Water can be a food’s best friend, but also worst enemy. Water keeps a cake moist, strawberries juicy and ice cream solid (thanks ice crystals). But water can also make a cookie soggy, make oil splash and lack of water can make an apple hard and chewy. Controlling the location and concentration of water in your food is one of the main challenges in food production.
Probably one of the most common examples of undesired presence of water are soggy pie crusts.You expect a pie crust to be crunchy and crackly, not soft and maybe rubbery. (If you’re a British Bake Off fan, you will know that soggy crusts is one of their pet peeves when judging the freshly baked pies and tarts!)
The problem becomes more apparent with moister fillings (what about fruits?) and becomes worse over time. It is really hard to keep that pie crust crispy a few days later. Of course, the easiest solution is to bake the crust well and eat it quickly. However, there are other ways to control movement of water, once you understand the driving forces behind the phenomenon.
Water in food
Water is all around you and sits in just about every food we eat (pure fats and oils are probably one of the few that don’t contain any water). Water is actually a very special molecule. It is made of only three atoms, in the center sits an oxygen atom to which two hydrogen atoms are attached.
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.
You can measure the concentration of water in a food. Scientists can do this by drying a food for instance and measuring how much moisture has left the product. The concentration says how much water actually sits in the food. However, not all water is equal here. Some water is more available than other water. For instance, when sugar is dissolved in water, some of the water isn’t ‘available’ anymore because it has to surround the sugar molecule.
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.
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. 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.
- 30g of basic unflavoured saltine crackers (search for saltine crackers on Wikipedia, in every country the shape is slightly different, but all are made with only flour, water and yeast and maybe some salt, so they're very neutral in flavour!)
- 30g sugar
- ½ tsp of cinnamon
- ½ tsp of anise seed powder (gives a wonderful light anise seed flavour to the pie!)
- ½ tsp of the spice that fits best with your pie
- Take the crackers and crush them by hand, they shouldn't become powders, they should be a collection of powdery and small bits and pieces of material. No need to use any brute force, you can break them well enough by hand.
- Mix through the sugar and flavours.
- Make your pie as described in your pie recipe. Just after adding the pie dough into the tin and before adding the fruits, cover the bottom with your mixture.
- The mixture makes enough for a tin of approx. 20 cm.
- Add the fruits on top.
- Bake in the oven as instructed in the pie recipe.