Strawberry pie moisture migration

Preventing soggy pie crust – On moisture migration

Water keeps a cake moist and strawberries juicy. But water can also make a cookie soggy, or ruin the crispiness of baked chicken skin. Water can be a food’s best friend or its worst enemy.

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.

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.

Chemical potential

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.

deep dish pizza with a cheese barrier
A deep dish pizza consists of pizza crust with an extensive layer of cheese and then a still slightly moist tomato sauce. That quantity of tomato sauce would ruin the crust of a pizza, if it weren’t for the cheese! The cheese, which contains a lot of fat, serves as a barrier for that water in the tomato sauce to make a soggy crust!

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.

Strawberry pie moisture migration

Anti-soggy crust mixture

Prep Time: 5 minutes
Total Time: 5 minutes

Ingredients

  • 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

Optional flavourants

  • 1/2 tsp of cinnamon
  • 1/2 tsp of anise seed powder (gives a wonderful light anise seed flavour to the pie!)
  • 1/2 tsp of the spice that fits best with your pie

Instructions

Making the mixture

  1. 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.
  2. Mix through the sugar and flavours.

Making the pie

  1. 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.
  2. The mixture makes enough for a tin of approx. 20 cm.
  3. Add the fruits on top.
  4. Bake in the oven as instructed in the pie recipe.

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5 Comments

    • Hi Rich,

      Thanks for sharing your thoughts! You’re right, blind baking can definitely help to prevent any moisture migration before we even add the filling. I guess it’s what I was referring to when mentioning that you should add the filling later, but it’s probably better to call it outright, thanks :-).
      With regards to the egg wash, I’m a little cautious as to whether it actually prevents or delays moisture migration. I know quite a lot of sources online do say so, but it’s just a thin layer of protein so can’t imagine it works as well as a fat. That said, in some cases just a little improvement is enough so it doesn’t have to be perfect. Will have to do a test one day in which I compare the two!

      Thank you!

    • Hi Rich,

      I was reading some more and just found another brilliant method that works according to Cook’s Illustrated ‘The science of good cooking’, which works better for savory pies: salting the vegetables. Salting them will drain out part of the moisture, preventing it from leaking into the crust. Haven’t tried it myself yet, but sounds like a smart way to remove some of that excess moisture without losing a ton of flavour & structure.

  1. How do you prevent moisture migration if you are deep frying the product instead of baking. Besides strategy 3 of adding a humectant, what other ways can be employed?

    • Hi Emily,
      Thanks for your question :-). Moisture migration can be quite of a challenge for frying as well indeed. Since frying occurs so much faster though the ways to prevent it are slightly different. In a very short time span the outside becomes crispy whereas the inside is still nice and moist. This is what you generally like to achieve and since you have a moist inside the moisture from the inside will move to the outside quite quickly. When the food comes out of the deep fryer it’s important to make sure that all the remaining vapor can escape easily so it doesn’t sit in the crispy crust. This is why french fries and fried chicken tend to be packaged in paper, to let the moisture out, but it’s not a long term solution. If you have some more time, just let them sit in the open (on a cooling rack) to let that steam escape. There’s not really a good way to keep the crispiness for a long period of time. However, you can often get the crispiness back by heating the fried foods again in an oven, just take care not to overcook them!

      If you want to read some more on frying, you might want to check out our post on the topic as well (or read about the oven vs fried snacks, there’s some tips how to recreate that crispy layer without frying).

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