For a cheap ingredient, water can cause a lot of expensive problems–microbial growth, mold, loss of texture, caking and clumping, rancidity, vitamin loss. The best way to understand water in your product is by learning how to measure water activity.
Calculating the change in energy
Let’s consider the reduction in vapor pressure. We can calculate the change in energy that accompanies a change in pressure using the first law of thermodynamics. If we let the symbol U represent the energy in a system and calculate the change in U that occurs when we change the volume, at constant pressure (we assume no heat is added or removed) we can write
What is water activity (aw)?
Water activity: it’s all about energy. Aw is the ratio between the vapor pressure of the food itself, when in a completely undisturbed balance with the surrounding air media.
Take a glass of water and a dry sponge. Dip the corner of the sponge into the glass of water. The water will move from the glass into the sponge.
Water activity is the force that causes the water to move into the sponge. To understand it better, think about how the water in the sponge is different from the water in the glass.
The water in the glass is free, but the water in the sponge is anything but free. It’s bound by hydrogen bonds, capillary forces and van der Waals–London forces. These are called matrix effects. The water in the sponge has a lower energy state than the water in the glass. Water will flow into the sponge, but to get it back out, we must do work by squeezing the sponge.
The water in the sponge has a lower vapor pressure, lower freezing point, and higher boiling point than the water in the glass. They are different in ways we can measure and quantify.
Water’s energy can also be decreased by diluting it with solutes. These are called osmotic effects. Since work is required to restore the water to its pure, free state, this also reduces the water activity. The total change in energy is the sum of matric and osmotic effects.