Water activity affects the physical properties of foods. Foods with a high water activity have a texture that is moist, juicy, tender, and chewy. When the water activity of these products is lowered, you get hardness, dryness, staleness, and toughness. Low water activity products have texture attributes described as crisp and crunchy, while these products at higher water activity levels change to a soggy texture.1
When water interacts with ionic and polar molecules, the closest water molecules are held or “tied up” making them unavailable for other uses. The remaining water is “free” or “bulk” water. Sugar molecules are excellent at tying up the water and holding onto it because of a sugar’s capacity to form many hydrogen bonds with water.
~An image of glucose hydrogen bonding with water from Lesson 3.
Water activity: it’s all about energy2
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. 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.
In your cookie, sugar is the “sponge” that is holding onto the water.
How does this impact my cookies?
In your lab, you examined the effect of fat on the cookie – butter vs. olive oil, or shortening.
Butter has ~15% water in it, ~80% fat and ~5% milk protein and sugars.3 This is because butter comes from milk, a watery liquid; it’s a normal part of the butter making process. Oils and Shortening however are 100% fat. What would you expect from two sugary baked goods that are identical, except one is made with a fat source that contains ~15% water, and the other uses a fat source that is 100% fat?
Borrowed from: Muhlenberg College