Water activity
Food safety |
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Terms |
Critical factors |
Bacterial pathogens |
Viral pathogens |
Parasitic pathogens |
Water activity (aw) is the
Water migrates from areas of high aw to areas of low aw. For example, if
Formula
The definition of aw is
An alternate definition can be
Relationship to
Uses
Water activity is an important characteristic for food product design and food safety.[citation needed]
Food product design
Food designers use water activity to formulate shelf-stable food. If a product is kept below a certain water activity, then mold growth is inhibited. This results in a longer shelf life.[citation needed]
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 migrates to the bran flakes over time, making the raisins hard and the bran flakes soggy. Food formulators use water activity to predict how much moisture migration affects their product.[citation needed]
Food safety
Water activity is used in many cases as a
For many years, researchers tried to equate bacterial growth potential with water content. They found that the values were not universal, but specific to each food product. W. J. Scott first established that bacterial growth correlated with water activity, not water content, in 1953. 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 never grow.[citation needed] Over time, bacterial levels decline.
Measurement
Water activity values are obtained by either a resistive electrolytic, a capacitance or a dew point hygrometer.
Resistive electrolytic hygrometers
Resistive electrolytic hygrometers use a sensing element in the form of a liquid electrolyte held in between of two small glass rods by capillary force. The electrolyte changes resistance if it absorbs or loses water vapor. The resistance is directly proportional to relative air humidity and therefore also to water activity of the sample (once vapor–liquid equilibrium is established). This relation can be checked by either verification or calibration using saturated salt-water mixtures, which provide a well-defined and reproducible air humidity in the measurement chamber.[citation needed]
The sensor does not have any physically given hysteresis as it is known from capacitance hygrometers and sensors, and does not require regular cleaning as its surface is not the effectively sensing element. Volatiles, in principle, influence the measurement performance—especially those that dissociate in the electrolyte and thereby change its resistance. Such influences can easily be avoided by using chemical protection filters that absorb the volatile compound before arriving at the sensor.[citation needed]
Capacitance hygrometers
Capacitance hygrometers consist of two charged plates separated by a
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 (+/- 0.015 aw). They should have regular calibration checks and can be affected by residual water in the polymer membrane (hysteresis).
Dew point hygrometers
The temperature at which
This method is theoretically the most accurate (+/- 0.003 aw) and often the fastest. The sensor requires cleaning if debris accumulates on the mirror.
Equilibration
With either method, vapor–liquid equilibrium must be established in the sample chamber. This takes place over time or can be aided by the addition of a fan in the chamber. Thermal equilibrium must also be achieved unless the sample temperature is measured.[citation needed]
Moisture content
Water activity is related to water 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.[citation needed]
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, an aqueous solution can be used to regulate humidity.[3]
Selected aw values
Substance | aw | Source |
---|---|---|
Distilled Water | 1.00 | [4] |
Tap water | 0.99 | [citation needed] |
Raw meats | 0.99 | [4] |
Milk | 0.97 | [citation needed] |
Juice | 0.97 | [citation needed] |
Salami | 0.87 | [4] |
Shelf-stable cooked bacon | < 0.85 | [5] |
Saturated NaCl solution | 0.75 | [citation needed] |
Point at which cereal loses crunch | 0.65 | [citation needed] |
Dried fruit | 0.60 | [4] |
Typical indoor air | 0.5 - 0.7 | [citation needed] |
Honey | 0.5 - 0.7 | [citation needed] |
Peanut Butter | ≤ 0.35 | [6] |
Microorganism Inhibited | aw | Source |
---|---|---|
Clostridium botulinum E | 0.97 | [7] |
Pseudomonas fluorescens | 0.97 | [7] |
Clostridium perfringens | 0.95 | [7] |
Escherichia coli | 0.95 | [7] |
Clostridium botulinum A, B | 0.94 | [7] |
Salmonella | 0.93 | [8] |
Vibrio cholerae | 0.95 | [7] |
Bacillus cereus | 0.93 | [7] |
Listeria monocytogenes | 0.92, (0.90 in 30% glycerol) | [9] |
Bacillus subtilis | 0.91 | [7] |
Staphylococcus aureus | 0.86 | [10] |
Most molds |
0.80 | [10] |
No microbial proliferation | <0.60 | [7] |
Solar planets habitability
Micro-organisms also require sufficient space to develop. In highly compacted bentonite and deep clay formations, microbial activity is limited by the lack of space and the transport of
At the surface of planets and in their atmosphere, space restrictions do not apply, therefore, the ultimate limiting factor is water availability and thus the water activity.[citation needed]
Most
Hallsworth et al. (2021) from the School of Biological Sciences at Queen's University Belfast have studied the conditions required to support the life of extremophile micro-organisms in the clouds at high altitude in the Venus atmosphere where favorable temperature conditions might prevail. Beside the presence of sulfuric acid in the clouds which already represent a major challenge for the survival of most micro-organisms, they came to the conclusion that the atmosphere of Venus is much too dry to host microbial life. Indeed, Hallsworth et al. (2021) have determined a water activity of ≤ 0.004, two orders of magnitude below the 0.585 limit for known extremophiles.[14] So, with a water activity in the Venus clouds 100 times lower than the threshold of 0.6 known in Earth conditions, the hypothesis envisaged by Greaves et al. (2020) to explain the biotic origin of phosphine in the Venus atmosphere is ruled out.[citation needed]
Direct measurements of the Venusian atmosphere by spatial probes point to very harsh conditions, likely making Venus an uninhabitable world, even for the most extreme forms of life known on Earth. The extremely low water activity of the desiccated Venusian atmosphere represents the very limiting factor for life, much more severe than the infernal conditions of temperature and pressure, or the presence of sulfuric acid.
References
- ISBN 978-0-632-05327-8.
- ISBN 978-0-834-21782-9.
- The Science Teacher. 51 (7): 29‑31.
- ^ ISBN 978-1-4327-3257-8.
- ^ "Bacon and Food Safety". United States Department of Agriculture Food Safety and Inspection Service. 2013-10-29. Retrieved 2017-06-18.
- PMID 23728806.
- ^ ISBN 9780470376454.
- ^ Shaw, Angela (2013). Salmonella: Create the most undesirable environment. Ames, IA: Iowa State University.
- ^ Ryser, Elliot T.; Elmer, Marth H. (2007). Listeria, Listeriosis and Food Safety (3rd ed.). CRC Press. pp. 173–174.
- ^ a b Marianski, 7
- S2CID 85250739.
- ISSN 0016-7037.
- S2CID 221655755.
- S2CID 237820246.
- ^ Timmer, John (28 June 2021). "Venus' clouds too dry, acidic for life". Ars Technica. Retrieved 1 July 2021.
- ^ Amos, Jonathan (29 June 2021). "Clouds of Venus 'simply too dry' to support life". BBC News. Retrieved 1 July 2021.
Further reading
- Reineccius, Gary (1998). Sourcebook of Flavors. Berlin: Springer. ISBN 978-0-8342-1307-4.
- Fennema, O.R., ed. (1985). Food Chemistry (2nd ed.). New York: Marcell Dekker, Inc. pp. 46–50.
- Bell, L.N.; Labuza, T.P. (2000). Practical Aspects of Moisture Sorption Isotherm Measurement and Use (2nd ed.). Egan, MN: AACC Egan Press.