A test method pioneered by Irish diagnostic technology company Megazyme International has become the global method of choice for ensuring dietary fibre content is not 'double counted' on nutritional labels.
The McCleary method, named after the company’s founder Barry McCleary, or the AOAC 2009.01 measures all components of dietary fibre, including resistant starch and non-digestible oligosaccharides. This avoids underestimates of dietary fibre values as well as ‘double counting’ - which can lead to inflated values.
Historically, food laboratories used the Prosky method (AOAC 985.29) for measuring dietary fibre. This uses bacterial α-amylase and harsh conditions (pH 8.2, 100°C) for the enzymatic incubation step - in which the sample is treated with enzymes that mimic the digestive process in the human small intestine.
However, in 2009, when CODEX Alimentarius set a new definition for dietary fibre, which acknowledged that non-digestible oligosaccharides and resistant starch also behave physiologically as dietary fibre, it became apparent that this method may not always be the most accurate, Megazyme International said.
The US is set to adopt the CODEX definition in 2016.
A partial picture
As David Mangan, R&D manager with Megazyme, explained: “The Prosky method measures some components of dietary fibre in an incomplete fashion. Most resistant starch and all non-digestible oligosaccharides are excluded which results in an underestimation of dietary fibre in many food types.”
To be precise, galacto-oligosaccharides, raffinose and stachyose are not measured while polydextrose, resistant maltodextrins, inulin, fructooligosaccharides, pectin, arabinogalactan and resistant starch are partially measured.
Mangan said this posed a challenge for analysts because, if, for example, resistant starch was measured using AOAC 2002.02 and the value obtained was added to the total dietary fibre value calculated using the Prosky method, this would mean resistant starch was ‘double counted’, resulting in an artificially high dietary fibre value.
The McCleary method, on the other hand, measured all components of dietary fibre as defined by CODEX. It used pancreatic α-amylase and conditions much closer to physiological (pH 6 and 37°C) for the enzymatic incubation step.
“Under these gentler conditions, the resistant starch is not hydrolysed as it is with the Prosky method,” explained Mangan.
Non-digestible oligosaccharides are measured using HPLC (high performance liquid chromatography).
Many national and international authorities, among them the EFSA (European Food Safety Authority), Food Standards Australia and New Zealand (FSANZ) and Health Canada, have accepted the CODEX definition of dietary fibre and the accompanying analytical method AOAC 2009.01.
In the USA, the FDA has proposed to adopt the CODEX definition and accompanying analytical method AOAC 2009.01 in a rule change to 21 CFR part 101 that will come into effect in 2016.
The UK is the only country to use a completely different definition – based on the use of the non-starch polysaccharide (NSP) method - to measure fibre.
“It is unclear at this time whether the UK will take this opportunity to harmonise with the rest of the world in this regard,” said Mangan.
Megazyme is the only commercial supplier of the reagents required to run AOAC 2009.01, sold in the form of its ‘Integrated Total Dietary Fiber Assay Kit’.