BS ISO 17997-2-2005 Milk - Determination of casein-nitrogen content - Direct method《乳 酪蛋白氮含量测定 直接方法》.pdf

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1、BRITISH STANDARD BS ISO 17997-2:2004 Milk Determination of casein-nitrogen content Part 2: Direct method ICS 67.100.10 BS ISO 17997-2:2004 This British Standard was published under the authority of the Standards Policy and Strategy Committee on 14 January 2005 BSI 14 January 2005 ISBN 0 580 45364 2

2、National foreword This British Standard reproduces verbatim ISO 17997-2:2004 and implements it as the UK national standard. The UK participation in its preparation was entrusted to Technical Committee AW/5, Milk and milk products, which has the responsibility to: A list of organizations represented

3、on this committee can be obtained on request to its secretary. Cross-references The British Standards which implement international publications referred to in this document may be found in the BSI Catalogue under the section entitled “International Standards Correspondence Index”, or by using the “

4、Search” facility of the BSI Electronic Catalogue or of British Standards Online. This publication does not purport to include all the necessary provisions of a contract. Users are responsible for its correct application. Compliance with a British Standard does not of itself confer immunity from lega

5、l obligations. aid enquirers to understand the text; present to the responsible international/European 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. Su

6、mmary of pages This document comprises a front cover, an inside front cover, the ISO title page, pages ii to v, a blank page, pages 1 to 7 and a back cover. The BSI copyright notice displayed in this document indicates when the document was last issued. Amendments issued since publication Amd. No. D

7、ate Comments Reference numbers ISO 17997-2:2004(E) IDF 29-2:2004(E)INTERNATIONAL NATSDRAD ISO 179972- FDI 92-2 tide tsriFino 0-400251-9 Milk Determination of casein-nitrogen content Part 2: Direct method Lait Dtermination de la teneur en azote de casine Partie 1: Mthode directe Referecne unbmers OSI

8、 -7997102:2)E(40 DI:2-92 F002)E(4 OSI DI dnaF 4002INTERNATIONAL STANDARD ISO 17997-2 IDF 29-2 First edition 2004-09-15 eD kliMimretnitao nof sacnie-rtinegon tnoctne :2 traP tem tceriDdoh tiaL D retmanitoid ne la enetru ne azeto c edaseni raPtie :1 Mtdohd eirecte BSISO179972:2004 ii BSISO179972:2004

9、iiiForeword 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

10、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 matt

11、ers of electrotechnical standardization. International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2. The main task of technical committees is to prepare International Standards. Draft International Standards adopted by the technical committees are circul

12、ated 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 elements of this document may be the subject of patent rights. ISO shall not be held responsib

13、le for identifying any or all such patent rights. ISO 17997-2IDF 29-2 was prepared by Technical Committee ISO/TC 34, Food products, Subcommittee SC 5, Milk and milk products, and the International Dairy Federation (IDF), in collaboration with AOAC International. It is being published jointly by ISO

14、and IDF and separately by AOAC International. ISO 17997IDF 29 consists of the following parts, under the general title Milk Determination of casein- nitrogen content: Part 1: Indirect method (Reference method) Part 2: Direct method BSISO179972:2004iv Foreword IDF (the International Dairy Federation)

15、 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 ISO and AOAC International in the development of standard m

16、ethods 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 IDF National Committees ca

17、sting a vote. ISO 17797-2IDF 29-2 was prepared by Technical Committee ISO/TC 34, Food products, Subcommittee SC 5, Milk and milk products, and the International Dairy Federation (IDF), in collaboration with AOAC International. It is being published jointly by ISO and IDF and separately by AOAC Inter

18、national. All work was carried out by the Joint ISO/IDF/AOAC Action Team on Nitrogen compounds, of the Standing Committee on Main components in milk. This first edition of ISO 17997-2IDF 29-2, together with ISO 17997-1IDF 29-1, cancels and replaces the first edition of IDF 29:1964, which has been te

19、chnically revised. ISO 17997IDF 29 consists of the following parts, under the general title Milk Determination of casein- nitrogen content: Part 1: Indirect method (Reference method) Part 2: Direct method BSISO179972:2004 vIntroduction This part of ISO 17997IDF 29 specifies a routine method for the

20、direct determination of the casein-nitrogen content of milk. The method was developed and optimized to provide results that agree well with results of the method described in ISO 17997-1IDF 29-1 when both are applied to fresh raw bovine milk. No collaborative study data were available for this metho

21、d when publishing its equivalent, IDF 29:1964. Recent research has been completed to develop the better defined indirect reference method given in ISO 17997-1IDF 29-1, and this routine method for the direct measurement of the casein-nitrogen content of milk. Both methods have been collaboratively st

22、udied and a reference to the obtained precision data is included in each part. BSISO179972:2004blankNITERNATNOIAL STANDARD IS-79971 O2:(4002E) ID-92 F2:(4002E)1Milk Determination of casein-nitrogen content Part 2: Direct method WARNING The use of the method and equipment described in this standard m

23、ay involve 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 appropriate safety and health practices and determine the applicability of local regula

24、tory limitations prior to use. 1 Scope This part of ISO 17997IDF 29 specifies a routine method for the direct determination of the casein-nitrogen content of bovine milk. The method can be modified for milk from other species or liquid dairy products. NOTE Casein nitrogen will decrease with milk sto

25、rage time due to casein breakdown even at 4 C. The casein nitrogen of heat-treated milk will be artificially high because of whey-protein denaturation. 2 Normative references The following referenced documents are indispensable for the application of this document. For dated references, only the edi

26、tion cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. ISO 648:1977, Laboratory glassware One-mark pipettes ISO 1042:1998, Laboratory glassware One-mark volumetric flasks ISO 8968-1IDF 20-1, Milk Determination of nitrogen content

27、 Part 1: Kjeldahl method ISO 8968-2IDF 20-2, Milk Determination of nitrogen content Part 2: Block-digestion method (Macro method) 3 Terms and definitions For the purposes of this document, the following term and definition applies. 3.1 casein-nitrogen content mass fraction of substances determined a

28、ccording to the procedures specified in this part of ISO 17997IDF 29 NOTE The casein-nitrogen content is expressed as a mass fraction in percent. BSISO179972:20042 4 Principle Casein is precipitated from a test portion of milk by the addition of acetic acid and sodium acetate solutions, such that th

29、e final pH of the mixture is approximately 4,6. The precipitated milk casein is separated by filtration. The filtrate will contain the non-casein-nitrogen components. The nitrogen content of the precipitate is determined by the procedure described in either ISO 8968-1IDF 20-1 or ISO 8968-2IDF 20-2.

30、5 Reagents Use only reagents of recognized analytical grade and glass-distilled water or water of at least equivalent purity. 5.1 Reagents for determination of total nitrogen. Use the reagents specified in ISO 8968-1IDF 20-1 or ISO 8968-2IDF 20-2. 5.2 Acetic acid solution, c(CH 3 CO 2 H) = 1,75 mol/

31、l. Using a volumetric pipette (6.5), add 10,0 ml of glacial acetic acid in a 100 ml volumetric flask (6.3). Dilute to the mark with water. 5.3 Sodium acetate solution, c(CH 3 CO 2 Na) = 1 mol/l. Dissolve 8,20 g of sodium acetate or 13,60 g of sodium acetate trihydrate in water in a 100 ml volumetric

32、 flask (6.3). Dilute to the mark with water. The sodium acetate solution may be stored at room temperature for one week or at between 0 C and 4 C for 6 months. 5.4 Buffer solution. Using a volumetric pipette (6.5), add 1,0 ml of acetic acid solution (5.2) to a 100 ml volumetric flask (6.3). Using an

33、other volumetric pipette, add 1,0 ml of sodium acetate solution (5.3) to the flask. Dilute to the mark with water. 6 Apparatus Usual laboratory equipment and, in particular, the following. 6.1 Apparatus for determination of total nitrogen. Use that specified in ISO 8968-1IDF 20-1 or ISO 8968-2IDF 20

34、-2. 6.2 Water bath, capable of maintaining a temperature of 38 C to 40 C. 6.3 One-mark volumetric flasks, with stoppers, of capacity 100 ml, conforming to ISO 1042:1998, class A. 6.4 Bottle-top dispensers, capable of delivering 70 ml of water and 30 ml of buffer solution (5.4), respectively. 6.5 One

35、-mark volumetric pipettes, of capacity 1 ml, 10 ml and 50 ml, conforming to ISO 648:1977, class A. 6.6 Filter funnel, made of glass or plastic, of diameter 75 mm. BSISO179972:2004 36.7 Filter paper, of diameter 15 cm, nitrogen-free (e.g. Whatman No. 1 1)or equivalent). Pleat before use. 6.8 Analytic

36、al balance, capable of weighing to the nearest 0,1 mg. 6.9 Automatic pipettor or adjustable micropipette, capable of delivering 0,75 ml. 7 Sampling It is important that the laboratory receive a sample which is truly representative and has not been damaged or changed during transport or storage. Samp

37、ling is not part of the method specified in this part of ISO 17997IDF 29. A recommended sampling method is given in ISO 707. 8 Preparation of test sample Warm the test sample in the water bath (6.2) set at between 38 C and 40 C to melt the milk fat so that a representative test portion of milk can b

38、e removed from the sample. Gently mix the sample immediately prior to removal and weighing of the test portion (9.1). 9 Procedure 9.1 Test portion Weigh, to the nearest 0,1 mg, approximately 5 g of the prepared test sample (Clause 8) directly into a Kjeldahl flask or digestion tube (6.1) or determin

39、e the difference between the mass of a small container containing approximately 5 g of test sample and the mass of the container after the test portion is poured into the flask or tube. Immediately add 70 ml of water, preheated to 38 C, using the bottle-top dispenser (6.4), while rinsing down any te

40、st sample remaining on the inside neck into the bottom of the flask or tube. Additional test samples may be weighed and water added at this point. However, care shall be taken to finish step 9.2.2 within 30 min after adding the test portion into the Kjeldahl flask or digestion tube. NOTE This 30-min

41、 time limit is to minimize proteolytic degradation of casein during sample preparation. 9.2 Determination 9.2.1 Using an automatic pipettor or adjustable micropipette (6.9), add 0,75 ml of acetic acid solution (5.2) to the Kjeldahl flask or digestion tube (9.1), taking care to avoid getting acetic a

42、cid solution on the neck of the flask or the tube. Swirl to mix and allow to stand at room temperature for 10 min. Using an automatic pipettor or adjustable micropipette (6.9), add 0,75 ml of sodium acetate solution (5.3) to the Kjeldahl flask or digestion tube. Again swirl to mix and let the precip

43、itate settle for 10 min. Pour the contents of the Kjeldahl flask or digestion tube through a filter funnel (6.6) containing pleated filter paper (6.7) to collect the filtrate. Some of the precipitate will remain in the Kjeldahl flask or digestion tube and some on the filter paper. It is not necessar

44、y to remove all the precipitate from the flask or tube. Filter completely before the next pour. 1) Whatman No. 1 is an example of a product available commercially. This information is given for the convenience of users of this part of ISO 17997IDF 29 and does not constitute an endorsement by ISO or

45、IDF of this product. BSISO179972:20044 Immediately after pouring the mixture, add 30 ml of buffer solution (5.4) with the bottle top dispenser (6.4) to the Kjeldahl flask or digestion tube, taking care not to allow any precipitate to dry on the neck of the Kjeldahl flask or digestion tube. Also use

46、the buffer solution to rinse any precipitate from the neck of the flask or tube down into the bottom. Swirl to mix the contents. After completion of the filtration of the first pour, pour the buffer rinse from the flask or tube through the same filter paper. Collect this second filtrate by adding it

47、 to the first one. Use the same side of the neck of the flask for all pours to reduce the area to which precipitate can adhere. Again, immediately rinse the neck of the flask or tube with an additional 30 ml of buffer solution (5.4), using the bottle-top dispenser (6.4). Swirl to mix the contents. A

48、fter completion of the filtration of the first buffer rinse (second pour), pour the second buffer rinse from the flask or tube through the same filter paper, adding the third filtrate to the first two. The thus-obtained filtrate shall be clear and free of particulate matter. If particles are present

49、, recycle the filtrate through the same filter paper or repeat the whole procedure from 9.1 onwards. NOTE 1 This method uses a fixed volume addition of acetic acid and sodium acetate solutions for every sample. This will not achieve an exact pH of 4,6 for every milk sample. However, it is a practical compromise that has been used traditionally for analysis of bovine milks, particularly when a large number of