Milk - Quantitative determination of microbiological quality - Guidance for establishing and verifying a conversion relationship between results of an alternative method and anchor method results (ISO 21187:2021)

This document gives guidelines for the establishment of a conversion relationship between the results
of an alternative method and an anchor method, and its verification for the quantitative determination
of the microbiological quality of milk.
NOTE The conversion relationship can be used a) to convert results from an alternative method to the anchor
basis or b) to convert results/limits, expressed on an anchor basis, to results in units of an alternative method.

Milch - Quantitative Bestimmung der bakteriologischen Qualität - Leitfaden für die Erarbeitung einer Übertragungsbeziehung zwischen den Messwerten von Routine- und Bezugsverfahren sowie deren Verifizierung (ISO 21187:2021)

Dieses Dokument gibt Anweisungen für die Erarbeitung einer Übertragungsbeziehung zwischen den Ergebnissen eines alternativen und eines Bezugsverfahrens sowie für deren Verifizierung für die quantitative Bestimmung der mikrobiologischen Qualität von Milch.
ANMERKUNG   Die Übertragungsbeziehung kann a) zur Übertragung von Ergebnissen von einem alternativen Verfahren auf die Bezugsbasis oder b) zur Übertragung von Ergebnissen/Grenzwerten, ausgedrückt auf einer Bezugsbasis, in Ergebnisse in Einheiten eines alternativen Verfahrens verwendet werden.

Lait - Mesure quantitative de la qualité bactériologique - Lignes directrices pour établir et vérifier une relation de conversion entre les résultats de la méthode alternatif et les résultats de la méthode d'ancrage (ISO 21187:2021)

Le présent document donne des lignes directrices pour l'établissement d'une relation de conversion entre les résultats d'une méthode alternative et les résultats d'une méthode d'ancrage, et sa vérification en vue de la détermination quantitative de la qualité microbiologique du lait.
NOTE       La relation de conversion peut être utilisée a) pour convertir les résultats d'une méthode alternative sur la base de la méthode d'ancrage ou b) pour convertir les résultats/limites exprimés sur la base de la méthode d'ancrage, en résultats exprimés dans les unités d'une méthode alternative.

Mleko - Kvantitativno določanje bakteriološke kakovosti - Navodilo za ugotavljanje in preverjanje konverzijske povezave med rezultati alternativne metode in rezultati uveljavljene metode (ISO 21187:2021)

General Information

Status
Published
Public Enquiry End Date
04-Jul-2019
Publication Date
24-May-2021
Current Stage
6060 - National Implementation/Publication (Adopted Project)
Start Date
22-Mar-2021
Due Date
27-May-2021
Completion Date
25-May-2021

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SLOVENSKI STANDARD
SIST EN ISO 21187:2021
01-julij-2021
Nadomešča:
SIST EN ISO 21187:2006
Mleko - Kvantitativno določanje bakteriološke kakovosti - Navodilo za ugotavljanje
in preverjanje konverzijske povezave med rezultati alternativne metode in rezultati
uveljavljene metode (ISO 21187:2021)
Milk - Quantitative determination of microbiological quality - Guidance for establishing
and verifying a conversion relationship between results of an alternative method and
anchor method results (ISO 21187:2021)
Milch - Quantitative Bestimmung der bakteriologischen Qualität - Leitfaden für die
Erarbeitung einer Übertragungsbeziehung zwischen den Messwerten von Routine- und
Bezugsverfahren sowie deren Verifizierung (ISO 21187:2021)
Lait - Mesure quantitative de la qualité bactériologique - Lignes directrices pour établir et
vérifier une relation de conversion entre les résultats de la méthode alternatif et les
résultats de la méthode d'ancrage (ISO 21187:2021)
Ta slovenski standard je istoveten z: EN ISO 21187:2021
ICS:
07.100.30 Mikrobiologija živil Food microbiology
67.100.10 Mleko in predelani mlečni Milk and processed milk
proizvodi products
SIST EN ISO 21187:2021 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST EN ISO 21187:2021

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SIST EN ISO 21187:2021


EN ISO 21187
EUROPEAN STANDARD

NORME EUROPÉENNE

March 2021
EUROPÄISCHE NORM
ICS 07.100.30; 67.100.01 Supersedes EN ISO 21187:2005
English Version

Milk - Quantitative determination of microbiological
quality - Guidance for establishing and verifying a
conversion relationship between results of an alternative
method and anchor method results (ISO 21187:2021)
Lait - Mesure quantitative de la qualité Milch - Quantitative Bestimmung der bakteriologischen
microbiologique - Lignes directrices pour établir et Qualität - Leitfaden für die Erarbeitung einer
vérifier une relation de conversion entre les résultats Übertragungsbeziehung zwischen den Messwerten von
de la méthode alternatif et les résultats de la méthode Routine- und Bezugsverfahren sowie deren
d'ancrage (ISO 21187:2021) Verifizierung (ISO 21187:2021)
This European Standard was approved by CEN on 4 December 2020.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this
European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.

This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and
United Kingdom.





EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2021 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 21187:2021 E
worldwide for CEN national Members.

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SIST EN ISO 21187:2021
EN ISO 21187:2021 (E)
Contents Page
European foreword . 3

2

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SIST EN ISO 21187:2021
EN ISO 21187:2021 (E)
European foreword
This document (EN ISO 21187:2021) has been prepared by Technical Committee ISO/TC 34 "Food
products" in collaboration with Technical Committee CEN/TC 302 “Milk and milk products - Methods of
sampling and analysis” the secretariat of which is held by NEN.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by September 2021, and conflicting national standards
shall be withdrawn at the latest by September 2021.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
This document supersedes EN ISO 21187:2005.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland,
Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of
North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the
United Kingdom.
Endorsement notice
The text of ISO 21187:2021 has been approved by CEN as EN ISO 21187:2021 without any modification.


3

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SIST EN ISO 21187:2021

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SIST EN ISO 21187:2021
INTERNATIONAL ISO
STANDARD 21187
IDF 196
Second edition
2021-02
Milk — Quantitative determination of
microbiological quality — Guidance
for establishing and verifying a
conversion relationship between
results of an alternative method and
anchor method results
Lait — Mesure quantitative de la qualité microbiologique — Lignes
directrices pour établir et vérifier une relation de conversion entre
les résultats de la méthode alternatif et les résultats de la méthode
d’ancrage
Reference numbers
ISO 21187:2021(E)
IDF 196:2021(E)
©
ISO and IDF 2021

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SIST EN ISO 21187:2021
ISO 21187:2021(E)
IDF 196:2021(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO and IDF 2021
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting
on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address
below or ISO’s member body in the country of the requester.
ISO copyright office International Dairy Federation
CP 401 • Ch. de Blandonnet 8 Silver Building • Bd Auguste Reyers 70/B
CH-1214 Vernier, Geneva B-1030 Brussels
Phone: +41 22 749 01 11 Phone: +32 2 325 67 40
Fax: +32 2 325 67 41
Email: copyright@iso.org Email: info@fil-idf.org
Website: www.iso.org Website: www.fil-idf.org
Published in Switzerland
ii © ISO and IDF 2021 – All rights reserved

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SIST EN ISO 21187:2021
ISO 21187:2021(E)
IDF 196:2021(E)
Contents Page
Foreword .v
Introduction .vii
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principles . 2
4.1 General . 2
4.2 Guidance for applied methods and laboratories . 2
4.3 Organizational set-up . 3
5 Consideration of factors influencing the conversion relationship . 3
5.1 General . 3
5.2 Environmental factors . 3
5.2.1 General. 3
5.2.2 Animal species . 4
5.2.3 Bulk milk storage conditions . 4
5.2.4 Seasonal variations . 4
5.2.5 Sampling and pre-treatment of the test samples . 4
5.2.6 Test sample preservation . 4
5.2.7 Milk production conditions . 4
5.3 Analytical factors . 5
5.3.1 Instrument make and model . 5
5.3.2 Chemicals . 5
5.3.3 High somatic cell counts . 5
6 Test samples . 5
6.1 Calculation of number of test samples . 5
6.2 Range of test samples . 6
6.3 Representativeness of samples . 6
6.4 Pre-treatment of test samples . 6
6.4.1 General. 6
6.4.2 Preparation of sub-samples . 6
6.4.3 Storage and transport of sub-samples . 7
7 Analysis . 7
8 Establishing a conversion relationship . 7
8.1 General . 7
8.2 Validity of results . 7
8.3 Conversion relationship . 8
8.4 Calculations . 8
8.4.1 General. 8
8.4.2 Removal of outliers . 8
8.4.3 Conversion relationship . 8
9 Verification of a conversion relationship . 9
9.1 Frequency of verification . 9
9.2 Calculation . 9
10 Test report . 9
Annex A (informative) Number of test samples for linear regression.11
Annex B (informative) Example identification of outliers and calculation of conversion
relationship.14
Annex C (informative) Example — Calculation of significance (verification of conversion
relationship) .15
© ISO and IDF 2021 – All rights reserved iii

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SIST EN ISO 21187:2021
ISO 21187:2021(E)
IDF 196:2021(E)
Bibliography .19
iv © ISO and IDF 2021 – All rights reserved

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SIST EN ISO 21187:2021
ISO 21187:2021(E)
IDF 196:2021(E)
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 of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www .iso .org/
iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 34, Food products, Subcommittee SC 5,
Milk and milk products, in collaboration with the European Committee for Standardization (CEN)
Technical Committee CEN/TC 302, Milk and milk products — Methods of sampling and analysis, in
accordance with the Agreement on technical cooperation between ISO and CEN (Vienna Agreement),
and the International Dairy Federation (IDF). It is being published jointly by ISO and IDF.
This second edition cancels and replaces the first edition (ISO 21187 | IDF 196:2004), which has been
technically revised. The main changes compared with the previous edition are as follows:
— the formula describing the conversion relationship has been based on grouped data rather than
data from individual samples;
— examples of how to perform outlier tests, and calculation and verification of conversion relationships
have been given in a spreadsheet.
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.
© ISO and IDF 2021 – All rights reserved v

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SIST EN ISO 21187:2021
ISO 21187:2021(E)
IDF 196:2021(E)
IDF (the International Dairy Federation) is a non-profit private sector organization representing the
interests of various stakeholders in dairying at the global level. IDF members are organized in National
Committees, which are national associations composed of representatives of dairy-related national
interest groups including dairy farmers, dairy processing industry, dairy suppliers, academics and
governments/food control authorities.
ISO and IDF collaborate closely on all matters of standardization relating to methods of analysis
and sampling for milk and milk products. Since 2001, ISO and IDF jointly publish their International
Standards using the logos and reference numbers of both organizations.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. IDF 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.
This document was prepared by the IDF Standing Committee on Statistics and Automation and ISO
Technical Committee ISO/TC 34, Food products, Subcommittee SC 5, Milk and milk products. It is being
published jointly by IDF and ISO.
The work was carried out by the IDF/ISO Action Team (S11) of the Standing Committee on Statistics
and Automation under the aegis of its project leaders, Ms B. Asmussen (DK), Ms V. Tzeneva (NL), Mr R.
Kissling (NZ) and Ms B. Müller (DE).
vi © ISO and IDF 2021 – All rights reserved

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SIST EN ISO 21187:2021
ISO 21187:2021(E)
IDF 196:2021(E)
Introduction
Conversion in quantitative microbiology means expressing the result of a quantitative determination
of the microbiological status of a test sample obtained with an alternative method in units of another
method, generally an anchor method. Through this, quantitative results obtained with alternative
methods can be compared to values or limits that are stated in anchor method units. For establishing
and applying a conversion relationship, a number of prerequisites should be met. These are referred to
in this document, but are generally described elsewhere.
Although a considerable part of the applied principles for conversion coincides with those applied for
the calibration of indirect or alternative methods against an anchor method, or by means of (certified)
reference materials, it is stressed that the background and aims for applying conversion are different
from those for calibration. Calibration involves the determination of the adjustment needed for each
level of an analyte to closely approximate the true value of its concentration or number. However, in
quantitative microbiology, a true value in its strict sense cannot be established and is only defined by the
method description applied. When applying alternative methods in the quantitative determination of
microbiological quality, one is often dealing with different methodological principles and therefore also
other units. Conversion is used to transfer results obtained with different methods to a common scale.
© ISO and IDF 2021 – All rights reserved vii

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SIST EN ISO 21187:2021

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SIST EN ISO 21187:2021
ISO 21187:2021(E)
INTERNATIONAL STANDARD
IDF 196:2021(E)
Milk — Quantitative determination of microbiological
quality — Guidance for establishing and verifying a
conversion relationship between results of an alternative
method and anchor method results
1 Scope
This document gives guidelines for the establishment of a conversion relationship between the results
of an alternative method and an anchor method, and its verification for the quantitative determination
of the microbiological quality of milk.
NOTE The conversion relationship can be used a) to convert results from an alternative method to the anchor
basis or b) to convert results/limits, expressed on an anchor basis, to results in units of an alternative method.
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 8196-1 | IDF 128-1, Milk — Definition and evaluation of the overall accuracy of alternative methods of
milk analysis — Part 1: Analytical attributes of alternative methods
ISO 8196-2 | IDF 128-2, Milk — Definition and evaluation of the overall accuracy of alternative methods of
milk analysis — Part 2: Calibration and quality control in the dairy laboratory
ISO 16140-1, Microbiology of the food chain — Method validation — Part 1: Vocabulary
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 8196-1 | IDF 128-1,
ISO 8196-2 | IDF 128-2, ISO 16140-1 and the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1
alternative method
method of analysis allowing quantification of the microbiological status of a test sample
Note 1 to entry: The method can be proprietary or non-commercial.
Note 2 to entry: The term “alternative” in this document refers to the entire method. It includes all aspects (such
as test sample pre-treatment, materials and instruments) required for the execution of the method.
© ISO and IDF 2021 – All rights reserved 1

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SIST EN ISO 21187:2021
ISO 21187:2021(E)
IDF 196:2021(E)
3.2
anchor method
method of analysis internationally recognized by experts or by agreement between parties, and used,
for instance, in legislation when expressing official limits for microbiological quality
Note 1 to entry: It is stressed that, in quantitative microbiology, any obtained value is only defined by the method
description applied. This applies to any alternative method as well as, for instance, to the standard plate count
for the enumeration of microorganisms.
3.3
analyte
component or property which is measured by the method of analysis
Note 1 to entry: The analyte can be the microorganism, stained particles (e.g. microscopic count), components
of microorganisms (e.g. lipopolysaccharides), the result of their ability to multiply (e.g. colony-forming units) or
their metabolic activity (e.g. change in conductivity/impedance).
3.4
organizing body
organization, possibly appointed by a competent authority, having the qualified staff and skills to
organize, coordinate and report on the outcome of the activities for the establishment and/or the
maintenance of a conversion relationship
3.5
measuring range
range wherein data with known precision and accuracy can be obtained
Note 1 to entry: Precision and accuracy data are determined in a validation study (e.g. by the instrument
manufacturer or a responsible organization).
3.6
range of interest
numerical values for alternative method results, typical of routine samples when analysing in a
laboratory
Note 1 to entry: If applicable, the range of interest shall include official limits and limits related to specific quality
schemes.
4 Principles
4.1 General
The establishment and verification of a conversion relationship is based on the examination of test
samples with an alternative method and the anchor method.
4.2 Guidance for applied methods and laboratories
For establishing and verifying a conversion relationship between the results of an alternative method
and the anchor method, the following prerequisites apply.
The alternative method should have been evaluated and validated in accordance with ISO 16140-2
and/or ISO 16297 | IDF 161. Procedures for sampling, test sample preservation, test sample transport,
test sample storage, sample pre-treatment, analysis and calculation of results should be documented,
[6]
strictly standardized and controlled in agreement with ISO/IEC 17025, the Eurachem Guide or
comparable standards.
Regular participation in proficiency tests and training according to the relevant standards, e.g.
ISO 4833-1, ISO 14461-2 | IDF 169-2, is strongly recommended.
2 © ISO and IDF 2021 – All rights reserved

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SIST EN ISO 21187:2021
ISO 21187:2021(E)
IDF 196:2021(E)
The anchor method should have been validated, documented, strictly standardized and controlled in
agreement with ISO/IEC 17025, the Eurachem Guide or comparable standards.
The protocol for the establishment of the conversion relationship and its verification should be
documented. It should follow the guidelines of this document.
4.3 Organizational set-up
There are various possible organizational set-ups, e.g. both the alternative and the anchor method are
fully carried out in the same laboratory, or several laboratories are involved in the trial.
Due to the instability and variability of the microbiological status of milk samples, the most robust
conversion relationships will be obtained w
...

SLOVENSKI STANDARD
oSIST prEN ISO 21187:2019
01-julij-2019
Mleko - Kvantitativno določanje bakteriološke kakovosti - Navodilo za ugotavljanje
in preverjanje konverzijske povezave med rezultati alternativne metode in rezultati
uveljavljene metode (ISO/DIS 21187:2019)
Milk - Quantitative determination of bacteriological quality - Guidance for establishing
and verifying a conversion relationship between results of an alternative method and
anchor method results (ISO/DIS 21187:2019)
Milch - Quantitative Bestimmung der bakteriologischen Qualität - Leitfaden für die
Erarbeitung einer Übertragungsbeziehung zwischen den Messwerten von Routine- und
Bezugsverfahren sowie deren Verifizierung (ISO/DIS 21187:2019)
Lait - Mesure quantitative de la qualité bactériologique - Lignes directrices pour établir et
vérifier une relation de conversion entre les résultats de la méthode alternatif et les
résultats de la méthode d'ancrage (ISO/DIS 21187:2019)
Ta slovenski standard je istoveten z: prEN ISO 21187
ICS:
07.100.30 Mikrobiologija živil Food microbiology
67.100.10 Mleko in predelani mlečni Milk and processed milk
proizvodi products
oSIST prEN ISO 21187:2019 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

---------------------- Page: 1 ----------------------
oSIST prEN ISO 21187:2019

---------------------- Page: 2 ----------------------
oSIST prEN ISO 21187:2019
DRAFT INTERNATIONAL STANDARD
ISO/DIS 21187
IDF 196
ISO/TC 34/SC 5 Secretariat: NEN
Voting begins on: Voting terminates on:
2019-04-02 2019-06-25
Milk — Quantitative determination of bacteriological
quality — Guidance for establishing and verifying a
conversion relationship between results of an alternative
method and anchor method results
Lait — Mesure quantitative de la qualité bactériologique — Lignes directrices pour établir et vérifier une
relation de conversion entre les résultats de la méthode alternatif et les résultats de la méthode d'ancrage
ICS: 67.100.01; 07.100.30
THIS DOCUMENT IS A DRAFT CIRCULATED
This document is circulated as received from the committee secretariat.
FOR COMMENT AND APPROVAL. IT IS
THEREFORE SUBJECT TO CHANGE AND MAY
NOT BE REFERRED TO AS AN INTERNATIONAL
STANDARD UNTIL PUBLISHED AS SUCH.
IN ADDITION TO THEIR EVALUATION AS
ISO/CEN PARALLEL PROCESSING
BEING ACCEPTABLE FOR INDUSTRIAL,
TECHNOLOGICAL, COMMERCIAL AND
USER PURPOSES, DRAFT INTERNATIONAL
STANDARDS MAY ON OCCASION HAVE TO
BE CONSIDERED IN THE LIGHT OF THEIR
POTENTIAL TO BECOME STANDARDS TO
WHICH REFERENCE MAY BE MADE IN
Reference numbers
NATIONAL REGULATIONS.
ISO/DIS 21187:2019(E)
RECIPIENTS OF THIS DRAFT ARE INVITED
IDF 196:2019(E)
TO SUBMIT, WITH THEIR COMMENTS,
NOTIFICATION OF ANY RELEVANT PATENT
RIGHTS OF WHICH THEY ARE AWARE AND TO
©
PROVIDE SUPPORTING DOCUMENTATION. ISO and IDF 2019

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oSIST prEN ISO 21187:2019
ISO/DIS 21187:2019(E)
IDF 196:2019(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO and IDF 2019
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting
on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address
below or ISO’s member body in the country of the requester.
ISO copyright office International Dairy Federation
CP 401 • Ch. de Blandonnet 8 Silver Building • Bd Auguste Reyers 70/B
CH-1214 Vernier, Geneva B-1030 Brussels
Phone: +41 22 749 01 11 Phone: +32 2 325 67 40
Fax: +41 22 749 09 47 Fax: +32 2 325 67 41
Email: copyright@iso.org Email: info@fil-idf.org
Website: www.iso.org Website: www.fil-idf.org
Published in Switzerland
ii © ISO and IDF 2019 – All rights reserved

---------------------- Page: 4 ----------------------
oSIST prEN ISO 21187:2019
ISO/DIS 21187:2019(E)
IDF 196:2019(E)
Contents Page
Foreword .iv
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principles . 2
4.1 General . 2
4.2 Requirements for applied methods and laboratories . 2
4.3 Organizational set-up . 3
5 Consideration of factors influencing the conversion relationship . 3
5.1 General . 3
5.2 Environmental factors . 3
5.2.1 Animal species . 4
5.2.2 High somatic cell counts . 4
5.2.3 Bulk milk storage conditions . 4
5.2.4 Sampling and pre-treatment of the samples . 4
5.2.5 Test sample preservation . 4
5.2.6 Milk production conditions . 4
5.2.7 Seasonal variations . 4
5.3 Analytical factors . 5
5.3.1 Instrument make and model . 5
5.3.2 Chemicals . 5
6 Test samples . 5
6.1 Calculation of number of test samples . 5
6.2 Range of samples. 6
6.3 Representativeness of samples . 6
6.4 Pretreatment of test samples . 6
6.4.1 General. 6
6.4.2 Preparation of sub-samples . 6
6.4.3 Storage and transport of sub-samples . 7
7 Analysis . 7
8 Establishing a conversion relationship . 7
8.1 Calculation . 7
8.1.1 General. 7
8.1.2 Validity of results . 7
8.1.3 Removal of outliers . 8
8.1.4 Conversion relationship . 8
8.1.5 Conversion equation . 8
9 Verification of a conversion relationship . 9
9.1 Frequency of verification . 9
9.2 Calculation . 9
10 Test report . 9
Annex A (informative) Number of samples for linear regression .10
Annex B (informative) Example – Identification of outliers and calculation of conversion
equation .13
Annex C (informative) Example – Calculation of significance (verification of conversion
relationship) .14
Bibliography .17
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oSIST prEN ISO 21187:2019
ISO/DIS 21187:2019(E)
IDF 196:2019(E)
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 of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www .iso
.org/iso/foreword .html.
This document was prepared by Technical Committee ISO/TC 34, Food products, Subcommittee SC 5,
Milk and milk products and the International Dairy Federation (IDF). It is being published jointly by ISO
and IDF.
This second edition cancels and replaces the first edition (ISO 21187 | IDF 196:2004), which has been
technically revised.
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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oSIST prEN ISO 21187:2019
ISO/DIS 21187:2019(E)
IDF 196:2019(E)
Foreword
IDF (the International Dairy Federation) is a non-profit organization representing the dairy sector
worldwide. IDF membership comprises National Committees in every member country as well as
regional dairy associations having signed a formal agreement on cooperation with IDF. All members of
IDF have the right to be represented at the IDF Standing Committees carrying out the technical work.
IDF collaborates with ISO in the development of standard methods of analysis and sampling for milk
and milk products.
The main task of Standing Committees is to prepare International Standards. Draft International
Standards adopted by the Standing Committees are circulated to the National Committees for
endorsement prior to publication as an International Standard. Publication as an International Standard
requires approval by at least 50 % of IDF National Committees casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. IDF shall not be held responsible for identifying any or all such patent rights.
ISO 21187|IDF 169 was prepared by the International Dairy Federation (IDF) and Technical Committee
ISO/TC 34, Food products, Subcommittee SC 5, Milk and milk products. It is being published jointly by IDF
and ISO.
All work was carried out by the Joint ISO-IDF Actio Team (S11) of the Standing Committee on Statistics
and automation under the aegis of its project leaders, Mrs. B. Asmussen (DK), Mr. R. Kissling (NZ) and
Mrs. B. Müller (DE).
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oSIST prEN ISO 21187:2019
ISO/DIS 21187:2019(E)
IDF 196:2019(E)
Introduction
Conversion in quantitative microbiology means expressing the result of a quantitative determination
of the bacteriological status of a test sample obtained with an alternative method in units of another
method, generally an anchor method. Through this, quantitative results obtained with alternative
methods can be compared to values or limits that are stated in anchor method units. For establishing
and applying a conversion relationship, a number of prerequisites should be met. These are referred to
in this International Standard, but are generally described elsewhere.
Although a considerable part of the applied principles for conversion coincides with those applied for
the calibration of indirect or alternative methods against an anchor method, or by means of (certified)
reference materials, it is stressed that the background and aims for applying conversion are different
from those for calibration. Calibration involves the determination of the adjustment needed for each
level of an analyte to closely approximate the true value of its concentration or number. However, in
quantitative microbiology, a true value in its strict sense cannot be established and is only defined by the
method description applied. When applying alternative methods in the quantitative determination of
bacteriological quality, one is often dealing with different methodological principles and therefore also
other units. Conversion is used to transfer results obtained with different methods to a common scale.
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oSIST prEN ISO 21187:2019
ISO/DIS 21187:2019(E)
DRAFT INTERNATIONAL STANDARD
IDF 196:2019(E)
Milk — Quantitative determination of bacteriological
quality — Guidance for establishing and verifying a
conversion relationship between results of an alternative
method and anchor method results
1 Scope
This document gives guidelines for the establishment of a conversion relationship between the results
of an alternative method and an anchor method, and its verification for the quantitative determination
of the microbiological quality of milk.
NOTE The conversion relationship can be used (1) to convert results from an alternative method to the anchor
basis or (2) to convert results/limits, expressed on a anchor basis, to results in units of an alternative method.
2 Normative references
The following documents are referred to in the text in such a way tht 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 8196-1, Milk — Definition and evaluation of the overall accuracy of alternative methods of milk
analysis — Part 1: Analytical attributes of alternative methods
ISO 8196-2, Milk — Definition and evaluation of the overall accuracy of alternative methods of milk
analysis — Part 2: Calibration and quality control in the dairy laboratory
ISO 11095, Linear calibration using reference materials
ISO 16140-1, Microbiology of the food chain — Method validation — Part 1: Vocabulary
ISO 16140-2, Microbiology of the food chain — Method validation — Part 2: Protocol for the validation of
alternative (proprietary) methods against a reference method
ISO 16297, │IDF 161, Milk — Bacterial count — Protocol for the evaluation of alternative methods
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 8196-1|IDF 128-1,
ISO 8196-2|IDF 128-2, ISO 16140-1 and the following apply.
3.1
alternative method
method of analysis allowing quantification of the microbiological status of a test sample
Note 1 to entry: The method can be proprietary or non-commercial.
Note 2 to entry: The term 'alternative' in this document refers to the entire method. It includes all aspects (such
as sample pretreatment, materials and instruments) required for the execution of the method.
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oSIST prEN ISO 21187:2019
ISO/DIS 21187:2019(E)
IDF 196:2019(E)
3.2
anchor method
method of analysis internationally recognized by experts or by agreement between parties, and used,
for instance, in legislation when expressing official limits for microbiological quality
Note 1 to entry: It is stressed that, in quantitative microbiology, any obtained value is only defined by the method
description applied. This applies to any alternative method as well as, for instance, to the standard plate count
for the enumeration of microorganisms.
3.3
analyte
component or property which is measured by the method of analysis
Note 1 to entry: The analyte may be the microorganism, stained particles (e.g. microsopic count), components
of microorganisms (e.g. lipopolysaccharides), the result of their ability to multiply (e.g. colony-forming units) or
their metabolic activity (e.g. change in conductivity/impedance).
3.4
organizing body
organization, possibly appointed by a competent authority, having the qualified staff and skills to
organize, to coordinate and to report on the outcome of the activities for the establishment of the
maintenane of a conversion relationship
3.5
measuring range
range in which reliable data can be obtained with an alternative method. Precision data for this
range were determined in a validation study (e.g. by the instrument manufacturer or a responsible
organization)
3.6
range of interest
numerical values in which the routine samples analysed in a laboratory can appear. This includes also
values which appear only infrequently. The range of interest also includes official limits and limits
related to specific quality schemes
4 Principles
4.1 General
The establishment and verification of a conversion relationship is based on the examination of test
samples with an alternative method and an anchor method.
4.2 Requirements for applied methods and laboratories
For establishing and verifying a conversion relationship between the results of an alternative method
and the anchor method, the following prerequisites apply.
The alternative method should have been evaluated and validated according to ISO 16140-2 and/
or ISO 16297│IDF 161. Procedures for sampling, test sample preservation, sample transport, sample
storage, sample pre-treatment, analysis and calculation of results should be documented, strictly
standardized and controlled in agreement with ISO/IEC 17025, Eurachem Guide ‘Accreditation for
1)
Microbiological Laboratories’ or comparable standards .
The anchor method should have been validated, documented, strictly standardized and controlled in
agreement with ISO/IEC 17025, Eurachem Guide ‘Accreditation for Microbiological Laboratories’ or
1)
comparable standards .
1) Regular participation in proficiency tests and training according to the relevant standards, e.g. ISO 4833-1 and
ISO 14461│IDF 169, is strongly recommended.
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oSIST prEN ISO 21187:2019
ISO/DIS 21187:2019(E)
IDF 196:2019(E)
The protocol for the establishment of the conversion relationship and its verification should be
documented. It should follow the guidelines of this International Standard.
4.3 Organizational set-up
There are a number of situations which can be distinguished, e.g. both the alternative and the anchor
method are fully carried out in the same laboratory, or several laboratories are involved in the trial.
Due to the instability and variability of the microbiological status of milk samples, the most robust
conversion relationships will be obtained where the alternative method and the anchor method are
undertaken on the same test samples, at the same place, at the same time. It shall be ensured that either
the sequence of testing do not impose significant influence on the test results or the method with the
lowest influence on the milk sample is applied first.
Subsampling should be avoided. However, in case of two or more participating laboratories subsamples
may be necessary.
In all cases, the organizational set-up should include all the necessary provisions to guarantee that the
obtained conversion relationship is representative of the circumstances under which the alternative
method is carried out and the resulting conversion relationship is later applied. Factors to consider are
listed in Clause 5.
The organizing body should provide guidance to the collaborating laboratories. Furthermore, it should
collect information on critical points in the procedure. All collaborators should be asked to record
relevant information, such as details on the method(s) used, details on the testing of samples, quality
control data, and possibly data about storage and transport conditions.
5 Consideration of factors influencing the conversion relationship
5.1 General
A number of factors can influence the outcome of alternative method or anchor method determinations,
or both. The relative magnitude of the effects can differ between test samples and is not necessarily
the same for both methods. This implies that certain factors can also influence the conversion
relationship. In the evaluation of an alternative method, all relevant factors should be identified and
should be considered since it is necessary to cover the consequences of their variation in one conversion
relationship, or otherwise to establish distinct conversion relationships.
In general, when distinction between samples cannot be made, or is not being made in routine
testing circumstances, the variation in the underlying variables should be covered in one conversion
relationship. Where a factor is shown to have a significant effect on the conversion relationship, more
than one conversion relationship may need to be established and applied.
Influencing factors are grouped into environmental factors affecting the milk sample e.g. content of
psychrotrophic bacteria or background noise from the sample matrix and analytical factors, which
relate to the analysis itself, e.g. reagents.
Below are listed some factors which can possibly influence the conversion relationship in raw milk
analysis. Some of these factors may be applied also to other situations.
5.2 Environmental factors
The microbiological flora of a milk sample, i. e. the type of microorganisms, their growth phase or
metabolic activity, influences the outcome of analytical methods depending on the principle of the
2)
measurement and thus can have a significant impact on the conversion relationship. The normal
variation of microbiological flora should be included in a conversion relationship.
2) For example plate count method contrary to the flow-cytometric method counts only aerobic microorganisms
while anaerobic strains cannot be determined.
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oSIST prEN ISO 21187:2019
ISO/DIS 21187:2019(E)
IDF 196:2019(E)
Microorganisms in milk originate from the udder, the teat skin, from the air and from contamination
from feedstuff, milking equipment and containers. The number and type of bacteria in milk may depend
on the general characteristics of milk production such as the method of milking, storage conditions and
collection intervals. The growth phase is dependent on the sample handling. Thus, there are numerous
environmental factors influencing the microbiological flora of a milk sample. Some of these factors,
which should be considered in the organizational set-up of the trial, are listed below.
5.2.1 Animal species
Certain components in milk from different animal species such as cow versus goat may impact the
analytical results and thus influence the conversion relationship.
NOTE Influencing components may also originate from species-related aspects such as level of milk
production, applied milking technique etc.
5.2.2 High somatic cell counts
Elevated somatic cell counts (e.g. > 1 000 000 cells/ml) may cause increased background noise and
higher count values.
5.2.3 Bulk milk storage conditions
The storage and shipping conditions of the bulk milk will affect the number of bacteria and their growth
phase. When official limits are given depending on the storage conditions (e.g. time, tempera
...

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