BS 5752-13-1995 Methods of test for coffee and coffee products - Roasted ground coffee - Determination of moisture content - Karl Fischer method (reference method)《咖啡和咖啡产品的试验方法 第13.pdf

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1、BRITISH STANDARD BS5752-13: 1995 ISO11817: 1994 Methods of test for Coffee and coffee products Part 13: Roasted ground coffee: Determination of moisture content Karl Fischer method (reference method)BS5752-13:1995 This British Standard, having been prepared under the directionof the Consumer Product

2、s and Services Sector Board, was published under theauthority of the StandardsBoard and comes intoeffect on 15 February1995 BSI10-1999 The following BSI references relate to the work on this standard: Committee reference AW/15 Draft for comment92/57071 DC ISBN 0 580 23812 1 Committees responsible fo

3、r this British Standard The preparation of this British Standard was entrusted to Technical Committee AW/15, Coffee, upon which the following bodies were represented: Association of Public Analysts British Soluble Coffee Manufacturers Association Coffee Trade Federation Ltd. Food and Drink Federatio

4、n Institute of Trading Standards Administration Ministry of Agriculture, Fisheries and Food Natural Resources Institute Amendments issued since publication Amd. No. Date CommentsBS5752-13:1995 BSI 10-1999 i Contents Page Committees responsible Inside front cover National foreword ii 1 Scope 1 2 Norm

5、ative reference 1 3 Definition 1 4 Principle 1 5 Reaction 1 6 Reagents and materials 1 7 Apparatus 1 8 Sampling 2 9 Preparation of test sample 2 10 Procedure 2 11 Expression of results 3 12 Test report 4 Annex A (informative) Results of interlaboratory test 5 Table A.1 Moisture content of roasted co

6、ffee 5 List of references Inside back coverBS5752-13:1995 ii BSI 10-1999 National foreword This Part of BS5752 has been prepared under the direction of the Consumer Products and Services Sector Board. It is identical with ISO11817:1994 Roasted ground coffee Determination of moisture content Karl Fis

7、cher method (Reference method), published by the International Organization for Standardization (ISO) and in the preparation of which the United Kingdom played a full part. This standard embodies an agreement, to which the UK was a party, reached in Subcommittee15, Coffee, of Technical Committee34,

8、Agricultural food products, of ISO. The Technical Committee has reviewed the provisions of ISO760:1978, to which reference is made in the text, and has decided that they are acceptable for use in conjunction with this standard. A related British Standard is BS2511:1970 Methods for the determination

9、of water (Karl Fischer method). NOTETypographical error. In clause5, line4, “sulfure” should be read as “sulfur”. A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a Bri

10、tish Standard does not of itself confer immunity from legal obligations. Cross-reference International Standard Corresponding British Standard ISO5725:1986 BS5497 Precision of test methods Part1:1987 Guide for the determination of repeatability and reproducibility for a standard test method by inter

11、-laboratory tests (Identical) Summary of pages This document comprises a front cover, an inside front cover, pagesi 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 am

12、endment table on the inside front cover.BS5752-13:1995 BSI 10-1999 1 1 Scope This International Standard specifies a method for the determination of moisture content of roasted ground coffee by the Karl Fischer titration method. Since it is precise, it is suitable as a reference method. 2 Normative

13、reference The following standard contains provisions which, through reference in this text, constitute provisions of this International Standard. At the time of publication, the edition indicated was valid. All standards are subject to revision, and parties to agreements based on this International

14、Standard are encouraged to investigate the possibility of applying the most recent edition of the standard indicated below. Members of IEC and ISO maintain registers of currently valid International Standards. ISO760:1978, Determination of water KarlFischer method (General method). 3 Definition For

15、the purposes of this International Standard, the following definition applies. 3.1 moisture content content of water, extracted with dried methanol in accordance with the procedure specified in this International Standard moisture content is expressed as a percentage by mass 4 Principle Extraction o

16、f the test portion with dried methanol at65 C (boiling temperature) under reflux with the exclusion of moisture. After cooling, titration of an aliquot part of the retained extract in a Karl Fischer apparatus until the end-point of the titration is reached according to the biamperometric method. 5 R

17、eaction During the determination of moisture content according to the Karl Fischer method, the water present in the sample reacts in the presence of an amine and an alcohol with iodine and sulfure dioxide: H 2 O+I 2 +SO 2 +ROH+3R n NH 3n F 2R n NH 3n HI+R n NH 3n HSO 4 R where R is an alkyl or alkox

18、yl group. The end-point of the reaction is obtained electrometrically by a surplus of iodine. 6 Reagents and materials Use only reagents of recognized analytical quality and distilled or demineralized water or water of equivalent purity. 6.1 Pyridine-free Karl Fischer reagent 1) , one- or two-compon

19、ent system. 6.2 Methanol, containing not more than0,01% (m/m) of water. 6.3 Molecular sieve, 0,3nm, pearl shaped; diameter approx.2mm; bulk density (loose) approx.75g per100ml. 7 Apparatus Usual laboratory apparatus and, in particular, the following. 7.1 Drying oven, capable of being operated at40 C

20、 5 C, for constant storage of glassware. 7.2 Analytical balances 7.2.1 Ultra-microbalance, capable of weighing to an accuracy of0,0001g, for the determination of the titre of the Karl Fischer reagent. 7.2.2 Microbalance, capable of weighing to0,001g, for weighing the test portion. 7.3 Reflux apparat

21、us 7.3.1 Heating device, suitable for a100ml round-bottomed flask; heating rate to be maintained by means of a thyristor regulator. 7.3.2 Round-bottomed flask, of100ml capacity, with conical sleeve. NOTE 1At least three flasks are required. 7.3.3 Reflux condenser, 25cm to30cm in length, with conical

22、 ground joint. 7.3.4 Drying tubes, filled with approx.40g of molecular sieve(6.3), changed daily. 7.4 Titration apparatus NOTE 2For the determination of the moisture content according to the Karl Fischer method, complete titration apparatuses which consist of the single parts given in7.4.1 to7.4.5 a

23、re commercially available. 7.4.1 End-point indicator, for titration according to the biamperometric method. 7.4.2 Platinium double electrode 7.4.3 Magnetic stirrer, fitted with a stirring rod covered with polytetrafluoroethylene. 7.4.4 Titration vessel, of approx.100ml capacity, with at least three

24、ground glass sockets. 1) Suitable products are commercially available.BS5752-13:1995 2 BSI 10-1999 Connect one of the ground glass sockets to the burette, place the platinium double electrode in the second one, and use the third one to add the reagents and the sample. A discharge tap at the bottom o

25、f the vessel is an advantage. Connect it to the vessel with a capillary tube having a volume of not more than0,5ml. Titration vessels without a discharge tap should have four ground glass sockets. Empty them by means of a glass tube drawn out up to a capillary reaching the bottom of the vessel. 7.4.

26、5 Burette, graduated in intervals of0,05ml, reagent bottle and drying tubes. When using automatic titration devices, the minimum interval depends on data from the manufacturer (e.g.0,02ml). Protect the reagent bottle, the burette and the KarlFischer reagent(6.1) from light. Close all the ventilation

27、 sockets in the titration vessel either with drying tubes which have been charged with an effective drying agent or molecular sieve, or with drying receivers filled with the reagent(6.1) in order to exclude the influence of moisture from the air. 7.5 Syringe, for measuring304l to404l of water for th

28、e determination of the titre, e.g.a0,05ml Hamilton microsyringe. 7.6 Desiccator, containing an effective drying agent, e.g.silica gel with moisture indicator, or molecular sieve. 7.7 Measuring cylinder, of capacity100ml, graduated in1ml. 7.8 Pipettes, of capacities5ml,10ml and50ml. 8 Sampling It is

29、important that the laboratory receive a sample which is truly representative and has not been damaged or changed during transport or storage. 9 Preparation of test sample 9.1 Roasted ground coffee Mix thoroughly the laboratory sample. 9.2 Vacuum-packed roasted ground coffee One original packing unit

30、 shall be considered as the test sample. 10 Procedure 10.1 Determination of titre of the Karl Fischer reagent Determine the titre of the reagent(6.1) in accordance with ISO760. For an exact dosage of smaller amounts of water, use the syringe(7.5), the discharge of which is obtained by difference wei

31、ghing with the ultra-microbalance(7.2.1). Determine the volume required by several titrations(10.6) of30mg to40mg of water and calculate the titre, T, according to the following formula: 10.2 Preparation of glassware Keep all required glassware in the oven(7.1) set at40 C. Prior to use, allow the gl

32、assware to cool to room temperature in the desiccator(7.6) and store it there. 10.3 Preparation of reflux apparatus Place75ml of methanol(6.2) in a100ml round-bottomed flask(7.3.2). Adjust the heating rate of the heating device(7.3.1), with the cooling water supply closed, by means of the thyristor

33、regulator so that the methanol condenses at the upper end of the reflux condenser(7.3.3). Note the adjustment. Then close the apparatus with the drying tube(7.3.4) and boil the methanol at an unchanged heating rate for30min under reflux. Open the cooling water supply and allow the apparatus to cool.

34、 Do not remove the flask until the apparatus has cooled to room temperature. After this preparation, remove any adhering water from the inner surface of the reflux condenser. Immediately after having removed the flasks, close the reflux apparatus thus prepared with a round-bottomed flask containing

35、an effective drying agent (molecular sieve or silica gel with humidity indicator). 10.4 Determination of blank value Pipette50,0ml of the dried methanol into a dried100ml round-bottomed flask(7.3.2) and boil with running cooling water for30min. Then allow the round-bottomed flask to cool, remove it

36、from the apparatus and close the flask immediately. Take an aliquot part of10,0ml for titration of the blank value and titrate it as described in10.5 and10.6. Record the volume of Karl Fischer reagent used. where m is the mass, in milligrams, of water used; V is the volume, in millilitres, of the re

37、agent(6.1) used. T m V - =BS5752-13:1995 BSI 10-1999 3 10.5 Extraction of samples Weigh, to the nearest0,001g, approx.3g of the test sample (clause9) into a100ml round-bottomed flask. Add50,0ml of dried methanol by means of a pipette and close the flask immediately. Join the round-bottomed flask to

38、the reflux condenser and heat the contents at the same heating rate as described in10.3 (noted thyristor regulation) with running cooling water up to the start of reflux. Boil for a further30min at reflux. Then allow the round-bottomed flask to cool to room temperature, remove it from the reflux con

39、denser and close the flask immediately. 10.6 Preparation of the titration apparatus Prepare the titration apparatus(7.4) in accordance with the manufacturers instructions. The air-tightness of the titration vessel is of special importance. NOTE 3A measure of lack of air-tightness is the leakage rate

40、(4g H 2 O per min) of the system. This can be between104g/min and604g/min depending on the ambient air humidity. The determination of the leakage rate is made in accordance with the instructions of the manufacturer of the KarlFischer titrator used and should not exceed604g/min for the determination.

41、 Place35ml of the dried methanol (receiver) in the titration vessel. Take care that the metallic parts of the platinum double electrode(7.4.2) are immersed completely. Before the first daily use, condition the system in a so-called stand-by titration for20min when the adhered water is removed comple

42、tely. 10.7 Determination Depending on the expected water content of the roasted ground coffee samples, take an aliquot part of10,0ml water content 3% (m/m) from the sample extracted in accordance with10.5 and pipette it rapidly into the titration vessel prepared in accordance with10.6, opening the v

43、essel for as short a time as possible. Start the automatic titration while stirring with the magnetic stirrer(7.4.3). The dosage of the reagent and the recognition of the end-point is controlled automatically by the apparatus. Adjust the apparatus so that the electronic titration ends if no potentia

44、l change of more than15mV occurs within20s. Carry out three titrations on each extract (or for the determination of the blank value). Do not change the receiver (see10.6). Use the arithmetic mean of the results of these three titrations as one value for the calculation(11.1). 11 Expression of result

45、s 11.1 Calculation The moisture content of roasted ground coffee, w wexpressed as a percentage by mass of the sample, is calculated according to the following formula: Round the result to one decimal place. 11.2 Precision Results of an interlaboratory test are given in Annex A. The interlaboratory t

46、est was carried out using vacuum-packed samples exclusively. 11.2.1 Repeatability The absolute difference between two independent test results, obtained using the same method on identical test material in the same laboratory by the same operator using the same equipment within a short interval of ti

47、me, should not be greater than0,14%. 11.2.2 Reproducibility The absolute difference between two single test results, obtained using the same method on identical test material in different laboratories with different operators using different equipment, should not be greater than0,25%. where T is the

48、 titre(10.1) of the Karl Fischer reagent, in milligrams of water per millilitre; m E is the mass of the test portion, in milligrams; V 1 is the volume of methanol, in millilitres, used for the extraction(10.5); V 2 is the aliquot volume of the extract, in millilitres, used for the titration; V 3 is

49、the volume, in millilitres, of the KarlFischer reagent(6.1) used for the titration of the blank value(10.4); V 4 is the volume, in millilitres, of the KarlFischer reagent(6.1) used for the titration of the sample extract(10.7); V 5 is the aliquot volume of methanol, in millilitres, used for the determination of the blank value(10.4). w w V 4 V 3 V 2 V 5 - T V 1 V 2 - 100 m E - =BS5752-13:1995 4 BSI 10-1999 12 Test report The test report shall specify the method used, the test result obtained,