1、BRITISH STANDARD BS1741-6.2: 1990 Methods for Chemical analysis of liquid milk and cream Part6: Determination of casein nitrogen content of liquid milk Section6.2 Routine methodBS1741-6.2:1990 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 30 September1990 BSI09-1999 The following BSI references relate to the work on this standard: Committee reference DAC/3 Draft for comment89/51964 DC ISBN 0 580 18789 6 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 CommentsBS1741-6.2:1990 BSI 09-1999 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 2 9 Precision 2 10 Test report 2 Publications referred to Inside back coverBS1741-6.2:1990 ii BSI 09-1999 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
8、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, pagesi andii, pages1 and 2, an inside bac
9、k 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.BS1741-6.2:1990 BSI 09-1999 1 1 Scope This Section of BS1741 describes a routine method for the determinatio
10、n of the casein nitrogen content of liquid milk. NOTE 1It is necessary to know the total nitrogen content of the milk NOTE 2BS1741-6.1 describes a reference method. NOTE 3The titles of the publications referred to in this standard are listed on the inside back cover. 2 Definition For the purposes of
11、 this Section of BS1741, the following definition applies. casein nitrogen content of milk the quantity of nitrogen corresponding to the difference between the total nitrogen content and the non-casein nitrogen content determined under the conditions specified in BS1741-5.2 and in this Section, expr
12、essed as gramsper100g of sample 3 Principle The casein and the fat are precipitated from a test portion by the addition of acetic acid and sodium acetate solution and removed by filtration. The nitrogen content of the resulting filtrate is then determined by the procedure described in BS1741-5.2. Th
13、e non-casein nitrogen content is then calculated and the casein nitrogen obtained by difference from the total nitrogen content. 4 Reagents 4.1 The reagents specified for the determination of total nitrogen in BS1741-5.2. 4.2 Acetic acid,100g/L solution. 4.3 Sodium acetate solution. Dissolve136.0g o
14、f sodium acetate trihydrate in water and dilute to the mark in a1000mL one-mark volumetric flask. 4.4 Sucrose solution. Dissolve25.0g of sucrose in water and dilute to the mark in a100mL one-mark volumetric flask. This solution shall be freshly prepared, immediately before use. 5 Apparatus 5.1 Ordin
15、ary laboratory apparatus and the apparatus specified in BS1741-5.2. 5.2 One-mark volumetric flasks, of capacity100mL, complying with class B of BS1792. 5.3 Pipettes, of capacity1mL and20mL, complying with class B of BS1583. 5.4 Water bath, capable of maintaining a temperature of40 1 C. 5.5 Filter fu
16、nnel, glass,100mm diameter. 5.6 Filter paper, diameter15cm, general purpose, ashless grade 1) . 5.7 Conical flasks, of capacity250mL. 6 Sampling Take a representative sample of the milk to be tested using, where appropriate, the procedures described in BS1741-1. 7 Procedure Transfer approximately10g
17、 of prepared sample to a100mL volumetric flask(5.2) and record the mass of one test portion to the nearest0.001g. Add75mL of water, raised to40 1 C using the water bath(5.4), and1.0mL of the acetic acid solution(4.2) using a1mL pipette(5.3) and gently mix the contents of the flask. Allow the flask t
18、o stand in the water bath(5.4) for10min and then add1.0mL of the sodium acetate solution(4.3) using a1mL pipette(5.3). Remove the flask from the water bath, mix the contents of the flask by swirling and allow the flask and contents to cool to20 C. Dilute the contents of the flask to100mL with water
19、and mix by gently inverting the flask. Allow the precipitate of casein and and fat to settle and then filter the contents of the flask through a pleated filter paper(5.6). Collect the filtrate in a clean, dry conical flask(5.7). Take20.0mL of the filtrate using a pipette(5.3) and determine its nitro
20、gen content by the procedure described in BS1741-5.2. NOTE 1It is often convenient to use a smaller burette than the25mL capacity burette specified in5.10 of BS1741-5.2. Carry out a blank determination of the nitrogen content using20.0mL of water,2.0mL of the sucrose solution(4.4),0.2mL of the aceti
21、c acid solution(4.2) and0.2mL of the sodium acetate solution(4.3). NOTE 2It is recommended that analysts periodically undertake determinations to verify that, results complying with the repeatability (see9.1) are being obtained. 1) Whatman No. 40 has been found suitable.BS1741-6.2:1990 2 BSI 09-1999
22、 8 Expression of results 8.1 Method of calculation Calculate the value of the non-casein nitrogen content, in grams per100g of sample, using the following expression: NOTEThe correction factor for skimmed milk is0.998. Calculate the casein nitrogen content of the sample by subtracting the value of t
23、he non-casein nitrogen content from the value of the total nitrogen content (determined in accordance with BS1741-5.2). 8.2 Result Express the result to the nearest0.01g/100g. 9 Precision NOTEThe precision data were obtained from a collaborative study undertaken in1990, involving11laboratories and t
24、wo skimmed and two whole milks. 9.1 Repeatability The difference between the values obtained from two determinations carried out simultaneously or in rapid succession by the same analyst using the same apparatus should not exceed0.02g/100g of test portion. 9.2 Reproducibility The difference between
25、the results obtained on samples of the same test material by two different analysts in different laboratories (where each result is the mean of two values that comply with the repeatability requirement) should not exceed0.04g/100g of test portion. 10 Test report The test report shall be in accordanc
26、e with BS1741-1. 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 volumetric solution of acid aused in the blank test (in mL); m is the mass of the test portion (ing); 0.994 is the correction factor for the volume
27、 of the precipitated casein in the100mL flask for milk of average composition. a See BS1741-5.2. 0.14 VV 0 ()5 0.994 m -BS1741-6.2:1990 BSI 09-1999 Publications referred to BS1583, Specification for one-mark pipettes. BS1741, Methods for chemical analysis of liquid milk and cream. BS1741-1, General
28、introduction including preparation of samples. BS1741-5.2, Routine method. BS1741-6.1, Reference method. BS1792, Specification for one-mark volumetric flasks. BS1741-6.2: 1990 BSI 389 Chiswick High Road London W4 4AL BSIBritishStandardsInstitution BSI is the independent national body responsible for
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