The AOAC Analytical Methods for Measuring Dietary Fiber

The analytical methods for measuring dietary fibre are regularly scrutinised with the view of making sure they remain compliant with current thinking on this valuable nutritional ingredient. Over many years there has been a continuous improvement in analytical methodology as greater understanding of dietary fibre and how it performs in the gut increase.

Not that long ago, the FDA adopted the basic tenets of the Codex Alimentarius definition of 2009 which explicitly stated that for ‘regulatory purposes, dietary fiber is the non-digestible carbohydrates (with DP ≥ 3) and lignin that are intrinsic and intact in plants, and/or added (isolated or synthetic) non-digestible carbohydrates (with DP ≥ 3) that have been determined by the FDA to have physiological benefits.’ 

In May 2016, the FDA then published two final rule changes to CFR21 Part101 which changed nutrition facts labelling and the official serving sizes. They were the first major changes to dietary fibre labelling for 20 years.  

By 2020 these new changes were made law. The FDA had now introduced new regulations and definitions for dietary fibre to try and improve consumption in the US diet. They revised the terms by redefining dietary fibre as non-digestible carbohydrates (NDCs) and with it changes in the way these were described and how it would impact nutritional labeling in the USA. (Please note we alternate between US and European spelling of fibre/fiber in this article). For brevity, the new dietary fiber daily value (DV) in the USA is 28g for a 2,000 calorie diet. The dietary fiber nutrient claims set by the FDA are based on the % DV present in a reference amount customarily consumed (RACC) for any given product. These are as follows: 

• 10-19% of DV (2.8-5.4 g for dietary fiber) in the RACC allows “good source” claim.
• 20+% of DV (> 5.5 g for dietary fiber) in the RACC allows “high” or “excellent” source claim.

Analysis of NDCs is very important because the changes meant that if a fiber has no physiological benefit then it is not classed as a dietary fiber for labelling purposes. A number of businesses have had to adjust their labels or remove claims altogether.

What is Dietary Fibre?

Dietary fiber is important nutritionally. We have discussed it at length in previous articles but it is worth reflecting on the definition changes that have occurred in the last five years. It is well established how dietary fibre is a major contributor to the health and well-being of all who consume it in their diet. There is no question of the physiological benefits!

Dietary fibre is made up of many components – it is in reality an heterogenous complex mix and needs to be not only clearly defined but with solid analytical methodology which supports that definition.

Dietary Fibre Acronyms And Terminology

Before wading into the methods it is useful to know what the various acronyms are for the forms of soluble and insoluble fibre. They are regularly used to describe the outcomes of analytical methods.

The key method to remember throughout analysis is that aqueous ethanol at 78% v/v precipitates higher molecule weight polysaccharides and oligosaccharides. The lower molecular weight oligosaccharides remain in solution.

Total dietary fibre (TDF) is the sum of high-molecular weight dietary fibre (HMWDF) and Soluble Dietary Fiber (SDFS). 

SDFS is dietary fiber soluble in water and also soluble in 78% aqueous ethanol. The SDFS is generally non-digestible oligosaccharides (NDOs).

SDFP is dietary fiber soluble in water but insoluble (i.e. precipitate) in 78% aqueous ethanol. It is also known as HMWSDF or high molecular weight soluble dietary fiber.

High molecular weight dietary fiber (HMWDF) is the sum of IDF which is water insoluble dietary fibre such as cellulose, lignin, hemicellulose (insoluble pentosans), insoluble pectin, yeast beta-glucan and SDFP. The SDFP can be hydrocolloids such as gums, (arabic, guar etc.), soluble pentosans, pectin and cereal based beta-glucan. 

The Soluble Dietary Fiber (SDF) is fiber that is soluble in water. It is composed of both SDFP and SDFS. In other words, soluble fibre that precipitates and remains in solution when 78% v/v ethanol is used.

The Original Definitions and Methods

In 1976, Trowell first defined dietary fibre as consisting of the plant polysaccharides and lignin which are resistant to hydrolysis by digestive enzymes of man. At that time a variety of methods were developed to support this definition. It is still the basis for the AOAC methodology.

One of the basic tenets of sample work-up before analysis is to get the sample into a form that can then be usefully analysed which will be a dried low-fat/fat-free sample that is then milled. In many cases, food samples are treated with solvents that remove fats and lipids as in ether extraction or a modified Gerber or Babcock method. High fat prevents proper milling of dried samples. This issue appears to be most significant when the fat content is above 10% w/w. Many dairy beverages though including milk have a fat content to a maximum of 4%. The defatting is done with petroleum ether usually three times in 25 ml portions per gram of sample before milling.  Samples are also dissolved  in water to remove soluble sugars and pigments. Many use phosphate buffers to modify pH because buffer systems are needed for optimal enzyme activity. The weight loss due to extraction of fat and moisture loss is necessary as a correction.

The most important method of the day has been the Prosky method (AOAC Method 985.29; AACC Method 32-05.01) which measured what was thought then to be total dietary fiber (TDF) where insoluble dietary fibre (IDF) and non-digestible soluble polysaccharides (SDFP) made up the TDF (Prosky et al., 1985;1988). The reality was that it only measured the high molecular weight component of dietary fiber (HMWDF).

A couple of other methods such as AOAC 991.43/AACC Method 32-20.01 measured total, soluble and insoluble dietary fibre (IDF) in foods. This too was and enzymatic-gravimetric method based on MES-Tris Buffer. There was also Method 993.19 (no corresponding AACC method) measured non-digestible soluble polysaccharides (SDFP) in foods. These were also acceptable but generally less robust.

The Lee method (AOAC 991.43/AACC method 32-07.01) was then developed to measure insoluble dietary fibre and non-digestible soluble polysaccharides as separate measures (i.e. IDF and SDFP separately). This method also measured the high molecular weight component of dietary fiber but it did separate insoluble and soluble fibres from each other. The soluble polysaccharides were ones that could be precipitated in alcohol.

A subsequent method was AOAC method 994.13 (AACC Method 32-25.01) which measured total dietary fiber as insoluble fibre plus non-digestible soluble polysaccharides to give a measure of sugar composition and what is called Klason lignin. This was HMWDF with sugar composition and Klason lignin. To give it its full title, the dietary fiber method was: ‘Total dietary fiber [determined as neutral sugar residues, uronic acid residues and Klason lignin] gas chromatographic colorimetric gravimetric method’ or the Uppsala method. This approach measures dietary fiber by enzymatically digesting non-fiber carbohydrates to stimulate human digestion, which also breaks the dietary fiber into its basic monomeric sugars. These are analysed by gas chromatography and the Klason lignin is determined by gravimetric means with the result added to the saccharide total to give total dietary fiber.

The next key modification was the Matsutani method (AOAC method 2001.03/AACC Method 32-41.01) which separately measured insoluble dietary fibre plus non-digestible soluble polysaccharides and non-digestible oligosaccharides. This was then a measure of HMWDF and SDFS in foods with no resistant starch.

Another less prominent method was AOAC method 993.21 (no corresponding AACC method) which measured insoluble dietary fibre plus non-digestible soluble polysaccharides in samples with over 10% fibre and less than 2% starch.

In 2012/2013 the Codex Committee on Methods of Analysis and Sampling recommended 14 methods for measurement of dietary fiber with eight of these being type 1 methods. Of these type 1 methods, one particular method, the AACC International Approved Method 32-45.01 (AOAC method 2009.01) came to prominence because it measured all of the dietary fiber components defined by the Codex Alimentarius. However, before then a group of methods were giving reasonably good quality information on dietary fiber from a physiological perspective. 

We start with the Prosky Method (AOAC 985.29) are ideal for measuring high molecular weight dietary fiber which are low in resistant starch.  Later on we will discuss AOAC 2009.01 and 2011.25. The AOAC 2009.01 method measures HMWDF and SDFS in all foods, whilst 2011.25 measures IDF, SDFP and SDFS in all foods. 

The Prosky Method (AOAC 985.29)/AACC 32-05 Total Dietary Fiber in Foods, Enzymatic-Gravimetric Method.

The first official method and one measuring total dietary fiber in foods using an enzymatic-gravimetric approach.

Consider a sample of non-starch polysaccharides, starches and proteins which is split into duplicate portions for analytical purposes. The food sample must be dried and excess fat removed with petroleum ether before analysis.  The first part of the method relies on incubation with carbohydrases (alpha-amylase and amyloglucosidase) and a protease. The carbohydrate degrading enzymes catalyse removal of the starches whilst proteases catalyse removal of protein to leave non-starch polysaccharides which are not degraded. Incidentally, non-digestible oligosaccharides are not measured at any point in this test and they remain a separate fraction. The hydrolysed sample is treated with ethanol to 86% final concentration to precipitate the carbohydrates. This precipitate is filtered. The filtrate used to be discarded but it too is now analysed for ethanol soluble dietary fibres in later modifications.

Total dietary fibre = weight (precipitate) – weight of ash and protein – blank.

The method in a little more detail is this:-

(1) Duplicate 1 gram samples are required. A blank must also be run along with the samples to check for any contribution form reagents to residue. 

(2) The sample, particularly mixed fibre samples,  needs to be defatted with petroleum ether (25ml per portion) three times if the fat content is over 10% because false high results can be generated. This is because fat adversely affects the consistency of the milling process.

(3) The samples are homogenised (milled) and dried, and for this freeze-drying is recommended.  Duplicate test portions are weighed and the difference in weight should not be more than 20mg.

(4) The starch in the sample is gelatinized and relies on thermostable alpha-amylase heated to 96-98ºC, for 30 minutes, pH 6.0. using phosphate buffer (50ml) for the adjustment.  The sample is cooled to room temperature.

(5) A protease is then added to produce peptides by catalysing hydrolysis of proteins. This protease is heated with the sample at 60ºC for 30 minutes and pH 7.5 +/- 0.2.

(6) Then the pH is made acidic to pH 4.0-4.6 and amyloglucosidase (60ºC, 30 minutes at pH 4.5) is added to catalyse hydrolysis of starch. This produces glucose, maltose but leaves some resistant starch. 

(7) The sample solution (about 70ml)  is then treated with ethanol (95%, 280ml) to precipitate any water soluble polysaccharides over a period of an hour. This is filtered leaving a residue of  IDF and SDFP. It is washed with pure ethanol and then acetone. This is dried.

(8) The protein and ash weights are determined and subtracted which leaves a value for HMWDF, a true measure of IDF plus SDFP.  The protein is determined from one set of duplicates using method AACC method 46-13 and using N x 6.25 as the conversion factor. The second portion for ash analysis is heated to 525°C for 5 hours.

The Matsutani modification of the Prosky method took the solution after filtration which was concentrated and then passed this through an HPLC column. This gave a further measure of the SDFS (soluble dietary fiber remaining in solution after 78% precipitation with ethanol). This figure together with the high molecular weight dietary fiber gave a more complete total dietary fiber figure.

Lee method (AOAC 991.43) Total, Soluble, and Insoluble Dietary Fiber in Foods with the 2001.03 modification for SDFS. 

The enzyme digestion step is performed as for the Prosky method (AOAC 985.29). The Lee method covers water-insoluble dietary fiber and ethanol precipitated fiber. Then follows the method part for AOAC2001.03/AACC 32-41 which is known as “Dietary Fiber Containing Supplemented Resistant Maltodextrin (RMD)”. The combination of methods determines TDF, SDF, IDF, and digestion-resistant oligosaccharides. The reason the latter modification is conducted is because only a portion of resistant maltodextrin is precipitated in the aqueous ethanol when AOAC 985.29 is applied to foods containing resistant maltodextrin. You notice that low molecular weight resistant maltodextrin is resistant maltodextrin that is soluble in the aqueous ethanol in AOAC 985.29.

This time after enzyming, there is filtration to remove a water insoluble fraction. This is collected, dried, weighed and the protein and ash measures taken before being subtracted.  This is a value for insoluble dietary fiber (IDF).

The filtrate (solution passing through the filter) is treated with ethanol which produces an alcohol insoluble fraction which is washed thoroughly. It too has its protein and ash determined. This remaining material is the SDFP or soluble dietary fiber that precipitates in 78% ethanol.

The final step is to put the AOAC 991.43 and Matsutani method together to measure  SDFS (or soluble dietary fiber in solution after ethanol precipitation)  which is also called NDO non-digestible oligosaccharides). To quantitate them, the filtrate is concetrated to 20 mL; deionized through a series of anion and cation exchange columns to remove interferences from buffers, salts, amino acids, peptides, and protein; and then reconcentrated to 10 mL and filtered. Size-exclusion high-pressure liquid chromatography (HPLC) separates the components by molecular weight. The peaks corresponding to oligosaccharides larger than two degrees of polymerization (2DP) represent the resistant oligosaccharides to be included in the dietary fiber quantity.

The total dietary fibre = weight of residue + weight lof low MW soluble fibre determined by HPLC- wt of protein, ash and blank.

We can also rephrase this as SDF is the sum of SDFS and SDFP whilst the TDF is the sum of the SDF and IDF.

The calculation of available carbohydrate when method 2001.03 is used is:-

Available carbohydrate = 100 – [moisture+ash+protein+fat+alcohol+(soluble dietary fiber+insoluble dietary fibre) + low molecular weight resistant maltodextrin].

The recoveries for specific functional fibres using AOAC 2001.03 is also variable. Higher values are recorded for polydextrose and galactooligosaccharides but glucooligosaccharides are only half what should be measured. Functional fibres do need to be inlcuded in the calculation of available carbohydrate. AOAC 2001.03 can produce higher  total dietary fiber values for higher testing costs.

Issues With Dietary Fiber Methods Generally.

All the main methods including the most recent ones have issues (Manas et al., 1994). We have mentioned some already such as complexity, ned for defatting etc.

All enzymes used must have defined activity and have no contamination issues. The main ones with enzyme purity are that the amyloglucosidase (AMG) contains cellulase, pectinase and xylanase which then hydrolyse these fibres respectively. That produces an underestimation of beta-glucan, arabinoxylan and pectin. Sometimes the thermostable alpha-amylase used is not consistent in strength and the protease used may also be contaminated with 1,3:1,4-beta-glucanase.  

One particular problem arose if protease treatment to remove protein in the sample was forgotten or left out. It led to measures of the Klason lignin fraction being overestimated, and the content and distribution of polysaccharides was also altered. Thee is also a possibility that some soluble dietary fiber (SDF) is left within the matrix of the insoluble dietary fibre (IDF) which then misleads the distribution of the soluble and insoluble fractions. Some of the protein, ash and blank measurements are so imprecise they lead to an under-or over-value for the actual dietary fiber elements. Finally, some of the Klason lignin fractions found by acid hydrolysis of dietary fibre are actually residues from other components and artifacts besides lignin.

It is now worth going back a step and understanding how the definition of a dietary fiber has come about. Without knowing the methods and terminology it becomes quite complex to explain. It then helps in understanding the situation with the new methods.

Development of the Definition Of Dietary Fiber

The Dietary fiber definition has developed over the years. The American Association of Cereal Chemists (2000) defined it as:

‘Dietary fiber is the edible parts of plants or analogous carbohydrates that are resistant to digestion and absorption in the human small intestine with complete or partial fermentation in the large intestine. Dietary fiber includes polysaccharides, oligosaccharides, ligin, and associated plant substances. Dietary fibers promote beneficial physiological effects including laxation, and/or blood cholesterol attenuation, and/or glucose attenuation.’

The key features were the carbohydrates had to be edible but resistant to digestion. It included non-digestible oligosaccharides (NDO).

The Food Nutrition Board of the Institute of Medicine of the National Academies (USA-2001) states:-

‘Dietary fiber consists of nondigestible carbohydrates and lignin that are intrinsic and intact in plants.

Added fiber consists of isolated, nondigestible carbohydrates that have beneficial physiological effects in humans.

Total fiber is the sum of dietary fiber and added fiber.’

The CODEX definition of Dietary Fiber was then developed at the 30th session of the Codex Committee on Nutrition and Foods for Special Dietary Uses (CCNFSDU; ALINORM 2009), Cape Town, South Africa 4th November 2008. This definition released in 2009 states:

Dietary fibre means carbohydrate in polymers with ten or more monomeric units which are not hydrolysed by the endogenous enzymes in the human small intestine of humans and belong to the following categories….

1. edible carbohydrate polymers naturally occurring in the food as consumed and includes especially Resistant Starch.
2. Carbohydrate polymers which have been obtained from raw materials by physical, enzymatic or chemical means and which have been shown to have a physiological; effect of benefit to health as demonstrated by generally accepted scientific evidence to competent authorities.
3. Synthetic carbohydrate polymer which have been shown to have a physiological effect of benefit to health as demonstrated by generally accepted scientific evidence to competent authorities

When derived from a plant origin, dietary fiber my include fractions of lignin and/or other compounds when associated with the polysaccharides in the plant cell walls and these compounds are quantified by the AOAC gravimetric analytical method for dietary fiber. These include the non-digestible oligosaccharides (NDOs)

The decision as to whether to include carbohydrates from 3 to 9 monomeric units should be left to national authorities. A method needed to be developed to include these 3 to 9 polymers in the dietary fiber determination.

Up to this point, total dietary fiber as measured by the AOAC Method 985.29 and 991.43 included cellulose, beta-glucan, galactomannan, arabinoxylan, pectin and arabinogalactan. It only partially measures fibers such as inulin which are broken down to fructooligosaccharides (FOS), some Resistant Starch and polydextrose.

To overcome these issues a number of methods were developed to analyse specific fiber components. The following were:

[1] AOAC Method 995.16 (AACC Method 32-23.01) CODEX Type II, measured (1-3)(1-4)-beta-glucan in cereals, feeds and foods (McCleary & Mugford, 1997).

[2] AOAC Method 997.08 (AACC Method 32-31.01) CODEX Type II for fructans and FOS.

[3] AOAC Method 999.03 (AACC Method 32-32.01) CODEX Type III for fructans and FOS but underestimates highly depolymeroised FOS).

[4] AOAC Method 2000.11 (AACC Method 32-28.01) CODEX Type II for polydextrose

[5] AOAC Method 2001.02 AACC Method 32-33.01) CODEX Type II Trans galacto-oligosaccharides

[6] AOAC method 2002.02 AACC Method 32-40.01 Codex Type II Resistant starch (RS2 and RS3).

Further questions were raised about measuring these individual components. For example, we can measure resistant starch and then add that to total dietary fiber but it’s not feasible because the method means that resistant starch would be included twice in the calculation. None of the methods were able to measure properly raffinose and stachyose or xylo-oligosaccharides. It needed a single method for all these different materials and so we come to the final and most recent set of AOAC methods.

McCleary Method: AOAC 2009.01/2011.25 Total Dietary Fiber (Codex Definition) by Enzymatic-Gravimetric Method and Liquid Chromatography

The measurement of TDF (AOAC 2009.01/2011.25) (Codex Type 1) method was developed along similar lines to Prosky and Lee but using more accurate physiological comparisons in the method. The method uses an enzyme-digestion protocol at 37°C to simulate human digestion. It is more consistent with the human digestive system than AOAC 991.43. It also uses HPLC to quantify the water:alcohol-soluble dietary fibers which are mainly resistant oligosaccharides. 

The method has also been referred to as the INTDF (Integrated total dietary fibre) method.  Here the dietary fibre sample is incubated on the following treatments:

1. Starch hydrolysis using PAA (Pancreatic alpha-amylase) (2 KU)/AMG (amyloglucosidase) 0.14 KU for 16 hours, 37 C, pH 6.0).
2. The pH is changed to 8.2 and heated to 95ºC. This denatures protein and inactivates the starch hydrolysing enzymes.
3. protein hydrolysis using a protease at 60ºC and pH 8.2
4. The pH s changed to 4.5 at 60ºC.
5. The sample is treated with ethanol to cause precipitation of dietary fibre. The precipitate is filtered, washed of solvent and dried. This fraction as its protein and ash levels determined. Two figures are generated: IDF and SDFP (HMWDF).
6. The ethanolic filtrate is concentrated, desalted on a column, reconcentrated and passed through HPLC using a waters Sugar Pak. The figure obtained is the SDFS which is all the non-digestible oligosaccharides. (In the next method, AOAC 2017.16, the Waters Sugar Pak column is replaced with an HPLC-TOSOH TSK).
7. The TDF (total dietary fibre) is the IDF+SDFP + SDFS. This figure includes all the high- and low-molecular weight dietary fibre along with resistant starches.

The benefits of this method meant that it more closely simulated the situation in the human digestive tract. In the Lee and Prosky methods, the sample was digested with enzyme at unrealistic temperatures closer to boiling water whereas mammalian pancreatic alpha-amylase used in this method is at human body temperature for a more typical 16 hours. The resistant starches with other fibres were hydrolysed and lost in the previous methods but are now recovered.

The new method also quantifies water:alcohol-soluble dietary fibers. In the older methods, only soluble fibers that precipitated with ethanol were recovered. The new method recovers all soluble fibers and all fibres of more than three or more carbohydrate units is classed as dietary fiber. It also quantifies more resistant starch and other soluble dietary fibers.

The AOAC2011.25 method is a modified version of AOAC 2009.01 and manages to differentiate between the 3 categories of dietary fibre. This includes the insoluble high molecular weight dietary fibres (iHMWDF) along with the resistant starch, a separate fraction called the soluble high-molecular weight dietary fibres (SHMWDF) and then the low molecular weight dietary fiber (LMWDF) with prebiotic fibres.

It also means that the traditional dietary fiber method AOAC 991.43 and the separate methods for resistant oligosaccharides need  be run. That means methods for maltodextrins (AOAC 2001.03), fructans (inulin and fructooligosaccharides (AOAC 997.08)), polydextrose (AOAC 2000.11), galactooligosaccharides (AOAC 2001.02), and resistant starch (AOAC 2002.02) are not necessary. They can be run however if you need to understand the make-up of a sample and for monitoring formulations.

Improving AOAC 2009.01 To Create AOAC Method 2017.16 (RINTDF)

Having released the method, a number of issues began to arise which challenged the efficacy of the application and needed sorting out (McCleary et al., 2013). 

Generally, the dietary fiber value for resistant starch type 4 (RS4) varies with the application of dietary fiber methods. Much lower values are obtained with AACCI Approved Method 32-45.01 than with 32-05.01. 

The 16 hour incubation time is not appropriate because it is not physiologically relevant as the usual time of residence of food in the small intestine is about 4 hours. Any phosphate cross-linked starches (RS4) are underestimated. Also, the differences result from the greater susceptibility of RS₄ to hydrolysis by pancreatic α-amylase than by bacterial α-amylase, and also a greater susceptibility to hydrolysis at lower temperatures.

On hydrolysis of samples high in starch in the assay format of AACCI Approved Method 32-45.01 (AOAC method 2009.01), resistant maltodextrins are produced from non-resistant starch. This overestimates the amount of dietary fiber in the sample. The major component is a heptasaccharide that is highly resistant to hydrolysis by most of the starch-degrading enzymes studied. However, it is hydrolyzed by the maltase/ amyloglucosidase/isomaltase enzyme complex present in the brush border lining of the small intestine. As a consequence, AOAC methods 2009.01 and 2011.25 (AACCI Approved Methods 32-45.01 and 32-50.01, respectively) must be updated to include an additional incubation with amyloglucosidase to remove these oligosaccharides. On that basis these resistant maltodextrins are not produced in the RIN-TDF Method.

Fructotriose is a component of commercial FOS and is not separated from disaccharides by chromatography on a Sugar-Pak® column. This trisaccharide is missed in the analysis because of co-elution resulting in an underestimation of FOS.  Incubation of the resistant oligosaccharides fraction with sucrase/β-galactosidase removes disaccharides that interfere with the quantitation of this fraction.

The use of sodium azide is poisonous and not desirable. The methodology for preparing the samples prior to HPLC is tedious and D-sorbitol is not a good internal standard because its found in fruits and various food products too.

The method was then revised further to become AOAC 2017.16; RINTDF. The key changes were to make the starch hydrolysis shorter by reducing the incubation time to 4 hours but using more concentrated forms of enzymes. The PAA went to 4 KU, AMG was 1.7 KU (from 0.14 KU) with pH and temperature remaining the same. Nothing else was altered until we come to the treatment of the ethanolic filtrate. The ethanolic filtrate was desalted in a tube for in-line HPLC and a TOSOH TSKgel® SEC HPLC  column used which replaced the Sugar-Pak® column. This was to give accurate measurement of fructooligosaccharides.

Desalting of the sample is simplified by using anion and cation exchange resins in a shaking tube followed by the use of an HPLC desalting pre-column. 

The sodium azide was removed because the incubation time for enzyming dropped from 16 hours to 4 hours.  Glycerol or diethylene glycol replaced sorbitol as the internal standard.

The choice of methods is important. If you have a sample of dietary fiber which does not contain resistant starch, then use the Prosky, Lee or the Lee+Matsutani modification. The Prosky method measures IDF+SDFP which is in fact HMWDF. The Lee method measures the IDF and SDFP separately. The Lee + Matsutani method measures IDF, SDFP and SDFS separately.

If the sample has resistant starch then use AOAC 2017.16  to get total dietary fiber otherwise use a modified form to get IDF, SDFP etc. It is to date the only recognised method that accurately measures all components of Total Dietary Fiber including all resistant forms of starch.

Pulses contain mostly galactans. These can be analysed using HPLC separation with electrochemical detection – the assay is generally for raffinose, stachyose and verbascose.

Decision Trees

Choice of a dietary fibre method will be suited to particular products. Many analysts use the original AOAC 985.29 or AOAC 991.43 methods as a first look assuming that there are no added dietary fibres or the sample is thought to be low or absent of the natural low molecular weight dietary fibres.

If they contain low molecular weight dietary fibres such as non-starch oligosaccharides and/or inulin then AOAC 2009.01 or AOAC 2011.25 is used. As in agreement with definitions of dietary fiber, only low molecular DF components of 3 or more monomeric units are quantified. (Remember, these two methods cannot pick up for example galacto-oligosaccharides (GOS) of 2 monomeric units neither will they pick up di-fructose FOS. 

If there are added fibres with natural high-molecular weight dietary fibre (HMWDF), resistant starch such as RS3 then AOAC 985.29/AOAC 991.43 is used. If the resistant starch is not RS3 but any of the others then use AOAC 2009.01 or AOAC 2011.25.

When the sample contains HMWDF but specifically also galacto-oligosaccharides (GOS) then AOAC 2009.01 is needed along with AOAC 2001.02 when a percentage value for the GOS is required.

If the sample contains known resistant maltodextrin, inulin, FOS and polydextrose then use either AOAC 2009.01 or AOAC 2011.25. 

References

Lee, S. C., Prosky, L., & Vries, J. W. D. (1992). Determination of total, soluble, and insoluble dietary fiber in foods—Enzymatic-gravimetric method, MES-TRIS buffer: Collaborative study. Journal of AOAC international, 75(3), pp. 395-416.

Manas, E., Bravo, L., & Saura-Calixto, F. (1994). Sources of error in dietary fibre analysis. Food Chemistry, 50(4), pp. 331-342 (Article).

McCleary, B.V., & Mugford, D.C. (1997) J. AOAC Int. 80, pp. 580–583

McCleary, B. V. (2014). Modification to AOAC official methods 2009.01 and 2011.25 to allow for minor overestimation of low molecular weight soluble dietary fiber in samples containing starch. Journal of AOAC International, 97(3), pp. 896-901  

___________., Sloane, N., Draga, A. Lazewska, I. (2013) Measurement of Total Dietary Fiber Using AOAC Method 2009.01 (AACC International Approved Method 32-45.01): Evaluation and Updates. Cereal Chem. 90(4) July/August pp. 396-414 (Article)

___________., Sloane, N., & Draga, A. (2015). Determination of total dietary fibre and available carbohydrates: A rapid integrated procedure that simulates in vivo digestion. Starch‐Stärke, 67(9-10), pp. 860-883

Prosky, L., Asp, N. G., Furda, I., DeVries, J. W., Schweizer, T. F. & Harland, B. F. (1985). Determination of total dietary fibre in foods and food products: Collaborative study. J. Assoc. Off. Anal. Chem., 68, 677.

Prosky, L., Asp, N. G., Schweizer, T. F., DeVries, J. W. & Furda, I. (1988). Determination of insoluble, soluble, and total dietary fibre in foods and food products. J. Assoc. Off. Anal. Chem., 71, 1017.