A timely question given the season, and an important one, reader Chris. Pick the right apple for, say, an upside-down cake and you get a firm flavorful apples inside a sweet, moist cake. Pick the wrong one and you get applesauce within a sodden mess.
In general bakers want firm apples that are also sweet. My go-to baking apples, because they’re so easy to find in the US, are Golden Delicious. These days Gala apples are popular for baking, though I have a hard time understanding why since they lose an awful lot of flavor in the oven. Other decent choices are Jonathans and Jonagolds, Winesaps and Newton Pippins. Best of all, if you can find them, are Red Romes, also known as Rome Beauties, probably the preeminent baking apples.
Avoid at all costs Red Delicious and McIntosh or anything labeled a “cooking apple” since these types break down to mush when heated. Granny Smiths, though they’re the first apples that pop into most peoples’ minds when they thing “firm apples”, are actually a rather so-so choice.
Why? Well, for as much as I love Granny Smith apples (my grandfather loved to eat them with sharp cheddar cheese) I rarely, if ever, use them for baking. The reason: when it comes to exposing apples to heat you want hard, sweet-tasting apples. Hard tart apples aren’t as good for baking, and are better suited to things like apple sauce.
But why is that? This is where we get into reader Dawn’s question. It all has to do with the behavior of starch molecules. 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 fruit flesh starches are generally quite rigid. But heat them up and a funny thing happens: they start absorbing moisture. This has the effect of engorging the apple’s cells to the point that they pop. They then collapse, and the flesh of the apple turns to mush. Here it helps to think of potato flesh: very starchy and 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 Granny Smiths are picked too early (which they often are, as they remain green at all stages of their development) the starch conversion process won’t be complete. The result will be cellular breakdown that I detailed above, but it will only occur in the apple’s outer flesh. The flesh in the center, where the starch-to-sugar conversion process will have been initiated, won’t get mushy. Thus it will appear to the cook or baker that the apple slice is “melting” from the outer edge of the apple slice toward the center.
So Granny Smiths are risky for baking. Not entirely unsuitable mind you, just…risky. So if you go the Granny route make sure to taste one first to make sure it’s not too tart. If so, head to the Golden Delicious pile.
Oh, and, in case you were wondering, mealiness in an apple is also related to moisture and cells, but in that case the cells are leaking water rather than soaking it up. This, as you probably already surmised, is what happens when an apple gets overripe. The walls of the flesh cells are simply breaking down and collapsing.
Thanks for great questions, readers Chris and Caitlin!