Water Activity Defined. Water Activity 101: The Basics Brady Carter. 34 th Annual Food Safety & Sanitation Workshop 11/11 11/12/14 1

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1 Water Activity 101: The Basics Research Scientist Decagon Devices, Inc. Water activity vs. Moisture content Moisture Content Quantitative amount of water in a sample on a wet or dry basis. An extensive property that depends on the amount of material. Water Activity A measure of the energy status of the water in a system (Qualitative). A intensive property that does not depend on the amount of material. Water Activity Defined Correct Definition: Water Activity is a measure of the energy status of the water in a system. Old Definition: Water activity is the amount of free or available water in a product as opposed to bound water. a w = f/f o = p/p o Vapor pressure of water above C a w = Vapor pressure of pure same C a w = ERH (%) /100 Workshop 11/11 11/12/14 1

2 Moisture Content Defined Quantitative measure of amount of water Empirical measurement with no standard Loss on Drying Fick s Equation Titration Cheese & Cracker System Experiment 1 - Cracker is equilibrated over saturated NaCl in a sealed container Cheese & Cracker System Initial Final Moisture Content 4% 20% Water Activity Workshop 11/11 11/12/14 2

3 Cheese & Cracker System Experiment 2 - Cheese is equilibrated over saturated NaCl in a sealed container Cheese & Cracker System Initial Final Moisture Content 60% 30% Water Activity Cheese & Cracker System Cracker and Cheese are placed together in a sealed container 20% Moisture 30% Moisture Which way does water move? Video Presentation Workshop 11/11 11/12/14 3

4 Moisture Analysis Comparison Water Activity Moisture Content Measures Energy of Water Amount of Water Standard Available Yes, salt standards No, empirical measurement Cost $1000-$10,000 $250-$25,000 Reporting Methods a w units Dry or wet basis % Methods 5 methods 35 methods Sample Preparation Little to none Depends on method Water Activity vs. Moisture Content Water Activity Measurement Models for water activity predictions Norrish Equation Grover Model Ross Equation Computer programs Water activity measurement methods Hair or Polymer Hygrometers Freezing Point Depression Isopiestic Equilibration Electric Hygrometers Chilled Mirror Dew Point Workshop 11/11 11/12/14 4

5 Chilled Mirror Dew Point Advantages Primary method of measuring vapor pressure (not calibrated) Highest accuracy ±0.003a w Rapid measurement <5 minutes Measures entire a w range ( a w ) High reliability Mirror Optical Sensor Infrared Sensor Sample Fan Disadvantages Need clean mirror Readings affected by alcohol and propylene glycol Accurate Measurements Calibration / Verification Standards Saturated Salt Slurries Unsaturated Salt Solutions Temperature Sample Preparation Need representative sample If slicing / grinding be consistent Prevent moisture exchange with environment Isotherm Methods Dynamic and Static Dynamic and static methods can be analyzed on 1 sample Dynamic isotherms for matrix transitions Static isotherms for kinetic research Run a sample initially with Dynamic isotherm to determine regions of interest Investigate the time dependency in the regions of interest using a Static isotherm Workshop 11/11 11/12/14 5

6 Static and Dynamic on 1 sample 25 Static Isotherm Dynamic Isotherm % Moisture Content Crystallization Glass Transition Inflection Point Water Activity * Data is for Spray Dried Milk Powder at 25C Applications for Complete Moisture Analysis Water Activity Can Help You: Control microbial growth Formulate profitable products Predict effects of temperature abuse Control chemical reaction rates Control moisture migration Avoid caking and clumping Predict packaging needs Microbial Growth Scott (1953 & 1957) showed that microorganisms have a limiting water activity level below which they will not grow. Water activity, not water content, determines the lower limit of available water for microbial growth. Scott,W.J Water relations of Staphylococcus aureus at 30ºC. Aust. J. Biol. Sci. 6: Scott,W.J Water relations of food spoilage microorganisms. Adv Food Res 7: Workshop 11/11 11/12/14 6

7 Microbial Growth Growth Limit Every microorganism has a water activity level below which it cannot grow. a w limit Microorganisms 0.91 Gram Negative Bacteria 0.86 Gram Positive Bacteria 0.88 Yeast (practical limit) 0.80 Production of mycotoxins 0.70 Molds (practical limit) 0.62 Osmophilic yeast 0.61 Xerophilic molds 0.60 Absolute limit for all growth Water Activity Mode of Action a w = 0.95 a w = 0.90 Water moves out to lower a w and turgor pressure is lost AA s Polyols Sugars a w = 0.93 a w = 0.90 AA s Polyols Sugars The microbe tries to adapt by altering its membrane make-up or by reducing its a w to maintain turgor pressure. It will produce or transport in small solutes to reduce its a w. a w = 0.93 a w = 0.90 Unable to reach equilibrium with its surroundings, the microbe will remain in the lag phase with no growth or will begin sporulation and go dormant, which can only be reversed by ideal environment conditions. Microbial Growth Hurdle Technology Symbols F = heating t = refrigeration a w = water activity ph = acidification Eh = redox potential pres.= preservatives K-F = competitive organisms V = vitamins N = nutrients Leistner, L. 1992, Food preservation by combined methods. Food Research International, 25: Workshop 11/11 11/12/14 7

8 Interaction Table A Table A. Interaction of ph and a w for control of spores in food heattreated to destroy vegetative cells and subsequently packaged. a w Values 0.92 or less Non-PHF*/non- ** > > 0.95 ph Values 4.6 or less > > 5.6 * PHF means Potentially Hazardous Food ** means Time/Temperature Control for Safety Food *** PA means Product Assessment Required 2005 Food Code PA PA*** PA Interaction Table B Table B. Interaction of ph and a w for control of vegetative cells and spores in food not heat-treated or heat-treated but not packaged. a w Values ph Values < > > 5.0 < 0.88 Non- PHF*/non- ** > > 0.92 Non-PHD/non- PA PA*** PA PA PA PA * PHF means Potentially Hazardous Food ** means Time/Temperature Control for Safety Food *** PA means Product Assessment Required 2005 Food Code Formulating for Water Activity Dehydrate Product Edible films and coatings Keep water from migrating between the different components in a composite samples. They are located on the surface or as thin layers between several parts within the product. Additives Ingredients involved in water binding Humectants: salt (NaCl) sugars (glucose, fructose, sucrose, syrups) glycols (glycerol, PEG, propylene glycol) amino acids (glycine, alanine) polymers (starch, gums) acids (citric acid, lactic acid) Anticaking agents Workshop 11/11 11/12/14 8

9 Product Formulation Snack Cake All 3 components have same water activity 3 components have very different moisture contents Each component has a unique texture Icing serves as a moisture barrier for the cake Modeling Temperature Abuse Water activity is temperature dependent Most products have a lower water activity value at lower temperature. Clausius- Clapeyron relationship: Granola Bar Isotherm at 3 different Temperatures Chemical/Biochemical Stability General Effect of Water Activity on Reaction Rates. Moisture content also influences the kinetics of reactions were water is a reactant. Shelf life due to temperature and water activity can be predicted using Q 10 and Q A models. 1.4 Reaction Rate m o Water Activity Workshop 11/11 11/12/14 9

10 Moisture Migration Moisture migration can lead to: texture changes microbial growth degradation reactions organoleptic changes Examples of Multi-domain systems cheese / cracker cereal with fruit pieces baked dough filling frozen pizza crust sauce ice cream in cone gelatin capsules Solutions to limit moisture migration make components to same water activity lower aw of high aw component raise aw of low aw component retard diffusion process within component (viscosity) edible barrier separate packaging Dry Ingredient Mixing Example What will the final water activity be of a mixture of 30% apples at 0.67 a w, and 70% granola at 0.18 a w. Moisture Content (% w.b.) Ingredient 1 Ingredient Water Activity Science of Powder Flow and Caking Explained Majority of food powders are spray dried and in amorphous glassy state Well established that caking is related to glass transition Pore size and shape has an impact Free flowing powder is transformed into lumps and eventually an agglomerated solid. Problem is ubiquitous in the food and pharmaceutical industries. Workshop 11/11 11/12/14 10

11 Water Activity and Glass Transition Moisture Content (% d.b.) Large number of water binding sites become available Caking, Clumping, 10 Crystallization, Loss Limited Water of Texture Binding Sites 5 RHc Critical Water Activity 0 Amorphous Metastable State Water Activity *Data is for Spray Dried Milk Powder Package Permeability Package Permeability can be determined using changes in water activity Moisture sorption isotherm slope needed for models Once package permeability is known, it can be used to predict shelf life in a package Models are also available to determine package needs to achieve a specific shelf life Cannot use WVTR values in models because it is not a fundamental physical property Fick s Law H 20 Evaporation a w out a w in H 20 H 20 k/x What You Need to Know to Predict Shelf Life Good Science, Not guessing Water Activity Temperature Time Hygrothermal Time (real time shelf life) Rate: the exponential of a polynomial in a w and T with empirical constants a, b, and c, specific to material Workshop 11/11 11/12/14 11

12 Conclusion Water Activity is a measure of the energy status of water. Decagon provides a range of products for measuring water activity Decagon is always happy to provide application and technical support Water Activity is important for Microbial Safety Chemical Stability Physical Stability Predicting Shelf Life Thank you Decagon Devices, Inc NE Hopkins Court Pullman, Washington Phone: (509) / (800) Fax: (509) brady@decagon.com Web: Workshop 11/11 11/12/14 12