A fructan is a
polymer
A polymer () is a chemical substance, substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeat unit, repeating subunits derived from one or more species of monomers. Due to their br ...
of
fructose
Fructose (), or fruit sugar, is a Ketose, ketonic monosaccharide, simple sugar found in many plants, where it is often bonded to glucose to form the disaccharide sucrose. It is one of the three dietary monosaccharides, along with glucose and gal ...
molecules. Fructans with a short chain length are known as
fructooligosaccharide
Fructooligosaccharides (FOS) also sometimes called oligofructose or oligofructan, are oligosaccharide fructans, used as an alternative sweetener. FOS exhibits sweetness levels between 30 and 50 percent of sugar in commercially prepared syrups. ...
s. Fructans can be found in over 12% of the
angiosperm
Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (). The term angiosperm is derived from the Greek words (; 'container, vessel') and (; 'seed'), meaning that the seeds are enclosed within a fruit ...
s including both
monocots
Monocotyledons (), commonly referred to as monocots, ( Lilianae '' sensu'' Chase & Reveal) are flowering plants whose seeds contain only one embryonic leaf, or cotyledon. A monocot taxon has been in use for several decades, but with various ranks a ...
and
dicots such as
agave
''Agave'' (; ; ) is a genus of monocots native to the arid regions of the Americas. The genus is primarily known for its succulent and xerophytic species that typically form large Rosette (botany), rosettes of strong, fleshy leaves.
Many plan ...
,
artichokes,
asparagus
Asparagus (''Asparagus officinalis'') is a perennial flowering plant species in the genus ''Asparagus (genus), Asparagus'' native to Eurasia. Widely cultivated as a vegetable crop, its young shoots are used as a spring vegetable.
Description ...
,
leek
A leek is a vegetable, a cultivar of ''Allium ampeloprasum'', the broadleaf wild leek (synonym (taxonomy), syn. ''Allium porrum''). The edible part of the plant is a bundle of Leaf sheath, leaf sheaths that is sometimes erroneously called a "s ...
s,
garlic
Garlic (''Allium sativum'') is a species of bulbous flowering plants in the genus '' Allium''. Its close relatives include the onion, shallot, leek, chives, Welsh onion, and Chinese onion. Garlic is native to central and south Asia, str ...
,
onion
An onion (''Allium cepa'' , from Latin ), also known as the bulb onion or common onion, is a vegetable that is the most widely cultivated species of the genus '' Allium''. The shallot is a botanical variety of the onion which was classifie ...
s (including
spring onions),
yacón
The yacón (''Smallanthus sonchifolius'') is a species of daisy traditionally grown in the northern and central Andes from Colombia to northern Argentina for its crisp, sweet-tasting, tuberous roots. Their texture and flavour are very similar t ...
,
jícama,
barley
Barley (), a member of the grass family, is a major cereal grain grown in temperate climates globally. It was one of the first cultivated grains; it was domesticated in the Fertile Crescent around 9000 BC, giving it nonshattering spikele ...
and
wheat
Wheat is a group of wild and crop domestication, domesticated Poaceae, grasses of the genus ''Triticum'' (). They are Agriculture, cultivated for their cereal grains, which are staple foods around the world. Well-known Taxonomy of wheat, whe ...
.
Fructans also appear in
grass
Poaceae ( ), also called Gramineae ( ), is a large and nearly ubiquitous family (biology), family of monocotyledonous flowering plants commonly known as grasses. It includes the cereal grasses, bamboos, the grasses of natural grassland and spe ...
, with
dietary implications for horses and other grazing animals (
Equidae
Equidae (commonly known as the horse family) is the Taxonomy (biology), taxonomic Family (biology), family of Wild horse, horses and related animals, including Asinus, asses, zebra, zebras, and many extinct species known only from fossils. The fa ...
).
Types
Fructans are built up of fructose residues, normally with a
sucrose
Sucrose, a disaccharide, is a sugar composed of glucose and fructose subunits. It is produced naturally in plants and is the main constituent of white sugar. It has the molecular formula .
For human consumption, sucrose is extracted and refined ...
unit (i.e. a
glucose
Glucose is a sugar with the Chemical formula#Molecular formula, molecular formula , which is often abbreviated as Glc. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae d ...
–fructose
disaccharide
A disaccharide (also called a double sugar or ''biose'') is the sugar formed when two monosaccharides are joined by glycosidic linkage. Like monosaccharides, disaccharides are simple sugars soluble in water. Three common examples are sucrose, ...
) 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.
Linkage normally occurs at one of the two primary
hydroxyl
In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...
s (
OH-1 or
OH-6), and there are two basic types of simple fructan:
* 1-linked: in
inulin
Inulins are a group of naturally occurring polysaccharides produced by many types of plants, industrially most often extracted from chicory. The inulins belong to a class of dietary fibers known as fructans. Inulin is used by some plants as a ...
, 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
A third type of fructans, the graminin type,
contains both β-2,1-linkages and 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
Functions
Fructans are important storage
polysaccharides
Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long-chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with wat ...
in the stems of many species of
grasses
Poaceae ( ), also called Gramineae ( ), is a large and nearly ubiquitous family of monocotyledonous flowering plants commonly known as grasses. It includes the cereal grasses, bamboos, the grasses of natural grassland and species cultivated in ...
and confer a degree of
freezing tolerance. A notable exception is
rice
Rice is a cereal grain and in its Domestication, domesticated form is the staple food of over half of the world's population, particularly in Asia and Africa. Rice is the seed of the grass species ''Oryza sativa'' (Asian rice)—or, much l ...
, which is unable to synthesise fructans.
In barley, fructan accumulates in the cell
vacuoles and acts as a
carbon sink
A carbon sink is a natural or artificial carbon sequestration process that "removes a greenhouse gas, an aerosol or a precursor of a greenhouse gas from the atmosphere". These sinks form an important part of the natural carbon cycle. An overar ...
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
Dietary fiber (fibre in English in the Commonwealth of Nations, Commonwealth English) or roughage is the portion of plant-derived food that cannot be completely broken down by human digestive enzymes. Dietary fibers are diverse in chemical co ...
.
Fructan content of various foods
See also
*
*
Notes
References
* ''Sugar – Chemical, Biological and Nutritional Aspects of Sucrose''. John Yudkin, Jack Edelman and Leslie Hough (1971, 1973). The Butterworth Group.
{{Carbohydrates
Polysaccharides
Prebiotics (nutrition)
Fructosides