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1、BRITISH STANDARD BS 684-2.25: 1997 ISO:6886:1996 Methods of analysis of Fats and fatty oils Part 2: Other methods Section 2.25 Determination of oxidative stability (accelerated oxidation test) IMPORTANT NOTICE: It is essential that BS 684-0.1 and BS 684-0.2 (BS EN ISO 661), which are published separ
2、ately, be read in conjunction with this Section. ICS 67.200.10 Licensed Copy: sheffieldun sheffieldun, na, Wed Dec 06 13:37:47 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 684-2.25:1997 This British Standard, having been prepared under the direction of the Consumer Products and Services Sector Boar
3、d, was published under the authority of the Standards Board and comes into effect on 15 March 1997 BSI 11-1998 First published August 1979 The following BSI references relate to the work on this standard: Committee reference AW/11 Draft for comment 89/54542 DC ISBN 0 580 26918 3 Committees responsib
4、le for this British Standard The preparation of this British Standard was entrusted to Technical Committee AW/11, Animal and vegetable fats and oils, upon which the following bodies were represented: Association of Public Analysts Department of Trade and Industry (Laboratory of the Government Chemis
5、t) Feed Fat Association FOSFA International International Federation of Inspection Agencies Lard Association Leatherhead Food Research Association Margarine and Shortening Manufacturers Association Ministry of Agriculture, Fisheries and Food National Edible Oil Distributors Association Seed Crushers
6、 and Oil Processors Association Soap and Detergent Industry Association Society of Chemical Industry Tropical Growers Association United Kingdom Renderers Association Ltd. Amendments issued since publication Amd. No.DateComments Licensed Copy: sheffieldun sheffieldun, na, Wed Dec 06 13:37:47 GMT+00:
7、00 2006, Uncontrolled Copy, (c) BSI BS 684-2.25:1997 BSI 11-1998i Contents Page Committees responsibleInside front cover National forewordii 1Scope1 2Definitions1 3Principle1 4Reagents and materials1 5Apparatus1 6Sampling2 7Preparation of test sample2 8Procedure2 9Calculation5 10Precision5 11Test re
8、port6 Annex A (informative) Summary of the method and examples of conductivity curves and the determination of induction time7 Annex B (informative) Bibliography9 Figure 1 Diagrammatic representation of the apparatus3 Figure 2 Diagrammatic representation of heating block, reaction vessel and measure
9、ment cell4 Figure A.1 Examples of conductivity curves8 List of referencesInside back cover Licensed Copy: sheffieldun sheffieldun, na, Wed Dec 06 13:37:47 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 684-2.25:1997 ii BSI 11-1998 National foreword This Section of BS 684 has been prepared by Technica
10、l Committee AW/11 and is identical with ISO 6886:1996 Animal and vegetable fats and oils Determination of oxidation stability (Accelerated oxidation test), published by the International Organization for Standardization (ISO) and in the preparation of which the United Kingdom played a full part. It
11、supersedes BS 684-2.25:1979 which was withdrawn in 1985, and from which it differs in that this edition has been technically updated. Additional information. ISO 5725:1986, to which informative reference is made in the text, has been superseded by ISO 5725-1:1994, ISO 5725-2:1994, ISO 5725-3:1994, I
12、SO 5725-4:1994 and ISO 5725-6:1994 which are identical with the following Parts of BS ISO 5725 Accuracy (trueness and precision) of measurement methods and results; BS ISO 5725-1:1994 General principles and definitions; BS ISO 5725-2:1994 Basic methods for the determination of repeatability and repr
13、odudibility of a standard measurement method; BS ISO 5725-3:1994 Intermediate measures of the precision of a standard measurement method; BS ISO 5725-4:1994 Basic methods for the determination of the trueness of a standard measurement method; and BS ISO 5725-6:1994 Use in practice of accuracy values
14、 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 British Standard does not of itself confer immunity from legal obligations. Cross-references Publication referred to
15、Corresponding British Standard Informative ISO 5555:1991BS EN ISO 5555:1995 Animal and vegetable fats and oils. Sampling (Identical) ISO 5725:1986BS 5497 Precision of test methods Part 1:1987 Guide for the determination of repeatability and reproducibility for a standard test method by inter-laborat
16、ory tests (Identical) Summary of pages This document comprises a front cover, an inside front cover, pages i and ii, pages 1 to 10, 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 amend
17、ment table on the inside front cover. Licensed Copy: sheffieldun sheffieldun, na, Wed Dec 06 13:37:47 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 684-2.25:1997 BSI 11-19981 1 Scope This International Standard specifies a method for the determination of the oxidation stability of fats and oils. It
18、is applicable to refined animal and vegetable fats and oils. NOTE 1The presence of volatile fatty acids and volatile acidic oxidation products makes accurate measurement impossible. 2 Definitions For the purposes of this International Standard, the following definitions apply. 2.1 induction period t
19、ime which passes between the moment when the sample reaches the desired temperature and the moment when the formation of oxidation products rapidly begins to increase 2.2 oxidation stability induction period determined according to the procedure specified in this International Standard. Oxidation st
20、ability is expressed in hours NOTE 2A temperature of 100 C is usually applied in the determination of oxidation stability. Depending on the oxidation stability of the sample under test, the determination may be carried out at a higher temperature, for example 110 C. The temperature should be chosen
21、so that an induction period of at least 5 h and at most 10 h is obtained. 3 Principle A stream of purified air is passed through the sample which has been brought to a specified temperature. The gases released during the oxidation process, together with the air, are passed into a flask containing wa
22、ter which has been demineralized or distilled and contains an electrode for measuring the conductivity. The electrode is connected to a measuring and recording device. It indicates the end of the induction period when the conductivity begins to increase rapidly. This accelerated increase is caused b
23、y the dissociation of volatile carboxylic acids produced during the oxidation process and absorbed in the water. 4 Reagents and materials Use only reagents of recognized analytical grade, and distilled or demineralized water. 4.1 Molecular sieve, with moisture indicator, 2 mbar, pore size 0,3 mm. Th
24、e molecular sieve should be dried in an oven set at 150 C and cooled down to room temperature in a siccative. 4.2 Potassium dichromate solution (optional), 20 g/l in 1 % sulfuric acid (V/V). 4.3 Petroleum ether, boiling range 40 C to 60 C, or acetone. 4.4 Cleaning agent, i.e. non-alkaline detergent
25、with strong fat removal qualities. NOTE 3 Dodecyl benzene sulfonate meets these requirements. 4.4.1 Cleaning solution A, for aeration vessels and connecting pipes, prepared from 100 g of cleaning agent (4.4) in 1 litre of water. 4.4.2 Cleaning solution B, for measurement cells, prepared from 20 g of
26、 cleaning agent (4.4) in 1 litre of water. 4.5 Glycerol 5 Apparatus Usual laboratory equipment and, in particular, the following. 5.1 Appliance for the determination of oxidation stability See Figure 1 and Figure 2 for diagrammatic representations. NOTE 4An appliance for determining oxidation stabil
27、ity can be obtained commercially under the tradename Rancimat, model 679, from Methrom-Herisau AG, Switzerland1). 5.1.1 Air filter, comprising a tube fitted with filter paper at the ends and filled with a molecular sieve (4.1), connected to the suction end of a pump. 5.1.2 Gas diaphragm pump, with a
28、n adjustable flowrate of between 3 l/h and 30 l/h and a maximum deviation of 0,03 l/h from the set value. 5.1.3 Needle valve 5.1.4 Wash bottles (four), of capacity 250 ml, connected as shown in Figure 1. Wash bottle A shall be empty. Wash bottles B, C and D shall be filled with 150 ml potassium dich
29、romate solution (4.2), 150 ml water and cotton wool, respectively, together with the molecular sieve (4.1). NOTE 5Rancimat 679 uses a molecular sieve prior to purification of the pumped air, and wash bottles are not used. The application of wash bottles is, however, strongly recommended. 5.1.5 Air-r
30、elief cock 5.1.6 Air circulator at each of the circulation points (usually six), fitted with a capillary tube of 5 mm external diameter, 0,6 mm internal diameter and 60 mm length. 5.1.7 Flowmeters (usually six), measuring range 0 l/h to 20 l/h, for connection to the air circulator (5.1.6). 1) Rancim
31、at, model 679, is an example of suitable equipment available commercially. This information is given for the convenience of users of this International Standard and does not constitute an endorsement by ISO of this equipment. Licensed Copy: sheffieldun sheffieldun, na, Wed Dec 06 13:37:47 GMT+00:00
32、2006, Uncontrolled Copy, (c) BSI BS 684-2.25:1997 2 BSI 11-1998 5.1.8 Cylindrical aeration vessels of borosilicate glass (usually six), of 25 mm external diameter and 200 mm height, connected to a sealing cap by means of a conical joint. The sealing cap shall be fitted with a gas inlet and outlet tu
33、be with a 13/5 spherical joint. The cylindrical part of the vessel shall preferably be narrower a few centimetres below the top in order to break any emerging foam. An artificial glass ring may also be used for this purpose. 5.1.9 Measurement cells (usually six), of approx. 150 ml capacity, total he
34、ight approx. 120 mm and external diameter approx. 56 mm, with a conical neck and gas inlet tube extending to the bottom inside of the vessel and fitted with a 13/5 spherical joint outside. The bottle shall be provided at the top with ventilation holes having a diameter of approx. 2 mm. 5.1.10 Electr
35、odes (usually six), for measuring conductivity, comprising double platinum electrodes with a measuring range of 0 S/cm to 300 S/cm, fitted with a 14/15 conical joint and aligned with the dimensions of the measurement cell (5.1.9). 5.1.11 Measuring and recording apparatus, comprising a) switches for
36、connecting each electrode (5.1.10) to a calibration potentiometer for setting the measuring signal at zero; b) an amplifier; c) a recorder for registering the measuring signal of each of the electrodes (5.1.10). 5.1.12 Thyristor and contact thermometer, with 150 mm insertion depth and attachments fo
37、r relay connection and an adjustable heating element; temperature scale 0 C to 150 C, graduated in 0,2 C. 5.1.13 Heating block, made of cast aluminium, of 200 mm height, adjustable to a temperature of 150 C 0,2 C. The block shall be provided with holes (usually six) for the aeration vessels (5.1.8),
38、 of 27 mm diameter and 140 mm depth, and an aperture for the contact thermometer (5.1.12) of 10 mm diameter and 140 mm depth. Alternatively, a heating bath may be used, filled with oil suitable for temperatures up to 150 C and adjustable to the nearest 0,2 C. 5.2 Thermometer, with a temperature scal
39、e of 70 C to 150 C, graduated in 0,2 C. 5.3 Measuring pipettes (two), of capacity 50 ml. 5.4 Oven, capable of being maintained at 150 C 3 C. 5.5 Oven, capable of being maintained at 500 C (optional). 5.6 Cooling bath, capable of being maintained at a steady temperature of approx. 15 C, using tap wat
40、er for example. 5.7 Connecting hoses, flexible and made of inert material polytetrafluoroethylene (PTFE) or silicone. 6 Sampling It is important that the laboratory receive a sample which is truly representative and has not been damaged or changed during transport and storage. Sampling is not part o
41、f the method specified in this International Standard. A recommended sampling method is given in ISO 55551. Store the sample in the dark at about 4 C. 7 Preparation of test sample In order to prevent the preparation of the test sample from influencing the test result, all handling of the laboratory
42、sample shall be restricted to the steps given in 7.1. and 7.2. 7.1 Light oils Remove the required quantity from the centre of the carefully homogenized sample using a pipette. 7.2 Semi-solid and solid fats Heat carefully until the fat just becomes fluid. Homogenize using a stirrer without beating ai
43、r in and then remove the required quantity from the centre of the sample using a pipette heated to the same temperature. 8 Procedure 8.1 Wash the aeration vessels (5.1.8), measurement cells (5.1.9) and their inlet and outlet tubes with petroleum ether (4.3) in order to remove as much of the organic
44、residue as possible. Rinse with hot water. Heat the aeration vessels (5.1.8) and their inlet and outlet tubes for 3 h in the cleaning solution A (4.4.1) at approx. 90 C. Soak the measurement cells (5.1.9) for 24 h in the cleaning solution B (4.4.2). Rinse the purified vessels and their inlet and out
45、let tubes thoroughly with tap water and finally with demineralized or distilled water. Dry them in the oven (5.4). NOTE 6If there is an oven (5.5) available which can be adjusted to 500 C, the following cleaning procedure can be used: rinse three times with acetone and leave them overnight in the ov
46、en set at 500 C. 8.2 Set up the apparatus as shown in Figure 1. If the apparatus is available commercially, follow the manufacturers instructions. Licensed Copy: sheffieldun sheffieldun, na, Wed Dec 06 13:37:47 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 684-2.25:1997 BSI 11-19983 Figure 1 Diagram
47、matic representation of the apparatus Licensed Copy: sheffieldun sheffieldun, na, Wed Dec 06 13:37:47 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 684-2.25:1997 4 BSI 11-1998 8.3 Attach the gas diaphragm pump (5.1.2) and adjust the flow to exactly 20 l/h using the needle valve (5.1.3) and flowmeter
48、s (5.1.7). Then switch the pump off again. 8.4 Bring the heating block (5.1.13) up to the desired temperature (usually 100 C, but see note 2) using the thyristor and contact thermometer (5.1.12). The temperature shall be maintained constant to within 0,2 C during the test period. Pour some glycerol
49、(4.5) into the holes of the heating block in order to promote heat transfer. Fill an aeration vessel (5.1.8) with 10 g of a thermostable oil and place the vessel without the sealing cap in one of the holes in the block. Place the thermometer (5.2) in the oil and check that the block is at the desired temperature. It is preferable that the temperature measurement be continued in this way during the test, even if this means that only five samples can be measured at the same time. If a heating bath (5.1.13) is used, bring i