Water Activity

What is Water Activity?

Water Activity Definitions

Wikipedia describes Water activity as an element amount used to symbolize the energy status of the water in a system. It is determent as the vapor pressure of water above a sample divided by that of pure water at the same temperature, consequently, pure distilled water has a water activity of exactly one. It is extensively used in food science as a easy, clear-cut measure of the dryness of food, foods characteristically have an optimum water activity at which they have the greatest shelf life. Water activity is also used in the pharmaceutical production and chemicals for moisture sensitive products.

There are few factors that decide the water activity of a material. Colligative effects of disband species (such as sugar & salt) cooperate with water during attraction, ionic bonds and hydrogen bonds. Capillary effects also manipulate water activity, since the vapor pressure of water above a curved liquid meniscus is less than that of pure water. Surface interactions, in which water interacts directly with chemical groups on un-dissolved ingredients (i.e. starches and proteins), can also have an effect on water activity. The interaction can be through ionic bonds, van der Waals forces, hydrophobic interaction and hydrogen bonds. These reasons combine to boost the energy required to cause the water to fade away, and as a result reduce the humidity above the sample at equilibrium. These factors can be grouped under two broad categories: osmotic and matrix effects.

Due to changeable degrees of osmotic and matrix interactions, water activity describes the continuum of energy states of the water in a system. The water appears “bounded” 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 method. Although these terms are easier to conceptualize, they fail to adequately define all aspects of the concept of water activity.

Water activity is very 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 (e.g. glassy vs. rubbery state) is dependent on temperature, one should not be surprised that temperature affects the water activity of the food. The temperature dependence of water activity varies between substances. Some substances have increased water activity with increasing temperature, while others show a decrease with increasing temperature. 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 substance.

For example, if honey (aw ≈ 0.6) is exposed to humid air (aw ≈ 0.7) the honey will absorb water from the air, the movement of water from soil to the leaves of plants, and cell turgor pressure. Since microbial cells consist of 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.

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Scientist Live: “Pharmaceutical Industry Trends”

June 22, 2021 – Scientist Live
Dr. Brady Carter discusses the new standard for water activity measurement
Water activity has been broadly used in the pharmaceutical industry since the publication in 2006 of USP <1112>, an informational chapter on the application of water activity in pharma. Although <1112> provided guidance for the utilization of water activity, it was not an official method. Now USP has developed USP <922> Water Activity as an official method that will hopefully further facilitate its implementation as an integral part of a pharmaceutical quality program.

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Water Activity Test Time: It’s the Sample, Not the Instrument

There can be an abundance of confusion with water activity instruments concerning test time. Some instruments claim a 5-minute test time while others offer fast or quick modes. The truth is that water activity test time is determined by the sample and not the instrument. Since water activity is an equilibrium measurement, a reading is not complete until vapor equilibrium has been achieved and this process cannot be sped up by an instrument (1). So, any claim to a specific test time is illogical and would only be true for select samples. The reality is that most types of samples require a minimum of 5 minutes or more to reach true equilibrium and test times that are faster than that are either using a prediction or the system uses end-of-test settings that are not stringent enough to achieve true vapor equilibrium.

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shelf-life products
Scientist Live: “Accuracy Needed In Shelf-life Modelling”

January 4, 2022 – Scientist Live
Dr. Brady Carter on modelling shelf life with water activity
The shelf life of a product is defined as the practical time that it remains desirable to consumers. It dictates the radius of distribution for the product, how it must be stored and its best by date. Failure to match this expected shelf life can result in customer complaints, product recalls and tarnished reputation. Consequently, correctly determining the optimal production process and handling that maximizes the shelf life and then monitoring to make sure those conditions are met is the difference between profitability and lost revenue.

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