1、BRITISH STANDARD Milk Determination of nitrogen content Part 5: Determination of pro tein-nitr ogen content The European Standard EN IS0 8968-5:2001 has the status of a British Standard ICs 67.100.10 BS EN IS0 8968-512002 Incorporating Corrigendum No. I Wk present to the responsible international/Eu
2、ropean committee any enquiries on the interpretation, or proposals for change, and keep the UK interests informed; monitor related international and European developments and promulgate them in the UK. - A list of organizations represented on this committee can be obtained on request to its secretar
3、y. Cr oss-r e fer enc e s The British Standards which implement international or European publications referred to in this document may be found in the BSI Standards Catalogue under the section entitled “International Standards Correspondence Index”, or by using the “Find” facility of the BSI Standa
4、rds Electronic Catalogue. A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations. 2 April 2002 In
5、corporating Annex ZA Summary of pages This document comprises a front cover, an inside front cover, the EN IS0 title page, the EN IS0 foreword page, the IS0 title page, pages ii tov, a blank page, pages 1 to 6, the Annex ZA and a back cover. This British Standard, having been prepared under the dire
6、ction-of the Consumer Policy and Strategy Committee, Products and Services Sector The BSI copyright date displayed in this document indicates when the document was last issued. was published under the authority of the Standards Policy and Strategy Committee on 28 January 2002 O BSI 2 April 2002 ISBN
7、 O 580 38874 3 EUROPEAN STANDARD NORME EUROPENNE EUROPISCHE NORM EN IS0 8968-5 December 2001 ICs 67.100.10 English version Milk - Determination of nitrogen content - Part 5: Determination of protein-nitrogen content (IS0 8968-52001) Lait - Dtermination de la teneur en azote - Partie 5: Dtermination
8、de la teneur en azote protique (IS0 8968- 52001) Milch - Bestimmung des Stickstoffgehaltes -Teil 5: Bestimmung des Proteinstickstoff-Gehaltes (IS0 8968- 52001) This European Standard was approved by CEN on 15 December 2001 CEN members are bound to comply with the CENKENELEC Internal Regulations whic
9、h 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 Management Centre or to any CEN member. This European Standa
10、rd 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 Management Centre has the same status as the official versions. CEN members are the national standard
11、s bodies of Austria, Belgium, Czech Republic, Denmark, Finland, France, Germany, Greece, Iceland, Ireland, Italy, Luxembourg, Netherlands, Norway, Portugal, Spain, Sweden, Switzerland and United Kingdom. EUROPEAN COMMITTEE FOR STANDARDIZATION EUROPISCHES KOMITEE FR NORMUNG COMIT EUROPEN DE NORMALISA
12、TION Management Centre: rue de Stassart, 36 B-1050 Brussels O 2001 CEN All rights of exploitation in any form and by any means reserved worldwide for CEN national Members. Ref. No. EN IS0 8968-52001 E EN IS0 8968-52001 I CORRECTED 2002-03-13 I Foreword This document (IS0 8968-52001) has been prepare
13、d by Technical Committee ISO/TC 34 “Agricultural 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 b
14、y publication of an identical text or by endorsement, at the latest by June 2002, and conflicting national standards shall be withdrawn at the latest by June 2002. According to the CENKENELEC Internal Regulations, the national standards organizations of the following countries are bound to implement
15、 this European Standard: Austria, Belgium, Czech Republic, Denmark, Finland, France, Germany, Greece, Iceland, Ireland, Italy, Luxembourg, Netherlands, Norway, Portugal, Spain, Sweden, Switzerland and the United Kingdom. Endorsement notice The text of the International Standard IS0 8968-52001 has be
16、en approved by CEN as a European Standard without any modifications. NOTE Normative references to International Standards are listed in annex ZA (normative). INTERNATIONAL STANDARD EN IS0 8968-52001 IS0 8968-5 IDF 20-5 First edition 2001-1 2-1 5 Milk - Determination of nitrogen content - Part 5: Det
17、ermination of protein-nitrogen content Laif - Dferminafion de la feneur en azofe - Partie 5: Dferminafion de la feneur en azofe profique Reference num bers IS0 8968-5:2001(E) IDF 20-5:2001(E) EN IS0 8968-52001 II EN IS0 8968-52001 Contents Page Foreword . iv 1 2 3 4 5 6 7 8 9 9.1 9.2 9.3 10 10.1 10.
18、2 11 11.1 11.2 11.3 12 . Normative references . . Term and definition . 1 . 2 . . Sampling. . . Preparation of test sample 3 . 3 Test portion . . 3 Direct determination . . 3 Indirect determination . 4 Calculation and expression of results Calculation of protein-nitrogen content . Calculation of tru
19、e protein content . Precis I on . . . . Test rep0 rt . . Bibliography 6 . 111 EN IS0 8968-52001 Foreword IS0 (the International Organization for Standardization) is a worldwide federation of national standards bodies (IS0 member bodies). The work of preparing International Standards is normally carr
20、ied out through IS0 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.
21、IS0 collaborates closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization. International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 3. The main task of technical committees is to prepare Internat
22、ional Standards. Draft International Standards adopted by the technical committees are circulated to the member bodies for voting. Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote. Attention is drawn to the possibility that some of the e
23、lements of this part of IS0 8968 IDF 20 may be the subject of patent rights. IS0 shall not be held responsible for identifying any or all such patent rights. International Standard IS0 8968-5 IDF 20-5 was prepared by Technical Committee ISO/TC 34, Food producfs, Subcommittee SC 5, Milk and milk prod
24、ucfs, and the International Dairy Federation (IDF), in collaboration with AOAC International. It is being published jointly by IS0 and IDF and separately by AOAC International. IS0 8968 IDF 20 consists of the following parts, under the general title Milk - Deferminafion of nifrogen confenf: - Par 1:
25、 Kjeldahl mefhod - Par 2: Block-digesfion mefhod (Macro mefhod) - Par 3: Block-digesfion mefhod (Semi-micro rapid roufine mefhod) - Par 4: Deferminafion of fhe non-profein-nifrogen confenf - Par 5: Deferminafion of fhe profein-nifrogen confenf iv EN IS0 8968-52001 Foreword IDF (the International Dai
26、ry Federation) is a worldwide federation of the dairy sector with a National Committee in every member country. Every National Committee has the right to be represented on the IDF Standing Committees carrying out the technical work. IDF collaborates with IS0 and AOAC International in the development
27、 of standard methods of analysis and sampling for milk and milk products. Draft International Standards adopted by the Action Teams and Standing Committees are circulated to the National Committees for voting. Publication as an International Standard requires approval by at least 50 % of National Co
28、mmittees casting a vote. International Standard IS0 8968-5 IDF 20-5 was prepared by Technical Committee ISO/TC 34, Food producfs, Subcommittee SC 5, Milk and milk producfs, and the International Dairy Federation (IDF), in collaboration with AOAC International. It is being published jointly by IS0 an
29、d IDF and separately by AOAC International. All work was carried out by the Joint ISO/IDF/AOAC Action Team, Nifrogen compounds, under the aegis of its project leader, Mr D.M. Barbano (US). V EN IS0 8968-52001 EN IS0 8968-52001 Milk - Determination of nitrogen content - Part 5: Determination of prote
30、in-nitrogen content WARNING- The use of this part of IS08968 IDF20 may involve the use of hazardous materials, operations, and equipment. This standard does not purport to address all the safety risks associated with its use. It is the responsibility of the user of this standard to establish appropr
31、iate safety and healthy practices and determine the applicability of local regulatory limitations prior to use. 1 Scope This part of IS0 8968 IDF 20 specifies a method for the direct determination of the protein-nitrogen content of liquid milk, whole or skimmed. An alternative indirect method using
32、calculations is also described. 2 Normative references The following normative documents contain provisions which, through reference in this text, constitute provisions of this part of IS0 8968 IDF 20. For dated references, subsequent amendments to, or revisions of, any of these publications do not
33、apply. However, parties to agreements based on this part of IS0 8968 IDF 20 are encouraged to investigate the possibility of applying the most recent editions of the normative documents indicated below. For undated references, the latest edition of the normative document referred to applies. Members
34、 of IS0 and IEC maintain registers of currently valid International Standards. IS0 8968-1 IDF 20-1 :2001, Milk - Deferminafion of nifrogen confenf - Par 1: Kjeldahl mefhod IS0 8968-2 IDF 20-2:2001, Milk - Deferminafion of nifrogen confenf - Par 2: Block-digesfion mefhod (Macro mefhod) IS0 8968-4 IDF
35、 20-4:2001, Milk - Deferminafion of nifrogen confenf - Par 4: Deferminafion of fhe non-profein- nifrogen confenf 3 Term and definition For the purposes of this part of IS0 8968 IDF 20, the following term and definition apply. 3.1 protein-nitrogen content mass fraction of substances determined by the
36、 procedure specified in this part of IS0 8968 IDF 20, directly or indirectly NOTE The protein-nitrogen content is expressed as a percentage by mass. 1 EN IS0 8968-52001 4 Principle Protein is precipitated from a test portion by addition of trichloroacetic acid solution such that the final concentrat
37、ion of trichloroacetic acid in the mixture is approximately 12 %.The protein precipitate is separated by filtration. (The filtrate contains the non-protein-nitrogen contents.) The nitrogen content of the filtrate is determined by the procedure described either in part 1 or part 2 of IS0 8968 IDF 20.
38、 5 Reagents Use only reagents of recognized analytical grade, unless otherwise specified, and distilled or demineralized water or water of equivalent purity. The reagents are as specified for the determination of total nitrogen described in part 1 or part 2 of IS0 8968 IDF 20, together with the foll
39、owing. A laboratory may decide which method to use. 5.1 Trichloroacetic acid solution (CC13COOH) Dissolve 15,O g of trichloroacetic acid in water in a 100 ml one-mark volumetric flask. Dilute to the mark with water. Do not use concentrations of trichloroacetic acid and volumes of solutions other tha
40、n those specified. The performance of the method with respect to mean value and between-laboratory performance characteristics will differ if other than the specified concentrations of trichloroacetic acid and volumes of solutions are used. 5.2 Hydrochloric acid standard volumetric solution, c(HCI)
41、= (0,l 0,000 5) mol/l. It is recommended that this material be purchased prestandardized by the manufacturer to meet or exceed the above specification. 6 Apparatus Usual laboratory apparatus and, in particular, the following, together with the apparatus specified in part 1 or part 2 of IS0 8968 IDF
42、20. 6.1 Water bath, capable of being maintained at 38 OC 2 OC. 6.2 Pipette, of capacities 5 ml 6.3 Filter funnel, made of glass, of diameter 75 mm 6.4 Filter paper, nitrogen free, of diameter 15 cm (e.g. Whatman No. 1) or equivalent. 6.5 Automatic pipette or piston pump, capable of delivering 10 ml.
43、 7 Sampling Sampling is not part of the method specified in this International Standard. A recommended sampling method is given in IS0 707. It is important that the laboratory receive a sample which is truly representative and has not been damaged or changed during transport or storage. 1) Whatman i
44、s an example of a suitable product available commercially. This information is given for the convenience of users of this part of IS0 8968 IDF 20 and does not constitute an endorsement by IS0 of this product. 2 EN IS0 8968-52001 8 Preparation of test sample Warm the test sample in the water bath (6.
45、1) set at 38 OC. Gently mix the test sample thoroughly by repeatedly inverting the sample bottle without causing frothing or churning. Cool the sample to room temperature immediately prior to weighing the test portion (9.1). 9 Procedure 9.1 Test portion Pipette 5,O ml 0,l ml of the prepared test sam
46、ple (clause 8) either into a dry and clean Kjeldahl flask or digestion tube, preweighed to the nearest 0,l mg. Weigh the test sample to the nearest 0,l mg. Immediately add 5 ml 0,l ml of water to the flask or tube, rinsing any test sample on its neck into the flask or tube. NOTE 1 The use of either
47、a Kjeldahl flask or digestion tube depends on the laboratorys choice of method. NOTE 2 For advice on test portion size to apply this method to dairy products other than milk, see annexA of IS0 8968-1 IDF 20-1 :2001. 9.2 Direct determination 9.2.1 Precipitation and filtration Add 40 ml 0,5 ml of tric
48、hloroacetic acid solution (5.1) to the Kjeldahl flask or digestion tube containing the test portion (9.1) and swirl to mix the contents. Let the flask or tube stand for approximately 5 min to allow the precipitate to settle. Pour the contents of the flask or tube through a filter paper (6.4) placed
49、in a filter funnel (6.3). Collect the filtrate in a clean conical flask. Some of the precipitate will remain in the Kjeldahl flask or digestion tube and some will be collected on the filter paper. It is not necessary to remove all of the precipitate from the flask or tube. Immediately after pouring the mixture and so as not to allow any precipitate to dry on the neck of the flask or tube, add by means of an automatic pipette (6.5), 10 ml of the trichloroacetic acid solution (5.1). Use the solution to rinse any precipitate from the neck of the flask or tube d
