Mock-apple pie filling is made, primarily, of crackers. There are no apples in it. Still, most people who taste it swear that they are eating real apple pie. What is the chemistry that tricks our senses?

If you want to make mock apple pie, here's what you need.

• 2 cups of water

• 1 ½ cups of sugar

• 1 ½ teaspoons cream of tartar

• 2 tablespoons butter

• 25-35 buttery crackers (try Ritz)

• ½ teaspoon cinnamon

Some fancier recipes include some lemon juice in there, but what are we, the Clampetts? To be honest, you could have stopped at just water, sugar, and cream of tartar. That's all you need to get an apple-ish taste to the pie. The crackers and the butter are just for bulking the recipe up, and the cinnamon is present because we're used to eating apple pie with cinnamon.

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Cream of tartar is a crystallized acidic compound that is precipitated from wine, and used to be found in the bottoms of wine barrels. As wine-makers were both accomplished chemists and drunkenly adventurous in the kitchen, they soon found uses for these white crystals. Cream of tartar, also known as potassium bitartrate, stabilizes egg whites when they're whipped into a foam, and prevents discoloration in cut fruit. It also has a slightly tart, fruity taste itself. Its acidity also makes it a mix-in ingredient in baking powder – which is basically baking soda with cream of tartar mixed in as an acid, so the the entire mixture foams the minute it hits liquid.

It is this fruity taste that tricks our tongues into thinking we're eating real fruit when we're not. To make mock-apple pie, people boil the sugar, water, and cream of tartar for a bit, then add the crackers, allowing the sweet and tart mixture to sink into the dough. They pour the entire thing into a pie crust, top it off with cinnamon and butter, and bake. It's supposed to taste pretty much like apple pie, except more gelatinous and consistent.

Are any io9ers acquainted with this mock-fruit pie? How does it stack up to the real thing?

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Image: National Cancer Institute.

[Sources: Smells Like Science, Chemical Apple Pie]