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Pawkit

Theory

43

Typically water activity is measured at atmospheric
pressure, so only the osmotic and matric effects are
important.

Osmotic effects

Osmotic effects are well known from biology and
physical chemistry. Water is diluted when a solute is
added. If this diluted water is separated from pure
water by a semi-permeable membrane, water tends
to move form the pure water side through the mem-
brane to the side with the added solute. If sufficient
pressure is applied to the solute-water mixture to
just stop the flow, this pressure is a measure of the
osmotic potential of the solution. Addition of one
mole of an ideal solute to a kilogram of water pro-
duces an osmotic pressure of 22.4 atm. This lowers
the water activity of the solution from 1.0 to 0.98

a

w

.

For a given amount of solute, increasing the water
content of the systems dilutes the solute, decreasing
the osmotic pressure, and increasing the water
activity. Since microbial cells are high concentra-
tions of solute surrounded by semi-permeable
membranes, the osmotic effect on the free energy of
the water is important for determining microbial
water relations and therefore their activity.

Matric effects

The sample matrix affects

a

w

by physically binding

water within its structure through adhesive and

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Содержание Pawkit

Страница 1: ...Portable Water Activity Measurement System Operator s Manual Ver 1 3 ...

Страница 2: ...Decagon Devices Inc 950 NE Nelson Court Pullman WA 99163 tel 509 332 5158 fax 509 332 5158 www decagon com pawkit pawkit decagon com Copyright 2001 Decagon Devices Inc All rights reserved ...

Страница 3: ...acy 5 Getting Started 6 Components of your Pawkit system 6 Preparing for Operation 6 3 Operation 8 Features 8 Sample Preparation and Insertion 9 Sample Preparation 9 Sample Insertion and chamber movement 11 Taking Measurements 15 Turning it off 18 Calibration 19 Calibration Standards 19 Steps in Calibration 20 Sampling Precautions 23 Pawkit and Temperature 24 4 Cleaning and Maintenance 26 ...

Страница 4: ...er Activity in Products 38 Water content 38 Water activity 39 Effect of Temperature on water activity 41 Water Potential 42 Factors in determining Water Potential 42 Sorption Isotherms relating aw to water content 44 7 Further Reading 47 Water Activity Theory and Measurement 47 Food Quality and Safety 48 Water Activity and Microbiology 49 Water Activity in Foods 50 Pharmaceuticals Cosmetics 54 Mis...

Страница 5: ... About this Manual This manual includes instructions on the operation calibration and maintenance of your Pawkit water activity system Please read these instructions care fully to ensure that your samples are measured accurately and that you can fully utilize the instru ment s potential Customer Service If you ever need assistance with your Pawkit or if you just have questions there are several wa...

Страница 6: ...support or if you have appli cation questions you can send us messages via email at pawkit decagon com Again please include as part of your message your Pawkit s serial number your name address phone fax number and return e mail address Warranty The Pawkit has a 30 day satisfaction guarantee and a one year warranty on parts and labor Note to our Users This manual is written to aid the end user in ...

Страница 7: ...d Material and equipment cov ered hereby which is not manufactured by Seller shall be covered only by the warranty of its manu facturer Seller shall not be liable to Buyer for loss damage or injuries to persons including death or to property or things of whatsoever kind including but not without limitation loss of anticipated prof its occasioned by or arising out of the installation operation use ...

Страница 8: ...Pawkit Introduction 4 out limitation the implied warranties of merchant ability and fitness for a particular purpose not expressly set forth herein ...

Страница 9: ...ween two porous electrodes in the headspace of a sealed chamber The electrical properties of the polymer change depending on the relative humidity of the chamber The electrodes give a signal based upon the relative humidity in the closed chamber This signal is then translated by the software and displayed as water activity on the instrument s screen At equilibrium the relative humidity of the air ...

Страница 10: ...pon the chilled mirror dewpoint method Contact Decagon for more details Getting Started Components of your Pawkit system Your Pawkit should have been shipped to you with the following items Pawkit main unit Durable carrying case 110 disposable Sample cups 3 spare sensor filters 1 cleaning kit 1 reusable stainless steel cup 2 vials each of the following calibration solu tions 6 0 molal NaCl 0 760 a...

Страница 11: ... This reduces the chance that sample material will spill inside the instrument To avoid inaccurate readings place your Pawkit in a location where the temperature remains fairly stable This location should be well away from air conditioner and heater vents open windows outside doors refrigerator exhausts or other items that may cause rapid temperature fluctuation ...

Страница 12: ...Once you have ensured that you have a stable working envi ronment you are ready to begin sampling Follow ing is a description of the features and operation of the instrument Features Diagram of Pawkit features LCD Sample Chamber Opening Tab Button I Button II Temp Sensor ...

Страница 13: ...onent samples e g muffins with raisins or samples that have out side coatings like deep fried breaded foods can be measured but may take longer to equili brate Samples like these may require additional preparation crushing or grinding to obtain a representative sample Completely cover the bottom of the cup with the sample if possible Pawkit is able to accurately measure a sample that leave small s...

Страница 14: ... are clean Wipe any excess sample material from the rim of the cup with a clean tis sue Material left on the rim or the outside of the cup will be transferred to subsequent samples and can affect the accuracy of your readings The rim of the cup forms a vapor seal with the sensor Therefore any sample material left on the cup rim may prevent this seal and contami nate future samples If a sample will...

Страница 15: ...holding the case near the LCD with one hand and pulling down on the metal tab with the other hand Flex the base of the instrument until it snaps open then lay it on a flat surface 2 Place your prepared sample cup inside the instrument in the cup holder as shown Insert cup into area shown ...

Страница 16: ...up is entirely within the chamber 3 Once the sample cup is properly inserted press down on the front end of the case to secure the cup inside the instrument You are now ready to take readings see Taking Measurements sec tion below ...

Страница 17: ...Pawkit Operation 13 4 When you are finished with the sample press on the back of the instrument above the LCD as illustrated to open the chamber and remove the cup ...

Страница 18: ...Pawkit Operation 14 Do not press on the middle to close the instru ment 5 To close the instrument turn it over then press on the two points shown with your thumbs until it snaps closed ...

Страница 19: ...scribed in the previous section 2 Press the left button I to turn on the instru ment It will display the last reading taken This allows you to begin a measurement and leave without having to attend the instrument through out the measurement If it is already on proceed to the next step ...

Страница 20: ...ter activity measurement 4 Once the measurement process has been started it will begin to display water activity measurements as well as temperature every 30 seconds During this time you will be able to see that it is measuring by looking at the sunburst icon to the right of the water activity value As it measures you will see the beams of the sun burst move from left to right 0 8924 6C aW ...

Страница 21: ...e sun burst disappears when the water activity reading is finished At this point you can either restart the measurement by pressing button I again or you can record the shown value and take the sample cup out 6 To remove the sample press on the back of the Pawkit to open the chamber then remove the cup as shown ...

Страница 22: ...g the sensors NOTE If you are done sampling Take the sample out You risk contaminating the chamber in transport or damaging sensors through extended contact with the sample Turning it off To turn off the Pawkit leave it idle for more than 5 minutes and it will shut off automatically If the Pawkit has automatically shut itself off pressing ...

Страница 23: ...rds The Pawkit uses 2 calibration standards 6 0 molal NaCl 0 760aw and 13 41 molal LiCl 0 250aw You received a small supply of these standards with your instrument These standards are specially prepared salt solutions at specific concentrations for constant and accurate water activity measurements They have been produced under a strict quality assurance regime and their accuracy is verified by an ...

Страница 24: ...e a vial of the 0 760 aw NaCl standard and empty the entire contents of the vial into a sam ple cup Place the sample cup into the Pawkit and close the chamber 2 Press the left button I to take a reading If it is reading the correct water activity 0 02 your Pawkit needs no calibration If not take a second reading After the second reading note the water activity value shown If it is reading the corr...

Страница 25: ...o scroll through the other selections They are d76 u25 d25 and Sto Note After scrolling past Sto you will come to two factory calibration settings that say u10 and d10 Ignore these settings and continue scrolling to return to the main menu The u and d before each number stand for up or down adjustment for each standard The num bers e g 25 and 76 correspond to the water activity of a calibration st...

Страница 26: ...er then press I This will store the new value you have set You will then return to the main screen and begin a new measure ment Note If you do not press Sto no change will be made to the calibration of the Pawkit 8 Repeat the above process with the 0 25 standard to set the calibration for the 0 25 water activity level 9 If you inadvertently enter the calibration routine keep pressing button II unt...

Страница 27: ...1 00 can shift the sensor calibration Therefore always remove samples as soon as the Pawkit is finished sampling beeps to avoid damage to the sensor If a sample is accidentally left in the chamber for an extended period of time be sure to check the calibration when the instrument is next used Measure 0 76 Standard OK to Sample Adjust 0 76 Calibration Measure 0 25 Standard Adjust 0 25 Calibration N...

Страница 28: ... removing the sample immediately after reading and allowing the Pawkit to stand open for a time between readings to allow the alcohol to diffuse out of the sensor chamber or by measuring a cup of activated charcoal Pawkit and Temperature Pawkit makes its most accurate measurements when the sample and instrument temperatures are within 1 C If the sample is too warm the thermometer icon on the left ...

Страница 29: ...ace the cup lid on the sample and wait until it has reached ambient temperature before attempting to read again If your sample is colder than the ambient tempera ture of the Pawkit the accuracy of your reading after 5 minutes may be questionable Wait until the sample s temperature is similar to that of the Pawkit ...

Страница 30: ...nly a soft cotton cloth to clean the LCD Tissues can scratch the plastic causing damage Use a moist cotton cloth to clean the rest of the outer case For cleaning inside the case and sample cham ber use a cotton swab moistened with water to clean sample residue If you have spilled sample material on the sensor screen and it doesn t come off clean or replace the screen as explained in the next secti...

Страница 31: ...tate cleaning and or replacing parts Following is a description on how to take apart and reassemble the instrument 1 With the Pawkit open lift the back of the foot plate up until the tabs on the foot plate clear the notches in the retainer and it clicks Next pull the whole foot plate forward until the bottom disengages and the foot plate comes free ...

Страница 32: ...rod in the retainer and the tabs in the foot plate must be re engaged in the notches in the retainer This is done by placing the foot plate over the retainer with the tabs aligned with the rod Press on the contact points while flexing the front tab upward so the springs will slide under the rod Slide the foot plate back until the tabs click into the notches in the retainer yy yyyy yyyyy yyyyy ...

Страница 33: ...Pawkit Cleaning and Maintenance 29 Align the tabs with the rod Press the contact points while lifting on the front tab of the foot plate Push while pressing to click in place ...

Страница 34: ...ate must be removed and reas sembled as described above Sensor Filter Cleaning Replacement You may periodically need to clean the porous white sensor filter if it becomes dirty To do so apply distilled water or isopropyl alcohol to a clean lint free cloth or Kimwipe and gently clean the surface of the filter Your Pawkit was shipped with 3 spare filters If the filter has become contaminated or requ...

Страница 35: ...The sensor is extremely fragile Do not touch it Chamber Cleaning Instructions 1 Use a moist cloth or Kimwipe to clean the sur rounding chamber area 2 Re assemble the instrument as described in the Disassembly section above yy yyyy yyyyy yyyyy ...

Страница 36: ...r right corner of the screen an occasional low battery indication does not mean the battery needs replacing To replace the battery follow these steps 1 Remove the Pawkit bottom completely as described in the Disassembly section above 2 Turn the top portion of the Pawkit over so you are looking at the underside Four tabs on the retainer engage the rim of the Pawkit case top Flex the front arms inwa...

Страница 37: ...omer circuit board and retainer from the stainless steel case Next remove the retainer and circuit board from the elastomer being careful not to kink the tempera ture sensor flex You will see the batteries on the upper portion above the LCD hole yy yyyy yyyyy yyyyy yyy yyyy yyyy yyyy Stainless Steel Case Elastomer Circuit Board Retainer ...

Страница 38: ...in place 5 Replace the circuit board and retainer in the elastomer Align the buzzer in the hole between the batteries and insert the retainer tabs in their holes in the elastomer 6 Place the elastomer assembly in the case top and seat it fully Slide the retainer forward and press on the back of it while sliding it back to engage the back two tabs Finally use a small screw driver or similar tool to...

Страница 39: ...Pawkit Cleaning and Maintenance 35 7 Attach the bottom of the Pawkit as described in the Disassembly section above ...

Страница 40: ...card number a repair budget and billing address Note If you purchased your instrument from one of our international distributors please contact them before contacting Decagon They will be able to provide you with local service and help you remedy the problem Shipping Directions When you ship your instrument back to us please include with it a document with the complete ship ping address name and d...

Страница 41: ...cking material to prevent damage in shipment Ship to Decagon Devices Inc ATTN Repair Department 950 NE Nelson Court Pullman WA 99163 Repair Costs Manufacturer s defects and instruments within the one year warranty will be repaired at no cost For non warranty repairs costs for parts labor and shipping will be billed to you We have a 50 mini mum repair charge An extra fee will be charged for rush wo...

Страница 42: ...total amount of water present in a sample The primary method for determining water content is by loss on drying but secondary methods such as infrared NMR or Karl Fisher titration are also used Moisture content determination is essen tial in meeting product nutritional labeling regula tions specifying recipes and monitoring processes However water content alone is not a reliable indi cator for pre...

Страница 43: ...y Water activity influences color odor flavor texture and shelf life of many products Most importantly it predicts product safety and stability with respect to microbial growth chemical and biochemical reaction rates and physi cal properties Therefore water activity is a far better indicator of perishability than water content Figure 1 shows how the relative activity of microorganisms lipids and e...

Страница 44: ... the Paw kit a sample is placed in a sample cup which is sealed inside a chamber Inside the sensor block is a dielectric humidity sensor Changes in the electrical conductance of the dielectric sensor occur as the relative humidity of the chamber changes By moni toring the change in electrical conductance the rel ative humidity of the headspace is computed When the water activity of the sample and ...

Страница 45: ...component food Initially out of the oven a baked good is not at internal equilibrium the outer surface is at a lower water activity than the center of the baked good One must wait a period of time in order for the water to migrate and the system to come to internal equilibrium It is impor tant to remember the restriction of the definition of water activity to equilibrium Effect of Temperature on w...

Страница 46: ...t µ is the same in both phases It is this fact that allows us to measure the water potential of the vapor phase and use that to determine the water potential of the liquid phase Gradients in µ are driving forces for moisture movement Thus in an isothermal system water tends to move from regions of high water potential high aw to regions of low water potential low aw Water content is not a driving ...

Страница 47: ...re is a measure of the osmotic potential of the solution Addition of one mole of an ideal solute to a kilogram of water pro duces an osmotic pressure of 22 4 atm This lowers the water activity of the solution from 1 0 to 0 98 aw For a given amount of solute increasing the water content of the systems dilutes the solute decreasing the osmotic pressure and increasing the water activity Since microbi...

Страница 48: ...ix by hydrogen bonding and van der Waal forces At higher water activity levels capillary forces and surface tension can also play a role Sorption Isotherms relating aw to water content Changes in water content affect both the osmotic and matric binding of water in a product Thus a relationship exists between the water activity and water content of a product This relationship is called the sorption...

Страница 49: ...herm for the particular product produced ideally using the process that brings the product to its final water content For example if one were to monitor the water con tent of dried potato flakes one would measure the water activity and water content of potato flakes dried to varying degrees using the standard drying process for those flakes An isotherm would be con structed using those data and th...

Страница 50: ...Pawkit Theory 46 tion to microbial growth chemical reactivity and stability over water content has been shown ...

Страница 51: ... of Research of the National Bureau of Stan dards A Physics and Chemistry 81A 89 96 Prior B A 1979 Measurement of water activity in foods A review Journal of Food Protection 42 8 668 674 Troller J A and J H B Christian 1978 Water Activ ity and Food Academic Press New York Troller J A and V N Scott 1992 Measurement of water activity aw and acidity In Compen dium of Methods for the Microbiological E...

Страница 52: ...l Foods World 27 9 403 407 Hardman T M 1988 Water and Food Quality Elseiver Press London Kress Rogers E 1993 Food quality measurement Food Industry News September 23 26 McMeekin T A and T Ross 1996 Shelf life pre diction Status and future possibilities Inter national Journal of Food Microbiology 33 65 83 Rockland L B and G F Stewart 1981 Water Activ ity Influences on Food Quality Academic Press Ne...

Страница 53: ...crobiology 15 103 105 Garcia de Fernando G D O Diaz M Fernandez and J A Ordonez 1992 Changes in water activity of selected solid culture media throughout incubation Food Microbiology 9 77 82 Kuntz L A 1992 Keeping microorganisms in con trol Food Product Design August 44 51 Miller A J 1992 Combined water activity and sol ute effects on growth and survival of Listeria monocytogenes Scott A Journal o...

Страница 54: ...ia for its recovery Applied and Environ mental Microbiology 62 2735 2740 Hand L 1994 Controlling water activity and pH in snack sticks Meat Marketing and Technol ogy May 55 56 Lee M B and S Styliadis 1996 A survey of pH and water activity levels in processed salamis and sausages in Metro Toronto Journal of Food Protection 59 1007 1010 Luecke F K 1994 Fermented meat products Food Research Internati...

Страница 55: ...raft goat s milk cheese Armada variety Food Chemistry 55 225 230 Kombila M E and C Lacroix 1991 The effect of combinations of salt lactose and glycerol on the water activity aw of cheese spreads Canadian Institute of Food Science and Tech nology Journal 24 233 238 Pisecky J 1992 Water activity of milk powders Milchwissenschaft 47 3 7 Vivier D R Ratomahenina and P Galzy 1994 Characteristics of micr...

Страница 56: ...ltini E D Torreggiani B R Brovetto and G Bertolo 1993 Functional properties of reduced moisture fruits as ingredients in food systems Food Research International 26 413 419 Zhang X W X Liu D X Gu W Zhou R L Wang and P Liu 1996 Desorption isotherms of some vegetables Journal of the Science of Food and Agriculture 70 303 306 Baked Goods and Cereals Aramouni F M K K Kone J A Craig and D Y C Fung 1994...

Страница 57: ...ife of bakery products FMBRA Bulletin April 2 71 74 Beverages Soups Sauces Preserves Carson K J J L Collins and M P Penfield 1994 Unrefined dried apple pomace as a potential food ingredient Journal of Food Science 59 1213 1215 Durrani M J R Khan M Saeed and A Khan 1992 Development of concentrated bever ages from Anna apples with or without added preservatives by controlling activity of water for s...

Страница 58: ...aceuticals Cosmetics Ahlneck C and G Zografi 1990 The molecular basis of moisture effects on the physical and chemical stability of drugs in the solid state International Journal of Pharmaceutics 62 87 95 Enigl D C and K M Sorrels 1997 Water Activity and Self Preserving Formulas In Preserva tive Free and Self Preserving Cosmetics and Drugs Principles and Practice Kabara J J and D S Orth ed Marcel ...

Страница 59: ... 1991 2027 Zografi G 1988 States of water associated with solids Drug Development and Industrial Pharmacy 14 14 1905 1926 Miscellaneous Bell L N and T P Labuza 1992 Compositional influence on the pH of reduced moisture solutions Journal of Food Science 57 732 734 Bell L N and T P Labuza 1994 Influence of the low moisture state on pH and its implication for reaction kinetics Journal of Food Engi ne...

Страница 60: ... Edible coat ings to inhibit lipid migration in a confec tionery product Journal of Food Science 58 1422 1425 Fernandez Salguero J R Gómez and M A Car mona 1993 Water activity in selected high moisture foods Journal of Food Composition and Analysis 6 364 369 ...

Страница 61: ...t Pawkit water activity meter Model Number Year of First Manufacture 2000 This is to certify that the Pawkit water activity meter manufactured by Decagon Devices Inc a corpora tion based in Pullman Washington USA meets or exceeds the standards for CE compliance as per the Council Directives noted above All instruments are built at the factory at Decagon and pertinent testing documentation is freel...

Страница 62: ...Pawkit Declaration of Conformity 58 ...

Страница 63: ...atteries 32 replacing 32 beeper 17 23 buttons 15 to begin measurement 16 C calibration 19 steps 20 calibration standards 6 19 LiCl 19 NaCl 19 cautions with sampling 23 CE compliance 57 chamber cleaning 31 contamination 18 cleaning 26 sensor filter 30 closing the Pawkit 14 ...

Страница 64: ... d76 21 Declaration of Conformity 57 disassembling 27 display 16 E e mail address 2 email address 2 environment for sampling 6 enzymes and water activity 39 equilibrium of sample aw and rh 40 of temperature 41 ethanol 24 F fax number 2 features 8 filter sensor 30 G Gibbs free energy 42 ...

Страница 65: ...samples 11 isotherms 44 L LCD cleaning 26 display 16 liability seller s 3 LiCl standards 19 lids for sample cups 10 lipids and water activity 39 liquid phase water 40 lithium ion batteries 32 location for sampling 7 low battery indicator 32 M maintenance 26 27 measurement time 17 measurements taking 15 ...

Страница 66: ...he chamber 11 operation environment 6 osmotic effects 43 P Pawkit accessories 6 closing 14 disassembly and re assembly 27 features 8 measurement technique 5 operation 8 pharmaceuticals 54 power shutoff 18 preparation for operation 6 of samples 9 R references 47 baked goods and cereals 52 beverages soups sauces preserves 53 ...

Страница 67: ...0 removing the sample 17 repair costs 37 shipping 36 repair instructions 36 S sample insertion 11 sample cups 6 9 filling level 10 stainless steel 9 samples multi component 9 seller s liability 3 sensor damage 23 sensor filter cleaning and replacing 30 shipping address 37 shipping for repair 36 sorption isotherms relating water activity to water content 44 Sto 21 ...

Страница 68: ...17 toll free number 2 U u25 21 u76 21 V vapor phase water 40 volatiles 23 W warranty 2 water activity definition 39 effect on food 38 39 related to microbial growth 39 stability diagram 39 water content definition 38 methods for determining 38 vs water activity 38 44 water potential factors in determining 42 matric effects 43 osmotic effects 43 ...

Страница 69: ...Pawkit Index 65 relation to water activity 42 wet samples cautions with 23 ...

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