Preventing soggy pie crust – On moisture migration

Water can be a food’s best friend but also worst enemy. Water keeps cake moist, strawberries juicy and ice cream solid. But water can also make a cookie soggy and lack of water can make an apple hard and chewy. It can also make a pie crust soggy, especially if it has a wonderfully moist filling on top. If nothing’s done, a soggy pie crust is unavoidable after a few days only.

So today, let’s tackle water! Or more specifically, movement of water or, more scientifically moisture migration.

Water migration

Water moves around in food a lot. We use this to our advantage when drying sausages or soaking raisins. The dry raisins will soak up the water, whereas the wet sausages will lose water to the air. However, there are a lot of cases that we don’t want this. So we have to know what causes this movement of water, without diving into too many sciency details.

Water activity

The 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 value of a moist pie filling will be higher than that of the crunchy pie crust.

The water activity is used by scientists in a lot of formulas, one of those is used to describe the so called ‘chemical potential’ of a food. A food with a higher water activity has a higher chemical potential. In simpler terms this means that there is a driving force for water to move from one place to the other. Water will always move from a spot with a high water activity to one with a lower water activity (thus: from soggy center to dry crust).

Just a side note for those new to the term water activity. 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, else, just scroll down and read through afterwards.

Examples of the movement of water

Let us have a look at two examples you might encounter: bread with an almond paste filling & strawberry pie with a crunchy crust.

Example 1

Moisture migration bread
This bread has a center filling made of almond paste and has a slice of cheese on top. The three different sections each have their own water activity as indicated. If the slide of bread with cheese is left long enough, the following will happen: the water from the bread will sit in the almond paste, making it softer and the moisture of the bread will go towards the cheese. Over time, the bread dries out considerably.

Example 2

Strawberry pie moisture migration
This pie contains moist strawberries and a creme patissier with a high water activity as well. The crust on the other hand has been dried in the oven thus has a lower water activity. Over time the moisture from the strawberries may leak into the creme patissier whereas the water in the creme patissier can migrate into the crust. In the end, you’re left with a soggy pie crust and a harder filling.

Preventing a soggy pie crust

Coming back to the initial discussion on soggy pie crusts: we want to prevent them. The way to do that is to prevent moisture from being able to travel from one side to the other. Here are a few tips, suitable both for cooks as well as food manufacturers.

Strategy 1: Time – speed up or delay

  1. The best and easiest is to store and keep pie with a dry crust and moist filling for only a short period of time. Once they have been put on top of one another, migration of water will start. The quicker you eat it, the less time moisture has to move.
  2. The next easiest is to delay the onset of water movement. In other words: delay putting the two layers on top of one another. Store the crust and filling separately and only mix when you plan upon eating it soon!

Strategy 2: Introduce a barrier

Another way to prevent moisture from travelling is to place a barrier in between the crust and filling which the water cannot go through. Most fats are a good example of such barriers.

  1. Spread a layer of chocolate over the crust before placing the filling on top. Chocolate contains enough fat to close of the crust and prevent that soggy pie crust. 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.
  2. Chocolate is pretty expensive so you will see that a lot of manufacturers do not use chocolate, instead they use cheaper coatings. These might still contain some cocoa, but also contain other fats besides milk fat and cocoa butter. They work the same as chocolate does though.

Strategy 3: Remove that water

If strategy 1 and 2 do not work for you it can be worthwhile to remove the water somehow. This is especially useful if you bake the crust and filling together. It you add something between the two layers that absorbs the moisture, it will not sit in your crust. Common examples are marzipan or bread crumbs. Instead of preventing the moisture migration, this smartly uses moisture migration and steers it in the right way.

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