prEN ISO 16634-2

SLOVENSKI STANDARD
01-december-2025
Živila - Določevanje celotnega dušika s sežigom po Dumasu in z izračunom deleža
surovih beljakovin - 2. del: Žito, stročnice in žitni proizvodi (ISO/DIS 16634-2:2025)
Food products - Determination of the total nitrogen content by combustion according to
the Dumas principle and calculation of the crude protein content -Part 2: Cereals, pulses
and cereal coproducts (ISO/DIS 16634-2:2025)
Lebensmittelerzeugnisse - Bestimmung des Gehaltes an Gesamtstickstoff mit dem
Verbrennungsverfahren nach Dumas und Berechnung des Gehaltes an Rohprotein - Teil
2: Getreide, Hülsenfrüchte und Getreideerzeugnisse (ISO/DIS 16634-2:2025)
Produits alimentaires - Détermination de la teneur en azote total par combustion selon le
principe Dumas et calcul de la teneur en protéines brutes - Partie 2: Céréales,
légumineuses et coproduits céréaliers (ISO/DIS 16634-2:2025)
Ta slovenski standard je istoveten z: prEN ISO 16634-2
ICS:
67.050 Splošne preskusne in General methods of tests and
analizne metode za živilske analysis for food products
proizvode
67.060 Žita, stročnice in proizvodi iz Cereals, pulses and derived
njih products
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

DRAFT
International
Standard
ISO/DIS 16634-2
ISO/TC 34/SC 4
Food products — Determination
Secretariat: SAC
of the total nitrogen content by
Voting begins on:
combustion according to the Dumas
2025-10-09
principle and calculation of the
Voting terminates on:
crude protein content —
2026-01-01
Part 2:
Cereals, pulses and cereal
coproducts
ICS: 67.050; 67.060
THIS DOCUMENT IS A DRAFT CIRCULATED
FOR COMMENTS AND APPROVAL. IT
IS THEREFORE SUBJECT TO CHANGE
AND MAY NOT BE REFERRED TO AS AN
INTERNATIONAL STANDARD UNTIL
PUBLISHED AS SUCH.
This document has not been edited by the ISO Central Secretariat.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL,
TECHNOLOGICAL, COMMERCIAL AND
USER PURPOSES, DRAFT INTERNATIONAL
STANDARDS MAY ON OCCASION HAVE TO
ISO/CEN PARALLEL PROCESSING
BE CONSIDERED IN THE LIGHT OF THEIR
POTENTIAL TO BECOME STANDARDS TO
WHICH REFERENCE MAY BE MADE IN
NATIONAL REGULATIONS.
RECIPIENTS OF THIS DRAFT ARE INVITED
TO SUBMIT, WITH THEIR COMMENTS,
NOTIFICATION OF ANY RELEVANT PATENT
RIGHTS OF WHICH THEY ARE AWARE AND TO
PROVIDE SUPPORTING DOCUMENTATION.
Reference number
ISO/DIS 16634-2:2025(en)
DRAFT
ISO/DIS 16634-2:2025(en)
International
Standard
ISO/DIS 16634-2
ISO/TC 34/SC 4
Food products — Determination
Secretariat: SAC
of the total nitrogen content by
Voting begins on:
combustion according to the Dumas
principle and calculation of the
Voting terminates on:
crude protein content —
Part 2:
Cereals, pulses and cereal
coproducts
ICS: 67.050; 67.060
THIS DOCUMENT IS A DRAFT CIRCULATED
FOR COMMENTS AND APPROVAL. IT
IS THEREFORE SUBJECT TO CHANGE
AND MAY NOT BE REFERRED TO AS AN
INTERNATIONAL STANDARD UNTIL
PUBLISHED AS SUCH.
This document has not been edited by the ISO Central Secretariat.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL,
© ISO 2025
TECHNOLOGICAL, COMMERCIAL AND
USER PURPOSES, DRAFT INTERNATIONAL
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
STANDARDS MAY ON OCCASION HAVE TO
ISO/CEN PARALLEL PROCESSING
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
BE CONSIDERED IN THE LIGHT OF THEIR
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
POTENTIAL TO BECOME STANDARDS TO
WHICH REFERENCE MAY BE MADE IN
or ISO’s member body in the country of the requester.
NATIONAL REGULATIONS.
ISO copyright office
RECIPIENTS OF THIS DRAFT ARE INVITED
CP 401 • Ch. de Blandonnet 8
TO SUBMIT, WITH THEIR COMMENTS,
CH-1214 Vernier, Geneva
NOTIFICATION OF ANY RELEVANT PATENT
Phone: +41 22 749 01 11
RIGHTS OF WHICH THEY ARE AWARE AND TO
PROVIDE SUPPORTING DOCUMENTATION.
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland Reference number
ISO/DIS 16634-2:2025(en)
ii
ISO/DIS 16634-2:2025(en)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 2
5 Reagents . 2
6 Apparatus . 3
7 Sampling . 3
8 Preparation of the test sample. 4
9 Procedure . 4
9.1 General .4
9.2 Test portion .4
9.3 Control of oxygen supply .4
9.4 Calibration .5
9.5 Determination .5
9.6 Detection and data processing .5
10 Calculation and expression of results . 6
10.1 Calculation .6
10.1.1 Nitrogen content .6
10.1.2 Crude protein content .6
10.2 Expression of results . .6
11 Precision . 6
11.1 Interlaboratory tests .6
11.2 Repeatability .7
11.3 Reproducibility .7
11.4 Critical difference .7
11.4.1 Comparison of two groups of measurements in the same laboratory .7
11.4.2 Comparison of two groups of measurements in two different laboratories .7
11.5 Uncertainty .8
12 Test report . 8
Annex A (informative) Flowchart for a basic Dumas apparatus . 9
Annex B (informative) Equipment calibration . 10
Annex C (informative) Results of interlaboratory tests .12
Bibliography . 19

iii
ISO/DIS 16634-2:2025(en)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee
has been established has the right to be represented on that committee. International organizations,
governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely
with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the different types
of ISO documents should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of any patent
rights identified during the development of the document will be in the Introduction and/or on the ISO list of
patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the meaning of ISO specific terms and expressions related to conformity assessment,
as well as information about ISO's adherence to the WTO principles in the Technical Barriers to Trade (TBT)
see the following URL: Foreword - Supplementary information
The committee responsible for this document is ISO/TC 34, Food products, Subcommittee SC 4, Cereals and pulses.
This edition cancels and replaces ISO/TS 16634-2:2016, which has been technically revised.
ISO 16634 consists of the following parts, under the general title, Food products — Determination of the total
nitrogen content by combustion according to the Dumas principle and calculation of the crude protein content:
— Part 1: Oilseeds and animal feeding stuffs
— Part 2: Cereals, pulses and cereal coproducts
Main modifications of this version are the following:
— Clarification of the scope, in particular for cereal coproducts;
— Introduction of Argon as a carrier gas;
— Introduction of near infrared spectroscopy to determine moisture content;
— Clarification of the grinding device’s check;
— Clarification of the nitrogen-free compound used for blank during the control of oxygen supply;
— Clarification of the use of the correction factor F ;
c
— Expression of results;
— Removal of Annex B presenting schematic diagrams of some apparatus.

iv
ISO/DIS 16634-2:2025(en)
Introduction
For a long time, the Kjeldahl method has been the most frequently used method for the determination of
the protein content of food products. In recent years, the Dumas method has gained importance compared
to the Kjeldahl method because it is faster and environment-friendly. Although the principles of the two
methods are different, both measure the nitrogen content of the product. Nitrogen content can be converted
into protein content by using an appropriate factor. The value of this factor varies depending on the relative
amounts of different proteins and their amino-acid composition in a given product.
Neither the Dumas nor the Kjeldahl method distinguishes between protein and non-protein nitrogen. In
most cases, results obtained by the Dumas method are slightly higher than those of the Kjeldahl method.
This is because the Dumas method measures almost all of the non-protein nitrogen, whereas the Kjeldahl
method measures only a part of it.
Taking into consideration that the protein content of a product calculated by both methods only approximates
to the true value, it is a matter of discretion which one is accepted. The best solution is to use a second
factor for the elimination of the systematic error caused by the non-protein nitrogen content of the different
products.
However, this second factor has to be determined for each product like the existing factors which indicate
the ratio of the protein content to the nitrogen content.

v
DRAFT International Standard ISO/DIS 16634-2:2025(en)
Food products — Determination of the total nitrogen
content by combustion according to the Dumas principle and
calculation of the crude protein content —
Part 2:
Cereals, pulses and cereal coproducts
1 Scope
This part of ISO 16634 specifies a method for the determination of the total nitrogen content and the
calculation of the crude protein content of cereals, pulses and cereal coproducts (e.g. wheat flour, wheat
starch, peas proteins, wheat gluten, semolina).
This method, like the Kjeldahl method (see References [1] and [6]), does not distinguish between protein
nitrogen and non-protein nitrogen. For the calculation of the protein content, various conversion factors are
used (see 3.2).
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO 712-1, Cereals and cereal products — Determination of moisture content — Part 1: Reference method
ISO 6540, Maize — Determination of moisture content (on milled grains and on whole grains)
ISO 24557, Pulses — Determination of moisture content — Air-oven method
ISO 12099, Animal feeding stuffs, cereals and milled cereal products — Guidelines for the application of near
infrared spectrometry
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
nitrogen content
mass fraction of the total nitrogen expressed as a percentage
Note 1 to entry: Determined by the procedure specified in this part of ISO 16634.
3.2
crude protein content
nitrogen content (3.1) multiplied by a factor
Note 1 to entry: A factor of 5,7 is used for wheat, rye, and their coproducts (e.g., flour, semolina). A factor of 6,25 is
applied to all other cereals, pulses, and cereal coproducts within the scope of this document.

ISO/DIS 16634-2:2025(en)
4 Principle
Samples are converted into gases by heating in a combustion tube. Interfering components are removed
from the resulting gas mixture. The nitrogen compounds in the gas mixture, or a representative part of
them, are converted to molecular nitrogen which is quantitatively determined by a thermal-conductivity
detector. The nitrogen content is calculated by a microprocessor.
5 Reagents
Use only reagents of recognized analytical grade or reagents of equivalent purity as specified by instrument
manufacturers. Except for the reference materials (5.12), all reagents shall be free from nitrogen.
5.1 Carrier gas(es), use either 5.1.1, 5.1.2 or 5.1.3.
5.1.1 Carbon dioxide, as pure as possible, but with a minimum CO volume fraction of 99,99 %.
5.1.2 Helium, as pure as possible, but with a minimum He volume fraction of 99,99 %.
5.1.3 Argon, as pure as possible and of minimum volume fraction ≥ 99,99 %.
The collaborative data were obtained in the 90’s with helium and carbon dioxide as carrier gazes. Some
apparatus (6.5) can be used with argon as carrier gas. For this type of argon apparatus, the results obtained
with argon shall be verified by the laboratory.
5.2 Oxygen, as pure as possible, but with a minimum O volume fraction of 99,99 %.
5.3 Sulfur dioxide and halogen absorbent, to eliminate any sulfur from the sample [e.g. lead chromate
(PbCrO ) or steel wool].
5.4 Copper oxide/platinum catalyst, for the post-combustion tube.
Platinum catalyst [5 % of Pt on alumina (Al O )] is blended with CuO in the ratio 1 part:7 parts or 1 part:
2 3
8 parts in accordance with the manufacturer's recommendations.
To prevent separation as a result of the different bulk densities of the two materials, it is recommended
not to prepare the mixture before filling the tube, but to pour the platinum catalyst and copper oxide
simultaneously into the post-combustion tube using a suitable funnel.
5.5 Silver or copper wool.
This shall be disaggregated before being inserted into the post-combustion or reduction tube.
5.6 Silica (quartz) or glass wool or cotton wool, as recommended by the instrument manufacturer.
5.7 Copper ortungsten (wire, cuttings, turnings or powder), for the reduction tube.
The use of copper or tungsten in one of these forms can improve the precision of analytical results for
samples with low nitrogen contents (about 1 % (m/m)).
5.8 Diphosphorus pentoxide (P O ) or granulated magnesium perchlorate [Mg(ClO ) ], or another
2 5 4 2
suitable drying agent, to fill the drying tubes.
5.9 Hollow corundum spheresor aluminium oxide pellets, for the combustion tube if recommended by
the instrument manufacturer.
ISO/DIS 16634-2:2025(en)
5.10 Copper oxide (CuO), as filling material for the combustion tube if recommended by the instrument
manufacturer.
5.11 Sodium hydroxide (NaOH), on a support material.
5.12 Aspartic acid (C H NO ) or ethylenediaminetetraacetic acid (C H N O ) or glutamic acid
4 7 4 10 16 2 8
(C H NO ) or hippuric acid (C H NO )standard, or other suitable reference materials with a known,
5 9 4 9 9 3
constant, certified nitrogen content.
The minimum recovery should preferably be 99 % (m/m).
5.13 Light petroleum, with a boiling range between 30 °C and 60 °C, or acetone or ethanol to clean
quartz tubes.
5.14 Nitrogen-free compound (ex. Sucrose)
6 Apparatus
Usual laboratory equipment and, in particular, the following.
6.1 Analytical balance, capable of weighing to the nearest 0,000 1 g.
6.2 Grinding device, appropriate to the nature of the sample to ensure homogeneity of ground sample
6.3 Sieves for grinding device control, of nominal opening adapted to grating of the used mill made of
non-ferrous material
6.4 Crucibles (e.g. made of stainless steel, quartz, ceramic material or platinum) or tin capsules or
tin foils or nitrogen-free filter paper, suitable for the Dumas apparatus used.
NOTE 1 Several instruments provided with an automatic sampler are commercially available.
NOTE 2 Some solid samples (e.g. powders) can be pressed to form pellets.
6.5 Dumas apparatus, fitted with a furnace able to maintain a given temperature greater than or equal
to 850 °C, with a thermal-conductivity detector and suitable device for signal integration.
Suitable Dumas apparatus operates according to the general flowchart given in Annex A, although different
arrangements and components may be used.
To avoid leaks, the sealing O-rings shall be slightly lubricated with high-vacuum grease prior to installation.
Experience has shown that it is important to clean all pieces of silicaware and glassware carefully and to
remove fingerprints from tubes, using a suitable solvent (5.13), before inserting them into the furnace.
7 Sampling
A representative sample should have been sent to the laboratory. This sample should not have been damaged
or changed during transport or storage.
Sampling is not part of the method specified in this part of ISO 16634. Recommended sampling methods are
given in ISO 24333 for cereals and cereal products or ISO 951 for pulses in bags

ISO/DIS 16634-2:2025(en)
8 Preparation of the test sample
The test sample shall be prepared from the laboratory sample in such a way that a homogeneous test sample
is obtained.
Using a suitable grinding device (6.2), grind the laboratory sample.
To check grinding device, particle size and homogeneity shall be controlled:
Pass the ground material through a sieve (6.3) of nominal opening adapted to the grating of the used mill.
Mills that produce particle sizes meeting the specifications given in Table 1 will give acceptable results.
Table 1 — Accepted particle size
Grinding device with a less than 1 mm sieve Grinding device with a 1 mm sieve
Nominal size of sieve Amount passing through Nominal size of sieve Amount passing through
openings sieve openings sieve
µm % (m/m) µm % (m/m)
710 100 > 1 000 100
500 95 to 100 1 000 95 to 100
200 85 or less 500 85 or less
Grinding may result in moisture loss and, therefore, the moisture content of the ground sample should
preferably also be determined when reporting nitrogen or protein contents on a dry-matter or constant-
moisture basis. Determination of the moisture content shall be carried out in accordance with ISO 712-1,
ISO 6540, ISO 24557 or ISO 12099 (on condition that the laboratory has validated the NIRS method according
to reference method and holds precision).
The homogenisation grinder efficiency shall also be verified. Check can be done by replicate preparation of
ground samples of a 2 + 1 mixture of maize and soya seeds and N determination. The expected coefficient of
variation should be less than 2 % (m/m).
9 Procedure
9.1 General
Carefully, follow the manufacturer's instructions for instrument set-up, optimization, calibration and
operation. Switch the instrument on and allow it to stabilize as defined in local procedures.
An instrument performance test should be carried out daily, using the reference material (5.12). The
recovery of nitrogen should be >99,0 % (m/m).
9.2 Test portion
Weigh, to the nearest 0,000 1 g, at least 0,1 g of the test sample into a crucible or tin capsule or nitrogen-free
filter paper (6.4). For samples low in protein (<1 % (m/m)), the amount of the test portion can be increased
up to 3,5 g, depending on the type of Dumas equipment being used, the nature of the sample, the volume (ex.
wheat bran) and the fouling capacity of the Crucibles or tin capsules or tin foils (6.4).
Lower test portions may be necessary for very high protein content samples or when only very small
amounts of sample are available. In the case of portions below 0,1 g, a validation shall be performed.
Depending on the type of equipment used, if the sample contains over 17 % (m/m)of moisture, drying may
be necessary before analysis.
9.3 Control of oxygen supply
Control the oxygen supply, in particular the flow, in accordance with the instructions of the material supplier.

-------------------
...