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| {{Food safety}}
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| ''This article is about Water activity in food science. For a more general discussion, see [[thermodynamic activity]].''
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| | | The feeling of freedom and self-control that you feel as you drive through the mountain trails can help you forget all the stress and worries of daily life. For more serious cyclists, next to bicycles themselves, proper shoes are the most important piece of equipment. While a well trained dolphin is not as intimidating as a close encounter with a 230 kg wild animal is a totally different experience. There should not be a strain and similarly you shouldn't find yourself catching your breath. In the event you liked this information and also you wish to acquire details with regards to [http://www.silverstoregames.com/profile/ardeasey Women mountain bike sizing.] generously visit our own web page. However, due to its high price tag and light weight, it is seldom used for jumping and downhill races. <br><br>However, you still have to choose the right Santa Cruz bikes for yourself. Now most people wear gloves whilst they are riding because they keep the hands warm. You can spend under $100 for a bargain bike at a department store, or lay down thousands for a professional model. “Whereas past research emphasized whether or not a relationship existed between bicycle riding on a saddle and erectile dysfunction, Schrader now says that the next step of contemporary research on the subject should focus on intervention. Titanium is very light and gives you an advantage and is usually used in racing bicycles. <br><br>If you are looking for the best safety mountain bike accessories then the first thing you need to look into is a helmet. The big three has a major part to play on this matter and so they should step up to be strong and mature leaders. Guided by the dolphin trainer, they will perform a series of tricks such as clapping, synchronized jumping, etc. Being humiliated by my buddies was not fun, but there were enough good things about skiing that I wanted to come back for more, just not with my buddies. If you happen to crash, then you will see that your hands are the first thing that will probably come into contact with the ground. <br><br>Read product reviews and cycling magazines, research online, and ask for advice at your local bike shop. Cold and long winters at high altitude are perfect conditions for winter recreation. This article is to help you when buying a new bike. " I want us to be able to chase one another around the room, have pillow fights, and wrestle. The biker will encounter numerous obstacles along the way which will make his trajectory an extreme adventure. <br><br>And there are some people who rightfully bring up the issue of the fuel costs and environmental impact of producing the electricty used to power the bike in the first place. Listed below are some of the common terms which you will come across. This is the major basis of differentiation between a road bike and a mountain bike. A twenty-inch wheeled bike are good for longer traveling time. Some switchbacks can be technically challenging and the narrow, winding trails can make for a few surprises when encountering other riders, but much of the system is gentle, rolling and easily managed by novices. |
| '''Water activity''' or a<sub>w</sub> 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.
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| Water activity is defined as the [[partial pressure|partial]] [[vapor pressure]] of water in a substance divided by the standard state partial vapor pressure of water. In the field of food science, the standard state is most often defined as the partial vapor pressure of pure water at the same [[temperature]]. Using this particular definition, pure [[distilled water]] has a water activity of exactly one.
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| As the temperature increases, a<sub>w</sub> typically increases, except in some products with crystalline [[salt]] or [[sugar]].
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| Higher a<sub>w</sub> substances tend to support more [[microorganisms]]. [[Bacterium|Bacteria]] usually require at least 0.91, and [[fungi]] at least 0.7.<ref>{{cite book |last1=Rockland |first1=L.B. |last2=Beuchat |first2=L.R. |year=1987 |title=Water Activity:Theory and Applications to Food |edition=2nd |location=New York |publisher=Marcell Dekker }}</ref> See also [[fermentation]].
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| Water migrates from areas of high a<sub>w</sub> to areas of low a<sub>w</sub>. For example, if [[honey]] (a<sub>w</sub> ≈ 0.6) is exposed to humid [[air]] (a<sub>w</sub> ≈ 0.7), the honey will [[hygroscopy|absorb water from the air]]. If [[salami]] (a<sub>w</sub> ≈ 0.87) is exposed to dry air (a<sub>w</sub> ≈ 0.5), the salami will [[desiccation|dry out]], which could [[drying (food)|preserve]] it or [[staling|spoil]] it.
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| ==Formulae==
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| Definition of a<sub>w</sub>:
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| :<math> a_w \equiv p / p_0 </math>
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| where p is the vapor pressure of water in the substance, and p₀ is the vapor pressure of pure water at the same temperature.
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| Alternate definition:
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| :<math> a_w \equiv l_w x_w </math>
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| where l<sub>w</sub> is the [[activity coefficient]] of water and x<sub>w</sub> is the mole fraction of water in the aqueous fraction.
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| Relationship to [[relative humidity]]:
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| The relative humidity of air in equilibrium with a sample is called the Equilibrium Relative Humidity (ERH).<ref name="isbn0-632-05327-5">{{cite book |last1=Young |first1=Linda |last2=Cauvain |first2=Stanley P. |title=Bakery food manufacture and quality: water control and effects |publisher=Blackwell Science |location=Oxford |year=2000 |pages= |isbn=0-632-05327-5 |oclc= |doi= |accessdate=}}</ref>
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| :<math> \mathrm{ERH} = a_w \times 100\% </math>
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| Estimated mold-free shelf life in days at 21° C:
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| :<math> \mathrm{MFSL} = 10^{7.91 - 8.1 a_w} </math><ref name="isbn0-834-21782-1">{{cite book |last1=Man |first1=C.M.D. |last2=Jones |first2=Adrian A. |title=Shelf Life Evaluation of Foods |publisher=Springer |location= |year=2000 |pages= |isbn=0-834-21782-1 |oclc= |doi= |accessdate=}}</ref>
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| ==Uses for water activity==
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| Water activity is an important consideration for food product design and food safety.
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| ===Food product design===
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| 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]].
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| Water activity values can also help limit [[moisture migration]] within a food product made with different [[ingredient]]s. 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.
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| ===Food safety===
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| Water activity is used in many cases as a [[critical control point]] 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.
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| 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.
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| 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.{{Cn|date=February 2013}} Over time, bacterial levels will decline.
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| ==Water activity measurement==
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| Water activity values are obtained by either a resistive electrolytic, a [[capacitance]] or a [[dew point]] [[hygrometer]].
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| ===Resistive electrolytic hygrometers===
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| Resistive electrolytic hygrometers respectively sensors consist of a sensing element in form of a liquid electrolyte which is held in between of two small glass rods by capillary forces. The electrolyte changes its resistance if water vapor is absorbed or desorbed. The resistance is directly proportional to relative air humidity and also to water activity of the sample once [[vapor–liquid equilibrium]] has been established. This relation can be checked on a regular base by either a [[verification and validation|verification]] or [[calibration]] using salt-water mixtures which provide a well-defined and reproducible air humidity in the measurement chamber.
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| 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 have in principle an influence on the measurement performance, especially those which [[dissociation (chemistry)|dissociate]] in the electrolyte and thereby change its resistance. Such influences can easily be avoided by using chemical protection filters which absorb the volatile compound before arriving at the sensor.
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| ===Capacitance hygrometers===
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| Capacitance hygrometers consist of two charged plates separated by a [[polymeric membrane|polymer membrane]] [[dielectric]]. As the membrane adsorbs water, its ability to hold a [[electric charge|charge]] increases and the capacitance is measured. This value is roughly proportional to the water activity as determined by a sensor-specific [[calibration]].
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| Capacitance hygrometers are not affected by most [[volatility (chemistry)|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 a<sub>w</sub>). They should have regular calibration checks and can be affected by residual water in the polymer membrane (hysteresis).
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| ===Dew point hygrometers===
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| [[File:Relative Humidity.png|thumb|400px|right|Red line shows saturation]]
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| 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 [[psychrometrics]] charts. | |
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| This method is theoretically the most accurate (+/- 0.003 a<sub>w</sub>) and often the fastest. The sensor requires cleaning if debris accumulates on the mirror.
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| ===Equilibration===
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| With either method, [[vapor–liquid 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.
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| ==Moisture content==
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| Water activity is related to [[water content]] in a [[nonlinear system|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.
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| ==Use in humidity control==
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| 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.<ref>{{cite journal |author=Demchick PH |year=1984 |title=Taking control of chamber humidity |journal=[[The Science Teacher]] |volume=51 |issue=7 |pages=29‑31}}</ref>
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| ==Selected a<sub>w</sub> values==
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| {| class="wikitable"
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| |+Food
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| ! Substance
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| ! a<sub>w</sub>
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| ! Source
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| |-
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| | Distilled Water || 1.00 || <ref name="Marianski5">''Marianski'', 5</ref>
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| |-
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| | Tap water || 0.99 || {{Cn|date=February 2013}}
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| |-
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| | Raw meats || 0.99 || <ref name="Marianski5"/>
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| |-
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| | [[Milk]] || 0.97 || {{Cn|date=February 2013}}
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| |-
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| | Juice || 0.97 || {{Cn|date=February 2013}}
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| |-
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| | [[Salami]] || 0.87 || <ref name="Marianski5"/>
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| |-
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| | Cooked [[bacon]] || < 0.85 || {{Cn|date=February 2013}}
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| |-
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| | Saturated [[Sodium chloride|NaCl]] solution || 0.75 || {{Cn|date=February 2013}}
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| |-
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| | Point at which cereal loses crunch || 0.65 || {{Cn|date=February 2013}}
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| |-
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| | Dried [[fruit]] || 0.60 || <ref name="Marianski5"/>
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| |-
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| | Typical indoor air || 0.5 - 0.7 || {{Cn|date=February 2013}}
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| |-
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| | Honey || 0.5 - 0.7 || {{Cn|date=February 2013}}
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| |}
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| {| class="wikitable"
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| +Microorganisms
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| ! Microorganism Inhibited
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| ! a<sub>w</sub>
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| ! Source
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| |-
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| |''[[Clostridium botulinum]]'' A, B || 0.97 || {{Cn|date=February 2013}}
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| |-
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| |''[[Clostridium botulinum]]'' E || 0.97 || {{Cn|date=February 2013}}
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| |-
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| |''[[Pseudomonas fluorescens]]'' || 0.97 || {{Cn|date=February 2013}}
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| |-
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| |''[[Clostridium perfringens]]'' || 0.95 || {{Cn|date=February 2013}}
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| |-
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| |''[[Escherichia coli]]'' || 0.95 || {{Cn|date=February 2013}}
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| |-
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| |''[[Salmonella]]'' || 0.93 || <ref name="Shaw">''Shaw'', 1</ref>
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| |-
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| |''[[Vibrio cholerae]]'' || 0.95 || {{Cn|date=February 2013}}
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| |-
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| |''[[Bacillus cereus]]'' || 0.93 || {{Cn|date=February 2013}}
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| |-
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| |''[[Listeria monocytogenes]]'' || 0.92 || {{Cn|date=February 2013}}
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| |-
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| |''[[Bacillus subtilis]]'' || 0.91 || {{Cn|date=February 2013}}
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| |-
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| |''[[Staphylococcus aureus]]'' || 0.86 || <ref name="Marianski7">''Marianski'', 7</ref>
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| |-
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| |Most [[mold]]s || 0.80 || <ref name="Marianski7"/>
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| |-
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| |No microbial proliferation || 0.50 || {{Cn|date=February 2013}}
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| |}
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| == Notes ==
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| {{reflist}}
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| ==References==
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| *{{cite book |last1=Rockland |first1=L.B. |last2=Beuchat |first2=L.R. |year=1987 |title=Water Activity:Theory and Applications to Food |edition=2nd |location=New York |publisher=Marcell Dekker }}
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| *{{cite book |title=The Art of Making Fermented Sausages |last1=Marianski |first1=Stanley |last2=Marianski |first2=adam |year=2008 |publisher=Outskirts Press |location=Denver, Colorado |isbn=978-1-4327-3257-8}}
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| *{{cite book |title=Salmonella: Create the most undesirable environment|last=Shaw|first=Angela|publisher=Iowa State University|location=Ames, IA|year=2013}}
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| *{{cite book |title=Sourcebook of Flavors|last=Reineccius|first=Gary |publisher=Springer |location=Berlin |year=1998|isbn=0-8342-1307-9}}
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| *{{cite book |editor-last=Fennema |editor-first=O.R. |year=1985 |title=Food Chemistry |edition=2nd |location=New York |publisher=Marcell Dekker, Inc. |pages=46–50}}
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| *{{cite book |last1=Bell |first1=L.N. |last2=Labuza |first2=T.P. |year=2000 |title=Practical Aspects of Moisture Sorption Isotherm Measurement and Use |edition=2nd |publisher=AACC Egan Press |location=Egan, MN}}
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| ==External links==
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| *[http://www.epluse.com/en/products/moisture-in-oil-instrumentation/ Moisture in Oil Sensors]
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| *[http://www.epluse.com/en/products/dew-point-measurement/ Dew Point Sensors]
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| [[Category:Food science]]
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| [[Category:Water]]
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The feeling of freedom and self-control that you feel as you drive through the mountain trails can help you forget all the stress and worries of daily life. For more serious cyclists, next to bicycles themselves, proper shoes are the most important piece of equipment. While a well trained dolphin is not as intimidating as a close encounter with a 230 kg wild animal is a totally different experience. There should not be a strain and similarly you shouldn't find yourself catching your breath. In the event you liked this information and also you wish to acquire details with regards to Women mountain bike sizing. generously visit our own web page. However, due to its high price tag and light weight, it is seldom used for jumping and downhill races.
However, you still have to choose the right Santa Cruz bikes for yourself. Now most people wear gloves whilst they are riding because they keep the hands warm. You can spend under $100 for a bargain bike at a department store, or lay down thousands for a professional model. “Whereas past research emphasized whether or not a relationship existed between bicycle riding on a saddle and erectile dysfunction, Schrader now says that the next step of contemporary research on the subject should focus on intervention. Titanium is very light and gives you an advantage and is usually used in racing bicycles.
If you are looking for the best safety mountain bike accessories then the first thing you need to look into is a helmet. The big three has a major part to play on this matter and so they should step up to be strong and mature leaders. Guided by the dolphin trainer, they will perform a series of tricks such as clapping, synchronized jumping, etc. Being humiliated by my buddies was not fun, but there were enough good things about skiing that I wanted to come back for more, just not with my buddies. If you happen to crash, then you will see that your hands are the first thing that will probably come into contact with the ground.
Read product reviews and cycling magazines, research online, and ask for advice at your local bike shop. Cold and long winters at high altitude are perfect conditions for winter recreation. This article is to help you when buying a new bike. " I want us to be able to chase one another around the room, have pillow fights, and wrestle. The biker will encounter numerous obstacles along the way which will make his trajectory an extreme adventure.
And there are some people who rightfully bring up the issue of the fuel costs and environmental impact of producing the electricty used to power the bike in the first place. Listed below are some of the common terms which you will come across. This is the major basis of differentiation between a road bike and a mountain bike. A twenty-inch wheeled bike are good for longer traveling time. Some switchbacks can be technically challenging and the narrow, winding trails can make for a few surprises when encountering other riders, but much of the system is gentle, rolling and easily managed by novices.