1、BRITISH STANDARD BS1741-6.1: 1990 Methods for Chemical analysis of liquid milk and cream Part6: Determination of casein nitrogen content of liquid milk Section6.1 Reference methodBS1741-6.1:1990 This British Standard, having been prepared under the directionof the Dairying Standards Policy Committee
2、, waspublished under the authorityof the Board of BSI andcomes into effect on 30September1990 BSI08-1999 The following BSI references relate to the work on this standard: Committee reference DAC/3 Draft for comment89/51965DC ISBN 0 580 18788 8 Committees responsible for this British Standard The pre
3、paration of this British Standard was entrusted by the Dairying Standards Policy Committee (DAC/-) to Technical Committee DAC/3, upon which the following bodies were represented: Association of British Preserved Milk Manufacturers Association of Public Analysts of Scotland Creamery Proprietors Assoc
4、iation Department of Trade and Industry (Laboratory of the Government Chemist) Intervention Board for Agricultural Produce Joint Committee of the Milk Marketing Board and the Dairy Trade Federation Milk Marketing Board Milk Marketing Board for Northern Ireland Ministry of Agriculture, Fisheries and
5、Food Royal Association of British Dairy Farmers Royal Society of Chemistry Society of Chemical Industry Society of Dairy Technology Amendments issued since publication Amd. No. Date CommentsBS1741-6.1:1990 BSI 08-1999 i Contents Page Committees responsible Inside front cover Foreword ii 1 Scope 1 2
6、Definition 1 3 Principle 1 4 Reagents 1 5 Apparatus 1 6 Sampling 1 7 Procedure 1 8 Expression of results 1 9 Precision 2 10 Test report 2 Publications referred to Inside back coverBS1741-6.1:1990 ii BSI 08-1999 Foreword This Section of BS1741 has been prepared under the direction of the Dairying Sta
7、ndards Policy Committee. The method was not described in the1963 edition of BS1741. NOTEThis Section should be read in conjunction with Part1 “General introduction including preparation of samples” published separately. A British Standard does not purport to include all the necessary provisions of a
8、 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, pagesi andii, pages1 and2, an inside back
9、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 the inside front cover.BS1741-6.1:1990 BSI 08-1999 1 1 Scope This Section of BS1741 describes a reference method for the determinatio
10、n of the casein nitrogen content of liquid milk. NOTE 1BS1741-6.2 describes a routine method. NOTE 2The titles of the publications referred to in this standard are listed on the inside back cover. 2 Definition For the purposes of this Section of BS1741, the following definition applies. casein nitro
11、gen content of milk the quantity of nitrogen contained within the precipitate obtained under the pH conditions specified in this Section of BS1741, expressed as grams per100g of sample 3 Principle The casein and the fat are precipitated from a test portion by the addition of acetic acid and sodium a
12、cetate solution and are then separated by filtration. The nitrogen content of the separated precipitate is then determined by the procedure described in BS1741-5.1. 4 Reagents 4.1 The reagents specified for the determination of total nitrogen in BS1741-5.1. 4.2 Acetic acid,100g/L solution. 4.3 Sodiu
13、m acetate solution. Dissolve136.0g of sodium acetate trihydrate in water and dilute to the mark in a1000mL one-mark volumetric flask. 5 Apparatus 5.1 Ordinary laboratory apparatus and the apparatus specified in BS1741-5.1. 5.2 Beakers, of capacity100mL. 5.3 Pipettes, graduated type1, of capacity1mL,
14、 complying with class B of BS700. 5.4 Water bath, capable of maintaining a temperature of40 1 C. 5.5 Filter funnel, glass,75mm diameter. 5.6 Filter papers, diameter9cm, general purpose, ashless grade 1) . 6 Sampling Take a representative sample of the milk to be tested using, where appropriate, the
15、procedures described in BS1741-1. 7 Procedure 7.1 Carry out duplicate determinations using two test portions taken from the same test sample. 7.2 Transfer approximately5g of the prepared sample to a tared100mL beaker(5.2) and record the mass of the test portion to the nearest0.001g. Add50mL of water
16、, raised to40 1 C by the water bath(5.4), and0.5mL of the acetic acid solution(4.2) using a graduated pipette(5.3) and gently mix the contents of the beaker. Allow the beaker to stand in the water bath(5.4) for10min and then add0.5mL of the sodium acetate solution(4.3) using a graduated pipette(5.3)
17、 and mix again. Allow the contents of the beaker to cool to about20 C. Filter the supernatant liquid in the beaker through a previously wetted pleated filter paper(5.6), held by a filter funnel(5.5). Wash the precipitate in the beaker with water three times by decantation through the filter paper. A
18、fter the third wash, transfer the precipitate completely to the filter paper using water to assist in the transfer. Wash the precipitate on the filter paper a further two times with water. Carefully remove the filter paper from the filter funnel, fold the paper to enclose the precipitate andthen det
19、ermine the nitrogen content of the precipitate and paper by the procedure described inBS1741-5.1. Carry out a blank determination of the nitrogen content of a filter paper(5.6) washed with water. 8 Expression of results 8.1 Method of calculation Calculate each duplicate value of the casein nitrogen
20、content, in grams per100g of sample, using the following expression: Round off the results to the nearest0.001g/100g. 1) Whatman No.40 has been found suitable. where V is the volume of the standard volumetric solution of acid aused in the determination (in mL); V 0 is the volume of the standard volu
21、metric solution of acid aused in the blank test (in mL); m is the mass of the test portion (in g). a See BS1741-5.1. 0.14VV 0 () m -BS1741-6.1:1990 2 BSI 08-1999 8.2 Result Take as the result the arithmetic mean of two determinations, provided that the requirement for repeatability(9.1) is satisfied
22、. Express the result to three decimal places. 9 Precision NOTEThe precision data were obtained from a collaborative study undertaken in1990, involving nine laboratories and two skimmed and two whole milks. 9.1 Repeatability The difference between the values obtained from two determinations carried o
23、ut simultaneously or in rapid succession by the same analyst using the same apparatus shall not exceed0.013g/100g of test portion. 9.2 Reproducibility The difference between the results obtained on samples of the same test material by two different analysts in different laboratories (where each resu
24、lt is the mean of two values that comply with the repeatability requirement) should not exceed0.018g/100g of test portion. 10 Test report The test report shall be in accordance with BS1741-1.BS1741-6.1:1990 BSI 08-1999 Publications referred to BS700, Graduated pipettes. BS1741, Methods for chemical
25、analysis of liquid milk and cream. BS1741-1, General introduction including preparation of samples. BS1741-5.1, Reference method. BS1741-6.2, Routine method. BS1741-6.1: 1990 BSI 389 Chiswick High Road London W4 4AL BSIBritishStandardsInstitution BSI is the independent national body responsible for
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