In its simplest terms, dietary fiber 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 and filtration. This mimics absorption of these sugars in the body.
The non-digestible precipitate contains the dietary fiber but also contains protein and inorganic material. These should not be included in dietary fiber so protein and inorganic material must be measured separately and subtracted from the weight.
1) Prosky Method (AOAC 985.29)1
Introduced in 1985. Uses bacterial α-amylase and harsh conditions (pH 8.2, 100ºC) for the enzymatic incubation step. This method does not measure all components of dietary fiber as currently defined by CODEX Alimentarius (international regulatory body for food ingredients). Most resistant starch and all non-digestible oligosaccharides are not included which results in an underestimation of dietary fiber.
2) McCleary Method (AOAC 2009.01)2-4
Introduced in 2009. Uses pancreatic α-amylase and conditions much closer to physiological (pH 6, 37ºC) for the enzymatic incubation step. This method measures all components of dietary fiber as currently defined by CODEX Alimentarius.
3) Rapid Integrated Total Dietary Fiber5
Introduced in 2015. Closely resembles AOAC 2009.01. This method addresses the minor limitations that have been identified in the McCleary Method (AOAC 2009.01) and it is envisaged that an interlaboratory evaluation will be conducted in 2016.
In certain cases, it is desirable to know what type of dietary fiber is present in a sample. Modifications to the standard methods (an additional filtration step) exist and these allow the dietary fiber content to be divided into soluble and insoluble dietary fiber (SDF and IDF). In both cases, all other steps in the method remain unchanged.
There are many different chemical entities that are classed as dietary fiber and there are distinct analytical methods to measure each of these. The methods approved by CODEX Alimentarius for the measurement of dietary fiber are shown below. Methods highlighted in green were developed by Megazyme (5 out of the 15 CODEX approved methods).
An inherent problem exists with the measurement of dietary fiber by the Prosky/Lee methods (AOAC 985.29/991.43). These methods measure some components of dietary fiber in an incomplete fashion.
The diagram below attempts to highlight the issue.
If Prosky/Lee Methods are used to measure dietary fiber, Galacto-oligosaccharides (GOS), Raffinose and Stachyose are not measured at all while Polydextrose, Resistant Maltodextrins, Inulin, FOS, Pectin, Arabinogalactan and Resistant Starch are partially measured. This poses a challenge for food science analysts because if, for example, Resistant Starch is measured using AOAC 2002.02 and the value obtained is added to the value for Total Dietary Fiber measured using AOAC 985.29, this results in a quantity of resistant starch being ‘double counted’ leading to an artificially high value for dietary fiber.
The solution to this problem is to replace the Prosky/Lee methods (AOAC 985.29/AOAC 991.43) with the McCleary methods (AOAC 2009.01/2011.25). These methods correctly measure all components of dietary fiber.
By examining the figure below we can identify the major similarities and differences between the Prosky and McCleary methods for total dietary fiber. Both methods involve the use of a 1 g sample and following an enzymatic digestion step, the non-digested material is precipitated and the protein and ash contents are determined separately and subtracted to calculate HMWDF.
The differences between the enzymatic treatment steps are evident. The starch content in a sample is gelatinised during the Prosky amylase incubation while this is avoided under the McCleary amylase incubation. This difference is crucial as gelatinisation removes the ‘resistant’ quality of resistant starch – an important component in dietary fiber. This leads to underestimation of total dietary fiber in certain samples using the Prosky method.
The other major difference between the Prosky and McCleary methods is the inclusion of the quantification of non-digestible oligosaccharides (NDOs) by HPLC. Non-digestible oligosaccharides are not measured in the Prosky method and this leads to underestimation of total dietary fiber in certain samples.
As a routine procedure, AOAC 2009.01 is not difficult to perform but analysts unfamiliar with the topic may find the prospect somewhat daunting. Megazyme provides an individual data booklet for every assay kit sold that outlines clear step-by-step guidelines on how to conduct the relevant experimental protocol. Useful tips and warnings on potential ‘stumbling blocks’ are provided at key points throughout.
K-TDFR can be used for the measurement of total dietary fiber according to AOAC methods 985.29, 991.43, 991.42 and 993.19. The data sheet for this product can also be found here.
K-INTDF can be used for the measurement of total dietary fiber according to AOAC methods 2009.01 and 2011.25. Furthermore, an instructional video has been prepared that clearly demonstrates every step in the process.
K-RINTDF is a new product launched in 2015 to enable customers to perform the Rapid Integrated Dietary Fiber method. This method addresses the minor limitations that have emerged in the use of AOAC 2009.901/2011.25. Interlaboratory evaluation of this method will be carried out in 2016. It is discussed in detail below the instructional video.
