Water Activity

Scientist Live: “Water Activity Test Time”

June 20, 2022 – Scientist Live
It’s the sample, not the instrument, explains Dr. Brady Carter

There can be an abundance of confusion with water activity instruments concerning test time. The truth is that the 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 vapour equilibrium has been achieved and this process cannot be sped up by an instrument. So, any claim to a specific test time is illogical and would only be true for select samples. Ultra-fast test time claims are based on either a prediction or end-of-test settings that are not stringent enough to achieve true vapour equilibrium. 

End of Test Requirements

To determine when vapor equilibrium has been achieved and a test should end, instrumentation looks for the rate of water activity change to fall below some setpoint. What setpoint is used and how it is determined will impact both test time and reliability. If the setpoint is set to too low of stringency, the test could end prematurely before true equilibrium has been achieved. The ideal system should allow the user to set the end-of-test setting to whatever best suits their situation and needs. The ambiguity in end-of-test settings and their potential impact on testing results even lead ISO to define the end-of-test requirements in the recently revised water activity method ISO18787. 

Comparison Testing on Real Samples

To illustrate the potential problems introduced by prematurely ending water activity tests before vapor equilibrium has been achieved, water activity tests were run on samples of cream filling, gummies and beef jerky. Subsamples taken from the sample sample were run in a Dewpoint instrument in continuous mode and in the Novasina LabMaster NEO with the stability set to fast (least stringent) and slow (most stringent). Fig. 1 provides and illustration of the results. The difference between the first and second result in the Dewpoint instrument was always greater than 0.003aw reported accuracy of the instrument for all products indicating the first test result was premature. Further, the graphs clearly indicate that the continuous tests are tracking the equilibrium curve, which suggests that the first two-three rest results were premature and vapor equilibrium had not been achieved. Alternatively, while the fast setting on the NEO also gave premature results, the slow stability setting fully reflected the true vapor equilibrium. 

The evidence presented here clearly shows that substantial time can be needed to achieve true vapor equilibrium during water activity testing and that end-of-test settings that are less stringent will give premature results that do not reflect the true water activity. For a product being made close to the cut-off for microbial growth, premature test data could result in releasing an unsafe product. There may be justification for using less stringent end-of-test settings for routine testing to improve test times, but this should never be done without first checking on how different these premature results will be from the true water activity. Then, an informed decision can be made on whether faster test times are acceptable without putting the company at risk for recalling the failed product. 

Fig. 1 Water activity testing results on A) Cream filling B) Gummies and C) Beef jerky using the continuous mode in a dewpoint system and the Fast and Slow end-of-test settings in the LabMaster NEO, compared to the true water activity equilibrium curve. For each of the products, the early test results from the dewpoint system are clearly premature results on the equilibrium curve. For each of the products, the early test results from the dewpoint system are clearly premature results on the equilibrium curve and six continuous tests are needed to reach true equilibrium. The fast setting of the NEO produces a premature result as well due to its lower stringency but is further along the equilibration curve than the early dewpoint readings. The only setting that produces a true equilibrium water activity is the NEO slow setting.  

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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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