Shelf Life: How is Water Activity (AW) Measured?

This sixth article in our guide to shelf life takes a look at one accurate and convenient way to measure water activity: an Aw meter. Subsequent articles take a deeper dive into ways to measure Aw, hygiene, and more.

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Shelf Life: Introduction to Water Activity

Each ingredient in a chocolate ganache has a unique capacity to bind with water. Learn how free water affects the shelf life of your final product.
Shelf Life: Introduction to Water Activity

See all of the articles in the Shelf Life series here

How is Aw Measured?

The typical system uses a sealed temperature-controlled chamber. A sample is placed in the chamber and sealed. The free water is allowed to escape into the air in the chamber. It remains there until all the free water has left the sample.

At equilibrium, the relative humidity of the air in the chamber is measured. The relation of this reading to pure water is the water activity measurement expressed as the term Aw. The range of water activity is from zero (no free water) to 1.0 (pure water).

A well balanced confection from Melissa Coppel — Tiramisu Bar from Melissa Coppel

Chef Coppel's recipes are balanced to offer the freshest, most premium experience possible while still offering a reasonable shelf life. You can find the recipe for this bar on the Cacao Barry website.

How to Measure the Aw of a Ganache

A specific machine with the ability to test for free water in the recipe is necessary, whether you have one in-house or send a sample out.

- Aw measuring instruments are expensive, and if it is not necessary to measure this value very often, such investment is not profitable.
For infrequent testing, it is usually more cost-effective to send samples to a lab for testing.
In the Chocolate Academy, we have a portable Aw measuring instrument at a much lower cost than the lab instruments. The results are not as accurate, however it is quick and easy to use.
Operation is easy. Once the ganache is prepared, we place it in a small cup designed for the machine, and we allow the ganache to crystallize at 20°C. We then place the cup in the machine and process the sample. The measurement usually takes from 15 to 30 minutes, depending on the sample.
If the Aw reading is higher than advisable, we must re-formulate the recipe and repeat the testing process.

Which Aw meter should you buy?

This depends on your region, needs, and budget. Many companies sell water activity devices, and all will offer detailed information about how their machines work and more information about the theory of water activity as it applies to their equipment.