Water activity, and why I measure every filling

AW stands for water activity (and not, as every Swede first assumes, after-work drinks). It's a measure of how much free water there is in a product. That could be a bonbon filling that needs analyzing, but it could just as well be a sponge cake that's baked, packaged and then sold in a store.

The natural follow-up question is: what does free water mean? Everything, to simplify a touch, contains water. The water in chocolate, in a carrot or in a jam is either bound to something, or it's free within the system it sits in. Bound means that ordinary granulated sugar (sucrose), for example, ties the water to itself so that nothing else can get at it. The problem, for anyone who wants their bonbons to keep for a long time, is that the sucrose can't bind all the water available in, say, a ganache. The water it fails to bind is what we call free water. Free water and moisture content are two different things. Honey contains around 18% water but has a low water activity (~0.6), while fresh bread is much higher, somewhere between 0.92 and 0.96.

Let's sit back down at the school desk and revise a little: water is the source of all life, which I think you'll recognize. The water that's free in a ganache simply becomes the source of life in the ganache. That is the problem. When there's free water, and above all too much of it, we open the door to life in the ganache, in the form of microorganisms. The common ones are mold, yeast and bacteria. One bacterium that turns up everywhere is Staphylococcus aureus, which lives naturally in the nose, on the skin and in the hair, among other places. So hygiene is everything when you make food.

Staphylococcus aureus is also a bacterium that can cause food poisoning. In the right environment, with enough free water, it multiplies quickly and produces a toxin. It's the toxin that makes you sick. If you make bonbons, or any other food, that's obviously something to avoid. By analyzing a filling meant for a bonbon, you can find out whether there's enough free water for Staphylococcus aureus to multiply and form its toxin. That's one of the most important reasons to measure the AW of a filling, and above all of a ganache.

When a filling is analyzed we learn its AW, and that gives a good indication of how long the filling keeps from a microbiological point of view. A little like magic! When you measure AW, the result is presented as a number from 0 to 1. Distilled water has the value 1 (very pure water, simply put). The less free water there is in whatever you're measuring, the lower the number. A well-formulated ganache can land at AW 0.80, while a pâte de fruit might land at 0.7765.

What the numbers, or the AW value, tell you is which kinds of microorganism can live and thrive in the filling. There are quite a few that can cause trouble for anyone handling food. Some have already been mentioned, but let's look at which of them could actually cause a problem. There are some big words involved, but the one thing to keep an eye on is the value 0.85. If the AW of a ganache is 0.85 or lower, Staphylococcus aureus can't, in practice, form its toxin, and that is also the limit the food industry tends to work from. In the EU there's no fixed AW limit written into law; it's the producer's responsibility to show that the product is safe. And that's where we want to end up, of course. A ganache with a higher value doesn't have to go straight in the trash, as long as you eat the bonbons quickly. I sometimes get asked whether a ganache can be made a certain way, and with my experience I can already tell from the recipe that it will come out well above 0.85. But if the bonbons are made today and eaten tomorrow, it matters less. You can't keep a ganache with a higher value at room temperature for more than a few days; in the fridge a little longer.

Let's look at what different AW values mean:

But AW isn't the only thing that decides: an intact shell with no cracks and a base that seals the bonbon properly matter too. Storage matters as well, to give bonbons, and chocolate in general, the best conditions for shelf life, but also for flavor and aroma. Chocolate is happiest stored in the dark at 12–18°C (54–64°F).

So how do you find out what AW a filling has?

I use a reference-grade AW meter. It's the same type of equipment you'll find at the world's biggest food manufacturers, or in laboratories. The meter I use works with a technique called dew point measurement, done with a chilled-mirror dew point hygrometer. It doesn't use a humidity sensor, as many simpler meters do. Instead it detects the moment condensation forms on a mirror sitting above the filling. The condensation comes from the water vapor (invisible to the naked eye) that the filling gives off into the air of the meter's sealed chamber. The mirror is cooled slowly, and with the help of a laser the meter detects exactly when the condensation forms. The mirror's temperature at that moment, the dew point, is then compared with the temperature of the filling, and from that the AW is calculated. Thanks to this technique we get a result with an accuracy of around ±0.003.

The method itself is simple:

To be sure the meter is reading correctly, I check it regularly with a verification. This is done the same way as when a filling is analyzed. But instead of filling a sample cup with a filling, we use a purpose-made solution of water and sodium chloride (salt), manufactured to have an exact AW value. I use a solution at AW 0.760, since that sits close to the values our fillings should have.

If the analysis of that solution comes out too high or too low, we can calibrate the meter using its calibration function. Then I run a new measurement to make sure the meter is reading exactly right.