A fructan is a polymer of fructose molecules. Fructans with a short chain length are known as fructooligosaccharides. Fructans can be found in over 12% of the angiosperms including both monocots and dicots such as agave, artichokes, asparagus, leeks, garlic, onions (including spring onions), yacón, jícama, barley and wheat.
Fructans are built up of fructose residues, normally with a sucrose unit (i.e. a glucose–fructose disaccharide) at what would otherwise be the reducing terminus. The linkage position of the fructose residues determine the type of the fructan. There are five types of fructans.
- 1-linked: in inulin, the fructosyl residues are linked by β-2,1-linkages
- 6-linked: in levan and phlein, the fructosyl residues are linked by β-2,6-linkages
Two more types of fructans are more complex: they are formed on a 6G-kestotriose backbone where elongations occur on both sides of the molecule. Again two types are discerned:
- neo-inulin type (also called "inulin neoseries"): predominant β-2,1-linkages
- neo-levan type (also called "levan neoseries"): predominant β-2,6-linkages
Fructans are important storage polysaccharides in the stems of many species of grasses and confer a degree of freezing tolerance. A notable exception is rice, which is unable to synthesise fructans.
In barley, fructan accumulates in the cell vacuoles and acts as a carbon sink within the cell to facilitate photosynthesis. Fructan reserves are transported to the reproductive tissue during grain filling, and to the vegetative tissues during periods of growth.
Chicory inulin-type fructans are used mainly as the raw materials for industrial production of fructans as food ingredients. Use in the food industry is based on the nutritional and technological properties of fructans as a prebiotic dietary fiber.
Fructan content of various foods
|Barley kernels (very young)||22%|
|Wheat bread (white)||0.7–2.8%|
|Look up fructan in Wiktionary, the free dictionary.|
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Fructans, on the basis of glycosidic linkage, are categorized into five groups: (a) inulin having β(2 → 1) linkage, (b) levan/phlein having β(2 → 6) linkage, (c) graminin (having inulin or levan backbone with ≥ 1 short branch), (d) inulin neoseries (like inulin but one glucose unit between two fructose moieties), and (e) levan neoseries (like levan but one glucose unit between two fructose moieties) (Figure 1).
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