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Decagon Devices, Inc.
Water Activity & pH Measurement for Food Safety
Retail Food Systems Research ConferenceIFT Foodservice Division
January 7, 2008
Anthony J Fontana Jr. Ph.D.Senior Research Scientist
Decagon Devices, Inc.
Decagon Devices, Inc.
Outline
IntroductionpH
What is it?pH Measurement
Water activityWhat is it?Water activity vs. Moisture contentWater activity Measurement
Water activity & pH for safetyConclusion
Decagon Devices, Inc.
Introduction
WaterUnique molecule - abundant in all 3 phasesEssential for LifeWater affects taste, structure, & susceptibility to spoilageFresh foods (meat, fruit, veg.) contain large amount of waterNeed to control water in food for safety and storageControl water through pH and/or water activity
Decagon Devices, Inc.
Introductionδ+H Hδ+
O 2δ- H2O + H2O <-> H3O+ + OH-
Waterable to Hydrogen bond 3-dimensionally with other water moleculestypes of bonding with other molecules: ionic bonds, hydrogen bonds, dipole-dipole, van der Waals
Decagon Devices, Inc.
Introduction
Food SafetyControl Pathogenic Bacteria:
pHWater ActivityTemperature
Potentially Hazardous Food (Time/Temperature Control for Safety Food).
Except a FOOD that because of the interaction of its aW and pH values is designated as not supporting pathogenic microorganism growth or toxin formation.
Decagon Devices, Inc.
Outline
IntroductionpH
What is it?pH Measurement
Water activityWhat is it?Water activity vs. Moisture contentWater activity Measurement
Water activity & pH for safetyConclusion
Decagon Devices, Inc.
pH
1901 – Fritz HaberGerman chemistVoltage at certain glass surfaces changed with the acidity of a solution
1909 – Soren SorensenDanish biochemistDeveloped the pH scaleDefined pH as minus logarithm base 10 of [H+]‘p’ – German word for ‘power’
Decagon Devices, Inc.
2005 Food Code Definition
"pH" means the symbol for the negative logarithm of the hydrogen ion concentration, which is a measure of the degree of acidity or alkalinity of a solution.Values between 0 and 7 indicate acidity and values between 7 and 14 indicate alkalinity. The value for pure distilled water is 7, which is considered neutral.
Decagon Devices, Inc.
pH Definition
pH scale from 0 to 14
pH 0 – means a very high acid activity[H3O+] > [OH-] acidic
Lemon juice & vinegar (pH 2 to 3), Stomach acid (pH 1), Nitric acid & Hydrochloric acid (pH 0)
pH 7 – neutral [H3O+] = [OH-] neutral
Water
pH 14 – very low active acid concentration[H3O+] < [OH-] basic
Sea water (pH 8), household ammonia (pH 11), oven cleaners (pH 13), sodium hydroxide (pH 14)
Decagon Devices, Inc.
pH Definition
pH = - log [H+]
More accurate:
pH = - log aH+
aH+ = hydrogen ion activityActivity = effective concentration of H+
Other ions shield H+ and affect their ability to participate in chemical reactions
Decagon Devices, Inc.
pH Measurement
Indicator DyeCertain organic dye solutions change color over a small pH rangeIndicate approximate pH of a solutionDyes are deposited on a strip of paper
pH Meter / Electrode SystemIn an unknown solution the voltage on the outer electrode surface changes proportionally to changes in [H+]Compares the potential of unknown [H+] to a known reference potentialpH meters convert the voltage ratio between a reference and sensing half-cells to pH values
Decagon Devices, Inc.
pH Measurement
Nernst equation
E = total potential difference (in mV)Eo = reference potentialR = gas constantT = Temperature in Kelvinn = number of electronsF = Faraday’s constant[H+] = hydrogen ion concentration
E = Eo + (2.3RT)/nF log{unknown [H+]/internal [H+]}
Decagon Devices, Inc.
pH ElectrodesGlass Electrodes
Combination electrode with both half cells in one body in either glass or epoxy body
General purpose glass – pH 0-14, temp to 100°CBlue glass – pH 0-13, temp to 110°CAmber glass – pH 0-14, temp to 110°C, low Na+ error
Types: Solution vs gel filledAg/AgCl
Most common - Suitable for almost all applicationsTemp limit 80°C
Calomel – Hg/Hg2Cl2For samples containing proteins, organics, or heavy metals that could react with silver and clog the reference junctionTemp limit 70°C
Storage – in pH storage solution (never dry)Not water as will cause ions to leach from glass bulb
Decagon Devices, Inc.
pH Electrodes
ISFET Electrodes Ion-specific field effect transistorNon-glass measuring surfaceSemiconductor electrodes on microchipStore dry
Decagon Devices, Inc.
pH Measurement
pH is a function of temperatureVoltage output from the electrode changes linearly in relation to pH and the temperature of the solution determines the slope of the graph
25°C = 59.16 mV / pH unit0°C = 54.20 mV / pH unit100°C = 74.04 mV / pH unit
Decagon Devices, Inc.
pH Measurement
Rinse electrode with dH2O and blot dryNote: Never wipe the electrode to remove excess water – wiping can create static charges that interfere with pH measurement
Place electrode in standard buffer – adjust pH meter to pH value of standard bufferRinse electrode with dH2O and blot dryPlace electrode in test solutionRecord pH of the test solutionRemove electrode from test solution and rinse with water
Decagon Devices, Inc.
Accurate pH Measurement
ComponentsElectrode
Calibration (one-point vs two-point)90% problems are related to the improper use, storage or selection of electrodesCleaning
MeterTemperature compensation (either automatic or manual)
BuffersUse fresh buffer solutionsBuffers should frame the range of pH for the samples being testedBuffers should be at the same temperature as the samples
Decagon Devices, Inc.
Outline
IntroductionpH
What is it?pH Measurement
Water activityWhat is it?Water activity vs. Moisture contentWater activity Measurement
Water activity & pH for safetyConclusion
Decagon Devices, Inc.
2005 Food Code Definition
"aw" means water activity which is a measure of the free moisture in a FOOD, is the quotient of the water vapor pressure of the substance divided by the vapor pressure of pure water at the same temperature, and is indicated by the symbol AW.
Decagon Devices, Inc.
Water Activity 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.
“Free”: same physical properties as pure water“Bound”: different physical properties
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Water Binding Properties
Factors that Control Water Activity
Colligative effects or Solute interactionsMatrix effects or Surface interactions Capillary effects
It is a combination of these factors in an ingredient or product that reduces the energy of the water as compared to pure water.
Decagon Devices, Inc.
Water Activity Definition
How does water get “Bound” in a product?
Ionic bondsHydroxyl groups of sugarsCarbonyl and amino groups of proteinsOther polar sites that hydrogen bondDipole-dipole forcesVan der Waals forces
Decagon Devices, Inc.
Water Activity Definition
aw = ———————————————
aw = p/po = %ERH/100
Range: 0 (bone dry) — 1.0 (pure water)
Vapor pressure of water above sample @ °C
Vapor pressure of pure water @ same °C
Decagon Devices, Inc.
Water activity vs. Moisture Content
Moisture ContentQuantitative amount of water in a sample on a wet or dry basis.An extensive property that depends on the amount of material.
Water ActivityA measure of the energy status of the water in a system (Qualitative).A intensive property that does not depend on the amount of material.
Decagon Devices, Inc.
Cheese and Cracker System
Experiment 1 - Cracker is equilibrated over saturated NaCl in a sealed container
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Cheese and Cracker System
Initial Final
Moisture Content 4% 20%
Water Activity 0.30 0.75
Decagon Devices, Inc.
Cheese and Cracker System
Experiment 2 - Cheese is equilibrated over saturated NaCl in a sealed container
Decagon Devices, Inc.
Cheese and Cracker System
Initial Final
Moisture Content 60% 30%
Water Activity 0.90 0.75
Decagon Devices, Inc.
Cheese and Cracker System
Experiment 3 - Cracker and Cheese are placed together in a sealed container
Which way does water move?
20% Moisture 30% Moisture
Decagon Devices, Inc.
Cheese and Cracker System
Equilibrium Conditions
aw cracker = aw cheese = aw air
At equilibrium the energy status (chemical potentials) are the same and thus no net exchange of water.
µcracker = µcheese = µair
Decagon Devices, Inc.
Water activity vs. Moisture content
Traditional Fruit Cakemake components to same water activity (flour, sugar, candied fruits, raisins, butter, eggs, nuts)
52.20.862Fruits (mixed)
24.50.857Dough
Moisture Content (% db)
Water activity
Decagon Devices, Inc.
Water activity vs. Moisture content
Differences in water activity between components, layers, or individual ingredients leads to moisture migration and potential problems within a product.
At equilibrium the energy status of the water is the same and thus, no net exchange of water.
aw component 1 = aw component 2
Decagon Devices, Inc.
Water Activity vs. Moisture Content
Decagon Devices, Inc.
Measurement of Water Activity
Water Activity is measured by equilibrating a product with the vapor phase and measuring the relative humidity of the vapor phase.
Sample
liquid water vapor(humidity)
Decagon Devices, Inc.
Water Activity Measurement
Water activity measurement methodsHair or Polymer HygrometersFreezing Point DepressionIsopiestic EquilibrationElectric HygrometersChilled Mirror Dew Point
Decagon Devices, Inc.
Electrical Properties Sensor
Moisture changes the electrical properties of a hygroscopic material in equilibrium with air above sample.Two types:
ResistanceCapacitance
Decagon Devices, Inc.
Electrical Properties Sensor
AdvantagesAccuracy = ±0.01awRelatively insensitive to volatilesMeasures entire aw range
DisadvantagesNeeds calibration (secondary method)Need temperature control or compensation of sensorSome sensor hysteresis
Decagon Devices, Inc.
Chilled Mirror Dewpoint
Mirror is chilled until dew is formed. The temperature at which saturation is achieved is determined by observing condensation on a chilled surface (mirror).
Infrared SensorMirror
Optical SensorFan
Sample
Decagon Devices, Inc.
Chilled Mirror Dewpoint
AdvantagesPrimary method of measuring vapor pressure (not calibrated)Highest accuracy ±0.003awRapid measurement <5 minutesMeasures entire aw range (0.03 – 1.0aw)High reliability
DisadvantagesNeed clean mirrorReadings affected by ethanol and propylene glycol (>1%)
Decagon Devices, Inc.
Accurate Measurements
Calibration / Verification StandardsSaturated Salt SlurriesUnsaturated Salt Solutions
Sample PreparationNeed representative sampleIf slicing / grinding – be consistentPrevent moisture exchange with environment
Temperature
Decagon Devices, Inc.
Outline
IntroductionpH
What is it?pH Measurement
Water activityWhat is it?Water activity vs. Moisture contentWater activity Measurement
Water activity & pH for safetyConclusion
Decagon Devices, Inc.
Microbial Growth
Scott (1953) 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.
Decagon Devices, Inc.
Microbial Growth
Growth LimitEvery microorganism has a water activity level below which it cannot grow.
aw Limit0.94 Growth and toxin production for all
types of Clostridium botulinum0.90 Growth of pathogenic bacteria
except for S. aureus (aerobic)0.88 “Practical” limit for yeast0.86 Staphylococcus aureus (aerobic)0.80 Production of mycotoxins0.70 “Practical” limit for the growth of mold0.60 Absolute limit for the growth of any
microorganism
Decagon Devices, Inc.
Microbial Growth
Minimum aw Limit of Growth for Bacteria
0.86Staphylococcus aureus (aerobic)
0.90Staphylococcus aureus (anaerobic)
0.91Bacillus subtilis
0.92Listeria monocytogenes
0.93Bacillus cereus
0.94Vibrio parahaemoliticus
0.95Salmonella spp.
0.94Clostridium botulinum A, B
0.95Clostridium perfringens
0.95Escherichia coli
0.97Pseudomonas fluorescens
0.97Clostridium botulinum E
Minimum awMicroorganism
* Optimum conditions for pH, temperature, etc.
Decagon Devices, Inc.
Water activity & pH for Safety
In the United States alone annually due to foodborne disease :
76 million illnesses325,000 hospitalizations5,000 deaths
Source: Centers for Disease Control (CDC)
The annual medical costs, productivity losses, and costs of premature deaths due to five major foodborne pathogens are estimated to be $ 6.9 billion.
(Crutchfield and Roberts, 2000 FoodReview 23(3):44-49)
Decagon Devices, Inc.
Microbial Growth
Hurdle technology combines existing and new preservation techniques to establish a series of preservative factors (hurdles) that the microorganisms in question are unable to overcome (jump over).
These hurdles may be: water activitypH / aciditytemperatureredox potentialpreservativesothers
Leistner, L., (1994) Further developments in the utilization of hurdle technology for food preservation.,Journal of Food Engineering, 22:421-432.
Decagon Devices, Inc.
Microbial Growth
Decagon Devices, Inc.
Interactive Table A
> 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.
Decagon Devices, Inc.
Interactive Table B
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 and spores in food not heat-treated or heat-treated but not packaged.
Decagon Devices, Inc.
ConclusionWater Activity and pH are critical for food safety.Both aw & pH are easily and accurately measured with instrumentation aw = p/po
pH = -log aH+
Decagon Devices, Inc.
Thank you
Anthony J. Fontana Jr. Ph.D. [email protected]
Decagon Devices, Inc.2365 NE Hopkins CourtPullman, Washington 99163Phone: (509) 332-2756 / (800) 755-2751Fax: (509) 332-5158Email: [email protected]: www.decagon.com/aqualab/
