Changing Methods for the Measurement of Dietary Fiber

In 2009, CODEX Alimentarius finally reached consensus upon a definition of dietary fiber (DF) after almost 20 years of deliberation:

Dietary fiber denotes carbohydrate polymers1 with 10 or more monomeric units2, which are not hydrolysed by the endogenous enzymes in the small intestine of humans and belong to the following categories:
• Edible carbohydrate polymers naturally occurring in the food consumed.
• Carbohydrate polymers, which have been obtained from food raw material by physical, enzymatic or chemical means and which have been shown to have a physiological benefit to health, as demonstrated by generally accepted scientific evidence to competent authorities.
• Synthetic carbohydrate polymers that have been shown to have a physiological benefit to health, as demonstrated by generally accepted scientific evidence to competent authorities.
NOTES:
(1) Includes also lignin and other compounds if quantified by AOAC method 991.43.
(2) Decision on whether to include carbohydrates with a degree of polymerization from DP 3 to 9 should be left to national authorities.

Lab Measurement

In its simplest terms, DF content in a sample is measured in the laboratory by what is called an enzymatic-gravimetric method. After defatting, a food sample is treated with enzymes that mimic the digestive process in the human small intestine. Digestible carbohydrates are broken down into simple sugars and removed from the sample by precipitation using ethanol followed by filtration. This process mimics the absorption of these sugars in the body. The non-digestible precipitate contains the DF but also contains protein and inorganic material. These should not be included in DF so protein and inorganic material must be measured separately and then subtracted from the weight.

Commonly Used Methods

Prosky Method (AOAC 985.29)—Introduced in 1985. Uses bacterial α-amylase and harsh conditions (pH 8.2, 100 degrees Celsius) for the enzymatic incubation step. Most resistant starch and all non-digestible oligosaccharides are not included, resulting in an underestimation of DF in many food types. This method does not measure all components of DF as currently defined by CODEX Alimentarius.

McCleary Method (AOAC 2009.01)—Introduced in 2009. Uses pancreatic α-amylase and conditions much closer to physiological (pH 6, 37 degrees Celsius) for the enzymatic incubation step. In addition, non-digestible oligosaccharides are measured using high-performance liquid chromatography, or HPLC, analysis. This method measures all components of DF as currently defined by CODEX Alimentarius.

‘Double Counting’ of DF Components

An inherent problem exists with the measurement of DF by the Prosky (AOAC 985.29). This method measures some components of DF in an incomplete fashion. For example, if AOAC 985.29 is used to measure DF, galacto-oligosaccharides (GOS), raffinose, and stachyose are not measured at all while polydextrose, resistant maltodextrins, inulin, fructooligosaccharides (FOS), pectin, arabinogalactan, and resistant starch (RS) are partially measured. This poses a challenge for food science analysts because if, for example, RS is measured using AOAC 2002.02 and the value obtained is added to the value for Total DF measured using AOAC 985.29, this results in a quantity of RS being “double counted,” leading to an artificially high value for DF.

The solution to this problem is to replace the Prosky method (AOAC 985.29) with the McCleary methods (AOAC 2009.01/2011.25). These methods correctly measure all components of DF and avoids the problem of “double counting.”

Regulation

Since CODEX Alimentarius published the new definition of DF in 2009, many national and international authorities have moved to redefine their regulations based on this. The European Food Safety Authority, Food Standards Australia and New Zealand (FSANZ), Health Canada, and equivalent bodies in China, Brazil, Chile, Mexico, Thailand, Korea, Malaysia, and Indonesia have all broadly accepted the CODEX definition and the accompanying analytical method AOAC 2009.01. South Africa is the only country that has decided not to include oligosaccharides with DP (degree of polymerization) 3-10 in their definition of DF which may prove somewhat problematic in terms of regulation as there is no analytical method available to meet that definition precisely.