🍎 Dietary fiber — enzymatic-gravimetric

Food Chemistry AOAC 991.43

Overview

STATUS (as of 03.11.2026). AOAC Official Method 991.43 “Total, Soluble, and Insoluble Dietary Fiber in Foods” (AOAC International; First Action in 1991); the corresponding method is AACC 32-07.01 (final approval 16.10.1991). The method is not a PN standard — the text of AOAC methods is available in the (paid) Official Methods of Analysis collection, which we did not have; the description is based on the procedure sheet of a kit for performing the method, manufacturer materials and regulations.

PRINCIPLE (kit procedure sheet). A dried and defatted sample (1 g) is digested with heat-stable α-amylase (30 min, 98–100 °C, pH 8.2), after cooling to 60 °C with protease (60 °C, pH 8.2, 30 min) and, after changing the pH to 4.5, with amyloglucosidase (60 °C, 30 min). The water-insoluble fraction is filtered off (insoluble dietary fibre, IDF); from the filtrate the soluble fraction is precipitated with ethanol and filtered off (soluble dietary fibre precipitating in 78 % ethanol, SDFP). Protein and ash are determined in both residues. Total dietary fibre in this method is high-molecular-weight dietary fibre (HMWDF), measured as the sum of IDF and SDFP — this distinguishes it from method AOAC 985.29, which measures it as a whole; because of the buffer used, the method requires only one pH change.

CORRECTION OF THE RESIDUES. In the AOAC methods of this family (description for AOAC 985.29 in training materials of the Centre for Food Safety of Hong Kong), the sample is analysed in two parallel test portions: one residue is used for the determination of protein, the other for the determination of ash, and fibre is calculated as the residue mass minus protein, ash and blank, relative to the mass of the test portion.

LIMITATIONS (kit procedure sheet). Methods AOAC 985.29 and 991.43 do not measure all dietary fibre components as defined by the Codex Alimentarius: they fail to measure most resistant starches accurately and exclude non-digestible oligosaccharides, which leads to an underestimation; for a complete measurement, the kit manufacturer points to the newer methods AOAC 2017.16 and 2022.01.

REGULATIONS. Regulation (EU) No 1169/2011 defines fibre as carbohydrate polymers with three or more monomeric units which are neither digested nor absorbed in the human small intestine and belong to the categories listed in Annex I point 12; fibre is not part of the mandatory nutrition declaration — the declaration may be supplemented with it (Article 30(1) and (2)(e)). The regulation does not specify a method for determining fibre.

WHAT WE DO NOT GIVE HERE. The composition and preparation of the buffer, amounts and activities of the enzymes, volumes of solutions and ethanol, the washing and drying of the residues, the conditions of the protein and ash determinations, the formula with the blank correction and the precision data — according to the text of the AOAC method, which we did not have.

Method principle

A dried and defatted sample is digested successively with heat-stable α-amylase at boiling temperature and with protease and amyloglucosidase at 60 °C, removing starch and protein. The insoluble fraction is filtered off as insoluble fibre, and soluble fibre is precipitated from the filtrate with ethanol and filtered off. The residues are weighed, and the result is corrected for protein and ash determined in the residues and for the blank; total fibre is the sum of the insoluble and soluble fractions.

Applications

• Total, insoluble (IDF) and soluble (SDFP) dietary fibre in foods
• Nutrition declaration — fibre as a supplementary component (Regulation (EU) No 1169/2011, Article 30(2)(e))

Key parameters

Standard

Standard number

AOAC Official Method 991.43

Title (PL)

Oznaczanie całkowitego, rozpuszczalnego i nierozpuszczalnego błonnika pokarmowego w żywności — Metoda enzymatyczno-grawimetryczna

Title (EN)

Total, Soluble and Insoluble Dietary Fiber in Foods — Enzymatic-Gravimetric Method

Step-by-step procedure

1. Sample preparation

   Use a dried and defatted sample; 1 g is taken for the test, in two parallel test portions (kit procedure sheet; CFS materials for AOAC 985.29).

2. α-Amylase digestion

   Incubate with heat-stable α-amylase for 30 min at 98–100 °C and pH 8.2.

3. Protease digestion

   Cool to 60 °C, add protease and incubate for 30 min at 60 °C and pH 8.2.

4. Amyloglucosidase digestion

   Change the pH to 4.5, add amyloglucosidase and incubate for 30 min at 60 °C.

5. Insoluble fraction

   Filter off the water-insoluble fraction — the residue is insoluble dietary fibre (IDF).

6. Soluble fraction

   Precipitate the soluble fraction from the filtrate with ethanol and filter it off — the residue is soluble fibre precipitating in 78 % ethanol (SDFP).

7. Correction and result

   Determine protein and ash in the residues; subtract protein, ash and blank from the residue mass and relate it to the mass of the test portion; total fibre = IDF + SDFP.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• The available sources contain no safety requirements — health and safety rules for digestion at 98–100 °C and work with ethanol are set by the laboratory

Frequently asked questions