Water activity
Encyclopedia
Water activity or aw was developed to account for the intensity with which water associates with various non-aqueous constituents and solids. Simply stated, it is a measure of the energy status of the water in a system. It is defined as the vapor pressure
of a liquid divided by that of pure water at the same temperature
; therefore, pure distilled water
has a water activity of exactly one.
As the temperature increases, aw typically increases, except in some products with crystalline salt
or sugar
.
Higher aw substances tend to support more microorganisms. Bacteria usually require at least 0.91, and fungi at least 0.7. See fermentation
.
Water migrates from areas of high aw to areas of low aw. For example, if honey
(aw ≈ 0.6) is exposed to humid air (aw ≈ 0.7) the honey will absorb water from the air.
where p is the vapor pressure of water in the substance, and p₀ is the vapor pressure of pure water at the same temperature.
Alternate definition:
where lw is the activity coefficient
of water and xw is the mole fraction of water in the aqueous fraction.
Relative humidity
:
Estimated mold-free shelf life in days at 21° C:
. If a product is kept below a certain water activity, then mold growth is inhibited. This results in a longer shelf-life
.
Water activity values can also help limit moisture migration within a food product made with different ingredients. If raisins of a higher water activity are packaged with bran flakes of a lower water activity, the water from the raisins will migrate to the bran flakes over time, resulting in hard raisins and soggy bran flakes. Food formulators use water activity to predict how much moisture migration will affect their product.
for Hazard Analysis and Critical Control Points
(HACCP) programs. Samples of the food product are periodically taken from the production area and tested to ensure water activity values are within a specified range for food quality and safety. Measurements can be made in as little as five minutes, and are made regularly in most major food production facilities.
For many years, researchers tried to equate bacterial growth potential with moisture content. They found that the values were not universal, but specific to each food product. W J Scott in 1953 first established that it was water activity, not water content
that correlated with bacterial growth
. It is firmly established that growth of bacteria is inhibited at specific water activity values. U.S. Food and Drug Administration
(FDA) regulations for intermediate moisture foods are based on these values.
Lowering the water activity of a food product should not be seen as a kill step. Studies in powdered milk
show that viable cells can exist at much lower water activity values, but that they will never grow. Over time, bacterial levels will decline.
or a dew point hygrometer
.
membrane dielectric
. As the membrane adsorbs water, its ability to hold a charge
increases and the capacitance is measured. This value is roughly proportional to the water activity as determined by a sensor-specific calibration
.
Capacitance hygrometers are not affected by most volatile
chemicals and can be much smaller than other alternative sensors. They do not require cleaning, but are less accurate than dew point hygrometers (+/- .015 aw). They should have regular calibration checks and can be affected by residual water in the polymer membrane (hysteresis).
The temperature at which dew
forms on a clean surface is directly related to the vapor pressure
of the air. Dew point hygrometers work by placing a mirror over a closed sample chamber. The mirror is cooled until the dew point temperature is measured by means of an optical sensor. This temperature is then used to find the relative humidity
of the chamber using psychrometric charts.
This method is theoretically the most accurate (+/- .003 aw) and often the fastest. The sensor requires cleaning if debris accumulates on the mirror.
must occur in the sample chamber. This will take place over time or can be aided by the addition of a fan in the chamber. Thermal equilibrium
must also take place unless the sample temperature is measured.
Vapor pressure
Vapor pressure or equilibrium vapor pressure is the pressure of a vapor in thermodynamic equilibrium with its condensed phases in a closed system. All liquids have a tendency to evaporate, and some solids can sublimate into a gaseous form...
of a liquid divided by that of pure water at the same temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
; therefore, pure distilled water
Distilled water
Distilled water is water that has many of its impurities removed through distillation. Distillation involves boiling the water and then condensing the steam into a clean container.-History:...
has a water activity of exactly one.
As the temperature increases, aw typically increases, except in some products with crystalline salt
Salt
In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...
or sugar
Sugar
Sugar is a class of edible crystalline carbohydrates, mainly sucrose, lactose, and fructose, characterized by a sweet flavor.Sucrose in its refined form primarily comes from sugar cane and sugar beet...
.
Higher aw substances tend to support more microorganisms. Bacteria usually require at least 0.91, and fungi at least 0.7. See fermentation
Fermentation (biochemistry)
Fermentation is the process of extracting energy from the oxidation of organic compounds, such as carbohydrates, using an endogenous electron acceptor, which is usually an organic compound. In contrast, respiration is where electrons are donated to an exogenous electron acceptor, such as oxygen,...
.
Water migrates from areas of high aw to areas of low aw. For example, if honey
Honey
Honey is a sweet food made by bees using nectar from flowers. The variety produced by honey bees is the one most commonly referred to and is the type of honey collected by beekeepers and consumed by humans...
(aw ≈ 0.6) is exposed to humid air (aw ≈ 0.7) the honey will absorb water from the air.
Formulae
Definition of aw:where p is the vapor pressure of water in the substance, and p₀ is the vapor pressure of pure water at the same temperature.
Alternate definition:
where lw is the activity coefficient
Activity coefficient
An activity coefficient is a factor used in thermodynamics to account for deviations from ideal behaviour in a mixture of chemical substances. In an ideal mixture, the interactions between each pair of chemical species are the same and, as a result, properties of the mixtures can be expressed...
of water and xw is the mole fraction of water in the aqueous fraction.
Relative humidity
Relative humidity
Relative humidity is a term used to describe the amount of water vapor in a mixture of air and water vapor. It is defined as the partial pressure of water vapor in the air-water mixture, given as a percentage of the saturated vapor pressure under those conditions...
:
- The relative humidity of air in equilibrium with a sample is called the Equilibrium Relative Humidity (ERH).
Estimated mold-free shelf life in days at 21° C:
Uses for water activity
Water activity is an important consideration for food product design and food safety.Food product design
Food designers use water activity to formulate products that are shelf stableShelf stable
Shelf stable food is food of a type that would normally be stored refrigerated but which has been processed so that it can be safely stored in a sealed container at room or ambient temperature for a usefully long shelf life...
. If a product is kept below a certain water activity, then mold growth is inhibited. This results in a longer shelf-life
Shelf life
Shelf life is the length of time that food, drink, medicine, chemicals, and many other perishable items are given before they are considered unsuitable for sale, use, or consumption...
.
Water activity values can also help limit moisture migration within a food product made with different ingredients. If raisins of a higher water activity are packaged with bran flakes of a lower water activity, the water from the raisins will migrate to the bran flakes over time, resulting in hard raisins and soggy bran flakes. Food formulators use water activity to predict how much moisture migration will affect their product.
Food safety
Water activity is used in many cases as a critical control pointCritical Control Point
Critical control point is a point, step or procedure at which controls can be applied and a food safety hazard can be prevented, eliminated or reduced to acceptable levels...
for Hazard Analysis and Critical Control Points
Hazard Analysis and Critical Control Points
Hazard analysis and critical control points, or HACCP , is a systematic preventive approach to food safety and pharmaceutical safety that addresses physical, chemical, and biological hazards as a means of prevention rather than finished product inspection...
(HACCP) programs. Samples of the food product are periodically taken from the production area and tested to ensure water activity values are within a specified range for food quality and safety. Measurements can be made in as little as five minutes, and are made regularly in most major food production facilities.
For many years, researchers tried to equate bacterial growth potential with moisture content. They found that the values were not universal, but specific to each food product. W J Scott in 1953 first established that it was water activity, not water content
Water content
Water content or moisture content is the quantity of water contained in a material, such as soil , rock, ceramics, fruit, or wood. Water content is used in a wide range of scientific and technical areas, and is expressed as a ratio, which can range from 0 to the value of the materials' porosity at...
that correlated with bacterial growth
Bacterial growth
250px|right|thumb|Growth is shown as L = log where numbers is the number of colony forming units per ml, versus T Bacterial growth is the division of one bacterium into two daughter cells in a process called binary fission. Providing no mutational event occurs the resulting daughter cells are...
. It is firmly established that growth of bacteria is inhibited at specific water activity values. U.S. Food and Drug Administration
Food and Drug Administration
The Food and Drug Administration is an agency of the United States Department of Health and Human Services, one of the United States federal executive departments...
(FDA) regulations for intermediate moisture foods are based on these values.
Lowering the water activity of a food product should not be seen as a kill step. Studies in powdered milk
Powdered milk
Powdered milk is a manufactured dairy product made by evaporating milk to dryness. One purpose of drying milk is to preserve it; milk powder has a far longer shelf life than liquid milk and does not need to be refrigerated, due to its low moisture content. Another purpose is to reduce its bulk for...
show that viable cells can exist at much lower water activity values, but that they will never grow. Over time, bacterial levels will decline.
Water activity measurement
Water activity values are obtained by either a capacitanceCapacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...
or a dew point hygrometer
Hygrometer
A hygrometer is an instrument used for measuring the moisture content in the environmental air, or humidity. Most measurement devices usually rely on measurements of some other quantity such as temperature, pressure, mass or a mechanical or electrical change in a substance as moisture is absorbed...
.
Capacitance hygrometers
Capacitance hygrometers consist of two charged plates separated by a polymerPolymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
membrane dielectric
Dielectric
A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...
. As the membrane adsorbs water, its ability to hold a charge
Charge
Charge or charged may refer to:* Charge , illegal contact by pushing or moving into another player's torso* Charge , a six-note trumpet or bugle piece denoting the call to rush forward...
increases and the capacitance is measured. This value is roughly proportional to the water activity as determined by a sensor-specific calibration
Calibration
Calibration is a comparison between measurements – one of known magnitude or correctness made or set with one device and another measurement made in as similar a way as possible with a second device....
.
Capacitance hygrometers are not affected by most volatile
Volatility (chemistry)
In chemistry and physics, volatility is the tendency of a substance to vaporize. Volatility is directly related to a substance's vapor pressure. At a given temperature, a substance with higher vapor pressure vaporizes more readily than a substance with a lower vapor pressure.The term is primarily...
chemicals and can be much smaller than other alternative sensors. They do not require cleaning, but are less accurate than dew point hygrometers (+/- .015 aw). They should have regular calibration checks and can be affected by residual water in the polymer membrane (hysteresis).
Dew point hygrometers
Dew
[Image:Dew on a flower.jpg|right|220px|thumb|Some dew on an iris in Sequoia National Park]]Dew is water in the form of droplets that appears on thin, exposed objects in the morning or evening...
forms on a clean surface is directly related to the vapor pressure
Vapor pressure
Vapor pressure or equilibrium vapor pressure is the pressure of a vapor in thermodynamic equilibrium with its condensed phases in a closed system. All liquids have a tendency to evaporate, and some solids can sublimate into a gaseous form...
of the air. Dew point hygrometers work by placing a mirror over a closed sample chamber. The mirror is cooled until the dew point temperature is measured by means of an optical sensor. This temperature is then used to find the relative humidity
Relative humidity
Relative humidity is a term used to describe the amount of water vapor in a mixture of air and water vapor. It is defined as the partial pressure of water vapor in the air-water mixture, given as a percentage of the saturated vapor pressure under those conditions...
of the chamber using psychrometric charts.
This method is theoretically the most accurate (+/- .003 aw) and often the fastest. The sensor requires cleaning if debris accumulates on the mirror.
Equilibration
With either method, vapor equilibriumEquilibrium
Equilibrium is the condition of a system in which competing influences are balanced. The word may refer to:-Biology:* Equilibrioception, the sense of a balance present in human beings and other animals...
must occur in the sample chamber. This will take place over time or can be aided by the addition of a fan in the chamber. Thermal equilibrium
Thermal equilibrium
Thermal equilibrium is a theoretical physical concept, used especially in theoretical texts, that means that all temperatures of interest are unchanging in time and uniform in space...
must also take place unless the sample temperature is measured.
Water activity and moisture content
Water activity is related to moisture content in a non-linear relationship known as a moisture sorption isotherm curve. These isotherms are substance and temperature specific. Isotherms can be used to help predict product stability over time in different storage conditions.Use in humidity control
There is net evaporation from a solution with a water activity greater than the relative humidity of its surroundings. There is net absorption of water by a solution with a water activity less than the relative humidity of its surroundings. Therefore, in an enclosed space, a solution can be used to regulate humidity.Example foods
| Substance | aw |
|---|---|
| Distilled Water | 1 |
| Tap water | 0.99 |
| Raw meats | 0.99 |
| Milk Milk Milk is a white liquid produced by the mammary glands of mammals. It is the primary source of nutrition for young mammals before they are able to digest other types of food. Early-lactation milk contains colostrum, which carries the mother's antibodies to the baby and can reduce the risk of many... |
0.97 |
| Juice | 0.97 |
| Salami Salami Salami is cured sausage, fermented and air-dried meat, originating from one of a variety of animals. Historically, salami has been popular among Southern European peasants because it can be stored at room temperature for periods of up to 10 years, supplementing a possibly meager or inconsistent... |
.87 |
| Cooked bacon Bacon Bacon is a cured meat prepared from a pig. It is first cured using large quantities of salt, either in a brine or in a dry packing; the result is fresh bacon . Fresh bacon may then be further dried for weeks or months in cold air, boiled, or smoked. Fresh and dried bacon must be cooked before eating... |
< 0.85 |
| Saturated NaCl Sodium chloride Sodium chloride, also known as salt, common salt, table salt or halite, is an inorganic compound with the formula NaCl. Sodium chloride is the salt most responsible for the salinity of the ocean and of the extracellular fluid of many multicellular organisms... solution |
0.75 |
| Point at which cereal loses crunch | 0.65 |
| Dried fruits | 0.60 |
| Typical indoor air | 0.5 - 0.7 |
| Honey | 0.5 - 0.7 |
| Dried fruit Fruit In broad terms, a fruit is a structure of a plant that contains its seeds.The term has different meanings dependent on context. In non-technical usage, such as food preparation, fruit normally means the fleshy seed-associated structures of certain plants that are sweet and edible in the raw state,... |
0.5 - 0.6 |
aw values of microorganism inhibition
| Microorganism Inhibited | aw |
|---|---|
| Clostridium botulinum Clostridium botulinum Clostridium botulinum is a Gram-positive, rod-shaped bacterium that produces several toxins. The best known are its neurotoxins, subdivided in types A-G, that cause the flaccid muscular paralysis seen in botulism. It is also the main paralytic agent in botox. C. botulinum is an anaerobic... A, B |
.97 |
| Clostridium botulinum Clostridium botulinum Clostridium botulinum is a Gram-positive, rod-shaped bacterium that produces several toxins. The best known are its neurotoxins, subdivided in types A-G, that cause the flaccid muscular paralysis seen in botulism. It is also the main paralytic agent in botox. C. botulinum is an anaerobic... E |
.97 |
| Pseudomonas fluorescens Pseudomonas fluorescens Pseudomonas fluorescens is a common Gram-negative, rod-shaped bacterium. It belongs to the Pseudomonas genus; 16S rRNA analysis has placed P. fluorescens in the P. fluorescens group within the genus, to which it lends its name.... |
.97 |
| Clostridium perfringens Clostridium perfringens Clostridium perfringens is a Gram-positive, rod-shaped, anaerobic, spore-forming bacterium of the genus Clostridium. C. perfringens is ever present in nature and can be found as a normal component of decaying vegetation, marine sediment, the intestinal tract of humans and other vertebrates,... |
.95 |
| Escherichia coli Escherichia coli Escherichia coli is a Gram-negative, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms . Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in humans, and are occasionally responsible for product recalls... |
.95 |
| Salmonella Salmonella Salmonella is a genus of rod-shaped, Gram-negative, non-spore-forming, predominantly motile enterobacteria with diameters around 0.7 to 1.5 µm, lengths from 2 to 5 µm, and flagella which grade in all directions . They are chemoorganotrophs, obtaining their energy from oxidation and reduction... |
.95 |
| Vibrio cholerae Vibrio cholerae Vibrio cholerae is a Gram-negative, comma-shaped bacterium. Some strains of V. cholerae cause the disease cholera. V. cholerae is facultatively anaerobic and has a flagella at one cell pole. V... |
.95 |
| Bacillus cereus Bacillus cereus Bacillus cereus is an endemic, soil-dwelling, Gram-positive, rod-shaped, beta hemolytic bacterium. Some strains are harmful to humans and cause foodborne illness, while other strains can be beneficial as probiotics for animals... |
.93 |
| Listeria monocytogenes | .92 |
| Bacillus subtilis Bacillus subtilis Bacillus subtilis, known also as the hay bacillus or grass bacillus, is a Gram-positive, catalase-positive bacterium commonly found in soil. A member of the genus Bacillus, B. subtilis is rod-shaped, and has the ability to form a tough, protective endospore, allowing the organism to tolerate... |
.91 |
| Staphylococcus aureus Staphylococcus aureus Staphylococcus aureus is a facultative anaerobic Gram-positive coccal bacterium. It is frequently found as part of the normal skin flora on the skin and nasal passages. It is estimated that 20% of the human population are long-term carriers of S. aureus. S. aureus is the most common species of... |
.86 |
| Most mold Mold Molds are fungi that grow in the form of multicellular filaments called hyphae. Molds are not considered to be microbes but microscopic fungi that grow as single cells called yeasts... s |
.80 |
| No microbial proliferation | .50 |
Reference
- Fennema, O.R., Ed. (1985). Food Chemistry - Second Edition, Revised and Expanded. New York: Marcell Dekker, Inc. pp. 46-50.
- Bell, L.N., and Labuza, T.P. 2000. Practical Aspects of Moisture Sorption Isotherm Measurement and Use. 2nd Edition AACC Egan Press, Egan, MN