ISO 937:2023

INTERNATIONAL ISO
STANDARD 937
Second edition
2023-08
Meat and meat products —
Determination of nitrogen content —
Reference method
Viandes et produits à base de viande — Détermination de la teneur en
azote — Méthode de référence
Reference number
ISO 937:2023(E)
© ISO 2023

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ISO 937:2023(E)
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ISO 937:2023(E)
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 1
5 Sampling . 1
6 Preparation of test sample . 2
7 Reagents and materials . 2
8 Apparatus . 3
9 Procedure .4
9.1 Test portion . 4
9.2 Determination . 4
9.2.1 Traditional Kjeldahl apparatus . 4
9.2.2 Automatic Kjeldahl meter. 5
9.3 Blank test . 5
9.4 Recovery test . 5
10 Expression of results . 6
11 Precision . 6
12 Notes on procedure . 7
13 Test report . 7
Bibliography . 8
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ISO 937:2023(E)
Foreword
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described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
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www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 34, Food products, Subcommittee SC 6,
Meat, poultry, fish, eggs and their products.
This second edition cancels and replaces the first edition (ISO 937:1978), which has been technically
revised.
The main changes are as follows:
— the automatic Kjeldahl method has been added;
— the order of the clauses of the document has been rearranged.
Any feedback or questions on this document should be directed to the user's national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
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INTERNATIONAL STANDARD ISO 937:2023(E)
Meat and meat products — Determination of nitrogen
content — Reference method
1 Scope
This document specifies a reference method for the determination of the nitrogen content of meat and
meat products by the Kjeldahl principle.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 385, Laboratory glassware — Burettes
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
nitrogen content
quantity of nitrogen corresponding to the ammonia produced
Note 1 to entry: As determined using the conditions specified in this document.
4 Principle
Digestion of a test portion with concentrated sulfuric acid, using copper(II) sulfate as a catalyst, to
convert organic nitrogen to ammonium ions; alkalization, distillation of the liberated ammonia into an
excess of boric acid solution, titration with hydrochloric acid or sulfuric acid to determine the ammonia
bound by the boric acid, and calculation of the nitrogen content of the sample from the amount of
ammonia produced.
5 Sampling
Sampling is not part of the method specified in this document. A recommended sampling method is
given in CAC/GL 50-2004.
It is important that the laboratory receive a sample which is truly representative and has not been
damaged or changed during transport or storage.
Start from a representative sample of at least 200 g.
Store the sample in such a way that deterioration and change in composition are prevented.
Preservatives, if any, should not contain nitrogen compounds in measurable amounts.
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ISO 937:2023(E)
6 Preparation of test sample
Homogenize the laboratory sample with the appropriate equipment (8.1). Take care that the
temperature of the sample material does not rise above 25 °C. If a mincer is used, pass the sample at
least twice through the equipment.
Fill a suitable airtight container with the prepared test sample, close the container and store in such
a way that deterioration and change in composition are prevented. Analyse the test sample as soon as
practicable, but always within 24 h after homogenization.
7 Reagents and materials
All reagents shall be of recognized analytical quality. The water used shall be distilled water or water of
at least equivalent purity.
7.1 Copper(II) sulfate pentahydrate (CuSO ·5H O).
4 2
7.2 Potassium sulfate (K SO ), anhydrous.
2 4
7.3 Sulfuric acid (H SO ), ρ 1,84 g/ml.
2 4 20
7.4 Ethanol (C H OH), 95 % (volume fraction).
2 5
7.5 Ammonium sulfate [(NH ) SO ], minimum purity 99,9 % (mass fraction) on dried material.
4 2 4
Immediately before use, dry the ammonium sulfate at 104 °C ± 4 °C for at least 2 h.
Allow it to cool at ambient temperature in a desiccator.
7.6 Tryptophan (C H N O ) or lysine hydrochloride (C H ClN O ) or acetanilide (C H NO),
11 12 2 2 6 15 2 2 8 9
minimum purity 99,9 % (mass fraction).
These reagents should be kept away from humidity.
WARNING — Do not dry these reagents in an oven before use.
7.7 Sucrose, with nitrogen content less than a mass fraction of 0,002 %.
WARNING — Do not dry sucrose in an oven before use.
7.8 Sodium hydroxide (NaOH) solution, carbonate-free, containing approximately 33 g of sodium
hydroxide per 100 g of solution.
Dissolve 500 g of sodium hydroxide in 1 000 ml of water.
7.9 Boric acid solution, c(H BO ) = 40,0 g/l.
3 3
Dissolve 40 g of boric acid in water and dilute to 1 000 ml.
7.10 Standard hydrochloric acid, c(HCl) = (0,1 ± 0,000 5) mol/l or sulfuric acid,
c(H SO ) = (0,05 ± 0,000 3) mol/l, the normality being known to four decimal places.
2 4
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ISO 937:2023(E)
7.11 Indicator solution, either 7.11.1 or 7.11.2.
7.11.1 Indicator solution A: mixed indicator (methyl red-methylene blue), prepared by dissolving
2 g of methyl red and 1 g of methylene blue in 1 000 ml of ethanol (7.4).
The colour change of this indicator solution occurs at a pH of 5,4.
7.11.2 Indicator solution B: mixed indicator (methyl red-bromocresol green), prepared by
dissolving 1 g of methyl red and 5 g of bromocresol green in 1 000 ml of ethanol (7.4).
The colour change of this indicator solution occurs at a pH of 5,2.
Store the indicator solution in a brown bottle in a dark and cool place.
7.12 Boiling regulators, either 7.12.1 or 7.12.2.
7.12.1 For the digestion: glass beads, silicon carbide or splinters of hard porcelain.
7.12.2 For the distillation: silicon carbide or freshly ignited pieces of pumice stone.
8 Apparatus
The usual laboratory apparatus and, in particular, the following shall be used.
8.1 Mechanical or electrical equipment capable of homogenizing the laboratory sample,
including a high-speed ro
...