1、 Reference numbers ISO 17997-2:2004(E) IDF 29-2:2004(E) ISO and IDF 2004INTERNATIONAL STANDARD ISO 17997-2 IDF 29-2 First edition 2004-09-15 Milk Determination of casein-nitrogen content Part 2: Direct method Lait Dtermination de la teneur en azote de casine Partie 1: Mthode directe ISO 17997-2:2004
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6、ion in writing from either ISO or IDF at the respective address below. ISO copyright office International Dairy Federation Case postale 56 CH-1211 Geneva 20 Diamant Building Boulevard Auguste Reyers 80 B-1030 Brussels Tel. + 41 22 749 01 11 Tel. + 32 2 733 98 88 Fax + 41 22 749 09 47 Fax + 32 2 733
7、04 13 E-mail copyrightiso.org E-mail infofil-idf.org Web www.iso.org Web www.fil-idf.org Published in Switzerland ii ISO and IDF 2004 All rights reservedISO 17997-2:2004(E) IDF 29-2:2004(E) ISO and IDF 2004 All rights reserved iiiForeword ISO (the International Organization for Standardization) is a
8、 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 represente
9、d 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. International Standards are draf
10、ted 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 circulated to the member bodies for voting. Publication as an International Sta
11、ndard 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 responsible for identifying any or all such patent rights. ISO 17997-2IDF 29-2 was
12、 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 International. ISO 17997IDF 29 consists of
13、 the following parts, under the general title Milk Determination of casein- nitrogen content: Part 1: Indirect method (Reference method) Part 2: Direct method ISO 17997-2:2004(E) IDF 29-2:2004(E) iv ISO and IDF 2004 All rights reservedForeword IDF (the International Dairy Federation) is a worldwide
14、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 methods of analys
15、is 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 casting a vote. IS
16、O 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 International. All wo
17、rk 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 technically revise
18、d. 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 ISO 17997-2:2004(E) IDF 29-2:2004(E) ISO and IDF 2004 All rights reserved vIntroduction This part of ISO 17997I
19、DF 29 specifies a routine method for the 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
20、 study data were available for this method 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.
21、Both methods have been collaboratively studied and a reference to the obtained precision data is included in each part. INTERNATIONAL STANDARD ISO 17997-2:2004(E) IDF 29-2:2004(E) ISO and IDF 2004 All rights reserved 1Milk Determination of casein-nitrogen content Part 2: Direct method WARNING The us
22、e of the method and equipment described in this standard may 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
23、 practices and determine the applicability of local regulatory 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
24、products. NOTE Casein nitrogen will decrease with milk storage 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 applic
25、ation 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 648:1977, Laboratory glassware One-mark pipettes ISO 1042:1998, Laboratory glassware One-mark volumetric flasks
26、ISO 8968-1IDF 20-1, Milk Determination of nitrogen content 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
27、-nitrogen content mass fraction of substances determined according to the procedures specified in this part of ISO 17997IDF 29 NOTE The casein-nitrogen content is expressed as a mass fraction in percent. ISO 17997-2:2004(E) IDF 29-2:2004(E) 2 ISO and IDF 2004 All rights reserved4 Principle Casein is
28、 precipitated from a test portion of milk by the addition of acetic acid and sodium acetate solutions, such that the 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 conte
29、nt of the precipitate is determined by the procedure described in either ISO 8968-1IDF 20-1 or ISO 8968-2IDF 20-2. 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 r
30、eagents 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/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) =
31、 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 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. Usin
32、g a volumetric pipette (6.5), add 1,0 ml of acetic acid solution (5.2) to a 100 ml volumetric flask (6.3). Using another 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 followi
33、ng. 6.1 Apparatus for determination of total nitrogen. Use that specified in ISO 8968-1IDF 20-1 or ISO 8968-2IDF 20-2. 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 Bo
34、ttle-top dispensers, capable of delivering 70 ml of water and 30 ml of buffer solution (5.4), respectively. 6.5 One-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. ISO 17997-2:2004(E) ID
35、F 29-2:2004(E) ISO and IDF 2004 All rights reserved 36.7 Filter paper, of diameter 15 cm, nitrogen-free (e.g. Whatman No. 1 1)or equivalent). Pleat before use. 6.8 Analytical balance, capable of weighing to the nearest 0,1 mg. 6.9 Automatic pipettor or adjustable micropipette, capable of delivering
36、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. Sampling is not part of the method specified in this part of ISO 17997IDF 29. A recommended sampling method is given in ISO 707. 8 P
37、reparation 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 be removed from the sample. Gently mix the sample immediately prior to removal and weighing of the test portion (9.1). 9 Procedur
38、e 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 determine the difference between the mass of a small container containing approximately 5 g of test sample and the mass of the container
39、 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 test sample remaining on the inside neck into the bottom of the flask or tube. Additional test samples may be weighed and water ad
40、ded 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 time limit is to minimize proteolytic degradation of casein during sample preparation. 9.2 Determination 9.2.1 Using an automat
41、ic 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 acid 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 auto
42、matic 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 precipitate settle for 10 min. Pour the contents of the Kjeldahl flask or digestion tube through a filter funnel (6.6) containing plea
43、ted 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 necessary to remove all the precipitate from the flask or tube. Filter completely before the next pour. 1) Whatman No. 1 is an example o
44、f 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 IDF of this product. ISO 17997-2:2004(E) IDF 29-2:2004(E) 4 ISO and IDF 2004 All rights reservedImmediately after pouring the mi
45、xture, 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 the buffer solution to rinse any precipitate from the neck of the flask
46、 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 to the first one. Use the same side of the neck of the flask for all p
47、ours 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. After completion of the filtration of the first buffer rinse (second pou
48、r), 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, recycle the filtrate through the same filter paper or repeat the whol
49、e 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 samples are to be analysed. pH variation in the range between 4,5 and 5,0 has been shown to have a negligible influence on the final result (see
