Water Activity

Today I measured water activity in my pollen substrates. I spent about half an hour with the AquaLab CX-2 in a food science lab near the Creamery, putting stuff into little plastic cups, loading them into the smooth black steel drawer where they fit snugly into a round opening, sliding the drawer closed and turning the instrument to "read," then waiting approximately 4 minutes per sample for the insistent beeping to notify me that my water activity measurement was finally ready.

"As you can see, taking the measurement is a trivial process," said the food scientist in a white lab coat. But the measurement of water activity is quite far from trivial when you consider its implications.

Water activity is a measurement of how much water is "available" in a substance. In technical terms, it's the partial pressure of water vapor in the headspace of a given substance- but let's talk about water "availability." What would this water be available for exactly?

I like to think of water activity a measurement of grooviness. Imagine your material is having a rave. There are lots of substances in attendance: sugars, lipids, proteins, inorganic compounds, organic compounds, water. Naturally, water is the life of the party and everyone wants to dance with it. In some materials, all the water is already locked into dancing with someone else at the party- some sugar or some random alcohol group or what have you. In other materials, someone forgot to invite the water so the party is lame and dry and no one is dancing with anyone. In materials like my pollen dough, the water is there, but only some of it is dancing with other attendees. So the water activity is measuring how much water present in the material is hanging around looking for someone to dance with. This water has a tendency to vaporize and hang out in the headspace or the air around the party, where it can be measured by the AquaLab CX-2 if you have 4 minutes to wait.

Water activity increases with water content, but also varies depending on the composition of the medium (whoever else it has to dance with), so the relationship between water activity and water content is seldom linear. Microbes like my yeast love water and will gladly cut a rug with that unoccupied water and have a grand old time. If there isn't enough water ready to dance, they'll leave the party. In fact, each microbial species has a threshold for water activity, below which they cannot grow. So this is why we measure water activity; it measures how much water is available -or ready to dance with microbes- while water content will only tell you how much water is there at the party.

My yeast seems to need a really groovy party to be happy. So far I have failed to see growth on pollen substrates with an A_w (fancy abbreviation for water activity) as much as 0.77, which should be close to the same as that of bee bread in the hive.