Water Activity in FoodsPage 1Safefood 360 is the provider of foodsafety management software forindustry leading businesses.W H I T E PA P E RWater Activity (aw) in FoodsCONTENTSSUMMARY1.Introduction2.The Structure of Water3.Water In Foods3.1Water and Food Safety3.2Water Activity (aw)3.3Effects of Reduced aw on Food Safety3.4To Reduce aw4.Measure of Water Content and aw4.1Relationship of Water Activity and Water4.2Analysis of Water Content4.3Analysis of Water Activity4.4Water Activity as a CCPSafefood 360, Inc. 2014Water is the most abundantconstituent of food and interms of food safety themost significant. Its presence, quantity and naturedetermines many chemicaland biochemical processesimportant for the control ofproduct safety and quality.A better understanding ofwater activity aw can assistin developing robust andscientifically supported foodsafety plans.Part of Our Professional Whitepapers Series

Water Activity in Foods1.IntroductionWater is the most abundant constituentof food and in terms of food safety themost significant. Its presence, quantityand nature determines many chemical andbiochemical processes important for thecontrol of product safety and quality.In many HACCP plans, water is frequentlyreferred to as an intrinsic parameterrequired in product safety and while mostof us understand its importance we oftenfail to understand the reasons why. Abetter understanding of water, and inparticular water activity (aw), can assistin developing robust and scientificallysupported food safety plans.Page 2At standard temperature and pressurewater is a liquid. It is also tasteless andodorless. The molecule itself is non-linearand polar with an electrical dipolemovement. It is a good polar solvent andis typically referred to as the universalsolvent.Hydrophilic (water-loving) substancesdissolve readily while hydrophobic (waterfearing) substances are immiscible. Atstandard temperature and pressure theboiling point of water is 100 C (212 F).The density of liquid water is 1,000 kg/m3(62.43 lb/cu ft) at 4 C. Ice has a density of917 kg/m3 (57.25 lb/cu ft).3.2.The Structure of WaterThe chemical structure of water is H2O. Itdoes not exist in an absolutely pure formin nature due to its chemical properties.One molecule of water is composed of twohydrogen atoms covalently bonded to asingle oxygen atom. It commonly occursin all three states, i.e. liquid, solid and gas.Water In FoodsWater in food is an important parameterin the field of food science and safety. Dueto its unique role in various chemical andbiochemical reactions in a food matrix anunderstanding of water is crucial.3.1Water and Food SafetyOne of the oldest forms of preserving foodsis drying or desiccation. When first appliedthere was little understanding of whyremoving moisture from food had the effectof extending its shelf life.The table on the following page showsthe typical water content of certain foodproduct and categories:Safefood 360, Inc. 2014Part of Our Professional Whitepapers Series

Water Activity in FoodsFoodPage 3% Water Food% WaterLettuce, tomatoes95Cabbage, broccoli92Carrots, potatoes90Citrus fruit87Apples, cherries85Raw poultry72Raw lean meat60Cheese37White bread35Salami30Preserves28Honey20Dried fruit18Butter18Wheat flour12Dried pasta12Milk powder4Beer90Fruit juices87Milk87Over time, an understanding emerged andtoday we have a solid scientificunderstanding of how water impacts onproduct quality and safety. We now knowthat the water content is less importantthan the specific behaviour of the water orWater Activity. Water activity is related towater content in a non-linear relationshipwhich is represented using as a moisturesorption isotherm curve.To understand this more fully, we mustrecognise that much of the water in foodis bound water, i.e. bound to ions as waterof hydration, or bound to surfaces of largemolecule or cell structures. This water isnot free to support microbial growth, orto participate in or support chemical orenzyme reactions and spoilage processes.The total amount of bound water in a foodhas no relation to food stability.3.2The free or available water in a foodsupports microbial growth, andparticipates in and supports chemical andenzymatic reactions and spoilage processes.It is this amount of free water which iscalled water activity, aw, and it is moreimportant for food stability, chemical andmicrobial, than total water content.Water Activity (aw)This is the most important parameterof water in terms of food safety. Wateractivity or aw is the partial vapor pressureof water in a substance divided by thestandard state partial vapor pressure ofwater.In the field of food science, the standardstate is most often defined as the partialvapor pressure of pure water at thesame temperature. Using this particulardefinition, pure distilled water has a wateractivity of exactly one.The factors that reduce the mobility ofwater in a food also reduce its tendencyto evaporate and its vapour pressure andthis provides a means of defining andmeasuring it.Safefood 360, Inc. 2014Part of Our Professional Whitepapers Series

Water Activity in FoodsPage 4Therefore, we can generally define WaterActivity (aw) as an indication of the amountof Free Water in a food.Specifically Water Activity (aw) is: Actual Equilibrium v.p. of air-spaceover the foodEquilibrium v.p. of pure water atsame temperatureAlternatively we can define it usingRelative Humidity (RH): Actual v.p. water vapour in air x 100Equilibrium v.p. of Pure Water atSame TemperatureWater Activity Relative Humidity of AirspaceOver the Food / 1003.3 PPoFoodTypical awFresh meat0.98Cheese0.97Preserves0.88Salami0.83Dried fruit0.76Honey0.75Pasta0.50Table: Water activity values of foodsThe ideal solution may be written as :AwAnother chemical reaction effect isMaillard Browning which is maximum at0.6-0.7. Most enzymes are inactivated at 0.85. At less than 0.75 bacterial growthis inhibited but some yeast and mouldsmay grow. At less than 0.6 all growth isinhibited. In terms of nutrients, reducedaw reduces losses of Vitamins C, E, B1.The following table indicates the aw ofcertain foods. %ERH100Effects of Reduced awon Food SafetyThe rate of certain chemical andbiochemical processes are effected by theamount of available water. One of theeffects of reducing the aw in a food productis to reduce the rate of these reactions. Theexception is the oxidation of fats wherethe rate decreases down to 0.4 to 0.5, thenincreases.Safefood 360, Inc. 2014In general, bacteria require highervalues of aw for growth than fungi, withgram-negative bacteria having higherrequirements than gram-positive.Organism groupawMost spoilage bacteria0.90Most spoilage yeasts0.88Most spoilage moulds0.80Halophilic bacteria0.75Xerophilic moulds0.61Osmophilic yeasts0.61Table (above): Minimum aw values forgrowth of microorganism groups in foodTable: Minimum aw values for growth ofspecific microorganisms in foodPart of Our Professional Whitepapers Series

Water Activity in FoodsOrganismPage 5awOrganismawClostridium botulinum, type E0.97Candida scottii0.92Pseudomonas spp.0.97Trichosporon pullulans0.91Acinetobacter spp.0.96Candida zaylanoides0.90Escherichia coli0.96Staphylococcus aureus0.86Enterobacter aerogenes0.95Alternaria citri0.84Bacillus subtilis0.95Penicillium patulum0.81Clostridium botulinum, types A and B0.94Aspergillus conicus0.70Candida utilis0.94Aspergillus echinulatus0.64Vibro parahaemolyticus0.94Zygosaccharomyces rouxii0.62Botrytis stolonifer0.93Rhizopus stolonifer0.93Mucor spinosus0.933.4To Reduce awReducing the aw of a food product may beachieved through a number of methods.The most obvious is by partial removal ofwater in the food product using a varietyof unit operations or processes. Theconcentration of water can also be reducedby the addition of other substancesincluding salt and sugar. Many of thesemethods have been used long before theconcept of water activity was understood.4. Measure of Water Contentand aw4.1 Relationship between WaterActivity and WaterThere is often confusion between wateractivity and water content measurements.In many sectors, water content is used tocontrol the amount of water present in aproduct for quantitative reasons.Safefood 360, Inc. 2014For example, where a product is sold byweight, controlling its water content maybe important to meet legal and commercialrequirements. Water activity is moresignificant for qualitative considerationssuch as product stability, shelf life (e.g.microbiological & enzymatic stability,aroma retention), handling characteristics,physical properties and chemical stability.Water activity and water content canbe related by a graph called a SorptionIsotherm (see diagram), so if the userhas the ability to measure bothparameters, the relationship can bedefined and each parameter derivedfrom the other (interpolation).In practice, the sorption isotherm may beimpractical to use because, not only doesthe relationship between Aw and moisturecontent change with temperature ofmeasurement, but also any variations inthe material composition has a modifyingeffect.Part of Our Professional Whitepapers Series

Water Activity in FoodsThe company should therefore decidewhich parameter of measurement is bestsuited to their products and processes. Forquality control purposes, moisture contentlimits are easily converted to water activitylimits by very simple comparative tests.Water activity measurement offers a nondestructive, easy-to-use measurement in awide range of convenient configurations forboth laboratory and on-site use.Picture: Sorption Isotherm4.2 Analysis of Water ContentGravimetric MethodWater content may be measured in foodusing a number of methods. The most basicof these are gravimetric methods. Thisinvolves drying a known quantity of thefood product in an oven until all moisturehas been evaporated. By measuring the drymatter content remaining, the watercontent can be determined. A vacuum ovencan be used for heat sensitive foods.Karl Fisher TitrationA more sophisticated method for theanalysis of water content is the Karl FisherTitration. The Karl Fisher titration isparticularly adaptable to food productsSafefood 360, Inc. 2014Page 6that show erratic results when heated orsubjected to vacuum, e.g., low moisturefoods such as dried fruits and vegetables,candies, chocolates, roasted coffee, oils andfats or any low moisture foods rich in sugaror protein. The method is based on thefundamental reaction described by Bunsenin 1853 involving the reduction of Iodineby SO2 in the presence of water.2H2O SO2 I2 C5H2SO4 2HIThis was modified to include methanol andpyridine in a four component system todissolve the Iodine and SO2. For eachmole of water, 1 mole of iodine, 1 mole ofSO2, 3 moles of pyridine, and 1 mole ofmethanol are used.In the titration, if a sample containingwater is titrated with a KF reagent, iodineis consumed while there is water in thesystem. When no water remains then freeiodine appears and can be detected usingtwo platinum electrodes immersed in thesolution being titrated.As water is titrated, the voltage of theelectrodes approaches virtually zero. Andwhere there is an excess of iodine (all thewater has been consumed) a current can bemeasured.4.3 Analysis of Water ActivityA number of methods can be employed tomeasure the water activity including aresistive electrolytic, a capacitance or a dewpoint hygrometer.Part of Our Professional Whitepapers Series

Water Activity in FoodsPage 7Resistive electrolytic hygrometersCapacitance hygrometersResistive electrolytic hygrometersuse a sensing element in the form of aliquid electrolyte held between two smallglass rods by capillary force. Theelectrolyte changes resistance if it absorbsor loses water vapor.Capacitance hygrometers consist of twocharged plates separated by a polymermembrane dielectric. As the membraneadsorbs water, its ability to hold a chargeincreases and the capacitance is measured.The resistance is directly proportional torelative air humidity, and also to thewater activity of the sample (once vapor–liquid equilibrium is established). Thisrelationship can be checked by either averification or calibration using saltwater mixtures, which provide a welldefined and reproducible air humidity inthe measurement chamber.The sensor does not have any physicallygiven hysteresis as it is known fromcapacitance hygrometers and sensors,and does not require regular cleaning asits surface is not the effectively sensingelement.Volatiles, in principle, influence themeasurement performance—especiallythose that dissociate in the electrolyteand thereby change its resistance. Suchinfluences can easily be avoided by usingchemical protection filters that absorb thevolatile compound before arriving at thesensor.This value is roughly proportional tothe water activity as determined by asensor-specific calibration. Capacitancehygrometers are not affected by mostvolatile chemicals and can be much smallerthan other alternative sensors.They do not require cleaning, but are lessaccurate than dew point hygrometers( /- 0.015 aw). They should have regularcalibration checks and can be affected byresidual water in the polymer membrane(hysteresis).Dew point hygrometersThe temperature at which dew forms on aclean surface is directly related to the vaporpressure of the air. Dew point hygrometerswork by placing a mirror over a closedsample chamber.The mirror is cooled until the dew pointtemperature is measured by means of anoptical sensor. This temperature is thenused to find the relative humidity of thechamber using psychrometrics charts.This method is theoretically the mostaccurate ( /- 0.003 aw) and often thefastest. The sensor requires cleaning ifdebris accumulates on the mirror.Safefood 360, Inc. 2014Part of Our Professional Whitepapers Series

Water Activity in Foods4.4 Water Activity as a CCPWater activity is frequently used as acritical control point for Hazard Analysisand Critical Control Points (HACCP)programs. Samples of the food product