1、BRITISH STANDARD CONFIRMED OCTOBER1993 BS770-8:1987 Methods for Chemical analysis of cheese Part 8: Determination of nitrogen content (reference method) NOTEThis Part should be read in conjunction with Part1 “General Introduction”, published separately. UDC 637.3.07:543.846BS770-8:1987 This BritishS
2、tandard, having been prepared under the directionof the Dairying Standards Committee, was published under the authority ofthe Board of BSI and comes intoeffect on 30 January1987 BSI11-1999 The Committees responsible for this BritishStandard are shown inPart1. The following BSI references relate to t
3、he work on this standard: Committee reference DAC/3 Draft for comment85/54445 DC ISBN 0 580 15683 4 Foreword This Part of BS770 has been prepared under the direction of the Dairying Standards Committee. It supersedes the method for determination of nitrogen given in clause7 of BS770:1963. Other new
4、Parts of BS770 supersede the remaining methods given in BS770:1963 and that standard is therefore withdrawn. The method described in this Part differs from the previous method chiefly in the use of copper sulphate rather than mercuric oxide or mercuric sulphate solution as the catalyst and in the us
5、e of boric acid as the collecting agent. A collaborative study organized on behalf of the committee established the precision values for the method as given in clause9. A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsib
6、le 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 cover and a back cover. This standard has been upda
7、ted (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover. Amendments issued since publication Amd. No. Date of issue CommentsBS770-8:1987 BSI 11-1999 i Contents Page Foreword Inside front cover 1 Scope 1 2 Definition 1
8、 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 coverii blankBS770-8:1987 BSI 11-1999 1 1 Scope This Part of BS770 describes a reference method forthe determination of nitrogen in chees
9、e and processed cheese of all types. NOTEThe titles of the publications referred to in this standard are listed on inside back cover. 2 Definition For the purposes of this Part of BS770, thefollowing definition applies. 2.1 nitrogen content the quantity of nitrogen corresponding to the ammonia produ
10、ced and determined under the conditions specified in this BritishStandard and expressed as a percentage by mass 3 Principle A test portion is digested with a mixture of concentrated sulphuric acid and potassium sulphate using copper(II) sulphate as a catalyst, to convert organic nitrogen present to
11、ammonium sulphate. Excess sodium hydroxide is added to the cooled digest to liberate ammonia from the ammonium sulphate. The ammonia is distilled into an excess ofboric acid solution, then titrated with standard sulphuric acid solution. The nitrogen content of the sample is calculated from the amoun
12、t of ammonia produced. 4 Reagents Use the following reagents, which shall be of recognized analytical grade and low in nitrogen. Water used shall comply with the requirements of BS3978. 4.1 Copper(II) sulphate solution. Dissolve25g of copper sulphate pentahydrate (CuSO 4 .5H 2 O) in10% (V/V) sulphur
13、ic acid solution and dilute to the mark with the acid in a500mL one-mark volumetric flask. 4.2 Potassium sulphate (K 2 SO 4 ), anhydrous. NOTECommercially available potassium sulphate tablets may be used. 4.3 Sulphuric acid, density1.84g/mL at20 C. 4.4 Sodium hydroxide solution, low in nitrogen, con
14、taining approximately40g of sodium hydroxide (NaOH) per100mL of solution. 4.5 Orthoboric acid solution. Dissolve40g of orthoboric acid (H 3 BO 3 ) in warm water and dilute to1L. 4.6 Sulphuric acid, standard volumetric solution, c(“ H 2 SO 4 )=0.10mol/L. 4.7 Mixed indicator solution. Dissolve0.01g of
15、methyl red,0.02g of bromothymol blue and0.06gof bromocresol green in100mL of ethanol70% (V/V). 4.8 Boiling regulators 4.8.1 Glass balls, undrilled, of diameter approximately5mm (for the digestion). 4.8.2 Anti-foaming emulsion (for the distillation). 5 Apparatus Ordinary laboratory apparatus and, in
16、particular, the following. 5.1 Balance, capable of weighing to an accuracy of 1mg. 5.2 Kjeldahl flasks, of capacity500mL. 5.3 Digestion apparatus, with heat supplied by a gasbunsen or electric element adjusted such that250mL of water in a500mL Kjeldahl flask plus two glass balls will come to a rolli
17、ng boil in approximately5min. 5.4 Effective suction device or efficient water scrubbing system, for attachment to the digestion range to allow for safe ventilation and extraction ofacid fumes evolved during digestion. 5.5 Distillation units, for use with a Kjeldahl flask fitted with a rubber stopper
18、 through which passes the lower end of a bulb-trap to prevent splash-over of the digest mixture during distillation, with the upper end of the bulb-trap connected to a condenser by rubber tubing. 5.6 Graduated dispenser or automatic pipettes, of capacity0.2mL,1mL and25mL, used to deliver the volumes
19、 of indicator solution, copper(II) sulphate solution and concentrated sulphuric acid. 5.7 Greaseproof paper, low in nitrogen. 5.8 Conical flasks, of capacity250mL. 6 Sampling Carry out the sampling in accordance with the methods described for cheese in BS809. 7 Procedure 7.1 Prepare a test sample in
20、 accordance with BS770-1. 7.2 Carry out duplicate determinations on two test portions taken from the same test sample. 7.3 Carry out two blank tests using the procedure described in7.4 but omitting the test portions.BS770-8:1987 2 BSI 11-1999 7.4 Weigh to the nearest1mg,1g 20mg oftheprepared sample
21、onto a piece of the greaseproof paper(5.7) of size approximately100mm 60mm. Transfer the greaseproof paper and the test portion to a Kjeldahlflask(5.2). NOTE 1The quantity of sample can be weighed by difference. This procedure is particularly useful in the case of soft cheeses. Place1mL of the coppe
22、r(II) sulphate solution(4.1), two glass balls(4.8.1),15g of the potassium sulphate(4.2) and25mL of the concentrated sulphuric acid(4.3) in the Kjeldahl flask. Slowly heat the flask on the digestion range(5.3) taking care to prevent the black froth entering the neck of the flask, until the digest has
23、 cleared. NOTE 2Once all the carbonaceous material has been destroyed the digest begins to clear. In clearing the digest shows a gradual change in colour from a dark blue green to a permanent pale blue green. The transition usually takes about8min. When the digest has cleared, adjust the heating to
24、give a gentle boiling and continue the boiling for1h. NOTE 3It is important that this1h period of further digestion is timed from the absolute clearing of the digest, i.e.obtainment of a permanent pale blue green colour. When the digest has cooled, add carefully250mL of water plus two or three drops
25、 of the anti-foaming emulsion(4.8.2). Mix the contents of the Kjeldahl flask thoroughly and allow the contents to cool. NOTE 4The addition of water to the digest, followed by mixing, generates a considerable amount of heat. NOTE 5Anti-bumping granules may also be added. Pour50mL of the orthoboric ac
26、id solution(4.5) and0.2mL of the mixed indicator solution(4.7) into a250mL conical flask(5.8) and place the flask on the condenser of the distillation unit(5.5) so that the end of the tube dips into the acid. Add90mL of the sodium hydroxide solution(4.4) by means of a measuring cylinder, pouring car
27、efully down the inclined neck of the Kjeldahl flask so that the two layers in the flask do not mix. Wash down any residual solution in the neck of the flask with a little water. Immediately connect the flask to the distillation unit and mix the contents gently by shaking. Apply heat to the flask to
28、obtain gentle boiling and collect approximately150mL of distillate over a period of about30min. Rinse down the condenser tube with water and then remove the receiving flask from the range. Titrate the distillate with the standard volumetric sulphuric acid solution(4.6) until the colour of the distil
29、late matches that of a freshly prepared solution consisting of150mL of water,50mL of the orthoboric acid solution(4.5) and0.2mL of the mixed indicator solution(4.7) in a250mL conical flask(5.8). 8 Expression of results Calculate the nitrogen content of the sample, expressed as a percentage by mass,
30、using the following formula: where v 0 is the volume of the standard volumetric sulphuric acid solution required for the blank test (in mL); v 1 is the volume of the standard volumetric sulphuric acid solution required for the determination (in mL); m is the mass of the test portion (in g). Take as
31、the result the arithmetic mean of the results of two determinations, provided that the requirement for repeatability (see9.1) is satisfied. Report the result to the nearest0.01g of nitrogen per100g of sample. 9 Precision NOTESee BS5497-1. The precision data were obtained from a collaborative study u
32、ndertaken in1985 and involving eight laboratories and six cheeses (Blue Stilton, Camembert, Cheddar, Cheshire and two varieties of full fat soft cheese, i.e.Blue Lymeswold and Melbury) having nitrogen contents in the range2.5g/100g to4g/100g of sample. 9.1 Repeatability The difference between the re
33、sults of two determinations carried out simultaneously or in rapid succession by the same analyst shall not exceed0.12g of nitrogen per100g of sample. 9.2 Reproducibility The difference between the single and independent results obtained by two analysts working in different laboratories on identical
34、 test material should not exceed0.18g of nitrogen per100g of sample. 10 Test report The test report shall include the information specified in BS770-1. 0.0014v 1 v 0 () 100 m - BS770-8:1987 BSI 11-1999 Publications referred to BS770, Methods for chemical analysis of cheese. BS770-1, General introduc
35、tion. BS809, Methods for sampling of milk and milk products. BS3978, Water for laboratory use. BS5497, Precision of test methods. BS5497-1, Guide for the determination of repeatability and reproducibility for a standard test method. BS770-8:1987 BSI 389 Chiswick High Road London W4 4AL BSIBritishSta
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