etymology The word "fructose" was coined in 1857 from the Latin for fructus (fruit) and the generic chemical suffix for sugars, -ose. It is also called fruit sugar and levulose or laevulose, due to its ability to rotate plane polarised light in a laevorotary fashion (anti-clockwise/to the left) when a beam is shone through it in solution. Likewise, dextrose (an isomer of glucose) is given its name due to its ability to rotate plane polarised light in a dextrorotary fashion (clockwise/to the right).
Chemical structure
Fructose adopts both cyclic six- and five-membered structure, The six membered ring can exist as either the β-d-fructopyranose and α-d-fructopyranose. The five-membered rings exists as either of two isomers β-d-fructofuranose and α-d-fructofuranose. Additionally, an acyclic (open-chain) form exists: keto-d-fructose. At 70% and 22% respectively, fructopyranose and fructofuranose are the dominant species in aqueous solution.
Chemical reactions
Fructose and fermentation Fructose may be anaerobically fermented by yeast and bacteria. Yeast enzymes convert sugar (sucrose, glucose, and fructose, but not lactose) to ethanol and carbon dioxide. Some of the carbon dioxide produced during fermentation will remain dissolved in water, where it will reach equilibrium with carbonic acid. The dissolved carbon dioxide and carbonic acid produce the carbonation in some fermented beverages, such as champagne.
Fructose and Maillard reaction Fructose undergoes the Maillard reaction, non-enzymatic browning, with amino acids. Because fructose exists to a greater extent in the open-chain form than does glucose, the initial stages of the Maillard reaction occur more rapidly than with glucose. Therefore, fructose has potential to contribute to changes in food palatability, as well as other nutritional effects, such as excessive browning, volume and tenderness reduction during cake preparation, and formation of mutagenic compounds.
Dehydration Fructose can be dehydrated to give hydroxymethylfurfural ("HMF", C6H6O3), which can be processed into liquid dimethylfuran (C6H8O). This conversion has long been proposed, not implemented, as a route to green fuels.
Physical and functional properties
Sweetness of fructose
The primary reason that fructose is used commercially in foods and beverages, besides its low cost, is its high relative sweetness. It is the sweetest of all naturally occurring carbohydrates. The relative sweetness of fructose has been reported in the range of 1.2–1.8 times that of sucrose. However, it is the 6-membered ring form of fructose that is sweeter; the 5-membered ring form tastes about the same as usual table sugar. Warming fructose leads to formation of the 5-membered ring form. Therefore, the relative sweetness decreases with increasing temperature. However, it has been observed that the absolute sweetness of fructose is identical at 5 °C as 50 °C and thus the relative sweetness to sucrose is not due to anomeric distribution but a decrease in the absolute sweetness of sucrose at higher temperatures.
The sweetness of fructose is perceived earlier than that of sucrose or glucose, and the taste sensation reaches a peak (higher than that of sucrose), and diminishes more quickly than that of sucrose. Fructose can also enhance other flavors in the system. Fructose exhibits a sweetness synergy effect when used in combination with other sweeteners. The relative sweetness of fructose blended with sucrose, aspartame, or saccharin is perceived to be greater than the sweetness calculated from individual components.
Fructose solubility and crystallization Fructose has higher water solubility than other sugars, as well as other sugar alcohols. Fructose is, therefore, difficult to crystallize from an aqueous solution. Sugar mixes containing fructose, such as candies, are softer than those containing other sugars because of the greater solubility of fructose.
Fructose hygroscopicity and humectancy Fructose is quicker to absorb moisture and slower to release it to the environment than sucrose, glucose, or other nutritive sweeteners. Fructose is an excellent humectant and retains moisture for a long period of time even at low relative humidity (RH). Therefore, fructose can contribute a more palatable texture, and longer shelf life to the food products in which it is used.
Freezing point Fructose has a greater effect on freezing point depression than disaccharides or oligosaccharides, which may protect the integrity of cell walls of fruit by reducing ice crystal formation. However, this characteristic may be undesirable in soft-serve or hard-frozen dairy desserts.
Fructose and starch functionality in food systems Fructose increases starch viscosity more rapidly and achieves a higher final viscosity than sucrose because fructose lowers the temperature required during gelatinizing of starch, causing a greater final viscosity. Although some artificial sweeteners are not suitable for home baking, many traditional recipes use fructose.
Sources
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