Reader Caitlin wants to know why, when she makes tarte Tatin with tart apples like Granny Smiths, they seem to melt away with heat.
That’s a great question, for as much as I love Granny Smith apples (my grandfather loved to eat them with sharp cheddar cheese) I never recommend them for baking. When it comes to exposing apples to heat you want hard, sweet-tasting apples. Tart apples are better for things like apple sauce.
It all has to do with the behavior of starch molecules, which are abundant in apples. Starches, as you’ll recall from other posts on the subject, are long molecules — chains of sugars — that plants use for structural purposes.
At room temperature these starch molecules are usually quite rigid. But heat them and a funny thing happens: they start absorbing moisture. This has the effect of engorging the apple’s cells, which are bags of mostly water. When that happens the cells blow up, then pop, then collapse, with the result being that the flesh turns to mush. Here it helps to think of potato flesh: very starchy, hard when cold, mushy when cooked. Starchy apples behave in the same way.
Granny Smith apples can be either sweet or starchy depending on when they’re picked. Like all apples, they convert starch to sugar as they ripen. This is a process that begins in the inside of the fruit and works its way outward.
If a Granny Smith is picked too early (which is easy to do since ripe or underripe they’re still green) the starch conversion process won’t be complete and the result will be breakdown as detailed above — but mostly around the outside of the apple slices, which is what gives the impression that the slices are “melting”.
That makes Granny Smiths risky for baking. Not entirely unsuitable mind you, just…risky. So of you go the Granny route make sure to taste one first to make sure it’s not too tart. Otherwise, well, you know what to do.
Oh, and, in case you were wondering, mealiness in an eating apple is also related to moisture and cells, but in that case the cells are leaking water as a result of overripeness. The cell walls are simply breaking down, which likewise causes them to collapse.
Thanks for a great question, Caitlin!