Fructans

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3.1.1 Definition

A fructan is any compound where one or more fructosyl-fructose linkages constitutes a majority of linkages.1 Even though according to classical rule, names of molecules ending with "an" should be used to designate polymers with DP > 10, no longer is there today a distinction between polymers and oligomers. Consequently, fructan is used to name molecules that have a majority of fructose residues whatever the number is. It even includes the disaccharide composed exclusively of two fructose residues, specifically the fructosyl-fructose or inulobiose but not sucrose, isomaltu-lose, and galactosucrose, etc.12 In addition, fructan is also sometimes either a cyclic or a branched molecule (Figure 3.1). Fructan is also known as polyfructosylfructose .3 All natural (plant and microbial) fructans are a mixture of oligomers or polymers or both, which is best described by the mean (or average) and the maximum number of fructose units, residues, or moieties,* known as the average and the maximum degree of polymerization (DPav and DPmax), respectively. More than 50 generic names of fructans have appeared in old literature including, to cite only a few, inulin, levan, and phlein (see definition below) but also fructoholoside, fructosan, graminin, inu-lenin, lavulan, levulosan, levosin, and pseudo-inulin, etc.,4 but their usage should be avoided.1

A FRUCTAN is:

A CARBOHYDRATE THAT CONSISTS MOSTLY OF FRUCTOSE (+ GLUCOSE)

A MOLECULE THAT HAS A MAJORITY OF FRUCTOSE RESIDUES (+ GLUCOSE)

A FRUCTAN can be:

-LINEAR: INULIN (2, 1 fructosyl-fructose) LEVAN (2, 6 fructosyl-fructose)

FIGURE 3.1 Definition of Fructan.

* These terms will be used interchangeably throughout the book to designate the fructose monomers.

3.1.2 Chemistry of Linear, Branched, and Cyclic Fructans

From a chemical point of view, the linear chain of fructans is either a D-D-glucopy-ranosyl-[-D-D-fructofuranosyl]n1-D-D-fructofuranoside (GpyFn) or a □-D-fructopyrano-syl-[-D-D-fructofuranosyl]n1-D-D-fructofuranoside (FpyFn). The fructosyl-glucose linkage is always D-(2<->1) as in sucrose,* but the fructosyl-fructose linkages are either □-(1i2) or □-(6i2). In branched fructans the branching linkages are usually □-(2""6). Fructans are mainly of plant origin, but they are also found in fungi and bacteria. In plant fructans the number of fructose monomers does not exceed 200, whereas in bacterial fructans it can be as high as 100,000, and it is highly branched.

The general terms to describe fructans are: inulin, levan, graminan, phlein, and kestoses (Table 3.1).1

• Inulin is a material that has mostly, or exclusively, the □-(1^2) fructosyl-fructose linkage, and glucose is allowed at the terminal position in the chain but is not necessary. Until recently, inulin was considered to be a linear molecule with □-(1^2) linkages exclusively. However, using optimized permethylation analysis, it has been possible to demonstrate that even native inulin has a very small degree (1-2%) of branching.5 All fructans in dicotyledons, but only part of the fructans in monocotyledons, are inulin-type fructans.4 Inulin exists also in a cyclic form that contains 6,7, or 8 fructofuranose rings.6

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