1、INTERNATIONAL STANDARD ISO 5519 Second edition 2008-02-15 Reference number ISO 5519:2008(E) ISO 2008 Fruits, vegetables and derived products Determination of sorbic acid content Fruits, lgumes et produits drivs Dtermination de la teneur en acide sorbiqueISO 5519:2008(E) ii ISO 2008 All rights reserv
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5、RIGHT PROTECTED DOCUMENT ISO 2008 All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below
6、 or ISOs member body in the country of the requester. ISO copyright office Case postale 56 CH-1211 Geneva 20 Tel. + 41 22 749 01 11 Fax + 41 22 749 09 47 E-mail copyrightiso.org Web www.iso.org Published in SwitzerlandISO 5519:2008(E) ISO 2008 All rights reserved iii Contents Page 1 Scope 1 2 Princi
7、ple 1 3 Reagents 1 4 Apparatus . 2 5 Sample 3 6 Procedure . 3 7 Calculation and expression of results . 7 8 Repeatability 7 9 Test report 8 Annex A (normative) Steam distillation apparatus . 9ISO 5519:2008(E) iv ISO 2008 All rights reserved Foreword ISO (the International Organization for Standardiz
8、ation) is a worldwide federation of national standards bodies (ISO member bodies). The work of preparing International Standards is normally carried out through ISO technical committees. Each member body interested in a subject for which a technical committee has been established has the right to be
9、 represented on that committee. International organizations, governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization. International Standar
10、ds are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2. The main task of technical committees is to prepare International Standards. Draft International Standards adopted by the technical committees are circulated to the member bodies for voting. Publication as an Intern
11、ational Standard requires approval by at least 75 % of the member bodies casting a vote. Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. ISO shall not be held responsible for identifying any or all such patent rights. ISO 5519 was
12、 prepared by Technical Committee ISO/TC 34, Food products, Subcommittee SC 3, Fruit and vegetable products. This second edition cancels and replaces the first edition (ISO 5519:1978), which has been technically revised.ISO 5519:2008(E) ISO 2008 All rights reserved v Introduction The determination of
13、 the sorbic acid content of fruits, vegetables and derived products has been studied in numerous projects during the acids use as a fungicide, especially in wines. Because of its great volatility (very similar to that of acetic acid), the simplest extraction process is its entrainment by steam. This
14、 method has the advantage of producing an almost pure aqueous solution of sorbic acid. Two techniques for the determination of the quantity of sorbic acid contained in this solution are described in this International Standard, namely: Technique A: spectrophotometry in the ultraviolet range, carried
15、 out after oxidation of sulfur dioxide, which would interfere. The oxidation occurs spontaneously in a few minutes in air, after the addition of a trace of a copper catalyst. The natural essential oils of citrus fruits do not interfere with the determination, provided that they are present in the sm
16、all quantities normal in juice not enriched with essential oils. When the quantities of essential oils are significant, they may be eliminated beforehand by the same method as that applied in technique B. Technique B: colorimetry based on Schmidts reaction, which requires the elimination of ethanol
17、and essential oils by the evaporation of an aliquot portion of the distillate. This technique, not as rapid as techniqueA, but giving comparable results, is provided for use when a spectrophotometer allowing measurements in the ultraviolet range is not available. The interference caused by essential
18、 oils of garlic, onion or leek may be eliminated, when using either technique, by the evaporation of an aliquot portion of the distillate vi INTERNATIONAL STANDARD ISO 5519:2008(E) ISO 2008 All rights reserved 1 Fruits, vegetables and derived products Determination of sorbic acid content 1S c o p e
19、This International Standard specifies a method for extracting the sorbic acid present in fruits, vegetables and derived products, and two techniques for determining the sorbic acid extracted. 2 Principle Homogenization of the product, followed by quantitative entrainment, by steam, of the sorbic aci
20、d present in a test portion. Determination of this acid in the distillate obtained, either by spectrophotometry in the ultraviolet range (technique A), or by measuring by photocolorimetry or by spectrophotometry, the pink colour obtained after oxidation by chromic acid and then treatment with thioba
21、rbituric acid (technique B). 3R e a g e n t s Use only reagents of recognized analytical quality, and distilled water or water of at least equivalent purity. 3.1 Tartaric acid COOH (CH OH) 2 COOH, crystalline. 3.2 Sorbic acid CH 3 (CH:CH) 2 COOH, standard solution, prepared by one of the following m
22、ethods (3.2.1 or 3.2.2). 3.2.1 Dissolve of sorbic acid in to of a sodium hydroxide solution. Transfer quantitatively into a volumetric flask, and dilute to the mark with water. Introduce of the solution obtained into a second flask, and dilute to the mark with water. 3.2.2 Dissolve of potassium sorb
23、ate, CH 3 (CH:CH) 2 COOK, (previously recrystallized and dried to constant mass in a drier at or in a desiccator over concentrated sulfuric acid) in water in a volumetric flask, and dilute to the mark with water. Introduce of the solution obtained into a secondflask, and dilute to the mark with wate
24、r. 3.3 Calcium hydroxide Ca(OH) 2 (if necessary), about ( ) solution. 3.4 Acetic acid (CH 3 COOH), solution. 3.5 Lactic acid CH 3 CH(OH)COOH, solution. 3.6 Copper, catalyst solution, for technique A. In a volumetric flask, dissolve, in a little water, of sodium hydrogen carbonate (NaHCO 3 ), andof p
25、ure copper(II) sulfate pentahydrate (CuSO 4 5H 2 O). Dilute to the mark with water. 3.7 Chromic/sulfuric acid solution, for technique B. Dissolve of potassium dichromate (K 2 Cr 2 O 7 ) in approximately of water. Transfer quantitatively into a volumetric flask. Add of a sulfuric acid (H 2 SO 4 ) sol
26、ution. Dilute to the mark with water. 1 litre of sulfuric acid solution contains of sulfuric acid, i.e. of sulfuric acid, . 0,010 g/l 0,100 g 10 ml 12 ml 0,1 N 1 000 ml 100 ml 1 000 ml 0,134 g 105 C 1 000 ml 100 ml 1 000 ml 0,04 N 1,48 g/l 0,1 N 1N 1 000 ml 0,5 g 0,001 g 0,050 g 90 ml 200 ml 100 ml
27、0,3 N 0,3 N 14,7 g 8,4 ml 20 = 1,84 g/mlISO 5519:2008(E) 2 ISO 2008 All rights reserved 3.8 Thiobarbituric acid solution, for technique B. Dissolve of thiobarbituric acid (C 4 H 4 N 2 O 2 S) in of water to which has been added of a sodium hydroxide (NaOH) solution. Transfer quantitatively into a vol
28、umetric flask, and add of a hydrochloric acid (HCl) solution. Dilute to the mark with water. This solution is not stable and must be used within following its preparation. 4 Apparatus Usual laboratory apparatus and, in particular, the following. 4.1 Analytical balance. 4.2 Homogenizer or mortar, as
29、appropriate. 4.3 Boiling water bath. 4.4 Steam distillation apparatus (see Figure A.1), comprising the items listed in 4.4.1 to 4.4.5. 4.4.1 Steam generator flask, of capacity to . 4.4.2 Bubbler, consisting of a cylindrical tube in diameter and in height, the lower part of which is closed and enlarg
30、ed into a sphere having a diameter of . The steam supply tube shall endabove the bottom of the bubbler. The spherical part, in which the product is placed, may be heated either electrically or by a flame. In the latter case, the burnt gases shall be deflected by a metal disc of diameter, having a ce
31、ntral orifice of approximately diameter in which the bottom of the bubbler is engaged. This device avoids the pyrogenation of materials which may be extracted from the product. The auxiliary heating shall be controlled so that the volume of the product placed in the bubbler neither decreases nor inc
32、reases by more than during the distillation. 4.4.3 Fractionating column, through which the vapour containing volatile acid passes. It may consist of a cylindrical tube of diameter and height , containing a corrugated helix of No. 100 stainless steel mesh, the helix having a pitch of , or a column of
33、 diameter and height , having glass internal points, or any other device having the same fractionating efficiency. The fractionation of the vapour is indispensable to retain hydroxymethylfurfural when it is present. This substance and its hydrolysis products absorb ultraviolet radiations at . The fr
34、actionating column may be reduced to in height or be replaced by a Kjeldahl flask when the product is free of hydroxymethylfurfural. 4.4.4 Condenser, of the West type, of effective length , placed vertically to ensure condensation of the vapour and complete cooling of the distillate. 4.4.5 Receiver
35、flasks, of capacities or : liquid products: flask with a graduation mark at ; thick or solid products: flask. 0,500 g 50 ml 10 ml 1N 100 ml 11 ml 1N 5h 1 000 ml 1 500 ml 30 mm 270 mm 60 mm 10 mm 150 mm 40 mm 5ml 20 mm 500 mm 15 mm 20 mm 600 mm 256 nm 200 mm 400 mm 200 ml 500 ml 200 ml 200 ml 500 mlI
36、SO 5519:2008(E) ISO 2008 All rights reserved 3 4.4.6 Checking of efficiency of steam distillation apparatus. The distillation apparatus shall allow of distillate to be collected in to , and shall also comply with the following minimal conditions. a) In normal distilling conditions, of a known quanti
37、ty of acetic acid added to the sample shall be found in the distillate, which shall be . For this test, use of a acetic acid solution (3.4). b) In the same distilling conditions, not more than 5 parts per thousand of a known quantity of lactic acid added to the sample shall be found in the distillat
38、e, which shall be . For this test, use of lactic acid solution (3.5). 4.5 Pipettes, of capacities , and . 4.6 Graduated pipettes, of appropriate capacities. 4.7 Evaporating dish. 4.8 Conical flasks, for technique A, of capacity . 4.9 Spectrophotometer, for technique A, allowing measurements at a wav
39、elength of (ultraviolet), with silica cells of optical path length. 4.10 Volumetric flasks, for technique B, of capacity . 4.11 Photocolorimeter, fitted with a green filter, or spectrophotometer allowing measurements at a wavelength of , for technique B. 5S a m p l e It is important that the laborat
40、ory receive a sample which is truly representative and has not been damaged or changed during transport or storage. 6 Procedure 6.1 Preparation of test sample 6.1.1 Liquid products (juices, pulpy fluid products, syrups, etc.) Thoroughly mix the laboratory sample. 6.1.2 Thick products (marmalades, ja
41、ms, etc.) Homogenize the laboratory sample after having carefully mixed it. 6.1.3 Solid products (fruits, vegetables) Cut a part of the laboratory sample into small pieces, and remove seeds, stalks and carpellary cells, if necessary, and carefully homogenize approximately of the sample. 6.1.4 Frozen
42、 or deep-frozen products After thawing the sample in a closed container and removing, if necessary, seeds, stalks and carpellary cells, mix the product with the liquid formed during the thawing process and proceed as described in 6.1.1, 6.1.2 or 6.1.3, as appropriate. 300 ml 12 min 15 min 99,5 % 200
43、 ml 20 ml 0,1 N 200 ml 20 ml 1N 10 ml 20 ml 25 ml 50 ml 256 nm 10 mm 25 ml 532 nm 40 gISO 5519:2008(E) 4 ISO 2008 All rights reserved 6.2 Test portion 6.2.1 Liquid products Using a pipette (4.5), take of the test sample (6.1) and introduce it into the bubbler (4.4.2). The test portion may also be ta
44、ken by mass, by weighing, to the nearest , approximately of the test sample. 6.2.2 Thick or solid products Weigh, to the nearest , approximately of the test sample (6.1) and introduce it into the bubbler (4.4.2) with the minimum of water necessary to entrain the whole of the test portion and to make
45、 the mixture sufficiently fluid. NOTE In certain cases, it is necessary to leave the test portion to soak in the water for to . 6.3 Distillation 6.3.1 General Introduce of the tartaric acid (3.1) into the bubbler (4.4.2) containing the test portion (6.2). Connect the bubbler to the steam generator f
46、lask (4.4.1) and to the condenser (4.4.4) and simultaneously heat the flask and the bubbler. Then carry out the distillation, making sure that the volume of the contents of the bubbler remains constant to within . 6.3.2 In the case of liquid products (6.2.1) Collect the distillate in the receiver fl
47、ask (4.4.5), stopping the distillation when the mark is reached. 6.3.3 In the case of thick or solid products (6.2.2) Collect, in the flask (4.4.5), a volume of distillate at least 20 times greater than the volume of the contents of the bubbler. Measure the volume ( ) collected, using a graduated cy
48、linder. 6.4 Technique A: determination by spectrophotometry in the ultraviolet range 6.4.1 Determination 6.4.1.1 If the initial product contains essential oils of garlic, onion or leek, the presence of these essential oils causes significant absorbance, especially in the case of garlic. Complete eva
49、poration 1)of the distillate, after its being made alkaline, allows the effect of this absorbance to be counteracted. When these essential oils are present, therefore, take, with a pipette (4.5), distillate (6.3) and transfer it to a small evaporating dish (4.7); make it alkaline with to of the calcium hydroxide solution (3.3), evaporate it to dryness on the boiling water bath (4.3) and reconstitute it with water to re-establish the initial volume. 1) The evaporation to dr
