1、BRITISH STANDARD BS ISO 21807:2004 Microbiology of food and animal feeding stuffs Determination of water activity ICS 07.100.30 BS ISO 21807:2004 This British Standard was published under the authority of the Standards Policy and Strategy Committee on 29 September 2004 BSI 29 September 2004 ISBN 0 5
2、80 44511 9 National foreword This British Standard reproduces verbatim ISO 21807:2004 and implements it as the UK national standard. The UK participation in its preparation was entrusted to Technical Committee AW/9, Microbiology, which has the responsibility to: A list of organizations represented o
3、n this committee can be obtained on request to its secretary. Cross-references The British Standards which implement international publications referred to in this document may be found in the BSI Catalogue under the section entitled “International Standards Correspondence Index”, or by using the “S
4、earch” facility of the BSI Electronic Catalogue or of British Standards Online. This publication does not purport to include all the necessary provisions of a contract. Users are responsible for its correct application. Compliance with a British Standard does not of itself confer immunity from legal
5、 obligations. aid enquirers to understand the text; present to the responsible international/European committee any enquiries on the interpretation, or proposals for change, and keep the UK interests informed; monitor related international and European developments and promulgate them in the UK. Sum
6、mary of pages This document comprises a front cover, an inside front cover, the ISO title page, pages ii to iv, pages 1 to 7 and a back cover. The BSI copyright notice displayed in this document indicates when the document was last issued. Amendments issued since publication Amd. No. Date Comments R
7、eference number ISO 21807:2004(E) OSI 4002INTERNATIONAL STANDARD ISO 21807 First edition 2004-09-01 Microbiology of food and animal feeding stuffs Determination of water activity Microbiologie des aliments Dtermination de lactivit de leau BSISO21807:2004IS:70812 O4002(E) DPlcsid Fremia ihTs PDF file
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11、to sswrehise specified, on trap fo this lbupictaion maeb y cudorperro de tuilizi den yna form ro na ybm ynae,s lecetrinoc ro mecinahcal, inclidung tohpcoiypodna gn micrfoilm, wittuoh repmissii non writign from ietI rehSa Ot tsserdda eh ebolw or ISOs memreb i ydobn the cnuotrfo y ttseuqer ehe.r ISO c
12、irypothg fofice saCe tsopale 65 eneG 1121-HC 02 av leT. 4 + 10 947 22 1 11 xaF0 947 22 14 + 9 74 E-mail coirypthgiso.o gr We bwww.is.o gro Pulbisdehi n Switlrez dnaii ISO 4002 Allr ithgsr esedevrBSISO21807:2004IS:70812 O4002(E) I SO 4002 All irthgs ersedevr iiiForeword ISO (the International Organiz
13、ation for Standardization) 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
14、 has the right to be 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. I
15、nternational Standards 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. Publ
16、ication as an International 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
17、rights. ISO 21807 was prepared by Technical Committee ISO/TC 34, Food products, Subcommittee SC 9, Microbiology. BSISO21807:2004IS:70812 O4002(E) iv I SO 4002 All irthgs ersedevrIntroduction Microorganisms require water for their metabolic activities, but only a certain fraction, the so-called “free
18、 water”, of the total water present in any foodstuff is available for this purpose. The amount of “free water”, termed the water activity, depends upon the nature and quantity of the components dissolved in the aqueous phase of the product (see reference 1). Various species of microorganisms tolerat
19、e only water activities that are within certain levels. Water activity can therefore be used to predict microbial growth and determine the microbiological stability of a food product. BSISO21807:2004INTENRATIONAL TSANDADR IS:70812 O4002(E)I SO 4002 All irthgs ersedevr 1Microbiology of food and anima
20、l feeding stuffs Determination of water activity 1 Scope This International Standard gives basic principles and requirements for physical methods of determining the water activity of products intended for human consumption and the feeding of animals. Water activity can be used to predict microbial g
21、rowth and determine the microbiological stability of a food product, and it also provides an important, quantitatively determinable criterion for estimating the times for which a foodstuff can be kept. 2 Normative references The following referenced documents are indispensable for the application of
22、 this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. ISO 7218, Microbiology of food and animal feeding stuffs General rules for microbiological examinations 3 Terms and definiti
23、ons For the purposes of this document, the following terms and definitions apply. 3.1 water activity a wratio of the water-vapour pressure in the foodstuff to the vapour pressure of pure water at the same temperature ( ) () F EM w S100 p T c a p T = where c EMis the relative equilibrium moisture con
24、tent of the atmosphere in contact with the foodstuff; p F (T) is the partial water-vapour pressure in equilibrium with the foodstuff at the temperature T (kept constant during measurement); p S (T) is the saturated partial pressure of pure water at the same temperature (T); this can be obtained from
25、 reference tables of water vapour pressure. NOTE The water activity is therefore a dimensionless quantity, with a completely anhydrous test specimen having a water activity of 0,0 and pure salt-free water having one of 1,0. The water activities of most foodstuffs are at the upper end of this scale,
26、and range from approximately 0,992 for untreated, raw meat down to about 0,800 for salted and dried products. BSISO21807:2004IS:70812 O4002(E) 2 I SO 4002 All irthgs ersedevr4 Measurement principles and apparatus A wide variety of measurement principles can be used to determine the water activity of
27、 foodstuffs (for details see references 2 and 3), including the direct or indirect determination of the equilibrium water-vapour pressure in closed systems. Examples of such methods are as follows: a) direct manometric pressure measurement; b) dew-point measurement c) determination of the change in
28、the capacitance of a capacitor; d) determination of the change in the electrical conductivity of an electrolyte; e) measurement of the change in length of a thread; f) determination of the increase in mass of a sorbent; g) determination of changes in temperature (micropsychrometry) when equilibrium
29、is established in closed systems; h) determination of the freezing point in an open system without establishing equilibrium. 5 Requirements for the measurement of water activity The general rules for microbiological examinations given in ISO 7218 shall be followed. Data on water activity reported in
30、 the literature are predominantly based on a measurement temperature of 25 C and this often applies also to most tables containing calibration standards for testing the measuring instruments. For methods a) to g) in Clause 4, it is therefore advisable to measure a wat 25 C. Variations of 1 C in the
31、actual measurement temperature will have no appreciable effect on the water activity. Methods for determining the a wvalue of foodstuffs shall comply with the following requirements. a) The method shall be accurate and reproducible and with a clear endpoint. The speed of measurement, ease of use and
32、 durability are also other important attributes relevant to the selection of a method. b) The method shall be capable of operating from the upper range of a wfrom 0,999 to 0,600. c) Calibration of a method shall be carried out, and the accuracy measured, using reference standards of either saturated
33、 salt solutions (see Annex A) or solutions of sodium chloride (see Annex B). d) The repeatability limit shall correspond to a standard deviation of 0,002 s n 1 for the a wrange from 0,999 down to 0,600. e) The method shall be capable of measuring a sufficiently large and, consequently, representativ
34、e sample. 6 Handling of instruments 6.1 The user shall always comply with the instructions given by the manufacturer of the measuring instrument concerned and shall check that the requirements specified in Clause 5 are fulfilled. Points 6.2 to 6.8 apply to methods carried out in closed systems i.e.
35、methods a) to g) in Clause 4, while points 6.9 to 6.11 apply to method h). BSISO21807:2004IS:70812 O4002(E) I SO 4002 All irthgs ersedevr 36.2 Before carrying out a single or series of measurements, the equipment shall be recalibrated (at least daily) using the salt standards given in Annex A or B.
36、If the device used cannot be calibrated internally, this may be done by plotting the experimental a wvalue determined using a particular salt solution on the x-axis and the associated theoretical a wvalue on the y-axis. (See Annex B for an example of such a curve.) At least three measurement points
37、shall be used for calibration, chosen is such a way that the measured sample value is within this range. Measured values for the salts/solutions may also be checked against existing calibrations. 6.3 Steps shall be taken to ensure that the temperature is constant while equilibrium is being establish
38、ed in the specimen chamber (measurement cell). The temperature shall not exceed 1 C. 6.4 The test specimen shall be conditioned at the temperature of the specimen chamber (measurement cell) beforehand. During this period, the test specimen shall be kept in an hermetically sealed container to prevent
39、 water vapour movement. This container shall only be opened shortly before the specimen is placed in the specimen chamber, which shall immediately be closed again. 6.5 To prevent any soiling of the sensor by the test specimen, it shall be checked before every measurement and cleaned if necessary, fo
40、llowing exactly the recommendations of manufacturer. 6.6 Gases emitted by the specimen material e.g ethanol, ammonia (in the case of fermented products) shall be prevented from affecting the measurement by selecting a suitable measurement method or by protective devices (e.g. active carbon filters).
41、 6.7 The measurement time will depend both on the test specimen and also on the measurement method used. For methods a) to g) in Clause 4, the average measurement time can range from several minutes to several hours because of the need to establish equilibrium. 6.8 In the case of methods in which th
42、e a wvalue is determined by using sorption processes methods c) to g) in Clause 4, the measurement shall always be carried out adsorptively because the measurement characteristic may be displaced by a hysteresis effect in the case of desorptive measurement. For this purpose, the measurement cell sha
43、ll be ventilated long enough for the next measurement to be started at as low a reading as possible (e.g. room humidity). 6.9 In all cases, salt solutions of fairly high concentration are inherently unsuitable for calibrating systems employing method h) since the salt may be precipitated when the ca
44、librating specimen is cooled as a result of the solution becoming more concentrated. Distilled water (a w= 1,000) and NaCI solutions with a concentration of up to about 8 % may be used for calibration. 6.10 Depending on the test specimen, the measurement time of method h) is roughly 6 min to 20 min,
45、 but this can be cut down further by cooling the specimen beforehand in a refrigerator (set to 2 C or above). 6.11 Method h) is not susceptible to interference effects due to the specimen and external factors. 7 Obtaining a representative specimen The distribution of water activity may be presumed t
46、o be largely homogeneous in virtually all foodstuffs. Homogenization using a mincer is therefore unnecessary. Such treatment is also inadvisable since the sample material may become hot during mincing and give off water, with the result that it will no longer be representative of the foodstuff under
47、 examination. Fermented meat products (e.g. sausage and ham) in which a water activity gradient is formed between the inside and outside regions because of drying are an exception. If necessary, the water activity conditions may be determined in the inside and outside regions, or even at points dist
48、ributed over the cross-section, so as to cover all the constituents by systematically selecting the measurement points. Another exception is water-in-oil emulsions (e.g. margarines) which have heterogeneous water activity even after homogenization. BSISO21807:2004IS:70812 O4002(E) 4 I SO 4002 All ir
49、thgs ersedevrAnnex A (normative) Water activity of saturated salt solutions at 25 C Salt a wSalt a wMgCl 20,328 KBr 0,809 K 2 CO 30,432 (NH 4 ) 2 SO 40,810 Mg(NO 3 ) 20,529 KCl 0,843 NaBr 0,576 Sr(NO 3 ) 20,851 CoCl 20,649 BaCl 20,902 SrCl 20,709 KNO 30,936 NaNO 30,743 K 2 SO 40,973 NaCl 0,753 NOTE Taken from Reference 4. BSISO21807:2004IS:70812 O4002(E) I SO 4002 All irthgs ersedevr 5Annex B (normative) Water activity of aqueous N
