1、BRITISH STANDARD BS 1741-7.2: 1989 Methods for Chemical analysis of liquid milk and cream Part 7: Determination of lactose content Section 7.2 Routine method UDC 637.12 + 637.148.074:543BS1741-7.2:1989 This British Standard, having been prepared under the directionof the Dairying Standards Committee
2、, was published under the authority ofthe Board of BSI and comes intoeffect on 31 July 1989 BSI 08-1999 The following BSI references relate to the work on this standard: Committee reference DAC/3 Draft for comment 86/54970 DC ISBN 0 580 17101 9 Committees responsible for this British Standard The pr
3、eparation 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 Manufacturers Association of Public
4、Analysts Association of Public Analysts of Scotland Creamery Proprietors Association Department of Trade and Industry (Laboratory of the Government Chemist) Ice Cream Alliance Limited Intervention Board for Agricultural Produce Joint Committee of the Milk Marketing Board and the Dairy Trade Federati
5、on Milk Marketing Board Milk Marketing Board for Northern Ireland Ministry of Agriculture, Fisheries and 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 of issue Comm
6、entsBS1741-7.2:1989 BSI 08-1999 i Contents Page Committees responsible Inside front cover Foreword ii 1 Scope 1 2 Definition 1 3 Principle 1 4 Reagents 1 5 Apparatus 1 6 Sampling 1 7 Procedure 2 8 Expression of results 2 9 Repeatability 2 10 Test report 2 Publications referred to Inside back coverBS
7、1741-7.2:1989 ii BSI 08-1999 Foreword This Section of BS 1741 has been prepared under the direction of the Dairying Standards Committee. It describes a method of determination essentially identical to standard method FIL-IDF 28:1974 of the International Dairy Federation, which itself is technically
8、equivalent to the method described in clause 9 of BS 1741:1963. Part 7 comprises two Sections, published separately, which together supersede clause 9 of BS 1741:1963, which is deleted by amendment. This Section describes a titrimetric method for use for routine purposes. Section7.1 describes a pola
9、rimetric method for use for reference purposes. NOTEThis Part should be read in conjunction with Part 1 “General introduction, including preparation of samples”, published separately. A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standard
10、s 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, pages i and ii, pages1 and 2, an inside back cover and a back cover. This stan
11、dard 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.BS1741-7.2:1989 BSI 08-1999 1 1 Scope This Section of BS 1741 describes a method for the routine determination of the lactose content of liquid
12、 milk of all types except those preserved with formaldehyde or any other substance which may interfere with the test. NOTEThe titles of the publications referred to in this standard are listed on the inside back cover. 2 Definition For the purpose of this Section of BS 1741 the following definition
13、applies. lactose content the lactose content, as determined under the conditions specified in this Section of BS 1741, expressed as a percentage by mass of the anhydrous lactose in the sample 3 Principle The method initially entails deproteinisation of the milk sample followed by oxidation of lactos
14、e with hypoiodite formed by the addition of potassium iodide and chloramine-T. After acidification, the remaining hypoiodite and chloramine-T are titrated, as iodine, with thiosulphate. 4 Reagents NOTESee BS 1741-1. 4.1 General. Use reagents of recognized analytical grade, where available. 4.2 Water
15、, complying with grade 3 of BS 3978. 4.3 Tungstic acid reagent. Dissolve 7 g of sodium tungstate dihydrate (Na 2 WO 4 .2H 2 O) in 870 mL of water (4.2), add 0.1 mL of orthophosphoric acid solution (85 % (m/m) minimum) and 70 mL of sulphuric acid solution (4.7) and mix. 4.4 Chloramine-T solution, app
16、roximately 0.02 mol/L. Dissolve approximately5.7 g of chloramine-T hydrate (N-chloro-p-toluene sulphonamide, sodium salt) in water in a 1000 mL one-mark volumetric flask(5.2), dilute to the mark and mix. 4.5 Potassium iodate solution, c(KIO 3) = 0.00667 mol/L. Dissolve exactly 1.427 g of pure potass
17、ium iodate in water in a 1000 mL one-mark volumetric flask (5.2), dilute to the mark and mix. NOTEThis solution is stable for up to 1 month. 4.6 Potassium iodide solution, 100 g/L. Prepare this solution freshly and in a colourless form. 4.7 Sulphuric acid solution, c(H 2 SO 4 ) = 0.5 mol/L. 4.8 Hydr
18、ochloric acid solution, c(HCl)=approximately 2 mol/L. 4.9 Soluble starch solution, approximately 10 g/L. 4.10 Sodium thiosulphate solution, c(Na 2 S 2 O 3 ) approximately 0.042 mol/L. Dissolve 10.50 g of sodium thiosulphate pentahydrate (Na 2 S 2 O 3 .5H 2 O) in water in a 1000 mL one-mark volumetri
19、c flask(5.2), dilute to the mark and mix. Determine the exact amount-of-substance concentration of the sodium thiosulphate solution (in mol/L) immediately before use as follows. Pipette10.0 mL of the potassium iodate solution(4.5) into a 250 mL conical flask, add 5 mL of potassium iodide solution (4
20、.6) and 5 mL of hydrochloric acid solution (4.8). Titrate the liberated iodine using the sodium thiosulphate solution until the solution is pale straw yellow in colour and then add approximately 1 mL of soluble starch solution (4.9), swirl to mix and then complete the titration, recording the volume
21、 used as S mL. Calculate the exact amount-of-substance concentration of the sodium thiosulphate solution as moles of Na 2 S 2 O 3per litre, 0.40 S. 5 Apparatus Ordinary laboratory apparatus including the following. 5.1 Analytical balance, capable of weighing to an accuracy of 0.1 mg. 5.2 One-mark vo
22、lumetric flasks, capacity 100 mL and 1000 L, complying with class B of BS 1792. 5.3 Pipettes, capacity 10 mL and 20 mL, complying with class B of BS 1583. 5.4 Burette, capacity, 10 mL graduated to 0.02 mL, complying with class B of BS 846. 6 Sampling Take a representative sample of the milk to be te
23、sted using, where appropriate, the procedures described in BS 1741-1.BS1741-7.2:1989 2 BSI 08-1999 7 Procedure 7.1 Pipette and weigh to the nearest 0.001 g 10 mL of the prepared sample (see clause 6) into a tared100 mL one-mark volumetric flask (5.2). Add25 mL of water, 40 mL of tungstic acid reagen
24、t(4.3) and mix gently. Dilute to the mark with water, mix and allow the precipitate to settle out. Filter the supernatant solution through a fluted filter paper into a clean dry flask, rejecting the first few millilitres of filtrate. Pipette 10 mL of the clear filtrate into a 250 mL capacity conical
25、 flask fitted with a ground glass stopper. Add 5 mL of potassium iodide solution (4.6) and, by pipette, 20 mL of chloramine-T solution (4.4) and mix. Close the flask with a stopper moistened with potassium iodide solution (4.6) and place in a dark cupboard at room temperature. After 90 min remove th
26、e stopper from the flask, rinse the stopper with water, allowing the rinses to run into the flask. Add 5 mL of the hydrochloric acid solution (4.8) and pipette into the flask 10 mL of the sodium thiosulphate solution(4.10). Titrate the remaining liberated iodine with the sodium thiosulphate solution
27、 (4.10) from the burette (5.4) until the titrated solution becomes pale straw yellow in colour. Add about1mL of the soluble starch solution (4.9) and complete the titration carefully until the first disappearance of the blue-black colour. NOTE 1The concentration of the sodium thiosulphate solution(4
28、.10) is such that a titration of 2 mL to 3.5 mL from the burette will be obtained for most milk samples and 9.5 mL to9.7mL for the blank determination. NOTE 2The time allowed for reaction with the chloramine-T solution (4.4) is critical and should be not less than 80 min and not more than 100 min fo
29、r all samples and blanks. NOTE 3When carrying out the analysis of a batch of samples, e.g. 10 or more, the timing of the manipulations can be improved by using a good quality dispenser for the chloramine-T solution, potassium iodide and hydrochloric acid solutions. NOTE 4It is recommended that analy
30、sts should periodically undertake duplicate determinations to verify that results within the repeatability of the method (see clause 9) are being obtained. 7.2 In accordance with 7.1, carry out a blank determination using 10 mL of water instead of the prepared sample. 8 Expression of results Calcula
31、te the anhydrous lactose content, expressed as a percentage by mass, using the following expression: where: C is the exact concentration of the thiosulphate solution, in mol/L (see 4.10); P is the correction for the volume occupied by the precipitate: for whole milk samples use 0.992, for skimmed mi
32、lk samples use 0.996; V 1is the total volume of thiosulphate solution used in the blank titration, in mL; V 2is the total volume of thiosulphate solution used in the sample titration, in mL; m is the mass of the test portion, in g. Express the result to two decimal places. NOTEThe lactose monohydrat
33、e content, expressed as a percentage by mass, can be obtained by multiplying the result by1.053. 9 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 exceed 0.06 %
34、(m/m). 10 Test report The test report shall be in accordance with BS 1741-1. 171 CPV 1 V 2 () m -BS1741-7.2:1989 BSI 08-1999 Publications referred to BS 846, Specification for burettes. BS 1741, Methods for chemical analysis of liquid milk and cream. BS 1741-1, General introduction, including prepar
35、ation of samples. BS 1741-7, Determination of lactose content. BS 1741-7.1, Reference method 1) . BS 1583, Specification for one-mark pipettes. BS 1792, Specification for one-mark volumetric flasks. BS 3978, Specification for water for laboratory use. 1) Referred to in the foreword only.BS 1741-7.2:
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