Operator`s Manual
Contents
1. 22 Pawkit and Temperature 23 5 Cleaning and Maintenance 24 Cleaning sree erra wr aE thet acne 24 Maintenance e eee eee 25 Sensor Filter Cleaning Replacement 25 Thermopile Sensor Cleaning 27 Chamber Cleaning Instructions 27 Battery Replacement 28 6 Verification and Calibration 30 Verification Standards 30 Steps to Verify Calibration 31 7 Support and Repair 35 Shipping Directions 35 Repair Costs i 2itiiaguctsseiee vindeass 36 Loaner Service 0005 37 8 Further Reading 38 Water Activity Theory amp Measurement 38 Food Quality and Safety 39 Water Activity and Microbiology 39 Water Activity in Foods 40 Pharmaceuticals Cosmetics 44 Miscellaneous piston ceinacerkl ees 45 Declaration of Conformity 46 Pawkit User Manual l Introduction 1 Introduction ee eee eee es _ I i Welcome to the Pawkit water activity measurement system The Pawkit allows you to make quick measurements of water activity to ensure the safety of your product We hope you find the contents of this manual useful in understanding your instrument and maximizing its benefit to you About this Manual This manual includes instructions on the operation calibra tion and maintenance of your Pawkit water activity system Please r2. Journal of Research of the National Bureau of Standards 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 Activity and Food Academic Press New York Troller J A and V N Scott 1992 Measurement of water activity a and acidity In Compendium of Meth ods for the Microbiological Examination of Foods Vanderzant C and D F Splittstoesser ed Ameri can Public Health Association Washington D C pp 135 151 van den Berg C 1985 Water activity In Concentration and Drying of Foods MacCarthy D ed Elsevier London pp 11 35 38 Pawkit User Manual 8 Further Reading Food Quality and Safety Brandt L 1996 Bound for success Controlling water activity gives technologists the edge in developing safe shelf stable foods Food Formulating Septem ber 41 48 Franks F 1982 Water activity as a measure of biological viability and quality control Cereal 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 prediction Status and future possibilities International Journal of Food Microbiology 33 65 83 Rockland L B and G F Stewart 1981 Water Activity Influence
3. of calibration standards 30 N NaCl standards 30 O Off turning off 22 Opening the chamber 17 Operation environment 6 Osmotic effects 11 P Pawkit accessories 5 features 14 operation 14 Pharmaceuticals 44 Power shutoff 22 Preparation for operation 6 of samples 15 R References 38 baked goods and cereals 42 beverages soups sauces preserves 43 dairy products 41 food quality and safety 39 fruits and vegetables 42 meat and seafood 40 microbiology 39 pharmaceuticals 44 Relative humidity 7 9 Repair costs 38 S Sample insertion 17 Sample cups 6 15 filling level 16 stainless steel 15 Samples multi component 15 Seller s liability 3 Sensor damage 23 Sensor filter cleaning and replacing 25 Sorption isotherms relating water activity to water content 12 Specification 4 Sto 32 T Temperature 23 effects on water activity 9 equilibrium 9 Theory 7 water activity 7 Time for measurement 21 U u25 32 u76 32 y Vapor phase water 8 Verification 30 steps 31 Verification standards 6 30 LiCl 30 NaCl 30 Volatiles 22 W Warranty 2 Water activity definition 7 effect on food 7 8 related to microbial growth 8 stability diagram 8 Water content definition 7 vs water activity 7 12 Water potential factors in determining 10 matric effects 11 osmotic effects 11 relation to water activity 10 Wet samples cautions with 22
4. ordinary wear and tear neglect misuse accident and exces sive deterioration due to corrosion from any cause are not to be considered a defect but Seller s liability for defective parts shall in no event exceed the furnishing of replacement parts EO B the factory where originally manufactured Material and equipment covered hereby which is not manu factured by Seller shall be covered only by the warranty of its manufacturer Seller shall not be liable to Buyer for loss damage or injuries to persons including death or to prop erty or things of whatsoever kind including but not with out limitation loss of anticipated profits occasioned by or arising out of the installation operation use misuse non use repair or replacement of said material and equipment or out of the use of any method or process for which the same may be employed The use of this equipment consti tutes Buyer s acceptance of the terms set forth in this war ranty There are no understandings representations or warranties of any kind express implied statutory or other wise including but without limitation the implied warran ties of merchantability and fitness for a particular purpose not expressly set forth herein Pawkit User Manual 2 About the Pawkit 2 About the Pawkit The Pawkit is designed to be a simple rapid and portable system for measurement of water activity It is easy to use durable and requires little maintenan
5. role Sorption Isotherms Relating Water Activity to Moisture Content Changes in moisture content affect both the osmotic and matric binding of water in a product Thus a relationship exists between the water activity and moisture content of a product This relationship is called the sorption isotherm and is unique for each product Besides being unique to each product the isotherm changes depending on whether it was obtained by drying or wetting the sample These fac tors need to be kept in mind if one tries to use moisture con tent to infer the stability or safety of a product Typically large safety margins are built in to moisture content specifi cations to allow for these uncertainties While the sorption isotherm is often used to infer water activity from moisture content one could easily go the other direction and use the water activity to infer the moisture content This is particularly attractive because water activity is much more quickly measured than moisture content This method gives particularly good precision in the center of the isotherm In order to infer moisture content from water activity one needs an isotherm for the particular product produced ideally using the process that brings the product to its final moisture content For example if one were to monitor the moisture content of dried potato flakes one would measure the water activity 12 Pawkit User Manual 3 Water Activity Theory and
6. tent Alimentaria 30 67 69 Kusumegi K T Takahashi and M Miyagi 1996 Effects of addition of sodium citrate on the pasteurizing conditions in Tuyu Japanese noodle soup Journal 43 Pawkit User Manual 8 Further Reading of the Japanese Society for Food Science and Tech nology 43 740 747 Sa M M and A M Sereno 1993 Effect of temperature on sorption isotherms and heats of sorption of quince jam International Journal of Food Science and Technology 28 241 248 Pharmaceuticals Cosmetics Ahlneck C and G Zografi 1990 The molecular basis of moisture effects on the physical and chemical stabil ity 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 Preservative Free and Self Preserving Cosmetics and Drugs Principles and Practice Kabara J J and D S Orth ed Marcel Dekker pp 45 73 Hageman MJ 1988 The role of moisture in protein sta bility Drug Development and Industrial Pharmacy 14 14 2047 2070 Heidemann D R and PJ Jarosz 1991 Performulation studies involving moisture uptake in solid dosage forms Pharmaceutical Research 8 3 292 297 Friedel R R and A M Cundell 1998 The application of water activity measurement to the microbiological attributes testing of nonsterile over the counter drug products Pharmacopeial Forum 24 2 6087 6090 Kontny M J 198
7. we recommend you use Decagon s AquaLab water activity meter which is a lab grade bench top instrument that has an accuracy of 0 003 aw and measures based upon 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 fol lowing items e Pawkit main unit e Durable carrying case Pawkit User Manual 2 About the Pawkit e 110 disposable Sample cups e 3 spare sensor filters e 1 reusable stainless steel cup e 2 vials each of the following verification standards 6 0 molal NaCl 0 760 a 1341 molal LiCl 02504 Preparing for Operation To ensure that your Pawkit operates correctly and consis tently always place it on a level surface when measuring 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 condi tioner and heater vents open windows outside doors refrig erator exhausts or other items that may cause rapid temperature fluctuation Pawkit User Manual 3 Water Activity Theory 3 Water Activity Theory Water is a major component of foods pharmaceuticals and cosmetics Water influences the texture appearance taste and spoilage of these products There are two basic types of water analysis moisture
8. 0 76 Calibration i o 6 If sample OK to i aw lt 0 50 Not Correct Sample Corr St Measure 0 25 Standard Not Correct Adjust 0 25 Calibration 34 Pawkit User Manual 7 Support and Repair 7 Support and Repair NOTE If you purchased your Pawkit from one of our inter national distributors please contact them They will be able to provide you with local support and service When encountering problems with your Pawkit that can t be resolved with the help of this manual please contact Decagon Customer Support at support decagon com 800 755 2751 US and Canada 509 332 2756 Interna tional or fax us at 509 332 5158 Please have the serial number and model of the instrument ready All Pawkits returning to Decagon for servicing must be accompanied with a Return Material Authorization RMA form Prior to shipping the instrument please contact a Decagon customer support representative to obtain an RMA Shipping Directions The following steps will help to ensure the safe shipping and processing of your Pawkit Pack the Pawkit in its carrying case securely in its original box If the original packaging is not available pack the box moderately tight with packing material e g styrofoam pea 35 Pawkit User Manual 7 Support and Repair nuts or bubble wrap ensuring the instrument is suspended in the packing material Use a box that has at least 2 inches of space between your instrumen
9. 1992 Relationship between water activity salts of lactic acid and growth of List eria monocytogenes in a meat model system Jour nal of Food Protection 55 574 578 Clavero M R S and L R Beuchat 1996 Survival of Escherichia coli O157 H7 in broth and processed salami as influenced by pH water activity and tem perature and suitability of media for its recovery 40 Pawkit User Manual 8 Further Reading Applied and Environmental Microbiology 62 2735 2740 Hand L 1994 Controlling water activity and pH in snack sticks Meat Marketing and Technology 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 EK 1994 Fermented meat products Food Research International 27 299 307 Minegishi Y Y Tsukamasa K Miake T Shimasaki C Imai M Sugiyama and H Shinano 1995 Water activity and microflora in commercial vacuum packed smoked salmons Journal of the Food Hygienic Society of Japan 36 442 446 Shimasaki T K Miake Y Tsukamasa M A Sugiyama Y Minegishi and H Shinano 1994 Effect of Water Activity and Storage Temperature on the Quality and Microflora of Smoked Salmon Nippon Suisan Gakkaishi 60 569 576 Dairy Products Fresno J M M E Tornadijo J Carballo P J Gonzalez and A Bernardo 1996 Characterization and bio chemical change
10. 8 Distribution of water in solid pharma ceutical systems Drug Development and Industrial Pharmacy 14 14 1991 2027 44 Pawkit User Manual 8 Further Reading 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 influ ence on the pH of reduced moisture solutions Jour nal 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 reac tion kinetics Journal of Food Engineering 22 291 312 Bell L N 1995 Kinetics of non enzymatic browning in amorphous solid systems Distinguishing the effects of water activity and the glass transition Food Research International 28 59 1 597 Brake N C and O R Fennema 1993 Edible coatings to inhibit lipid migration in a confectionery product Journal of Food Science 58 1422 1425 Fernandez Salguero J R G mez and M A Carmona 1993 Water activity in selected high moisture foods Journal of Food Composition and Analysis 6 364 369 45 Pawkit User Manual Declaration of Conformity Declaration of Conformity Application of Council 89 336 EEC Directive Standards to which EN55022 1987 conformity is declared EN500082 1 1992 Manufacturer s Name Decagon Devices Inc 2365 NE Hopkins Court Pullman WA 99163 USA Type of Equipment Paw
11. Manual 5 Cleaning and Maintenance 5 Cleaning and Maintenance Cleaning The accuracy of your Pawkit is dependent on keeping your instrument clean Dust and sample debris can contaminate the sampling chamber and must therefore be regularly cleaned out To clean your instrument carefully follow the instructions in this chapter Here are some tips for keeping your Pawkit clean e Use only a soft cotton cloth to clean the LCD Tissues can scratch the plastic causing damage e Use moist lint free tissues to clean the rest of the outer case e To clean the case and sample chamber use a moistened lint free tissue to clean sample residue If you have spilled sample material on the sensor filter and it doesn t come off clean or replace the filter as explained in the next section It is important that contamination of this filter is minimized as the relative humidity of the sam ple is measured via the filter 24 Pawkit User Manual 5 Cleaning and Maintenance Maintenance Sensor Cover Sensor Filter Thermopile Sensor 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 to a clean lint free cloth or Kimwipe and gently clean the surface of the filter remember to allow adequate time for filter to dry before use If the filter has become contaminated and will not clean with a moistened tissue it wi
12. ce PawkKit Specifications Water Activity Range 0 00 to 1 00 a Water Activity Accuracy 0 02 a Water Activity Resolution 0 01 a Read time 5 min Sample Temperature Range NA Sample Temperature accuracy NA Sample Temperature resolution NA Sample Dish Capacity 7 ml recommended 15 ml full Operating Environment 4 to 50 C 0 to 90 Relative Humidity non condensing Case Dimensions 6 6 x 10 7 x 2 0 cm Weight 115 g 4 oz Case Material Stainless Steel and Valox 325 Plastic Display 6 digit custom LCD with symbols Data Communication NA Power 2 3 Volt 16 mm coin cell batteries 3 years Warranty 1 year parts and labor Pawkit User Manual 2 About the Pawkit How Pawkit works The Pawkit uses a capacitance humidity sensor to measure the water activity of a sample The sensor is suspended in the headspace of the chamber and uses a special polymide mate rial sandwiched between two electrodes to sense humidity changes The sensor converts the humidity value into a spe cific capacitance which is then measured electronically by the circuit 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 in the chamber is the same as the water activity of the sample Accuracy The Pawkit is accurate to 0 02 a For many applications this accuracy is more than adequate If you require higher accuracy in your measurements
13. content and water activity Moisture Content The meaning of the term moisture content is familiar to most people It implies a quantitative analysis to determine the total amount of water present in a sample Primary methods for determining moisture content are loss on dry ing and Karlf Fisher titration but secondary methods such as infrared and NMR are also used Moisture content deter mination is essential in meeting product nutritional labeling regulations specifying recipes and monitoring processes However moisture content alone is not a reliable indicator for predicting microbial responses and chemical reactions in materials The limitations of moisture content measurement are attributed to differences in the intensity with which water associates with other components Water Activity Water activity a is a measurement of the energy status of the water in a system It indicates how tightly water is bound structurally or chemically within a substance Water activity is the relative humidity of air in equilibrium with a sample in a sealed measurement chamber The concept of Pawkit User Manual 3 Water Activity Theory water activity is of particular importance in determining product quality and safety 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 reac tion rat
14. e In addition to equilibrium between the liquid phase water in the sample and the vapor phase the internal equilibrium of the sample is important If a system is not at internal equilib rium one might measure a steady vapor pressure over the period of measurement which is not the true water activity of the system An example of this might be a baked good or a multi 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 Temperature Effects Temperature plays a critical role in water activity determina tions Most critical is the measurement of the difference between sample and dew point temperature Best accuracy is Pawkit User Manual 3 Water Activity Theory therefore obtained when the sample is near chamber tem perature Water Potential Some additional information may be useful for understand ing what water activity is and why it is such a useful measure of moisture status in products Water activity is closely related to a thermodynamic property called the water poten tial or chemical potential u of water which is the change in Gibbs free energy G when water concentration changes Equilibriu
15. e vegeta bles 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 Growth of Clostridium sporogenes PA 42 Pawkit User Manual 8 Further Reading 3679 in home style canned quick breads Journal of Food Protection 57 882 886 Clawson A R and AJ Taylor 1993 Chemical changes during cooking of wheat Food Chemistry 47 337 343 Gomez R Fernandez Salguero J M A Carmona and D Sanchez 1993 Water activity in foods with inter mediate moisture levels Bakery and confectionery products Miscellany Alimentaria 30 55 57 Michniewicz J C G Biliaderis and W Bushuk 1992 Effect of added pentosans on some properties of wheat bread Food Chemistry 43 25 1 257 Seiler D A L 1979 The mould free shelf life of bakery products FMBRA Bulletin April 2 71 74 Beverages Soups Sauces Preserves Carson K J J L Collins and M P Penfield 1994 Unre fined dried apple pomace as a potential food ingre dient Journal of Food Science 59 1213 1215 Durrani M J R Khan M Saeed and A Khan 1992 Development of concentrated beverages from Anna apples with or without added preservatives by con trolling activity of water for shelf stability Sarhad Journal of Agriculture 8 23 28 Ferragut V J A Salazar and A Chiralt 1993 Stability in the conservation of emulsified sauces low in oil con
16. ead these instructions carefully to ensure that your samples are measured accurately and that you can fully uti lize the instrument s potential Customer Support If you ever need assistance with your Pawkit or if you just have questions there are several ways to contact us Our Customer Support and Sales Representatives are available Monday thru Friday E mail support decagon com Please include your name contact information instrument serial number s and a description of your problem or ques tion Pawkit User Manual l Introduction sales decagon com Please include your name address phone number the items you wish to order and a purchase order number Credit card numbers should always be called in Phone 1 800 755 2751 USA and Canada Only 1 509 332 2756 International Fax 1 509 332 5158 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 understanding the basic concepts of water activity enabling them to use our instrument with confidence Every effort has been made to ensure that the content of this manual is correct and scientif ically sound Pawkit User Manual l Introduction Seller s Liability Seller warrants new equipment of its own manufacture against defective workmanship and materials for a period of one year from date of receipt of equipment the results of
17. ectly pro ceed to the next step Note An error code of 9 99 at any time during the process indicates that the sensor has failed and that the instrument needs to be serviced Refer to Chapter S for shipping instructions Once the reading is finished the right button II will be active Button II is only active until the Pawkit shuts itself off Press it once and you will see the following screen 31 Pawkit User Manual 6 Verification and Calibration 0 76 u 6 4 This screen shows that you are in the calibration mode This one in particular shows that you are ready to adjust calibration upwards for the 0 76 standard The numbers in the upper right corner indicate the water activity measurement that your Pawkit just read Press the II button to scroll through the other selections They are u76 d76 u25 d25 Sto u10 and d10 The u and d before each number stand for up or down adjust ment for each standard The numbers e g 25 and 76 correspond to the water activity of a verification stan dard 0 76 and 0 25 a The Sto position stores a read ing The d10 and u10 are for factory adjustment only and should not be used 5 As an example if your NaCl reading is lower than it should be press the II button to scroll to u76 adjust up for 0 76 standard If it is higher than it should be scroll to d76 adjust down for 0 76 standard Note If you accidentally sc
18. es and physical properties Therefore water activity is a far better indicator of perish ability than moisture content Figure 1 shows how the rela tive activity of microorganisms lipids and enzymes relate to water activity While other factors such as nutrient avail ability and temperature can affect the relationships water activity is the best single measure of how water affects these processes WATER ACTIVITY STABILITY DIAGRAM Solute amp Capillary Tonle Covalent lt gt p gt Moisture Content Relative Reaction Rate 04 es Water Activi Figure 1 Water Activity Diagram adapted from Labuza Water activity of a system is measured by equilibrating the liquid phase water in the sample with the vapor phase water Pawkit User Manual 3 Water Activity Theory in the headspace and measuring the relative humidity of the headspace In the Pawkit a sample is placed in a sample cup which is sealed inside a chamber Inside the sensor block is a capacitive humidity sensor Changes in the electrical capaci tance of the polymide layer of the sensor occur as the relative humidity of the chamber changes By monitoring the change in electrical capacitance the relative humidity of the headspace is computed When the water activity of the sam ple and the relative humidity of the air are in equilibrium the measurement of the headspace humidity gives the water activity of the sampl
19. is a measure of the osmotic potential of the solution Addition of one mole of an ideal solute to a kilogram of water produces an osmotic pressure of 22 4 atm This lowers the water activity of the solution from 1 0 to 0 98 a For a given amount of solute increasing the moisture content of the systems dilutes the solute decreasing the osmotic pressure and increasing the water activity Since microbial cells are high concentrations 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 water activity by physically binding water within its structure through adhesive and cohesive forces that hold water in pores and capillaries and to particle surfaces If cellulose or protein were added to water the energy status of the water would be reduced Work would need to be done to extract the water from this matrix This reduction in energy status of the water is not osmotic because the cellulose or protein concentrations are far too low to produce any sig nificant dilution of water The reduction in energy is the result of direct physical binding of water to the cellulose or protein 11 Pawkit User Manual 3 Water Activity Theory matrix by hydrogen bonding and van der Waal forces At higher water activity levels capillary forces and surface tension can also play a
20. kit water activity meter Model Number N A Year of First Manufacture 2000 This is to certify that the Pawkit water activity meter manu factured by Decagon Devices Inc a corporation 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 freely available for verification This certification applies to all Pawkit models 46 Index A Accuracy 5 AquaLab 5 B Batteries 28 replacing 28 Beeper 21 22 Buttons 19 to begin measurement 20 C Calibration 30 Cautions with sampling 22 CE compliance 46 Cleaning 24 sensor filter 25 Closing the chamber 17 Cold samples 23 Contact information 1 Customer service D d25 32 d76 32 Declaration of Conformity 46 Display 20 E Environment for sampling 6 Enzymes and water activity 8 Equilibrium of sample aw and rh 9 of temperature 9 F Features 14 Filter sensor 25 G Gibbs free energy 10 H High temperature 23 Hot samples 23 Humidity related to water activity 5 I Inserting samples 17 L LCD cleaning 24 display 20 Liability seller s 3 LiCl standards 30 Lids for sample cups 16 Lipids and water activity 8 Liquid phase water 8 Lithium ion batteries 28 Location for sampling 6 Low battery indicator 28 M Maintenance 24 25 Measurement taking 19 time 21 Molality
21. ll require removal To remove the sensor filter use a knife or needle point to gently pry up the edge of the filter The removed fil ter may be washed with soap and water in a sink dried and 25 Pawkit User Manual 5 Cleaning and Maintenance pressed back into the recess in the Pawkit Your Pawkit was shipped with 3 spare filters and can be replaced at any time Removal of filter AN Note The sensor is extremely fragile Do not touch it 26 Pawkit User Manual 5 Cleaning and Maintenance Thermopile Sensor Cleaning The lens of this sensor must be free of all dirt and lint Use a Kimwipe tissue moistened with isopropyl alcohol or water to clean the thermopile sensor Chamber Cleaning Instructions Use a moist Kimwipe tissue to clean the surrounding cham ber area The chamber area especially where the cup seals must be free of all contamination 27 Pawkit User Manual 5 Cleaning and Maintenance Battery Replacement The Pawkit uses two Lithium ion battery cells and they should last for several years If the battery charge is low you will see a low battery indicator icon appear in the lower right corner of the screen an occasional low battery indica tion does not mean the battery needs replacing To replace the battery follow these steps 1 Remove the Pawkit bottom by unscrewing the two screws Screw Locations 2 Separate the stainless steel top and elast
22. m occurs in a system when p is the same every where in the system Equilibrium between the liquid and the vapor phases implies that u 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 p are driving forces for mois ture movement Thus in an isothermal system water tends to move from regions of high water potential high a to regions of low water potential low a Moisture content is not a driving force for water movement and therefore can not be used to predict the direction of water movement except in homogeneous materials Factors in Determining Water Potential The water potential of the water in a system is influenced by factors that effect the binding of water They include osmotic matric and pressure effects Typically water activity is measured at atmospheric pressure so only the osmotic and matric effects are important 10 Pawkit User Manual 3 Water Activity Theory 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 perme able membrane water tends to move from the pure water side through the membrane to the side with the added sol ute If sufficient pressure is applied to the solute water mix ture to just stop the flow this pressure
23. moisture content of potato flakes dried to varying degrees using the standard drying process for those flakes An isotherm would be constructed using those data and the moisture content would be inferred using the measured water activity of samples and that isotherm The importance of the concept of water activity of foods pharmaceuticals and cosmetics cannot be overly emphasized Water activity is a measure of the energy status of the water in a system More importantly the usefulness of water activity in relation to microbial growth chemical reactivity and stability over moisture content has been shown 13 Pawkit User Manual 4 Operation 4 Operation Operation of the Pawkit is very simple Once you have ensured that you have a stable working environment you are ready to begin sampling Following is a description of the features and operation of the instrument Features Sample Cup Diagram of Pawkit features 14 Pawkit User Manual 4 Operation Sensor Cover Sensor Filter Thermopile Sensor Bottom of Pawkit Sample Preparation and Insertion Your Pawkit system comes with 110 disposable plastic sam ple cups and 1 stainless steel sample cup If you run out additional cups can be purchased from Decagon Sample Preparation Special care should be taken in preparing the sample in order to get the best readings possible Follow these guidelines when preparing samples Make
24. ngs 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 contaminate future samples e Ifasample will be read at some other time put the sam ple cup s disposable lid on the cup to restrict water trans 16 Pawkit User Manual 4 Operation fer To seal the lid place tape or Parafilm completely around the cup lid junction It is necessary to seal the cup if it will be a long time before the measurement is made Sample Placement 1 Open the Pawkit by holding the case near the LCD with one hand and pulling down on the plastic sensor cover tab with the other hand The sensor cover will rotate and snap into the open posi tion as shown in the illustration below 17 Pawkit User Manual 4 Operation 2 Place your prepared sample cup onto a level surface a Next place the opened Pawkit onto the prepared sample cup The cup will fit under the sensors into a recess in the bottom of the Pawkit A correctly positioned cup will result in the Pawkit being level on the bench when sitting on the cup and the sensor cover legs Ensure the cup is entirely within the recess Otherwise the Pawkit will not be level on the bench and the cup will not make a vapor seal with the sensor Final sample cup placement Pawkit User Manual 4 Operation Once the Pawkit is properly positioned over the sample c
25. omer which contains the batteries from the white plastic bottom 28 Pawkit User Manual 5 Cleaning and Maintenance Remove the old Lithium ion batteries Replace with new CR1632 or equivalent 3V lithium coin cells Make sure to orient the batteries so the positive contact is facing down into the elastomer pocket Make sure the two small springs which make contact between the battery terminal and the circuit board are in place Replace the circuit board plastic bottom in the elas tomer top Align the buzzer in the hole between the bat teries Tighten the two screws to complete the assembly of the Pawkit 29 Pawkit User Manual 6 Verification and Calibration 6 Verification and Calibration E As mentioned earlier the Pawkit takes water activity mea surements by measuring the change in electrical properties of a special polymer held between two electrodes Due to the nature of the capacitance humidity sensor there may be times when you may need to adjust the calibration This sec tion explains how to do so Calibrations should be verified frequently with salt standards and adjusted as needed Verification Standards The Pawkit uses 2 calibration standards 6 0 molal NaCl 0 760a and 13 41 molal LiCl 0 250a 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 wate
26. ot be dis played until the instrument beeps and the sunburst icon disappears from the screen Mod ct oc I l a Pad W If you receive an error code of 9 99 at any time during the process it indicates that the sensor has failed and that the instrument needs to be serviced Refer to Chapter 7 for instructions on how to return your Pawkit for repair NOTE DO NOT lift or move the instrument during the measurement You risk contaminating the chamber and you will break the vapor seal of the chamber and invalidate the water activity measurement 5 After 5 minutes the instrument will display the final water activity and beep 5 times The sunburst disappears when the water activity reading is finished At this point 21 Pawkit User Manual 4 Operation you can either restart the measurement by pressing but ton I again or you can record the shown value and end the measurment procedure 6 Remove the sample cup by lifting the Pawkit Lift the Pawkit straight up as shown to avoid spilling the sample cup The sample may now be discarded or covered with a lid if it is to be re measured at a later time 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 automati cally shut itself off pressing button I will wake up the instrument and display the last water activity measurement Sampling Precautions Long exposure
27. pay kit water activity meter Portable Water Activity Measurement System Operator s Manual Version 8 Decagon Devices Inc 2365 NE Hopkins Court Pullman WA 99163 tel 509 332 2756 fax 509 332 5158 www decagon com aqualab decagon com Copyright 2007 2011 Decagon Devices Inc All rights reserved Pawkit User Manual Contents l Introduction 4 003 c0500 bowen tense 1 About this Manual o n aana 1 Customer Support 000 1 Warranty ental ace ie ee nem wepte en 2 Note to our Users 2 Seller s Liability 5 3 2 About the Pawkit 4 PawKit Specifications 5 4 How Pawkit works 00005 5 ACCONOCY eiin ran ETE cae an 5 Getting Started c 0cdisetoe tos ieneaes 5 Components of your Pawkit System 5 Preparing for Operation 6 3 Water Activity Theory 7 Moisture Content 7 Water Activity gocc0cesuaciey sane van 7 Temperature Ettects 0i4 cae seseuwes ex 9 Water Potential na nn 10 Factors in Determining Water Potential 10 Sorption Isotherms 12 4 Operation sssssssessessee 14 Features ore a E a aan 14 Sample Preparation and Insertion 15 Sample Preparation n aana 15 Sample Placement o nananana 7 Pawkit User Manual Taking Measurements 19 Turning it off aaa 22 Sampling Precautions
28. r activity mea surements They have been produced under a strict quality assurance regime and their accuracy is verified by an inde pendent third party instrument They are very accurate easy to use and readily available from Decagon Devices Most importantly they greatly reduce preparation errors Because of these reasons we recommend using these standards for the most accurate calibration of your Pawkit The verifica tion standards are shelf stable for one year If these standards are not available you can make a saturated Sodium Chloride NaCl slurry with a water activity value of 0 75 a To make a salt slurry of NaCl add water until the salt can absorb no more water as evidenced by the presence 30 Pawkit User Manual 6 Verification and Calibration of free liquid The slurry should take the shape of the cup and flow when tipped with the amount of free liquid at a minimum Steps to Verify Calibration l Take a vial of the 0 760 a NaCl standard and empty the entire contents of the vial into a sample cup Place the Pawkit over the sample cup as described in the previous section Press the left button I to take a reading If it is reading the correct water activity 0 02 your Pawkit needs no adjustment If not take a second reading After the sec ond reading note the water activity value shown If it is reading the correct water activity 0 02 your Pawkit needs no adjustment If it is not reading corr
29. roll past your desired adjust ment screen simply keep pressing the II button until you cycle back to the right screen 6 Once you have scrolled to the proper screen for calibra tion adjustment press the I button to adjust the value to 32 Pawkit User Manual 6 Verification and Calibration what it should be Each time you press the I button the value in the corner will change by an increment of 0 01 7 When you have it set to the correct value press the II button to scroll until Sto appears in the lower right corner then press I This will store the new value you have set You will then return to the main screen and begin a new measurement Note Ifyou 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 Ifyou inadvertently enter the calibration routine keep pressing button II until you scroll back to the main screen Note The 0 76 standard adjustment adjusts the calibra tion intercept while the 0 25 adjusts the slope Changes in the intercept are more likely to occur than changes in the slope so the 0 76 verification check is the most important and should be done more frequently Following is a graphical representation of the calibration routine 33 Pawkit User Manual 6 Verification and Calibration Measure 0 76 Standard Correct gare Adjust
30. s during the ripening of a Spanish craft goat s milk cheese Armada variety Food Chemistry 55 225 230 Kombila M E and C Lacroix 1991 The effect of combi nations of salt lactose and glycerol on the water activity a of cheese spreads Canadian Institute of Food Science and Technology Journal 24 233 238 41 Pawkit User Manual 8 Further Reading Pisecky J 1992 Water activity of milk powders Milchwis senschaft 47 3 7 Vivier D R Ratomahenina and P Galzy 1994 Charac teristics of micrococci from the surface of Roque fort cheese Journal of Applied Bacteriology 76 546 552 Fruits and Vegetables Beveridge T and S E Weintraub 1995 Effect of blanch ing pretreatment on color and texture of apple slices at various water activities Food Research Interna tional 28 83 86 Kiranoudis C T Z B Maroulis E Tsami and K D Mari nos 1993 Equilibrium moisture content and heat of desorption of some vegetables Journal of Food Engineering 20 55 74 Makower B and G L Dehority 1943 Equilibrium mois ture content of dehydrated vegetables Industrial and Engineering Chemistry 35 2 193 197 Maltini 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 som
31. s on Food Quality Academic Press New York Seow C C T T Teng and C H Quah 1988 Food Pres ervation by Moisture Control Elsevier New York Taoukis P W Breene and T P Labuza 1988 Intermedi ate moisture foods Advances in Cereal Science and Technology 9 91 128 Water Activity and Microbiology Beuchat L R 1981 Microbial stability as affected by water activity Cereal Foods World 26 7 345 349 Chen H C 1995 Seafood microorganisms and seafood safety Journal of Food and Drug Analysis 3 133 144 39 Pawkit User Manual 8 Further Reading Farber J M F Coates and E Daley 1992 Minimum water activity requirements for the growth of Liste ria monocytogenes Letters In Applied Microbiol ogy 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 control Food Product Design August 44 51 Miller A J 1992 Combined water activity and solute effects on growth and survival of Listeria monocy togenes Scott A Journal of Food Protection 55 414 418 Tokuoka K and T Ishitani 1991 Minimum water activi ties for the growth of yeasts isolated from high sugar foods Journal of General and Applied Microbiol ogy 37 111 119 Water Activity in Foods Meat and Seafood Chen N and L A Shelef
32. sure that the sample to be measured is homoge neous Multi component samples e g muffins with rai sins or samples that have outside coatings like deep fried breaded foods can be measured but may take longer to equilibrate Samples like these may require 15 Pawkit User Manual 4 Operation additional preparation crushing or grinding to obtain a representative sample e Completely cover the bottom of the cup with the sam ple if possible The Pawkit is able to accurately measure a sample that leave small spaces of the cup bottom exposed For example raisins only need to be placed in the cup and not flattened to cover the bottom A larger sample surface area increases instrument efficiency by shortening the time needed to reach vapor equilibrium e Fill the cup no more than half full of the sample The Pawkit does not require a large sample size to make its reading As long as the bottom of the cup is covered by the sample and that the sample is representative of the product you wish to measure you should be able to make accurate readings If the sample cup is too full you risk contaminating the sensor which will lead to inaccu rate readings e Make sure that the rim and outside of the sample cup are clean Wipe any excess sample material from the rim of the cup with a clean tissue Material left on the rim or the outside of the cup will be transferred to subsequent samples and can affect the accuracy of your readi
33. t and each wall of the box Include a copy of the RMA form in the shipment Please verify the ship to and bill to information contact name and problem description If anything is incorrect please contact a Decagon representative Tape the box in both directions for added support Ship to Decagon Devices Inc ATTN Repair Department 2365 NE Hopkins Court Pullman WA 99163 Repair Costs Manufacturer s defects and instruments within the one year warranty will be repaired at no charge Non warranty repair charges for parts labor and shipping will be billed to you An extra fee may be charged for rush work Decagon will provide an estimated repair cost if requested 36 Pawkit User Manual 7 Support and Repair Loaner Service Decagon has loaner instruments to keep you measuring water activity while your instrument is being serviced If your Pawkit is still under warranty or you have a service plan with your instrument there is no charge for the loaner ser vice Decagon Devices Inc ATTN Repair Department 2365 NE Hopkins Court Pullman WA 99163 37 Pawkit User Manual 8 Further Reading 8 Further Reading Water Activity Theory amp Measurement Duckworth R 1975 Water Relations of Foods Academic Press New York Gomez Diaz R 1992 Water activity in foods Determi nation methods Alimentaria 29 77 82 Greenspan L 1977 Humidity fixed points of binary satu rated aqueous solutions
34. to a variety of volatile substances or to sam ples with water activities near 1 00 can shift the sensor cali bration 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 22 Pawkit User Manual 4 Operation If sensor damage occurs an error code of 9 99 will be dis played on the screen Refer to chapter 7 for further instruc tions on how to return your Pawkit for repair 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 of the screen will appear You will see the mercury go up the thermometer and pop out of the top and the instrument will beep indicating that the sample s temperature is too high and there is danger of condensing water in the sample chamber and on the sensor If you get this warning while sampling remove the sample place 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 temperature 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 23 Pawkit User
35. up you are now ready to take readings see next section titled Taking Measurements To close the instrument reverse the opening procedure With one hand holding the case near the LCD pull down on the plastic sensor cover tab with the other hand and rotate until it snaps into the closed position cover ing the sensors Taking Measurements 1 Make sure the sample cup is positioned as described in the previous section Press the left button I to turn on the instrument It will display the last reading taken This allows you to begin a measurement and leave without having to attend 19 Pawkit User Manual 4 Operation the instrument throughout the measurement If it is already on proceed to the next step 3 Press button I to begin the water activity measurement The LCD display will be reset to 0 004 NOTE Pressing button I any time during a measure ment will restart the water activity measurement 4 Once the measurement process has been started the Pawkit will begin to display water activity measurements as well as temperature after 5 seconds and update the display every second 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 mea sures you will see the beams of the sunburst move from left to right 20 Pawkit User Manual 4 Operation The final water activity measurement will n
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