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Controlling just where water sits and how much is in your food, is one of the main challenges in maintaining the quality of so many foods. Luckily, the movement of water in your food can be described by a few ‘basic’ formulas and principles. Once you understand the science of those, you’ll be able to predict how the water will move and how you can prevent it from doing so!
We’ll be using a pie crust turning soggy as an example here, but the theory below applies to a lot of other foods!
Joys of a freshly baked pie
The joy of most freshly made pies, apart from their delicious smell, is the contrast in texture of the juicy, moist filling with a crunchy outer crust. Whether it’s an apple pie, a strawberry version, or a meat pie, they all have this texture contrast.
So what makes for this contrast in texture? A difference in moisture content. The outside of your pie, the crust, has been exposed to the heat of an oven directly. The heat has evaporated moisture from the crust and that lack of moisture has caused it to become crunchy (as we discussed in more detail here). The inside on the other hand, if it was baked along as well, is still very moist. Water couldn’t escape as easily, being captured within the crust.
It’s not just pies that struggle with maintaining the delicious contrasting textures. Chicago deep dish pizza, a special type of pizza, can struggle with this as well!
The loss of crunch
Unfortunately, this contrast in textures doesn’t last. Over time, the crust will lose some or all of its crunch. Moisture migrates from the moist filling into the dry crunchy crust, causing it to become soggy. Technically, the filling becomes a little drier as well. However, over a time frame of just a few days, you won’t notice these changes since the resulting texture change is minor. It’s the crust that gives the movement away.
Driving force for the movement of water
Simply said, water moves from moist to dry areas. Chemists can measure just how moist and dry different components are by measuring the water activity. Water activity, as discussed in more detail elsewhere, is a measure for the amount of ‘available’ water.
The amount of available water is less than the actual overall water content. This is important, since the non-available water, is bound or held up and can’t move around freely. For instance, sugar could be dissolved in this water, holding the water in its place.
This availability of water in a food is described by the term water activity. The water activity of a food is a number between 0 and 1. If the water activity of a food is 1, it is pure water, in a food with a water activity of 0, there is no water available at all. In food, both these extreme values are rare, most foods lie somewhere in between.
The water activity is used in a lot of different calculations. It can be used to describe the so called ‘chemical potential’ of a food. In a food with a higher water activity the water has a higher chemical potential.
So what does that mean in real life? Particles want to move from a higher chemical potential to a lower chemical potential, to even out differences. In food that translates into water moving from a region with a high water activity to a region with a low water activity.
A pie crust will have quite a low water activity, maybe around 0,4 (the exact value depends on your recipe and how it has been prepared). However, a filling like a creme patissiere will have a water activity of 0,9. As a result, the moisture in the filling will move into the crust, evening out the water activity values. However, the higher moisture content of that crust will make it soggy.
Water activity vs. concentration
The water activity is not the same as the concentration of water. Other ingredients in food (salt and sugar for instance), may interfer with the water and make it less ‘available’ than other ingredients. So two foods with equal amounts of water will have the same concentration, but may have a different water activity.
Want to understand the details better? Have a look at this video.
Water in food
Water is all around you and sits in just about every food we eat. Some foods contain a lot of it, such as milk or orange juice, which are mostly water. Others contain very little. A dry biscuit or potato chip are examples of those. The amount of water in a food determines the texture of a lot of foods. Only foods with very little moisture tend to be crispy and crunchy for instance.
Water is only a very small molecule, made up of just three atoms (2x hydrogen + 1x oxygen) with a unique chemistry. Water can diffuse and migrate through most foods very easily thanks to its small size. Its chemistry makes it attracted to some types of molecules and less so to others (hydrophobicity vs hydrophilicity).
Water is what enables life on this earth, not just humans need it, most bacteria depend on it as well. Water is a very special molecule made of only three atoms. In the center sits an oxygen atom to which two hydrogen atoms are attached. These two hydrogen atoms don’t sit exactly opposite one another though, they sit at a bit of an angle and this is partially what makes water so interesting to a chemist. It enables all sorts of reactions to occur.
Water is one of the main solvents in nature, meaning that a lot of components can dissolve in water. Sugar is a great example of that. Since the water molecule is so small, it can also travel around quite easily. We’ve seen this in fruits and vegetables where travel of the water through the cell wall is important to maintain the firm structure because of turgor.
What’s more, water can evaporate into a gas slowly at room temperature (you probably haven’t seen this happen for oils!). This is what we use when drying sausages. The temperature at which they’re dried isn’t very high, but water still slowly leaves the sausage, drying it out over time. We use the reverse effect when soaking raisins, there we want water to go into the raisins and we do that by placing them in water for a while.
So how do we know where the water will travel to? For that we need some thermodynamics and a concept called water activity.
Preventing moisture migration
In the case of a pie crust with the soft moist filling, you want to prevent moisture migration. So how to do that. There are actually various tactics and methods to do so. Food manufacturers use a lot of these solutions all the time to keep their products fresh.
Strategy 1: Speed up or wait
The best and easiest is to store and keep pie with a dry crust and moist filling for only a short period of time is to either eat it fast or wait with combining the two layers until just before you want to eat them. Of course, the second method only works if you don’t have to bake the filling.
Movement of water is always time bound. The longer you give it a chance to move through your food, the more severe the effect becomes, until it has reached an equilibrium.
Strategy 2: Introduce a barrier
Another way to prevent moisture from migrating is to place a barrier in between the crust and filling that prevents the movement of the water. Most fats are a good example of such barriers. Water cannot travel through a solid fatty layer.
A common ingredient to make this barrier is chocolate. You spread melted chocolate thinly onto the bottom of the pie and wait for it to set before adding the moist filling.The solid chocolate will be a great barrier for migration of moisture. A great advantage of using chocolate is that the pie will still taste good or even better! A disadvantage is though that you can only use it when you bake your filling and crust separately, the crust should be cooled down when adding the chocolate.
If you want to prevent moisture migration in a product that still has to be baked as a whole (e.g. an apple pie or a pear pie) you have to use another type of barrier. In these case you will often use something that actually absorbs some of the moisture that is released during baking. You want to prevent the moisture from even having a chance to move into the crust to start with. If it would, the crust would never have a chance to crisp up. A common method is to add breadcrumbs at the bottom of a pie or some marzipan or almond paste.
Strategy 3: Even out water activity
If the water activity of both components is the same, the moisture will not move between the two layers. So manufacturers might try to make them the same, it’s what they tend to do with cereals that contain raisins. That isn’t always possible since a change in water activity will also affect the texture of the food.
A common way to even the water activity out in the example of our pies would be to add more sugar or another ‘humectant’ to the filling. Sugar holds on to water, making it less available, so decreasing the water activity!
Recipe for a super crunchy crust in a fruit pie
This is a simple filling that doesn’t make a pie by itself, but helps to get that super crunchy bottom. Make the pie crust, then cover with this mixture before adding your moist filling and baking it in the oven. It’s not suitable if you don’t bake your filling. The reason this mixture works is that during baking it absorbs a lot of the moisture of the (fruity) filling and mixes completely with the filling an crust. After it’s baked you won’t be able to see that this mixture was originally there!
This trick works great with our basic pie crust, simply add a mixture of fruits with some sugars mixed through as a topping. You can also use it with apple pie, adding some spices like cinnamon or anise seed works especially well here.