Fundamentals of Aw-Decagon

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CONTENTS

What is Water Activity?Page 2-3n DefinitionnMeasurement Methods

Water activity for productsafety and qualityPage 4-5nMicrobial Growthn Chemical/Biochemical Reactivityn Physical PropertiesnMoisture Migration

Government CompliancePage 67

Microbial Growth Limits TablePage 8

Fundamentals ofWater Activity

Itis now generally accepted that awis more closely related tothe physical, chemical, and biological properties of foods andother natural products than is total moisture content. Specificchanges in color, aroma, flavor, texture, stability, and

acceptability of raw and processed food products have beenassociated with relatively narrow awranges.

Rockland LB & Nishi SK, Food Tech 34:42-59 (1980).

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Water Activity is MostRelevant for Quality andSafety IssuesAnother more important type ofwater analysis is water activity (aw).Water activity describes the energy

status or escaping tendency of thewater in a sample. It indicates howtightly water is bound, structurallyor chemically, in products. Both thewater content and the water activityof a sample must be specified to fullydescribe its water status. However,water activity is the property mostrelevant for quality and safety issues.Water activity is closely related to thepartial specific Gibbs free energy ofthe system. Thus, water activity is athermodynamic concept and has

requirements for measurements.These requirements are that thesystem be in equilibrium, thetemperature defined, and a standardstate specified. Pure water is taken asthe reference or standard state fromwhich the energy status of water infood systems is measured. The Gibbsfree energy of free water is zero; thus,the water activity is 1.0.

Water Activity is a Ratioof Vapor Pressures

Water activity is the ratio of thevapor pressure of water in a material(p) to the vapor pressure of purewater (po) at the same temperature.Relative humidity of air is the ratio ofthe vapor pressure of air to itssaturation vapor pressure. Whenvapor and temperature equilibriumare obtained, the water activity of thesample is equal to the relative

humidity of air surrounding thesample in a sealed measurementchamber. Multiplication of wateractivity by 100 gives the equilibriumrelative humidity (ERH) in percent.

aw= p/po= ERH (%) / 100

As described by the above equation,water activity is a ratio of vaporpressures and thus has no units. Itranges from 0.0aw(bone dry) to1.0aw(pure water).

Bound Water isNot Totally ImmobilizedWater activity is sometimesdescribed in terms of the amounts ofbound and free water in a

product. Although these terms areeasier to conceptualize, they fail todefine all aspects of the concept ofwater activity. Free water is notsubjected to any force that reduces itsenergy; therefore, all water in food isbound water. The issue is notwhether water is bound, but howtightly it is bound. Water activityis a measure of how tightly water isbound and related to the workrequired to remove water from thesystem. Water that is bound

should not be thought of as totallyimmobilized. Microbial and chemicalprocesses are related to this boundenergy status in a fundamental way.Because water is present in varyingenergy states, analytical methods thatattempt to measure total moisture insamples dont always agree or relateto safety and quality. Water activitytells the real story.

Water is recognized as beingvery important, if notcritical, to the stability of

most products. Controlling the waterwithin a product, by some method ofdrying or by chemically/structurally

binding (salting or sugaring) has longbeen used by man for preservation.This not only controls microbialspoilage, but also chemical andphysical stability.

Water Content Alone isNot a Reliable PredictorTraditionally, discussions aboutwater in products or ingredients focuson moisture or water content, whichis a quantitative or volumetricanalysis that determines the total

amount of water present. Watercontent of a product is a familiarconcept to most people. Onemeasures the water content by loss ondrying, infrared, NMR or Karl Fishertitration. Moisture contentdetermination is essential in meetingproduct nutritional labelingregulations, specifying recipes andmonitoring processes. However,water content alone is not a reliablepredictor of microbial responses andchemical reactions in materials.

Chemically Bound Water isUnavailable to MicrobesThe limitations of water contentmeasurement as an indicator of safetyand quality are attributed todifferences in the intensity whichwater associates with othercomponents in the product. Thewater content of a safe product variesfrom product to product and fromformulation to formulation. One safe,stable product might contain 15%water while another containing just8% water is susceptible to microbialgrowth. Although the wetter productcontains proportionally more water,its water is chemically bound byother components, making itunavailable to microbes. Using onlywater content values, its impossibleto know how available the waterin the product is to support microbialgrowth or influence product quality.

What is Water Activity?

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Bound or Free Water areNot Very Useful DescriptionsThere are several factors (osmotic,matrix, and capillary) that controlwater activity in a system. It is acombination of these factors in a

product that reduces the energy of thewater and thus reduces the vaporpressure above the sample ascompared to pure water. Due tovarying degrees of osmotic andmatrix interactions, water activitydescribes the continuum of energystates of the water in a system ratherthan a static boundness. Boundor free are not a very usefuldescription since it is an attempt toclassify a continuum in terms ofdiscrete states.

Two Methods ofWater Activity Instrumentsare AvailableThere is no device that can be putinto a product that directly measuresthe water activity. Rather, awismeasured with an indirect method.Water activity is measured byequilibrating the liquid phase waterin the sample with the vapor phasewater in the headspace of a closedchamber and measuring the relative

humidity of the headspace. Methodsfor water activity determinations aredetailed in the Official Methods ofAnalysis of AOAC International(1995). New instrument technologieshave vastly improved speed, accuracyand reliability of measurements.Reliable laboratory instrumentationis required to guarantee the safety of

products and enforce governmentregulations. Two different types ofwater activity instruments arecommercially available. One useschilled mirror dewpoint technologywhile the other utilizes relative

humidity sensors that changeelectrical resistance or capacitance.Each has advantages anddisadvantages. The methods vary inaccuracy, repeatability, speed ofmeasurement, stability in calibration,linearity, and convenience of use.

Chilled Mirror DewpointIn a chilled mirror dewpoint system,a sample is placed in a sample cupwhich is sealed against a sensorblock. Inside the sensor block is a

dewpoint sensor, an infraredthermometer, and a fan. Thedewpoint sensor measures thedewpoint temperature of the air, andthe infrared thermometer measuresthe sample temperature. From thesemeasurements the relative humidityof the headspace is computed as theratio of dewpoint temperaturesaturation vapor pressure tosaturation vapor pressure at thesample temperature. When the wateractivity of the sample and the relative

humidity of the air are in equilibrium,the measurement of the headspacehumidity gives the water activity ofthe sample. The fan is to speedequilibrium and to control theboundary layer conductance of thedewpoint sensor.

There is now wide agreement that awis the most useful expression of the water requirements for microbial growth andenzyme activity. The alternatives of solute concentration and water content have been shown very clearly by Scott (1962) tobe inadequate for describing the availability of water for the multiplication of certain bacteria.Troller JA, & Christian JHB, Water ActivityBasic Concepts. in Water Activity and Food , (Academic Press, New York, 1978),Chap. 1, pp. 112.

Water Activity Measurementin Less than 5 MinutesThe major advantages of the chilledmirror dewpoint method are speedand accuracy. Chilled mirrordewpoint is a primary approach to

measurement of relative humiditybased on fundamentalthermodynamic principles. Chilledmirror instruments make accurate(0.003aw) measurements in less than5 minutes. Since the measurement isbased on temperature determination,calibration is unnecessary, butrunning a standard salt solutionchecks proper functioning of theinstrument. If there is a problem, themirror is easily accessible and can becleaned in a few minutes. For some

applications, fast readings allowmanufacturers to perform at-linemonitoring of a products wateractivity.

Electric HygrometersOther water activity instruments useresistance or capacitance sensors tomeasure relative humidity. Thesesensors are made from a hygroscopicpolymer and associated circuitry thatgives a signal relative to theequilibrium relative humidity (ERH).

Commercially available instrumentsmeasure over the entire awrange withan accuracy of 0.015aw. Since theseinstruments relate an electrical signalto relative humidity, the sensor mustbe calibrated with known saltstandards. In addition, the ERH isequal to the sample water activityonly as long as the sample and sensortemperatures are the same. Accuratemeasurements require goodtemperature control or measurement.Advantages of capacitance sensorsinclude simple design and inexpensiveimplementation.

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SafeStorage

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Water activitys usefulness asa quality and safetymeasurement was

suggested when it became evidentmoisture content could notadequately account for microbial

growth fluctuations. Water activity isa measure of the energy status of thewater in a system. The water activity(aw) concept has served themicrobiologist and food technologistfor decades and is the mostcommonly used criterion for safetyand quality. Its usefulness cannot bedenied.

Predicting Safetyand Stability

Water activity

predicts safety andstability with respectto microbial growth,chemical andbiochemical reactionrates, and physicalproperties. Figure 1shows stability interms of microbial

growth limits and rates ofdegradative reactions as a function ofwater activity. Therfore, bymeasuring and controlling the water

activity, it is possible to: a) predictwhich microorganisms will bepotential sources of spoilage andinfection, b) maintain the chemicalstability of products, c) minimizenonenzymatic browning reactionsand spontaneous autocatalytic lipidoxidization reactions, d) prolong theactivity of enzymes and vitamins, ande) optimize the physical properties ofproducts such as moisture migration,texture, and shelf life.

MicrobialGrowthMicroorganismshave a limiting wateractivity level belowwhich they will notgrow. Water activity, not moisturecontent, determines the lower limit ofavailable water for microbialgrowth. Since bacteria, yeast, andmolds require a certain amount ofavailable water to support growth,

designing a product below a criticalawlevel provides an effective meansto control growth. Water may bepresent, even at high content levels, ina product, but if its energy level issufficiently low the microorganisms

cannot remove the water to supporttheir growth. This desert-likecondition creates an osmoticimbalance between themicroorganisms and the localenvironment. Consequently, themicrobes cannot grow and itsnumbers will decline until iteventually dies.

LimitingMicroorganism GrowthWhile temperature, pH, and several

other factors can influence whetheran organism will grow in a productand the rate at which it will grow,water activity is often the mostimportant factor. Water activity maybe combined with other preservativefactors (hurdles), such astemperature, pH, redox potential,etc., to establish conditions thatinhibit microorganisms. The wateractivity level that limits the growth ofthe vast majority of pathogenicbacteria is 0.90aw, 0.70awforspoilage molds, and the lower limitfor all microorganisms is 0.60aw.Table 1 (back page), lists the wateractivity limits for growth ofmicroorganisms significant to publichealth and examples of products inthose ranges.

Chemical/BiochemicalReactivityWater activity influences not onlymicrobial spoilage but also chemicaland enzymatic reactivity. Water mayinfluence chemical reactivity indifferent ways; it may act as asolvent, reactant, or change themobility of the reactants by affectingthe viscosity of the system. Wateractivity influences nonenzymaticbrowning, lipid oxidization,degradation of vitamins and othernutrients, enzymatic reactions,protein denaturation, starchgelatinization, and starchretrogradation (see Figure 1).

Typically, as the water activity level is

lowered, the rate ofchemical degradativereactions decreases.

Physical PropertiesBesides predicting the

rates of various chemicaland enzymatic reactions,water activity affects thetextural properties offoods. Foods with highawhave a texture that isdescribed as moist, juicy,tender, and chewy. Whenthe water activity ofthese products islowered, undesirabletextural attributes, suchas hardness, dryness,

staleness, and toughness,are observed. Low awproducts normally havetexture attributesdescribed as crisp andcrunchy, while theseproducts at higher awlevels change to soggytexture. Critical wateractivities determinewhere products become unacceptablefrom a sensory standpoint.

Caking, Clumping, Collapseand StickinessWater activity is an important factoraffecting the stability of powders anddehydrated products during storage.Controlling water activity in apowder product maintains properproduct structure, texture, stability,density, and rehydration properties.Knowledge of the water activity ofpowders as a function of moisturecontent and temperature is essentialduring processing, handling,packaging and storage to prevent thedeleterious phenomenon of caking,clumping, collapse and stickiness.Caking is water activity, time, andtemperature dependent and is relatedto the collapse phenomena of thepowder under gravitational force.

Moisture MigrationBecause water activity is a measureof the energy status of the water,differences in water activity between

components is the driving force for

Water activity forproduct safety and quality. D

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moisture migration as the systemcomes to an equilibrium. Thus, wateractivity is an important parameter incontrolling water migration ofmulticomponent products. Somefoods contain components atdifferent wateractivity levels, such asfilled snacks orcereals with driedfruits. By definition,water activity dictatesthat moisture willmigrate from a regionof high awto a region of lower aw,but the rate of migration depends onmany factors. Undesirable texturalchanges can result from moisturemigration in multicomponent foods.For example, moisture migratingfrom the higher awdried fruit into thelower awcereal causes the fruit tobecome hard and dry while the cerealbecomes soggy.

The practical applications of awthroughout the food industry have generatedcreative new food products worth many billions of dollars since the 1960s. Perhapsof even greater potential value is the pool of basic information about awand itsconnection to moisture relations and food qualities that has been generated sincethe 1960s.Bone DP, Practical Applications of Water Activity and Moisture Relations in Foods. inWater Activity: Theory and Applications to Food , L. B. Rockland, L. R. Beuchat, Eds.(Marcel Dekker Inc., New York, 1987), Chap. 15, pp. 369-395.

Water Adjusts BetweenMaterials Until Water

Activity EquilibriumDifferences in water activity levelsbetween components or a componentand environmental humidity are adriving force for moisture migration.Knowledge of whether water willabsorb or desorb from a particularcomponent is essential to preventdegradation, especially if thesubstance is moisture sensitive. Forexample, if equal amounts ofcomponent 1 at 2% and component2 at 10% moisture content must be

blended together, will there bemoisture exchange between thecomponents? The final moisturecontent of the blended materialwould be 6%, but did any moistureexchange between component 1 and2? The answer depends on the wateractivities of the two components. Ifthe water activities of the twocomponents are the same, then nomoisture will exchange between thetwo components. Also, two

ingredients at the same moisturecontent may not be compatible when

mixed. If two materials of differingwater activities but the same watercontent are mixed, the water willadjust between the materials until anequilibrium water activity isobtained.

Shelf-life/PackagingWater activity is a critical factor indetermining the shelf life of products.Critical upper and lower wateractivity levels can be established withrespect to microbial, texture, flavor,

appearance, aroma, nutritional, andcooking qualities for food products.Rates of exchange of moisturethrough the package and the rate ofchange in awof the food towards acritical limit will determine the shelflife of a product. Knowledge of thetemperature, ambient relativehumidity and critical awvalues willaid in selection of a package with thecorrect barrier properties to optimizequality and shelf life.

Figure 1

Water Activity - Stability Diagram

Water Activity

MoistureContent

Relat

iveReactionRate

Modern water activity instrumentsensure safe quality food products.

0.1

!

Loss of Crispness

Powder Caking

Collapse

MoistureSorptionIs

otherm

MoldGro

wthYe

ast

Bacte

ri a

Browning

Reactions

LipidO

xidatio

n

EnzymeActi

vity

Non-PHF PHF

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10


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The U.S. food, drug, andcosmetic laws are intended toassure the consumer that

foods are pure, wholesome, safe toeat, and produced under sanitaryconditions; that drugs and medical

devices are safe and effective for theirintended uses; that cosmetics are safeand made from appropriateingredients; and that all labeling andpackaging is truthful, informative,and not deceptive. Modern scientificmethods are required to enforce theFederal Food, Drug, and CosmeticAct. Laws to ensure thewholesomeness of foods and thesafety and efficacy of drugs would beimpractical without reliable methodsof laboratory analysis to determine

whether products are up to astandard.

Good Manufacturing PracticeThe United States has one of thesafest food supplies of any country inthe world. The FDAs GoodManufacturing Practice Regulationsincorporates water activity guidelinesin defining food safety regulations.GMP regulations detail the specificrequirements and practices to befollowed by industry to assure that

foods are produced under sanitaryconditions and are pure, wholesome,and safe to eat. Specific parts andparagraphs of applicable GMPregulations from Title 21 of the Codeof Federal Regulations use awinrelation to control measures and foodsafety. However, neither GMPs alonenor activities of regulatory agenciesalone can guarantee a completely safe

food supply. Consequently, a sciencebased system, Hazard Analysis andCritical Control Points (HACCP), hasbeen created to improve food safetyand reduce the incidence offoodborne illness.

In-process DetectionHACCP is a way for industry tocontrol and prevent problems, andensure safe food by controlling theproduction process from beginning toend, rather than detecting problemsat the end of the line. It identifieswhere hazards might occur in thefood production process and putsinto place actions to prevent thehazards from occurring. For example,a target water activity must be

established to prevent hazardousorganisms from growing. Bycontrolling major food risks, such asmicrobiological, chemical andphysical contaminants, the industrycan better assure consumers that itsproducts are safe.

Potentially Hazardous FoodThe term potentially hazardous food(PHF) was developed by the U.S.Public Health Service during the lasthalf of the twentieth century to

regulate perishable food. Potentiallyhazardous food means a food thatrequires time/temperature control forsafety (TCS) to limit pathogenicmicroorganism growth or toxinformation. Foods were considerednon-PHF when their aw 0.85, whichis below the water activity forStaphylococcus aureus growth andtoxin production or the pH 4.6,

which is below the pH for proteolyticClostridium botulinum growth andtoxin production.

New Scientific Based CriteriaThe 2005 version of the Food Code

updates the definitions of PHF byusing a scientific based criteria thatconsiders the interaction of awandpH in determining if a food isdesignated as a non-PHF/non-TCSfood. Interaction Tables A & Bconsider the interaction of awand pHunder certain conditions of heat-treatment and packaging. The hurdleeffect will control or eliminatepathogens that would otherwise beineffective when used alone. Theeffect of a heat treatment which

destroys vegetative cells and the effectof packaging which preventrecontamination is considered.

0.85 or Less awWill NotSupport PathogenicBacterial GrowthThe USDA and FSIS also use wateractivity in Generic HACCP Model 10for Heat-treated, Shelf-stable Meatand Poultry Products. The sciencestates and verifies that all pathogenicbacteria stop growing at a water

activity of 0.86. The model states thatManufacturers should not use themoisture protein ratio (MPR) as ameasure of proper drying for shelf-stability or safety. It is product wateractivity that is best correlated toinhibition of each pathogensgrowth. Thus, if you produce jerkyto a water activity of 0.85 or less,then the product will not support the

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PAPAPANon-PHF/non-

TCS

> 0.92

PA

PA***

Non-PHF/non-

TCS

> 5.0> 4.6 5.04.2 4.6< 4.2

* PHF means Potentially Hazardous Food

** TCS means Time/Temperature Control for Safety Food

*** PA means Product Assessment Required

PANon-PHF/non-

TCS

Non-PHF/non-

TCS

> 0.90 0.92

Non-PHF/non-

TCS

Non-PHF/non-

TCS

Non-PHF/non-

TCS

0.88 0.90

Non-PHD/non-

TCS

Non-PHF/non-

TCS

Non-PHF*/non-

TCS**

< 0.88

pH Valuesaw Values

Table B. Interaction of pH and aw for control of vegetative cells andspores in food not heat-treated or heat-treated but not packaged.

> 5.6> 4.6 5.64.6 or less

* PHF means Potentially Hazardous Food

** TCS means Time/Temperature Control for Safety Food

*** PA means Product Assessment Required

PAPANon-PHF/non-

TCS

> 0.95

PA***Non-PHF/non-TCS

Non-PHF/non-TCS

> 0.92 0.95

Non-PHF/non-

TCS

Non-PHF/non-

TCS

Non-PHF*/non-

TCS**

0.92 or less

pH Valuesaw Values

Table A. Interaction of pH and aw for control of spores in food heat-

treated to destroy vegetative cells and subsequently packaged.

growth of any pathogenic bacteria.Drying your product to a wateractivity of 0.80 or less does not makethe product any safer. The productwill have less consumer appeal(because it will be tougher and

chewier) and because jerky is sold ona weight basis, you will be losingprofit.

Pharmaceuticaland CosmeticsCurrently there are no water activityregulations for pharmaceutical orcosmetic products. Although, there isa proposed USP (United StatesPharmacopeial) Method using water activity. USP Method Microbiological Attributes of

Nonsterile Pharmaceutical Products Application of Water ActivityDetermination provides guidance onthe influence of water activity as itpertains to product formulationsusceptibility to microbialcontamination. The chapter discussesthe potential for improving productpreservation by maintaining lowwater activity. The determination ofthe water activity of nonsterilepharmaceutical dosage forms aids inthe decisions about the following:

Optimizing product formulationsto improve antimicrobial effectivenessof preservative systems.

Reducing the degradation of activepharmaceutical ingredients withinproduct formulations susceptible tochemical hydrolysis.

Reducing the susceptibility offormulations (especially liquids,ointments, lotions, and creams) tomicrobial contamination.

Providing a tool for the rationalefor reducing the frequency ofmicrobial limit testing and screeningfor objectionable microorganisms forproduct release and stability testingusing methods contained in thegeneral test chapter Microbial LimitTests .

Table B can be used to determine if a food which is not heat-treated or heat-

treated but not packaged is PHF, Non-PHF or Requires Product Assessment.

Food not heat-treated may contain vegetative cells and pathogenic spores.

Food that was heat-treated but not packaged may become re-contaminated.

pH values considered in Table B must include 4.2 because Staphylococcus

aureuscan grow at that level.

Table A can be used to determine if a food which is heat-treated and

packaged is PHF, Non-PHF or Requires Product Assessment. Food must meetcooking requirements of Food Code section 3-401.11 (no partial cooks) to

eliminate vegetative pathogens. Spore forming pathogens are the only

remaining biological hazards of concern. Food is packaged to prevent re-

contamination. Therefore, higher pH & awcan be safely tolerated.

Water activity plays an important role in the safety, quality, processing, shelf life, texture and sensory properties of foods.Fontana AJ & Campbell CS, Water Activity. in Handbook of Food Analysis, Physical Characterization and Nutrient Analysis , L.M. L. Nollet, Ed. (Marcel Dekker, New York, 2004), Chap. 3, pp. 39-54.

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Interaction Table A

Interaction Table B


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The importance of the water activitymoisture content concept of foods cannot be overemphasized. More importantly, thevalue of water activity has been shown to control the stability of dehydrated and semi-moist foods.Labuza TP, Sorption Phenomena in Foods: Theoretical and Practical Aspects. in Theory, Determination and Control of PhysicalProperties of Food Materials , C. Rha, Ed. (D. Reidel Publishing Co., Dordrecht-Holland, 1975), Chap. 10, pp. 197-219.

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Fundamentals of Aw-Decagon