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A lot of pie recipes will tell you to take out the liquid in juicy fruits, like apple and peaches, for pie baking.

The purpose is to avoid a soggy crust with too much liquid.

Some people even put tapioca starch to thicken the liquid.

Fruits must be macerated to take out the liquid. This is done by slicing and then adding sugar but also lemon juice.

But what is the purpose of the lemon juice? I know that it lowers the PH and increases the acidity.

But how does this assist in taking out the liquid from the fruit (I"m assuming this is the purpose).

Why don't just skip it?

I googled it but the answers didn't seem scientific or accurate? E.g., lemon juice is to add flavor or it is to speed up the process (but how?)

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    Why do you think that "to add flavor" is not scientific or accurate?
    – rumtscho ♦
    Commented Dec 12, 2017 at 19:46
  • The taste of the lemon tart flavor is absent when (1) the quantity of lemon juice is too slight and (2) the pie is baked at a very high temperature for a long period. I know that certain spices, like cinnamon, baked in high heat will lose a lot of flavor (so I'm assuming, but not certain, this extends to other things like lemon). So if it's just flavor, it seems abortive. I'm guessing it's done for another reason (not flavor) but to draw out the juices and this might have something to do with the acid??
    – user62973
    Commented Dec 12, 2017 at 19:50
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    lemon will degrade in flavor from cooking heat, but enough heat to destroy the acidic property would only be found on the surface of non covered fruit/filling in a pie being baked - a core temperature of 170°C would leave the fruit thoroughly candied and/or caramelized, probably unpalatably hard, and without much thermal buffer to keep it from outright burning.
    – rackandboneman
    Commented Dec 12, 2017 at 20:18

4 Answers 4

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While there might be an acceleration effect, sugar on its own is very capable of drawing out liquids quickly. Two other effects are probably much more important:

  • lemon juice is very effective in stopping cut fruit from oxidizing/enzymatic browning.

  • taste balancing. Fruit is naturally sweet and sour, adding heavy amounts of sugar can upset that balance too far into the sweet region, resulting in a cloying/one dimensional/stodgy character

  • in some recipes, keeping the sugar/fruit mixture from caramelizing/browning too much. Acid inhibits maillard reactions to a degree.

  • acid promotes sugar inversion when combined with heat, which slightly changes the sweetness profile (halfway to honey, more or less) and potentially significantly the texture (there will be no crystallization of loose sugar-water mixtures upon cooling).

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The lemon juice increases the acidity of the mixture. Sugar is hygrophilic (meaning is draws water) This acidic sugar bath thins cell membranes so the juices can flow more easily, while still maintaining a solid piece of fruit. After 2 to 4 hours of maceration, you can strain the mixture. This is how you draw the flavor and color of the fruit out into what has now become a flavored invert syrup for making say strawberry or peach ice cream. In the case of pies, it's to dessicate the fruit some, i.e. dry it out a bit.

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  • I read in Saveur, Food52 etc. that the lemon juice "speeds up" the process and your explanation is a good idea of how. Do you have any links referring source so I can take a deeper look? Cooks Illustrated mentions vinegar/acid will make meat mushy and dissolve but the opposite happens with vegetables - where it strengthens the pectin. So acid has opposite effects. Assuming fruit is similar to veggies with pectin, i wonder if acid strengthens rather than dissolves.
    – user62973
    Commented Dec 12, 2017 at 22:38
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The main effect is in my opinion that of preventing oxidative browning of the fruit. This is by virtue of the antioxidant ascorbic acid ( vitamin C ) in the lemon juice.

Obviously it has welcomed or just side effects too, at minimum it imparts acidity.

Let me note here that, let us say we do not want to alter taste of the ingredients, we could use a bit of vit C in water. It is still a bit sour, but not lemon taste.

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The answer to your question is related to the PH balance of your fruit filling. When you heat your filling after adding sugar, it releases pectin which is negatively charged. This allows the fruit to release flavor but these negativley charge pectin strands repel each other and so the lemon being acidic adds a positively charge which allows the pectin strands to bond. These bonded pectin strands are what allows your juice to thicken. Many add geletin, tapioca, corn starch or flour to assist with this but the acidic lemon juice is what balances the PH of the filling and keeps it from being too juicy.

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  • Hello, and welcome to Stack Exchange. pH and charge aren't directly related, and I doubt that pectin is "negatively charged". Where did you get this information?
    – Daniel Griscom
    Commented Aug 21, 2018 at 18:23

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