1、BRITISH STANDARD CONFIRMED FEBRUARY1991 BS770-9:1985 ISO4099:1984 Methods for Chemical analysis of cheese Part9: Determination of nitrate and nitrite contents Cadmium reduction and photometry method NOTEThis Part should be read in conjunction with Part1 “General introduction”, published separately.
2、UDC 637.3.07:546.173+546.175BS770-9:1985 This BritishStandard, having been prepared under the directionof the Dairying Standards Committee, was published under the authority ofthe Board of BSI and comes intoeffect on 30August1985 BSI 01-2000 First published January1980 First revision August1985 The
3、following BSI references relate to the work on this standard: Committee reference DAC/3 Draft for comment 84/53993 DC ISBN 0 580 14525 5 Committees responsible for this BritishStandard The preparation of this BritishStandard was entrusted by the Dairying Standards Committee(DAC/-) to Technical Commi
4、ttee DAC/3, upon which the following bodies were represented: Association of British Preserved Milk Manufacturers Association of Public Analysts Association of Public Analysts of Scotland Creamery Proprietors Association Dairy Trade Federation Department of Trade and Industry(Laboratory of the Gover
5、nment Chemist) Ice Cream Alliance Limited 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 Food National Institute for Research
6、in Dairying Royal Association of British Dairy Farmers Royal Society of Chemistry Society of Chemical Industry Society of Dairy Technology University of Reading Amendments issued since publication Amd. No. Date of issue CommentsBS770-9:1985 BSI 01-2000 i Contents Page Committees responsible Inside f
7、ront cover National foreword ii 1 Scope and field of application 1 2 Reference 1 3 Definition 1 4 Principle 1 5 Reagents 1 6 Apparatus 2 7 Sampling 2 8 Procedure 2 9 Expression of results 4 10 Test report 5 Figure Apparatus for nitrate reduction 6 Publications referred to Inside back coverBS770-9:19
8、85 ii BSI 01-2000 National foreword This revision of this Part of BS770, which has been prepared under the direction of the Dairying Standards Committee, is identical with ISO4099:1984 “Cheese Determination of nitrate and nitrite contents Method by cadmium reduction and photometry”, prepared by ISO/
9、TC34, Agricultural food products, of the International Organization for Standardization(ISO). It supersedes BS770-9:1980, which is withdrawn. Terminology and conventions. The text of the International Standard has been approved as suitable for publication as a BritishStandard without deviation. Some
10、 terminology and certain conventions are not identical with those used in BritishStandards; attention is drawn especially to the following. The comma has been used as a decimal marker. In BritishStandards it is current practice to use a full point on the baseline as the decimal marker. Where the wor
11、ds “International Standard” appear, referring to this standard, they should be read as “British Standard”. The spelling “sulfur” is used throughout for that element and its derivatives. In BritishStandards it is current practice to use the spelling “sulphur”. Cross-reference. The Technical Committee
12、 has reviewed the provisions of ISO707:1984, to which reference is made in the text, and has decided that they are acceptable for use in conjunction with this standard. A related standard is BS809 “Methods for sampling milk and milk products”, which is in course of revision. Additional information.
13、With reference to8.4, for some hard cheeses, it is often advantageous to grate the cheese, rather than grinding, prior to mixing(kneading). NOTETypographical error. In 5.1, the chemical formula for cadmium sulfate octahydrate should read “3CdSO 4 8H 2 O”. A British Standard does not purport to inclu
14、de all the necessary 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, pagesi
15、 andii, pages1 to6, 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 the inside front cover.BS770-9:1985 BSI 01-2000 1 1 Scope and field of application This Internatio
16、nal Standard specifies a method by cadmium reduction and photometry for the determination of the nitrate and nitrite contents of cheese. The method is suitable for hard, semi-hard and soft cheeses of various ages and for processed cheese. The detection limits of the method are5mg of nitrate per kilo
17、gram and0,5mg of nitrite perkilogram. 2 Reference ISO707, Milk and milk products Methods of sampling. 3 Definition nitrate and nitrite contents of cheese the contents of substances determined by the procedure specified in this International Standard and expressed respectively as milligrams of nitrat
18、e ionand of nitrite ionper kilogram 4 Principle Extraction of the cheese with warm water, precipitation of the fat and proteins, and filtration. Reduction of the nitrate in a portion of the filtrate to nitrite, by means of copperized cadmium. Development of a red colour, in portions of both unreduce
19、d filtrate and of the reduced solution, by addition of sulfanilamide and N-1-naphthyl-ethylenediamine dihydrochloride, and spectrometric measurement at a wavelength of538nm. Calculation of the nitrite content of the sample and of the total nitrite content after reduction of nitrate, by comparing the
20、 measured absorbances with those of a series of standard sodium nitrite solutions; calculation of the nitrate content from the difference between these two contents. 5 Reagents All reagents shall be of recognized analytical grade. The water used shall be distilled or deionized water, free from nitra
21、te and nitrite. NOTEIn order to avoid possible inclusion of small gas bubbles in the copperized cadmium column(6.10), the distilled or deionized water used for the preparation of the column(8.1), for checking the reducing capacity of the column(8.2), and for regeneration of the column(8.3) should pr
22、eferably be freshly boiled and afterwards cooled to room temperature. 5.1 Cadmium, granules, diameter0,3 to0,8mm. If cadmium granules are not available commercially, they may be prepared as follows. Place a suitable number of zinc rods in a beaker and cover with a40g/l solution of cadmium sulfate oc
23、tahydrate (CdSO 4 8H 2 O). From time to time, scrape the cadmium sponge from the rods over a period of24h. Remove the zinc rods and decant the liquid until only sufficient remains to cover the cadmium. Wash the sponge two or three times with water. Transfer the cadmium to a laboratory blender togeth
24、er with400ml of0,1mol/l hydrochloric acid, and blend for a few seconds to obtain granules of the required size. Return the contents of the blender to the beaker and leave to stand for several hours, occasionally stirring to remove bubbles. Decant most of the liquid and immediately copperize as descr
25、ibed in8.1.1 to8.1.5. 5.2 Copper(II) sulfate, solution. Dissolve20g of copper(II) sulfate pentahydrate (CuSO 4 5H 2 O) in water and dilute to1000ml. 5.3 Buffer solution, pH9,6 to9,7. Dilute50ml of concentrated hydrochloric acid 20 1,19g/ml; about38%(m/m) hydrogen chloride solution with600ml of water
26、. After mixing, add135ml of ammonium hydroxide 20 0,91g/ml; about25%(m/m) ammonia solution. Dilute to1000ml with water and mix. NOTEIf ammonium hydroxide of this concentration is not available, an equivalent amount of a more concentrated solution may be used, for example100ml of35%(m/m) solution( 20
27、 0,88g/ml). Adjust the pH to9,6 to9,7 if necessary. 5.4 Hydrochloric acid, about2mol/l. Dilute160ml of concentrated hydrochloric acid( 20 1,19g/ml) to1000ml with water. 5.5 Hydrochloric acid, about0,1mol/l. Dilute50ml of the hydrochloric acid(5.4) to1000ml with water. 5.6 Solutions for precipitation
28、 of proteins and fat 5.6.1 Zinc sulfate, solution. Dissolve53,5g of zinc sulfate heptahydrate (ZnSO 4 7H 2 O) in water and dilute to100ml. 5.6.2 Potassium hexacyanoferrate(II), solution. Dissolve17,2g of potassium hexacyanoferrate(II) trihydrate K 4 Fe(CN) 6 3H 2 O in water and dilute to100ml. ( NO
29、3 ) ( NO 2 )BS770-9:1985 2 BSI 01-2000 5.7 EDTA, solution. Dissolve33,5g of disodium ethylenediaminetetraacetate dihydrate (Na 2 C 10 H 14 N 2 O 8 2H 2 O) in water and dilute to1000ml. 5.8 Solutions for colour development 5.8.1 Solution I Dissolve, by heating on a water-bath,0,5g of sulfanilamide (N
30、H 2 C 6 H 4 SO 2 NH 2 ) in a mixture of75ml of water and5ml of concentrated hydrochloric acid( 20 1,19g/ml). Cool to room temperature and dilute to100ml with water. Filter if necessary. 5.8.2 Solution II Dilute450ml of concentrated hydrochloric acid( 20 1,19g/ml) to1000ml with water. 5.8.3 Solution
31、III Dissolve0,1g of N-1-naphthyl-ethylenediamine dihydrochloride (C 10 H 7 NHCH 2 CH 2 NH 2 2HCl) in water. Dilute to100ml with water. Filter if necessary. The solution may be stored for up to1 week in a well-stoppered brown bottle in a refrigerator. 5.9 Sodium nitrite, standard solution correspondi
32、ng to0,001g of nitrite per litre. On the day of use, dissolve in water0,150g of sodium nitrite (NaNO 2 ), dried to constant mass at110 to120 C, dilute to1000ml with water in a one-mark volumetric flask and mix. Dilute10ml of this solution with20ml of the buffer solution (5.3) and dilute further to10
33、00ml with water in a one-mark volumetric flask. Mix. 1ml of this standard solution contains1,004g of 5.10 Potassium nitrate, standard solution corresponding to0,0045g of nitrate per litre. Dissolve in water1,468g of potassium nitrate(KNO 3 ), dried to constant mass at110 to120 C, dilute to1000ml wit
34、h water in a one-mark volumetric flask and mix. On the day of use, dilute5ml of this solution with20ml of the buffer solution(5.3) and dilute further to1000ml with water in a one-mark volumetric flask. Mix. 1ml of this standard solution contains4,504g of 6 Apparatus All glassware shall be thoroughly
35、 cleaned and rinsed with distilled water to ensure that it is free from nitrate and nitrite. Usual laboratory apparatus, and in particular 6.1 Analytical balance 6.2 Appropriate grinding device 6.3 Suitable laboratory mixer/homogenizer, with glass containers of capacity250 or400ml. 6.4 Conical flask
36、s, of capacity250ml. 6.5 Volumetric flasks, of capacities100;500 and1000ml, complying with the requirements of ISO1042, class B. 6.6 Pipettes, to deliver2;4;5;6;8;10;12;20 and25ml, complying with the requirements of ISO648, class A, or ISO835-1. NOTEWhere appropriate, burettes may be used instead of
37、 pipettes. 6.7 Graduated cylinders, of capacities 5; 10; 25; 100; 250; 500 and1000ml. 6.8 Glass funnels, of diameter about7cm, with short stem. 6.9 Filter paper, medium grade, of diameter about15cm, free from nitrate and nitrite. 6.10 Reduction column (for example as shown in theFigure). 6.11 Spectr
38、ometer, suitable for measuring absorbance at a wavelength of538nm, with cells of optical path length1 to2cm. 7 Sampling 7.1 SeeISO707. 7.2 Store the sample in such a way that deterioration and change in composition are prevented. 8 Procedure 8.1 Preparation of the copperized cadmium column 8.1.1 Tra
39、nsfer the cadmium granules(5.1) (approximately40 to60g for each column) into a conical flask(6.4). 8.1.2 Add sufficient of the hydrochloric acid(5.4) to cover the cadmium. Swirl for a few minutes. 8.1.3 Decant the solution and wash the cadmium in the flask with water, until it is free from chloride.
40、 8.1.4 Copperize the cadmium granules by adding the copper(II) sulfate solution(5.2)(about2,5ml pergram of cadmium) and swirling for1min. 8.1.5 Decant the solution and wash the copperized cadmium immediately with water, taking care that the cadmium is continuously covered with water. Terminate the w
41、ashing when the wash water is free from precipitated copper. NO 2 . NO 3 .BS770-9:1985 BSI 01-2000 3 8.1.6 Fit a glass wool plug to the bottom of the glass column intended to contain the copperized cadmium(seethe Figure). Fill the glass column with water. 8.1.7 Transfer the copperized cadmium into t
42、he glass column with minimum exposure to air. The height of the copperized cadmium should be15 to20cm. NOTE 1Avoid trapping air bubbles between the copperized cadmium granules. NOTE 2Take care that the level of the liquid does not fall below the top of the copperized cadmium. 8.1.8 Condition the new
43、ly prepared column by running through it a mixture of750ml of water,225ml of the standard potassium nitrate solution(5.10),20ml of the buffer solution (5.3) and20ml of the EDTA solution(5.7), at a flow rate not exceeding6ml/min, then wash the column with50ml of water. 8.2 Checking the reducing capac
44、ity of the column Carry out this check at least twice a day, at the beginning and at the end of a series of determinations. 8.2.1 Pipette20ml of the standard potassium nitrate solution (5.10) into the reservoir on top of the column. Immediately add5ml of the buffer solution(5.3) to the contents of t
45、he reservoir. Collect the eluate in a100ml volumetric flask. The flow rate shall not exceed6ml/min. 8.2.2 When the reservoir has nearly run empty, wash the walls of the reservoir with about15ml of water and, when this has run off, repeat the same treatment with another15ml portion of water. After th
46、is second portion of water has run into the column as well; completely fill the reservoir with water and allow it to pass through the column at maximum flow rate. 8.2.3 After nearly100ml of eluate has been collected, remove the volumetric flask, make up to the mark with water and mix well. 8.2.4 Pip
47、ette10ml of the eluate into a100ml volumetric flask. Add water to obtain a volume of about60ml. Proceed as specified in8.8.2, 8.8.3 and8.8.4. 8.2.5 From the nitrite content(in micrograms of nitrite ion per millilitre) of the diluted eluate(8.2.4), determined from the calibration curve(8.10), calcula
48、te the percentage reducing capacity of the column(0,0674g of per millilitre corresponds to100%reducing capacity). If the reducing capacity is less than95%, the column should be regenerated. 8.3 Regeneration of the column Regenerate the column as follows, at the end of each day after use, or more fre
49、quently if the check(8.2) indicates a loss of efficiency. 8.3.1 Add about5ml of the EDTA solution(5.7) and2ml of the hydrochloric acid(5.5) to100ml of water. Run the mixture through the column at a flow rate of about10ml/min. 8.3.2 When the reservoir has run empty, wash the column with water, the hydrochloric acid(5.5) and water successively. 8.3.3 If the column still does not show a satisfactory efficiency, repeat the procedure specified in8.1.8. 8.4 Preparation of the test sample Prior to analysis, remove the rind or mouldy surface laye
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