Water activity - Citizendia

Water activity or a_w is a measurement of the energy status of the water in a system. It is defined as the vapor pressure of water divided by that of pure water at the same temperature; therefore, pure distilled water has a water activity of exactly one. Vapor pressure (also known as equilibrium vapor pressure or saturation vapor pressure) is the Pressure of a Vapor in equilibrium Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature Purified water is water from any source that is physically processed to remove impurities

There are several factors that control water activity in a system. Colligative effects of dissolved species (e. g. salt or sugar) interact with water through dipole-dipole, ionic, and hydrogen bonds. Capillary effect where the vapor pressure of water above a curved liquid meniscus is less than that of pure water because of changes in the hydrogen bonding between water molecules. Surface interactions in which water interacts directly with chemical groups on undissolved ingredients (e. g. starches and proteins) through dipole-dipole forces, ionic bonds (H3O+ or OH-), van der Waals forces (hydrophobic bonds), and hydrogen bonds. It is a combination of these three factors in a food product that reduces the energy of the water and thus reduces the relative humidity as compared to pure water. These factors can be grouped under two broad categories osmotic and matric effects.

Due to varying degrees of osmotic and matric interactions, water activity describes the continuum of energy states of the water in a system. The water appears “bound” by forces to varying degrees. This is a continuum of energy states rather than a static “boundness”. Water activity is sometimes defined as “free”, “bound”, or “available water” in a system. Although these terms are easier to conceptualize, they fail to adequately define all aspects of the concept of water activity.

Water activity is temperature dependent. Temperature changes water activity due to changes in water binding, dissociation of water, solubility of solutes in water, or the state of the matrix. Although solubility of solutes can be a controlling factor, control is usually from the state of the matrix. Since the state of the matrix (glassy vs. rubbery state) is dependent on temperature, one should not be surprised that temperature affects the water activity of the food. The effect of temperature on the water activity of a food is product specific. Some products increase water activity with increasing temperature, others decrease aw with increasing temperature, while most high moisture foods have negligible change with temperature. One can therefore not predict even the direction of the change of water activity with temperature, since it depends on how temperature affects the factors that control water activity in the food.

As a potential energy measurement it is a driving force for water movement from regions of high water activity to regions of low water activity. For example, if honey (a_w ≈ 0. Honey is a sweet and Viscous fluid produced by Honey bees (and some other species and derived from the nectar of Flowers According to the 6) is exposed to humid air (a_w ≈ 0. 7) the honey will absorb water from the air. Temperature and layers The temperature of the Earth's atmosphere varies with altitude the mathematical relationship between temperature and altitude varies among five Other examples of this dynamic property of water activity are; moisture migration in multidomain foods (e. g. cracker-cheese sandwich), the movement of water from soil to the leaves of plants, and cell turgor pressure. Since microbial cells are high concentrations of solute surrounded by semi-permeable membranes, the osmotic effect on the free energy of the water is important for determining microbial water relations and therefore their growth rates.

Higher a_w substances tend to support more microorganisms. A microorganism (also spelled micro organism or micro-organism and also called a microbe) is an Organism that is Microscopic (usually Bacteria usually require at least 0. The Bacteria ( singular: bacterium) are a large group of unicellular Microorganisms Typically a few Micrometres in length bacteria have 91, and fungi at least 0. A fungus (ˈfʌŋgəs is a eukaryotic Organism that is a member of the kingdom Fungi (ˈfʌndʒaɪ 7. See fermentation. Fermentation is the process of deriving energy from the oxidation of organic compounds such as carbohydrates using an endogenous electron acceptor which is

1 Formulae
2 Uses for Water Activity
- 2.1 Food Product Design
- 2.2 Food Safety
3 Water Activity Measurement
4 Water Activity and Moisture Content
5 Selected a_w values
- 5.1 Example Foods
- 5.2 a_w Values of Microorganism Inhibition
6 References
7 External links

Formulae

Definition of a_w:

$a_w \equiv p / p_0$

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:

$a_w \equiv l_w x_w$

where l_w is the activity coefficient of water and x_w is the mole fraction of water in the aqueous fraction. An activity coefficient is a factor used in Thermodynamics to account for deviations from ideal behaviour in a Mixture of Chemical substances In an

Relative humidity:

$\mathrm{RH} = a_w \times 100\%$

Estimated mold-free shelf life in days at 21° C:

$\mathrm{MFSL} = 10^{7.91 - 8.1 a_w}$

Uses for Water Activity

Water activity is an important consideration for food product design and food safety. Relative humidity is a measurement of the amount of Water vapor that exists in a gaseous mixture of air and water

Food Product Design

Food designers use water activity to formulate products that are shelf stable. In the Food processing industry shelf stable means that a typically refrigerated product has been altered so it can be safely stored and sold in sealed container at room temperature 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. This article is about ingredients in general There is also an American soul and R&B group called The Main Ingredient. 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.

In addition, water activity helps limit or slow certain undesirable reactions, such as non-enzymatic browning, fat oxidation, vitamin degradation, enzymatic reactions, protein denaturation, starch gelatinization and starch retrogradation. This too maintains product quality and extends shelf life.

Food Safety

Water activity is used in many cases as a Critical Control Point for Hazard Analysis and Critical Control Points (HACCP) programs. Critical Control Point (CCP 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 (critical levels Hazard Analysis and Critical Control Points (HACCP is a systematic preventive approach to Food safety and pharmaceutical safety that addresses physical chemical, Hazard Analysis and Critical Control Points (HACCP is a systematic preventive approach to Food safety and pharmaceutical safety that addresses physical chemical, Samples of the food product are periodically taken from the production area and tested to ensure that 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. Water content or moisture content is the quantity of Water contained in a material such as Soil (called soil moisture) rock, They found that the values were not universal, but specific to each food product. WJ Scott in 1953 first established that it was water activity, not water content that correlated with bacterial growth. Water content or moisture content is the quantity of Water contained in a material such as Soil (called soil moisture) rock, Bacterial growth is the division of one Bacterium into two idential daughter cells during a process called Binary fission. It is firmly established that growth of bacteria is inhibited at specific water activity values. 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. Powdered milk is a manufactured dairy product made by evaporating Milk to dryness. Over time bacterial levels will decline.

Water Activity Measurement

Water activity values are obtained by either a capacitance or a dew point hygrometer. Capacitance is a measure of the amount of Electric charge stored (or separated for a given Electric potential. Hygrometers are instruments used for measuring Humidity. A simple form of a hygrometer is specifically known as a "psychrometer" and consists of two Thermometers

Capacitance Hygrometers

Capacitance hygrometers consist of two charged plates separated by a polymer membrane dielectric. A polymer is a large Molecule ( Macromolecule) composed of repeating Structural units typically connected by Covalent Chemical bonds A dielectric is a nonconducting substance ie an insulator. The term was coined by William Whewell in response to a request from Michael Faraday. 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. Calibration is the process of establishing the relationship between a measuring device and the units of measure

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 a_w). They require regular calibration and can be affected by residual water in the polymer membrane.

Dew Point Hygrometers

Red line shows saturation

The temperature at which dew forms on a clean surface is directly related to the vapor pressure of the air. Formation Water will condense into droplets depending on the temperature Vapor pressure (also known as equilibrium vapor pressure or saturation vapor pressure) is the Pressure of a Vapor in equilibrium 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. A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument This temperature is then used to find the relative humidity of the chamber using psychrometric charts. Relative humidity is a measurement of the amount of Water vapor that exists in a gaseous mixture of air and water Psychrometrics or psychrometry are terms used to describe the field of engineering concerned with the determination of physical and thermodynamic properties of gas-vapor mixtures

This method is the most accurate (+/- . 003 a_w) and often the fastest. The sensor requires cleaning if debris accumulates on the mirror. . .

Equilibration

With either method, vapor equilibrium 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. In Thermodynamics, a thermodynamic system is said to be in thermodynamic equilibrium when it is in thermal equilibrium Mechanical equilibrium, and

Water Activity and Moisture Content

Water activity is related to moisture content in a non-linear relationship known as a moisture sorption isotherm curve. Water content or moisture content is the quantity of Water contained in a material such as Soil (called soil moisture) rock, This article describes the use of the term nonlinearity in mathematics These isotherms are substance and temperature specific. Isotherms can be used to help predict product stability over time in different storage conditions.

Selected a_w values

Example Foods

Substance	a_w
Distilled Water	1
Tap water	0. 99
Raw meats	0. 97 - 0. 99
Milk	0. Milk is an opaque white liquid produced by the Mammary glands of female Mammals (including Monotremes. 97
Juice	0. 97
Cooked bacon	< 0. Bacon is a cut of Meat taken from the sides belly or back of a Pig that has been cured, smoked, or both 85
Saturated NaCl solution	0. For sodium chloride in the diet see Salt. Sodium chloride, also known as common salt, table salt, or Halite, is a 75
Point at which cereal loses crunch	0. 65
Typical indoor air	0. 5 - 0. 7
Honey	0. 5 - 0. 7
Dried fruit	0. The term fruit has different meanings dependent on context and the term is not synonymous in Food preparation and Biology. 5 - 0. 6

a_w Values of Microorganism Inhibition

Microorganism Inhibited	a_w
Clostridium botulinum E	. Clostridium botulinum is a Gram-positive, rod shaped bacterium that produces the Neurotoxin Botulin, which causes the flaccid 97
Pseudomonas fluorescens	. Pseudomonas fluorescens is a common Gram-negative, rod-shaped Bacterium. 97
Escherichia coli	. 95
Clostridium perfringens	. Clostridium perfringens (formerly known as Clostridium welchii) is a Gram-positive, rod-shaped anaerobic, spore-forming Bacterium 95
Salmonella	. Salmonella is a Genus of rod-shaped Gram-negative enterobacteria that causes Typhoid fever, Paratyphoid fever 95
Vibrio cholerae	. Vibrio cholerae (also Kommabacillus) is a Gram negative curved-rod shaped Bacterium with a polar Flagella that causes Cholera 95
Clostridium botulinumA, B	. Clostridium botulinum is a Gram-positive, rod shaped bacterium that produces the Neurotoxin Botulin, which causes the flaccid 97
Bacillus cereus	. Bacillus cereus is an endemic, soil-dwelling Gram-positive, rod-shaped, beta hemolytic bacteria that causes Foodborne 93
Listeria monocytogenes	. Listeria monocytogenes is a Gram-positive Bacterium, in the division Firmicutes, named for Joseph Lister. 92
bacillus subtilis	. Bacillus subtilis Sporejpg\|thumb\|right\|Sporulating Bacillus subtilis ]] Bacillus subtilis is a Gram-positive, Catalase -positive 91
Staphylococcus aureus	. Staphylococcus aureus (ˌstæfɨləˈkɒkəs ˈɔriəs literally "Golden Cluster Seed" and also known as golden staph) is the most common cause of 87
Most Fungi	. A fungus (ˈfʌŋgəs is a eukaryotic Organism that is a member of the kingdom Fungi (ˈfʌndʒaɪ 70
No microbial proliferation	. 60

References

Fennema, O. R. , Ed. (1985). Food Chemistry - Second Edition, Revised and Expanded. New York: Marcell Dekker, Inc. pp. 46-50.
[1], 13 April 2008

External links

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