1、BRITISH STANDARD BS 1741-5.1: 1988 Methods for Chemical analysis ofliquid milk and cream Part 5: Determination of the nitrogen content of liquid milk Section 5.1 Reference method UDC 637.12 + 637.148.074:543BS1741-5.1:1988 This British Standard, having been prepared under the directionof the Dairyin
2、g Standards Committee, was published under the authority ofthe Board of BSI and comes intoeffect on 29 February 1988 BSI 08-1999 The following BSI references relate to the work on this standard: Committee reference DAC/3 Draft for comment 86/53331 DC ISBN 0 580 16388 1 Committees responsible for thi
3、s British Standard The preparation of this British Standard was entrusted by the Dairying Standards Committee (DAC/-) to Technical Committee DAC/3, upon which the following bodies were represented: AFRC Institute of Food Research, Reading Laboratory Association of British Preserved Milk Manufacturer
4、s Association of Public Analysts Association of Public Analysts of Scotland Creamery Proprietors Association Dairy Trade Federation Department of Trade the connecting tubing and stopper(s) shall be close fitting and preferably made of neoprene. 5.8 Anti-bumping granules, e.g. of fused alumina. 5.9 P
5、ipette or automatic pipette, for delivering0.10mL portions of the indicator solution(4.7). 5.10 Conical flasks, of capacity500mL, graduated at200mL. 5.11 Burette of capacity50mL, class A, complying with BS846. 6 Sampling Take a representative sample of the milk to be tested using, where appropriate,
6、 the procedures described in BS1741-1.BS1741-5.1:1988 2 BSI 08-1999 7 Procedure 7.1 Carry out duplicate determinations using two test portions taken from the same test sample. 7.2 To the Kjeldahl flask (5.2) add three glassballs(5.3), 15g of the potassium sulphate(4.1), 1.0mL of the copper sulphate
7、solution (4.2), approximately5g of prepared sample, weighed to the nearest0.001g and25mL of the sulphuric acid (4.3), using the acid to wash down any copper sulphate solution, potassium sulphate or sample left on the neck of the flask. Gently mix the contents of the flask. NOTEBecause sulphuric acid
8、 is consumed in the digestion of organic matter, use30mL of the sulphuric acid (4.3) instead of25mL for digestion, if the sample contains more than5.0%(m/m) of fat. Use the same volume of sulphuric acid in the blank test (see7.7). 7.3 Heat each Kjeldahl flask on the digestion apparatus (5.6), very g
9、ently at first, taking care to prevent the black froth entering the neck of the flask. When the initial frothing has ceased and copious white vapour appears, boil vigorously (acidvapour condensing half-way up the neck of the flask) until no black particles remain and until the digest becomes a clear
10、 pale blue-green in colour. On reaching this stage adjust the heating to give gentle boiling and continue the heating for1h. If black particles enter the neck of the flask and these are not all washed down into the bulb by the refluxing acid during the initial stages of the vigorous boiling period (
11、this is facilitated for example by rotating the flask) allow the flask to cool sufficiently and wash the particles into the bulb with the minimum of water. Then continue the digestion as described above. The timing of the1h period of gentle boiling shall not be started until a pale green tint become
12、s visible in the digest. 7.4 When the Kjeldahl flasks are cool add210mL of water plus three or four drops of the antifoaming agent (4.4), using the water to wash down the neck of the flask. Mix the contents thoroughly and ensure that any crystals which separate out are dissolved. Add some anti-bumpi
13、ng granules (5.8) and then add100mL of the sodium hydroxide solution (4.5) by gently pouring the solution down the inclined neck of the flask to form a layer at the bottom of the bulk of the flask. NOTEIf the digest solidifies on cooling the test should be repeated using a new test portion. 7.5 Imme
14、diately after the addition of the sodium hydroxide solution to each Kjeldahl flask, connect it to a distillation apparatus (5.7), the tip of whose condenser outlet tube is immersed in50mL of the boric acid solution (4.6) plus0.10mL of the indicator solution (4.7), contained in a conical flask(5.10).
15、 Swirl the contents of each Kjeldahl flask to mix thoroughly and boil, gently at first to prevent excessive frothing. When100mL to125mL of distillate have been collected, lower each conical flask until the tip of the condenser outlet tube is approximately40mm above the200mL mark. Continue each disti
16、llation until irregular boiling (bumping) starts and then immediately stop the heating. Disconnect each Kjeldahl flask and rinse the tip of each condenser outlet tube with a little water, collecting the rinsings in the conical flask. The distillation rate shall be such that approximately150mL of dis
17、tillate are collected when irregular boiling (bumping) starts; the volume of the contents of each conical flask will be approximately200mL, i.e. approximately150mL of the distillate will have been collected as required. The efficiency of each condenser shall be such that the temperature of the conte
18、nts of each conical flask does not exceed25 C during the distillation. 7.6 Titrate each distillate with the standard volumetric sulphuric acid solution (4.8) until the colour matches that of a freshly prepared solution consisting of150mL of water,50mL of the boric acid solution and0.10mL of the indi
19、cator solution, contained in a conical flask (5.9). Take each burette reading to0.01mL. 7.7 Carry out a blank test following the procedure described in7.2 to7.6, taking5mL of distilled water with about0.1g of sucrose (4.9) instead of the prepared sample. NOTE 1The titration of the blank distillate w
20、ill require only a very small volume of the standard volumetric solution, normally not more than0.15mL. NOTE 2The accuracy of the procedure should be checked regularly by means of the following recovery trials, carried out in accordance with7.2 to7.6. a) Check that the digestion procedure is efficie
21、nt by using a test portion of approximately0.35g of phenacetin (4.11) weighed to the nearest0.001g. The recovery of nitrogen from the phenacetin should be not less than99.0%. b) If that recovery is not obtained go on to check that no loss of nitrogen occurs as a result of excessive heat or mechanica
22、l leaks during digestion or distillation, by using a test portion of approximately0.15g of ammonium sulphate (4.10), weighed to the nearest0.001g along with0.1g of sucrose(4.9) and5mL of distilled water. Results outside the limits99.0% to100.0% nitrogen recovered are indicative of failures in the pr
23、ocedure and/or inaccuracies in the titration procedure.BS1741-5.1:1988 BSI 08-1999 3 8 Expression of results 8.1 Method of calculation Calculate each duplicate value of the nitrogen content, in grams of nitrogen per100g of sample, by means of the formula where Round the results to the nearest0.001g
24、per100g. 8.2 Result Take as the result the arithmetic mean of two determinations provided that the requirement for repeatability (see9.1) is satisfied. Express the result to three decimal places. 9 Precision NOTEThe precision data were obtained from an international collaborative study undertaken in
25、1985 and involving25 laboratories and six samples of milk having nitrogen contents in the range0.35g per100g to0.68g per100g. 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 reag
26、ents and apparatus shall not exceed0.007g per100g. 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 result is the mean of two values that comply with the repeatability requirement) sho
27、uld not exceed2.7% of the mean value of the results. 10 Test report The test report shall be in accordance with BS1741-1. V is the volume of the standard volumetric solution of acid used in the determination (inmL); V o is the volume of the standard volumetric solution of acid used in the blank test
28、 (inmL); m is the mass of the test portion (in g). 0.14 VV o () m -4 blankBS1741-5.1:1988 BSI 08-1999 Publications referred to BS 604, Specification for graduated glass measuring cylinders. BS 846, Specification for burettes. BS 1741, Methods for chemical analysis of liquid milk and cream. BS 1741-1
29、, General introduction including preparation of samples. BS 1741-5, Determination of nitrogen content. BS 1741-5.2, Routine method 1) . 1) In preparation.BS 1741-5.1: 1988 BSI 389 Chiswick High Road London W4 4AL BSIBritishStandardsInstitution BSI is the independent national body responsible for pre
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