Why do some apples get mushy when you bake them?

I got a very interesting email on this subject recently, from a pastry student by the name of Zach. It reads:

An interesting phenomenon has been occurring in class for the last 2 weeks. We are making apple tarte tatins with granny smith apples. These granny smiths seem to have been picked a bit too early. The sugar is not quite fully developed and there is a bit of astringent puckering in the mouth sensation (like from an unripe hachiya persimmon). However, the apples are not mealy. Now the strange part: the apples have been breaking down significantly and getting mushy during cooking/final baking of the tatins. Our instructor is great – certified master pastry chef who is very familiar with this recipe and technique – so the technique is there – just the apples are not behaving. We assume these are last years crop of apples, but it still doesn’t account for their mealiness only after cooking. We are trying to research if there is any correlation between the age of an apple and its tendency to mush out after being cooked. It seems that if the apples were stored incorrectly that the mealy texture would be evident before cooking. I personally think it may have something to do with the possible premature picking of the apples. Anyway, if you have any ideas on this matter (scientific and/or anecdotal), your input would be much appreciated.

A very interesting conundrum indeed. And in fact the taste and texture problems are related, for it’s sweet-tasting apples that tend to bake up well, while milder/tarter ones are better for things like apple sauce. Why? It all has to do with the behavior of starch molecules, which are of course 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. Cold, they’re usually quite rigid. Heat them though, and a funny thing happens: they start absorbing moisture. In a fruit, this has the effect of deflating cells (which are themselves bags of mostly water), which causes them to collapse and turn the flesh to mush. Here it helps to think of a potato: very starchy, hard when cold, mushy when cooked. Starchy apples behave in much 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, a process that begins in the inside of the fruit and works its way outward. If the apple is picked early and the process isn’t complete, the result will be breakdown in the oven (especially around the outside of the apple pieces, giving the impression that they’ve “melted”). That makes Granny Smiths something of a dicey proposition for baking. If you go that route — as I did with my Tarte Tatin yesterday — make sure they’re sweet-tasting and not too tart. Otherwise, it’s probably safer to just stick with the tried-and-trues that I listed below.

Oh, and, in case you were wondering, mealiness in an eating apple is also related to moisture, but in that case the cells are leaking water (due to overripeness), which likewise causes them to collapse.