Water Activity

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. 

However, correctly determining the shelf life of a product can be a challenging endeavor, often due to a lack of resources and the time to conduct full shelf-life studies. One option to speed up the shelf-life testing is to derive a model that can predict the shelf life based on the expected storage conditions of the product. To be effective, these models need to account for the effect of both water activity and temperature and not just one of these as they both impact shelf life. 

The method

While there are examples of shelf-life models in scientific literature, the most well-known being the Arrhenius equation, the only fundamental model that includes both water activity and temperature is hygrothermal time.(1) It is derived from a form of the Eyring (2) equation for rate change and Gibbs equation for free energy and is given by the equation shown above. 

Where T is the temperature (K), R is the gas constant (J mol-1 K-1), Ea is the activation energy (J mol-1), B is the molecular volume ratio, aw is the water activity, and r0 is the rate at the standard state. In practice, the values for B, Ea/R and r0 will be unique to each situation and are derived empirically through least squares iteration. Once the constants are known, any temperature and water activity can be used with the hygrothermal time model to determine the reaction rate at those conditions and hence the shelf life that the product will remain acceptable to the consumer. 

The significance

Hygrothermal time has proven to be effective at predicting the shelf life of products where the mode of failure is related to the rate of the chemical reaction such as lipid oxidation or vitamin loss. The shelf-life model is unique because it accounts for both water activity and temperature effects. The model can predict the change in shelf life that would occur due to processing to an incorrect water activity specification or subjecting a product to abuse storage conditions. When combined with Fickian based packaging models, it can predict the change in shelf life experienced by a product in the package when stored at different ambient conditions. 


1. Carter, B. P., Syamaladevi, R. M., Galloway, M. T., Campbell, G. S., & Sablani, S. S. 2017. A Hygrothermal Time Model to Predict Shelf Life of Infant Formula. In U. Klinkesorn (Ed.), Proceedings for the 8th Shelf Life International Meeting (pp. 40-45). Bangkok, Thailand: Kasetsart Univeristy. 

2. Eyring, H. 1936. Viscosity, plasticity, and diffusion as examples of absolute reaction rates. J. Chem. Phys. 4:283

A Robot in a chemical laboratory assists humans in the most dangerous operations.
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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water in a flask in the laboratory
Water Activity Standard Methods and Testing Regulations

Water activity testing, gauged by using a water activity meter, is most often used to determine the shelf life of a food product. It is important to gauge the amount of water in a food as it is known that water activity above 0.95 a w will provide sufficient moisture to support the growth of bacteria, yeast, and mold. Because it predicts stability relative to microbial growth, their rates of deteriorative reaction, and physical properties, water activity is considered to be an important property in the field of microbiology. In fact, regulatory agencies such as the Association of Official Analytical Chemists (AOAC), International Organization for Standardization (ISO), American Society for Testing and Materials, (ASTM) United States Pharmacopeia (USP) and a host of American, Canadian and International governments, have incorporated water activity standards into their safety regulations. If the water activity of food is controlled to 0.85 a w or less in a finished product, it is considered regulation-compliant.

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