Prepared by Assoc Prof Darryl Small (v4, Sept 2012) questions/feedback to <darryl.small@rmit.edu.au>

Molecule of the week Fructans

Carbohydrate molecules are extremely diverse from

many points of view- chemical, structural, solubility,

taste, functionality in foods as well as health and

nutritional aspects. The fructans are quite unusual and

there is considerable current interest in their use as food

ingredients because of their unusual properties.

Where are fructans found? This group of

polysaccharides occurs in a variety of different plant

species. Included are some common foods, with the best

known being onions and related species (garlic,

shallots, chives). Wheat plants and other cereals contain

these polymers in low concentrations. Over recent years

fructans have been extracted for use as a food

ingredient and the plant used for this has been chickory,

reflecting the higher amounts present.

Molecular properties The name gives us some

clues on the structure: fruct- firstly tells us that fructose

units (the monomer) are linked covalently into a bigger

molecular structure. Secondly, the latter part of the

name -an indicates a polymeric molecule - hence

fructans are polysaccharides primarily consisting of

fructose.

Typically, fructans found so far in most plants are

similar to each other, but they are unlike most other

polysaccharides in all living things. Usually natural

polysaccharides have at least one hundred sugar groups

linked covalently to give the complete structure. Many,

including starches in many food ingredients have

thousands but the fructans are most commonly made up

of around twenty and this is known as the degree of

polymerisation (abbreviated DP or dp). Therefore

fructans are unusually small compared to most

polysaccharides.

Properties of fructans As with carbohydrates

generally, fructans are hydrophilic, attracting a crowd

of water molecules which form hydrogen bonds with

the many free hydroxyl groups exposed on the surface

of the chain. This, in combination with the bonding

pattern between the fructose units, and particularly the

low average degree of polymerisation, mean that

fructans are very readily soluble in water. The

shortness of the chains restricts the impact of adding

fructans on viscosity and mouthfeel of foods, unlike

many of the other polysaccharides added into foods

(such as guar gum and xanthan).

Structure of fructans As with most

polysaccharides, fructans are linear chains of the

monomeric units (see the diagram). For most of the

different plant sources that have been studied there is

one glucose unit also present at one end of the chain

(this is non-reducing).

In the structure of the molecule, n can vary:

1. for different plant

sources;

2. within any one

sample/source

because fructans are

polydisperse;

3. often within the range

of 20 to 40.

The presence of fructose

and the dp helps us to

understand some

significant aspects of

fructan behaviour and

applications

CH2

CH2OHO

O

CH2OH

O

CH2OH

CH2OHO

On

Fructans as prebiotics This term refers to molecules

which are not hydrolysed by the enzymes we produce in

our digestive tract. They are, however, utilised by the

“friendly” bacteria (referred to as probiotics) that

populate our lower digestive tract. Hence prebiotics can

be considered as dietary fibre. The ability of the bacteria

to use fructans as a food source benefits us as the good

bacteria promote our health and wellbeing.

RMIT research on fructans The source from

which fructans have been extracted in Europe is chickory

and the resultant food ingredient is often referred to as

inulin. In seeking an alternative, the plant Jerusalem

artichoke was selected as this is suited to production in

Victoria. The fructans from this source have recently

been extracted and characterised [1]. In a related study,

options for synthesis of lactulose (another prebiotic) were

investigated [2]. Finally, the fructan was shown to be

effective as an ingredient in the formulation of yoghurt

products [3].

References and further reading

1] Paseephol T, Small DM, Sherkat F. 2007. Process

optimisation for fractionating Jerusalem artichoke

fructans with ethanol using response surface

methodology. Food Chem 104(1):73-80.

2] Paseephol T, Small DM, Sherkat F. 2008. Lactulose

production from milk concentration permeate using

calcium carbonate-based catalysts. Food Chem

111(2):283-90.

3] Paseephol T, Small DM, Sherkat F. 2008. Rheology

and texture of set yoghurt as affected by inulin

addition. J Texture Studies 39(6):617-34.