1、October 2004DEUTSCHE NORM English price group 7No part of this translation may be reproduced without prior permission ofDIN Deutsches Institut fr Normung e. V., Berlin. Beuth Verlag GmbH, 10772 Berlin, Germany,has the exclusive right of sale for German Standards (DIN-Normen).ICS 67.100.20!$QT“146974
2、9www.din.deDDIN 10349Determination of pH of butter plasmaBestimmung des pH-Wertes im ButterplasmaSupersedesDIN 10349:1985-08www.beuth.deTranslation by DIN-Sprachendienst.In case of doubt, the German-language original should be consulted as the authoritative text.Document comprises 6 pagesDIN 10349:2
3、004-10 2 Foreword This standard has been prepared by Technical Committee Chemische und physikalische Milchuntersuchung of the Normenausschuss Lebensmittel und landwirtschaftliche Produkte (Food and Agricultural Products Standards Committee). A relationship exists between DIN 10349 and International
4、Standard ISO 7238:1983 Butter Determination of pH of the serum Potentiometric method published by the International Organization for Standardization (ISO), but differs in that: a) the specified buffer solutions are slightly different and specific examples have not been included; b) there are differe
5、nces in the centrifuging equipment needed; c) there are differences in the procedure mainly as a result of specifying (20 1) C as the test temperature rather than applying temperature compensation with the test temperature unspecified; d) due to the procedural differences, the interlaboratory test a
6、s in DIN ISO 5725 yielded slightly different values for the precision of the method. Amendments This standard differs from DIN 10349:1985-08 as follows: a) The title has been changed from butter serum to butter plasma, and the definition of butter plasma from DIN 10474:2003-02 has been adopted. b) T
7、he standard has been editorially revised and harmonized with current rules of presentation. Previous editions DIN 10349: 1985-08 1 Scope This standard specifies a potentiometric method of determining the pH of butter plasma. 2 Normative references This standard incorporates, by dated or undated refe
8、rence, provisions from other publications. These normative references are cited at the appropriate places in the text, and the titles of the publications are listed below. For dated references, subsequent amendments to or revisions of any of these publications apply to this standard only when incorp
9、orated in it by amendment or revision. For undated references, the latest edition of the publication referred to (including any amendments) applies. DIN 10474:2003-02, Determination of butter type by neural network analysis of compositional parameters using the chemometric method DIN 10349:2004-10 3
10、 DIN EN ISO 707, Milk and milk products Guidance on sampling (ISO 707:1997) DIN ISO 5725, Precision of test methods Determination of repeatability and reproducibility for a standard test method by inter-laboratory tests; Identical with ISO 5725, edition 19861)3 Terms and definitions For the purpose
11、of this standard the following terms and definitions apply. 3.1 butter plasma fat-free aqueous phase that is mixed thoroughly after being separated from the butter oil by centrifuging butter melted at 65 C from DIN 10474:2003-02 NOTE In DIN 10474 butter serum is referred to as deproteinized butter p
12、lasma. 3.2 pH of butter plasma pH determined by the method described in this standard NOTE See DIN 19260 for terminology associated with pH-measurement. 4 Principle After melting the butter sample and separating off the butter plasma, the potential difference (cell voltage) between a glass pH electr
13、ode and a reference electrode (measuring cell consisting of individual electrodes or a combination cell) immersed in the plasma is measured. 5 Reagents 5.1 General Analytically pure reagents shall be used and the water used shall be doubly distilled or water of at least equivalent purity. 5.2 Two bu
14、ffer solutions, having pH values at 20 C that are known to two decimal places. One of the buffer solutions shall have a pH that is as close as possible to neutral (7,00), while the second solution shall have a pH in the vicinity of the expected pH of the butter plasma. NOTE Suitable ready-to-use buf
15、fer solutions are commercially available. 5.3 Cleaning liquid for removing protein and fatty residues from the electrodes. NOTE Suitable cleaning liquids are recommended by electrode manufacturers. 1) DIN ISO 5725:1988-04 (since withdrawn) was used to determine precision data. DIN 10349:2004-10 4 5.
16、4 Electrode storage liquid NOTE Suitable storage liquids are recommended by electrode manufacturers. 6 Apparatus In addition to standard laboratory equipment, the following shall be used. 6.1 Centrifuge, capable of being adjusted to (350 50) g (e.g. Gerber centrifuge for determining milk fat as in D
17、IN 10479-2). 6.2 Glass tubes, for centrifuging (e.g. DIN 12217 glass tubes). When using Gerber centrifuges, e.g. centrifuge tubes with a length of 150 mm to 180 mm, an outside diameter of about 24 mm and a wall thickness of 1,5 mm. 6.3 Elastomer stoppers (e.g. elastomer stoppers as in DIN 12871). 6.
18、4 Test tubes or beakers, of nominal capacity 25 ml (e.g. beakers as in DIN 12331). 6.5 Ice/water bath or suitable cooling device. 6.6 pH measuring equipment, consisting of a pH meter having a maximum error of pH 0,01, and of a glass electrode and a reference electrode (e.g. calomel electrode). If a
19、potentiometer with temperature correction is not available, the equipment shall be suitable for measure-ments at 20 C. NOTE Glass and reference electrodes may also be combined as a combination electrode. 6.7 Thermometer, having scale divisions of 0,5 C (e.g. laboratory thermometer as in DIN 12775).
20、6.8 Water bath, capable of being maintained at about 65 C. 7 Sampling As specified in DIN EN ISO 707. 8 Procedure 8.1 General Perform two determinations on every analytical sample. 8.2 Separation of butter plasma To extract the butter plasma, press a heated glass tube (6.2) into the butter until it
21、is about three-quarters filled. Then seal the tube with an elastomer stopper (6.3) and melt the sample by immersing the tube in the water bath (6.8) maintained at about 65 C. As soon as the butter has melted, place the tube with the elastomer stopper facing down in the centrifuge (6.1) and centrifug
22、e the sample for five minutes. Then immediately DIN 10349:2004-10 5 transfer the tube, with the elastomer stopper still facing down, to the ice/water bath or cooling device (6.5) and leave it there until the fat has completely solidified. Decant the butter plasma into a test tube or beaker (6.4). 8.
23、3 Calibration of pH measuring equipment Calibrate the pH measuring equipment (6.6) by the two-point method at (20 1) C using the buffer solutions (5.2). NOTE If a series of samples is to be examined, check the calibration of the equipment using the two buffer solutions at intervals of about 30 minut
24、es. 8.4 Measurement Immerse the measuring cell in the butter plasma at a temperature of (20 1) C (check the temperature using the thermometer (6.7) and, once the reading is constant, read off the pH directly to an accuracy of 0,01. 8.5 Cleaning and storing the measuring cell After every measurement,
25、 rinse the measuring cell with water and wipe it with a paper tissue. After completing the measurements, clean the measuring cell with the cleaning liquid (5.3), rinse it with water and immerse it in the storage liquid (5.4). 9 Expression of results Calculate the arithmetic mean of the two values an
26、d report the pH to an accuracy of 0,01. 10 Precision 10.1 General The precision data were determined in an interlaboratory test performed as specified in DIN ISO 57251)on butter plasma having a pH of 4,66. 10.2 Repeatability limit (same operator, same equipment) The absolute difference between two s
27、uccessive results obtained under repeatability conditions will not exceed the repeatability limit, r, of 0,02 pH units in more than 5 % of cases on average. If the repeatability limit is exceeded, both results shall be regarded as suspect and the determination shall be repeated. 1) See page 3. DIN 1
28、0349:2004-10 6 10.3 Reproducibility limit (different operators, different equipment) The absolute difference between two individual results obtained under reproducibility conditions will not exceed the reproducibility limit, R, of 0,07 pH units in more than 5 % of cases on average. If the reproducib
29、ility limit is exceeded, both results shall be regarded as suspect and the determination shall be repeated. 11 Test report a) b) nature, origin and designation of sample; c) type and date of sampling; d) date of receipt and testing; e) pH of butter plasma; f) any reason for deviating from this stand
30、ard. Bibliography DIN 10479-2, Butyrometric determination of fat content of milk and milk products Part 2: Testing of specific dairy products DIN 12217, Laboratory glassware Tubes made of borosilicate glass 3.3 DIN 12331, Laboratory glassware Beakers DIN 12775, Laboratory glassware Laboratory thermometers with 0,1 C, 0,2 C and 0,5 C scale intervals DIN 12871, Laboratory apparatus in rubber and plastics Elastomer stoppers DIN 19260, pH measurement General terms and definitions The test report shall include the following details as a minimum: a reference to this standard;
