1、 Reference numbers ISO 17997-1:2004(E) IDF 29-1:2004(E) ISO and IDF 2004INTERNATIONAL STANDARD ISO 17997-1 IDF 29-1 First edition 2004-09-15 Milk Determination of casein-nitrogen content Part 1: Indirect method (Reference method) Lait Dtermination de la teneur en azote de casine Partie 1: Mthode ind
2、irecte (Mthode de rfrence) ISO 17997-1:2004(E) IDF 29-1:2004(E) PDF disclaimer This PDF file may contain embedded typefaces. In accordance with Adobes licensing policy, this file may be printed or viewed but shall not be edited unless the typefaces which are embedded are licensed to and installed on
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7、98 88 Fax + 41 22 749 09 47 Fax + 32 2 733 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 17997-1:2004(E) IDF 29-1:2004(E) ISO and IDF 2004 All rights reserved iiiForeword ISO (the Internati
8、onal 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
9、 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 standa
10、rdization. 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 circulated to the member bodies for
11、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 responsible for identifying any or all
12、such patent rights. ISO 17997-1IDF 29-1 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
13、 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 ISO 17997-1:2004(E) IDF 29-1:2004(E) iv ISO and IDF 2004 All rights reservedForeword IDF (the Inte
14、rnational Dairy 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 ISO and AOAC International in th
15、e development 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
16、 IDF National Committees casting a vote. ISO 17797-1IDF 29-1 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 a
17、nd separately by AOAC International. 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 edition of ISO 17997-1IDF 29-1, together with ISO 17997-2IDF 29-2, cancels and replaces the first edition of IDF 29:19
18、64, which has been technically 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 ISO 17997-1:2004(E) IDF 29-1:2004(E) ISO and IDF 2004 All rights reserved
19、vIntroduction This part of ISO 17997IDF 29 is a classical reference method for the indirect determination of the casein- nitrogen content of milk. No collaborative study data were available for this method when publishing the first edition of IDF 29:1964. Recent research has been completed to develo
20、p a better defined indirect reference method. A routine method for the direct measurement of the casein-nitrogen content of milk is given in ISO 17997-2IDF 29-2. Both parts of ISO 17997IDF 29 have been collaboratively studied and a reference to the obtained precision data is now included in each par
21、t. INTERNATIONAL STANDARD ISO 17997-1:2004(E) IDF 29-1:2004(E) ISO and IDF 2004 All rights reserved 1Milk Determination of casein-nitrogen content Part 1: Indirect method (Reference method) WARNING The use of the method and equipment described in this standard may involve hazardous materials, operat
22、ions 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 regulatory limitations prior to use. 1 Scope
23、 This part of ISO 17997IDF 29 specifies a reference method for the indirect 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 storage time due to casein breakdown
24、 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 edition cited applies. For undated r
25、eferences, 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 Part 1: Kjeldahl method ISO 8968
26、-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 terms and definitions apply. 3.1 non-casein-nitrogen content mass fraction of substances determined according to the procedures sp
27、ecified in this part of ISO 17997IDF 29 ISO 17997-1:2004(E) IDF 29-1:2004(E) 2 ISO and IDF 2004 All rights reserved3.2 casein-nitrogen content mass fraction of substances determined according to the procedures specified in this part of ISO 17997IDF 29 NOTE Both the non-casein-nitrogen content and th
28、e casein-nitrogen content are expressed as a mass fraction in percent. 4 Principle The total-nitrogen content of a test sample is determined by the method of either ISO 8968-1IDF 20-1 or ISO 8968-2IDF 20-2. Casein is precipitated from a separate test portion of the same milk by the addition of aceti
29、c acid and sodium acetate solutions, such that the final pH of the mixture is approximately 4,6. The precipitated milk casein is removed by filtration, so the remaining filtrate contains the non-casein-nitrogen components. The nitrogen content of the filtrate is determined by the procedure described
30、 in ISO 8968-1IDF 20-1 or ISO 8968-2IDF 20-2. The casein-nitrogen content is calculated as the difference between the total-nitrogen content and the non-casein-nitrogen content of the milk. 5 Reagents Use only reagents of recognized analytical grade and glass-distilled water or water of at least equ
31、ivalent 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/l. Using a volumetric pipette (6.6), add 10,0 ml of glacial acetic acid in a 100 ml volumetric flask (6.3). Di
32、lute 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 flask (6.3). Dilute to the mark with water. The sodium acetate solution may be stored at room temperature for
33、 one week or at between 0 C and 4 C for 6 months. 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 20-2. 6.2 Water bath, capable of maintaining a temperature of 38 C
34、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 dispenser, capable of delivering 75 ml of water (optional). 6.5 Graduated measuring cylinder, of capacity up to 100 ml. ISO 17997-1:2004(E) IDF 29-1:2004(E) ISO and IDF 20
35、04 All rights reserved 36.6 One-mark volumetric pipettes, of capacity 1 ml, 10 ml and 50 ml, conforming to ISO 648:1977, class A. 6.7 Filter funnel, made of glass or plastic, of diameter 75 mm. 6.8 Filter paper, of diameter 15 cm, nitrogen-free (e.g. Whatman No. 1 1)or equivalent). Pleat before use.
36、 6.9 Conical flasks, or equivalent, of capacity 100 ml. 6.10 Analytical balance, capable of weighing to the nearest 0,1 mg. 6.11 Automatic pipettor or (adjustable) micropipette, capable of delivering 1,0 ml (optional). 7 Sampling It is important that the laboratory receive a sample which is truly re
37、presentative 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 Preparation of test sample Warm the test sample in the water bath (6.2) set at between 38 C
38、 and 40 C to melt the milk fat so that a representative test portion of milk can be removed from the test sample. Gently mix the sample immediately prior to removal and weighing of the test portion (see 9.1 and 9.2). 9 Procedure 9.1 Determination of total nitrogen Determine the total-nitrogen conten
39、t of the test sample (Clause 8), w N , expressed as a mass fraction in percent, by using the procedure as described in ISO 8968-1IDF 20-1 or ISO 8968-2IDF 20-2. 9.2 Determination of non-casein nitrogen 9.2.1 Test portion 9.2.1.1 Weigh, to the nearest 0,1 mg, approximately 10 g of the prepared test s
40、ample (Clause 8) into a 100 ml volumetric flask (6.3). Immediately add 75 ml of water, preheated to 38 C, using a bottle-top dispenser (6.4) or a measuring cylinder (6.5). Additional samples may be weighed and water added at this point. However, care shall be taken to finish step 9.2.1.2 within 30 m
41、in after adding the test portion to the volumetric flask. NOTE This 30-min time limit is to minimize proteolytic degradation of casein during sample preparation. 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
42、 ISO 17997|IDF 29 and does not constitute an endorsement by ISO or IDF of this product. ISO 17997-1:2004(E) IDF 29-1:2004(E) 4 ISO and IDF 2004 All rights reserved9.2.1.2 Using a volumetric pipette (6.6) or an automatic pipettor or (adjustable) micropipette (6.11), add 1 ml of acetic acid solution (
43、5.2) to the flask. Stopper the flask and swirl to mix the contents. Place the flask for 10 min in the water bath (6.2) set at between 38 C and 40 C. Using another volumetric pipette (6.6) or an automatic pipettor or (adjustable) micropipette (6.11), add 1 ml of sodium acetate solution (5.3) to the f
44、lask and swirl to mix. Cool the flask and its contents to 20 C. Dilute to the mark with water at 20 C. Stopper the flask and invert to mix. Let the precipitate settle. Filter the contents of the flask using a filter funnel (6.7) and a pleated filter paper (6.8). Collect the entire filtrate in a 100
45、ml conical flask (6.9). The filtrate shall be clear and free of particulate matter. If not, repeat the procedure starting from 9.2.1 with a new test portion. NOTE This method uses a fixed volume addition of acetic acid and sodium acetate solutions for every sample. This will not achieve an exact pH
46、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
47、result (see Reference 8). The alternative is to monitor the pH as acid is added to the sample with the appropriate temperature adjustments for pH-meter calibration. In this case, the exact dilution of each test portion should be measured and a different dilution factor should be taken into account f
48、or each test portion analysed. 9.2.1.3 Swirl the clear filtrate to ensure it is mixed well. Add 50 ml of filtrate with a volumetric pipette (6.6) to a Kjeldahl flask or digestion tube containing boiling aids, potassium sulfate and copper sulfate solution, as specified in ISO 8968-1IDF 20-1 or ISO 89
49、68-2IDF 20-2. 9.2.1.4 Add the appropriate amount of sulfuric acid to the Kjeldahl flask or digestion tube. Digest, distil and titrate as specified for milk in ISO 8968-1IDF 20-1 or ISO 8968-2IDF 20-2. When using ISO 8968-2IDF 20-2, take care to apply gradual heating if the digest foams excessively. The total digestion time may be longer than for the determination of the total-nitrogen content of milk. The flask or tube may be stoppered after the addition of sulf
