Dietary fiber can generally be described as that portion of food that is not digested in the human small intestine. It passes into the large intestine where it is partially or fully fermented. These characteristics of dietary fiber are associated with its numerous well documented health benefits.
CODEX Alimentarius was founded in 1963 as part of the World Health Organisation Food and Agriculture Organisation (WHO/FAO) and is the international body that sets guidelines for national regulatory authorities. In 2009, they finally reached consensus upon a definition of dietary fiber after almost 20 years of deliberation:
CODEX Alimentarius definition of dietary fiber:
This definition of Dietary Fiber required an accompanying analytical method that was capable of measuring dietary fiber in a simple, robust, reproducible manner. Megazyme fulfilled this requirement with the development of the McCleary Method Integrated Dietary Fiber Assay Kit (AOAC 2009.01/AOAC 2011.25).
For further information on this aspect of dietary fiber, see measurement of dietary fiber.
Types of Dietary Fiber
Dietary fiber can be classified in a number of different ways.
Bulk properties can in certain cases provide useful information:
Dietary fiber can also be classified by its ingredient components. These are wide ranging but very well defined classes of chemical entities.
Polymers of glucose containing various bond types between individual glucose units including β-1,4, β-1,3 and β-1,6.
β-Glucan obtained from barley, oats and yeast is available from Megazyme. High, medium and low viscosity grades are available and these are indicative of the molecular weight range in each product.
Polymers of β-1,4 linked mannose units with varying degrees of α-1,6 galactosyl substitution. The ratio of galactose to mannose determines the polysaccharide identity.
Galactomannan obtained from carob, guar and konjac is available from Megazyme. High, medium and low viscosity grades are available and these are indicative of the molecular weight range in each product.
Polymers consisting of a β-1,4 linked xylose backbone with the xylose monomers bearing a mixture of either 2, or 3, or 2 and 3 substituted α-L-arabinose residues.
Arabinoxylan obtained from rye and wheat is available from Megazyme. High, medium and low viscosity grades are available and these are indicative of the molecular weight range in each product.
Pectin describes a range of diverse structural polysaccharides that are found in many fruits and can be subdivided into two broad classes, namely galacturonans (consisting of homo-galacturonans, unsubstituted and substituted galacturonans) and rhamnogalacturonans (consisting of rhamnogalacturonan-I and rhamnogalacturonan-II).
Homogalacturonan is a linear chain of 1,4-linked-α-D-galacturonic acid residues in which some of the carboxylic acid groups are present as methyl esters. Substituted galacturonans include xylo-galacturonan and apio-galacturonan which exhibit a degree of branching of the respective monosaccharides on the polygalacturonic acid backbone.
Rhamnogalacturonan-I (RG-I) contains a repeating disaccharide subunit of galacturonic acid/methyl ester and rhamnose [→2)-rhamnose-α-(1→4)-galacturonic acid-α-(1→]. The 4- position of the rhamnose residues bear long side chains consisting of β-1,4 linked galactose and/or α-1,5 linked arabinose and/or arabinogalactan. Rhamnogalacturonan-II (RG-II) is a complex, structurally diverse heteropolymer made up of 11 different monosaccharides with extensive branching on a linear backbone of α-1,4 linked galacturonic acid.
Pectic polysaccharides including polygalacturonic acid, rhamnogalacturonan I (obtained from potato) and rhamnogalacturonan (obtained from soy bean) are available from Megazyme.
A polysaccharide based on a β-1,3 linked linear galactosyl backbone. The 4- or 6- positions can act as branch points for short chains of galactose or arabinose residues. The overall ratio of galactose to arabinose is in the region of 6:1.
Arabinogalactan obtained from larch wood is available from Megazyme.
This is a term given to any starch that is resistant to hydrolysis by the enzymes of the small intestine. There are four different types of resistant starch (RS1-4).
Inulin is a polysaccharide found in nature containing long chain fructose monomers linked β-1,2 and a terminal glucose residue. Fructo-oligosaccharides (FOS) are the β-1,2 linked fructose oligomers lacking the terminal glucose residue. They are usually formed through the degradation of inulin although they can also be manufactured commercially using enzymatic transglycosylation.
Inulin will soon be available from Megazyme.
Oligomers formed by transgalactosylation reactions with DP usually in the range of 2-8 and possessing either β-1,3, β-1,4 or β-1,6 linkages.
A mixture of β-1,3 and β-1,4 linked galactobiose (DP2) is available from Megazyme.
In terms of industrial production, these are formed from starch using heat/acid/enzymatic treatment to rearrange the standard α-1,4 bonds between glucose monomers to a random mixture of either α- or β- 1-2, 1-3 or 1-4 bonds. The resulting oligomers/polymers are resistant to digestion.
Polydextrose is formed industrially by the forced condensation of glucose, sorbitol and citric acid, polydextrose is a synthetic polymer of glucose that is resistant to digestion.
A small proportion of dietary fiber is made up of the waxes of plant source ingredients. These waxes are generally polymers of long chain aliphatic acid monomers like 18-hydroxy-oleic acid, or aromatic acid monomers like the hydroxy-cinnamic acids.