BS 5086-7-1991 Analysis of butter - Method for determination of pH of butter serum《黄油分析 第7部分 pH值测定方法》.pdf

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1、BRITISH STANDARD BS5086-7: 1991 Analysis of butter Part 7: Method for determination of pH of butter serum NOTEThis Part should be read in conjunction with Part1 “General introduction including preparation of samples” published separately.BS5086-7:1991 This British Standard, having been prepared unde

2、r the direction of the Agriculture andFood Standards Policy Committee,was published underthe authority of the Standards Board and comes intoeffect on 29 November 1991 BSI 03-1999 First published January1985 Second edition November1991 The following BSI references relate to the work on this standard:

3、 Committed reference AFC/3 Draft for comment91/51728DC ISBN 0 580 20015 9 Committees responsible for this British Standard The preparation of this British Standard was entrusted by the Agricultural and Food Standards Policy Committee (AFC/-) to Technical Committee AFC/3, upon which the following bod

4、ies were represented: Association of British Preserved Milk Manufacturers Association of Public Analysts of Scotland Creamery Proprietors Association Department of Trade and Industry (Laboratory of the Government Chemist) Intervention Board for Agricultural Produce Joint Committee of the Milk Market

5、ing Board and the Dairy Trade Federation Milk Marketing Board Milk Marketing Board for Northern Ireland Ministry of Agriculture, Fisheries and Food Royal Association of British Dairy Farmers Royal Society of Chemistry Society of Dairy Technology Amendments issued since publication Amd. No. Date Comm

6、entsBS5086-7:1991 BSI 03-1999 i Contents Page Committees responsible Inside front cover Foreword ii 1 Scope 1 2 Principle 1 3 Sampling 1 4 Reagents 1 5 Apparatus 1 6 Procedure 1 7 Expression of results 1 8 Precision 2 9 Test report 2 Publication(s) referred to Inside back coverBS5086-7:1991 ii BSI 0

7、3-1999 Foreword This Part of BS5086 has been prepared under the direction of the Agriculture and Food Standards Policy Committee, at the request of Technical Committee AFC/3. It supersedes the1985 edition, which is withdrawn. This edition introduces technical changes but it does not reflect a full r

8、eview or revision of the standard, which will be undertaken in due course. Correspondence with international standards. This Part of this British Standard is related to ISO7238:1983 “Butter Determination of pH of the serum Potentiometric method” published by the International Organization for Standa

9、rdization (ISO). It differs from that method principally in that it requires the use of a larger test portion (100 g in place of 50 g), and sets a standard test temperature of20C, where the ISO method permits the use of any appropriate temperature. BS5086-7:1985 permitted measurements at other tempe

10、ratures to be corrected to20C, but methods of calculating corrections have proved unreliable, and this new edition requires determinations to be at20 1 C. Attention is drawn to BS1647 which gives further information on determination of pH. A British Standard does not purport to include all the neces

11、sary 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. Summary of pages This document comprises a front cover, an inside front cover, pages i and ii, pages1

12、 and2, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on theinside front cover.BS5086-7:1991 BSI 03-1999 1 1 Scope This Part of BS5086 describes a method for the determ

13、ination of the pH of the serum from butter of all types. NOTEThe titles of the publications referred to in this standard are listed on the inside back page. 2 Principle The serum is separated from the melted butter and the potential difference at20C between a glass electrode and a reference electrod

14、e immersed in the serum is measured and expressed in pH units. 3 Sampling Take a representative sample of the butter to be tested using, where appropriate, the procedure described in BS5086-1. 4 Reagents 4.1 General The reagents shall be of analytical reagent quality and the water used in their prep

15、aration shall be recently distilled water, complying with BS3978, that has been protected from absorption of carbon dioxide. 4.2 Two buffer solutions, each of pH value known to0.01pH units at20 C and chosen to bracket the pH value of the serum under test. NOTEThe following buffer solutions may be us

16、ed. a) Buffer solution of pH4.00 at 20C. Dissolve in water10.12g of potassium hydrogen phthalate (KHC 8 H 4 O 4 ), which has been previously dried to constant mass at120C. Make up to 1000 mL with water at20 1 C and mix well. Preserve the solution by adding approximately2mL of chloroform or carbon te

17、trachloride. b) Buffer solution of pH6.88 at 20C. Dissolve in water3.388g of potassium dihydrogen orthophosphate (KH 2 PO 4 ) and3.533g of disodium hydrogen orthophosphate(Na 2 HPO 4 ), both compounds having been previously dried to constant mass at120C. Make up to1000mL with water at20 1 C and mix

18、well. Preserve the solution by adding approximately2mL of chloroform or carbon tetrachloride. 5 Apparatus Usual laboratory apparatus, and in particular the following. 5.1 pH meter, with minimum sensitivity 0.01pH units, with a glass electrode and a suitable reference electrode. NOTE 1The glass and r

19、eference electrodes may be assembled into a system of combined electrodes. The use of a combined glass-reference electrode is recommended. NOTE 2Use of a pH meter with temperature compensation is recommended. NOTE 3Attention is drawn to BS3145 and BS2586. 5.2 Water bath, controlled at approximately6

20、5C. 5.3 Centrifuge 1) ,capable of attaining a relative radial acceleration of approximately 375 g. 5.4 Centrifuge tubes 1) , of capacity approximately50mL with tight fitting stoppers of rubber or similar material. 6 Procedure 6.1 Test portion Take approximately100g of the sample. 6.2 Separation of t

21、he serum Separate the serum from the butter. Transfer the serum (including the protein) to a beaker of capacity25mL and bring it to a temperature of20 1 C. NOTEThere are many methods for the separation of the serum from the butter, one of which is as follows. Transfer the test portion to a beaker or

22、 other suitable glass vessel and heat the vessel in the water bath(5.2). When the test portion is fully melted, mix and transfer equal quantities of the test portion to each of two centrifuge tubes(5.4). Close the centrifuge tubes with the stoppers, place them with the stoppered ends downwards in th

23、e tube holders and centrifuge for5min. Immediately transfer the centrifuge tube, with stoppered ends downwards, into an ice water bath, such that the tubes are totally immersed and leave until the fat has completely congealed. When all the fat has congealed, remove the tubes from the ice water bath

24、and still holding the stoppered ends downwards, carefully remove each stopper in turn and collect the serum from each tube, in the same beaker. Mix the contents of the beaker. 6.3 Calibration of the pH meter Adjust the temperature of the buffer solutions(4.1) to20 1 C and calibrate the pH meter in a

25、ccordance with the manufacturers instructions. NOTEIf a series of samples is being tested, check the calibration of the pH meter with one or both buffer solutions at least every30min. 6.4 Determination Introduce the electrodes into the serum at a temperature of20 1 C. Carry out the determination usi

26、ng the procedure appropriate to the pH meter used. When the reading becomes constant, read the pH directly from the scale of the instrument, to the nearest0.01pH unit. 6.5 Cleaning the electrodes Clean the electrodes by rinsing consecutively with acetone at room temperature and water at30C to35C. 7

27、Expression of results Record the measured pH value at20C, expressing the result to the nearest 0.01pH unit. 1) This apparatus is required if the method of separation of the serum from the butter described in the note to6.2 is used.BS5086-7:1991 2 BSI 03-1999 8 Precision 8.1 Repeatability The differe

28、nce between two single results obtained on the same laboratory sample by one operator using the same apparatus within a short time interval should not exceed0.03pH unit on average more than once in20 cases in the normal and correct operation of the method. 8.2 Reproducibility The difference between

29、two single and independent results obtained by two operators working in different laboratories on duplicate laboratory samples should not exceed0.10pH unit on average more than once in20 cases in the normal and correct operation of the method. 9 Test report The following information shall be include

30、d in the test report: a) the result, expressed in accordance with clause7; b) the number of this British Standard, i.e.BS5086-7:1991; c) details of the sample identity.BS5086-7:1991 BSI 03-1999 Publication(s) referred to BS1647, pH measurement 2) . BS2586, Specification for glass and reference elect

31、rodes for the measurement of pH. BS3145, Specification for laboratory pH meters. BS3978, Specification for water for laboratory use. BS5086, Analysis of butter. BS5086-1, General introduction including preparation of samples. ISO7238, Butter Determination of pH of the serum Potentiometric method 2)

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