I saw somewhere they rubbed sliced eggplants in salt and leaving them out up to an hour before putting them in the oven.
From what I understood it had to do something with the eggplant's bitter taste...
I want to cook sliced eggplants in the oven myself now, and some in with a pasta sauce.
Should I rub them in salt? Why do people do that?
Eggplants differ in bitterness. You can cook some of them and never notice a problem. But other exemplars are quite bitter, and can overwhelm a dish. That's why it is a good idea to preemptively do something to remove their bitterness.
I have read dozens of suggestions how to do it. Some are OK, others are downright terrible. Have you tried soaking eggplant in water with lemon juice? An eggplant being basically a sponge, you end up with pieces bloated to three times their original size, dripping water into whatever dish you try to use them in. Also, they stay rather bitter.
The logic behind the salt is that it will suck out the bitter juice of the eggplant through osmosis. You can then remove the mushy salt and use the eggplant pieces. You can certainly use it if you want, but it isn't the best method. First, osmosis does occur when you salt food, but nowhere near the rates claimed. You don't end up with a dessicated steak if you salt it 10 minutes before frying, and if you salt a slice of eggplant, you have to make thin slices (doesn't work for ratatouile 2-cm-cubes), give it some help (press the slices with a weight so the juice will be pressed out) and wait a long time. Even then, the results can be inconsistent.
The good news is that one of the few veggie-themed Food labs posts is dedicated to the problem of disembittering eggplant. The tests seem to be as rigorous as usuall, and the best practice recommendation is to not salt the eggplant, but bake it.
Microwave is the best method, as it dehydrates well and does so quickly. If you don't have a microwave, the second-best method is the oven.
Obviously, this isn't good for all applications, because it dehydrates the eggplant. For example, I don't think you can make a good imam bayildi with the dry roasted eggplant (but then, maybe it will be OK, I've never tried). But for the typical uses in Western cuisine, it should go fine.
The original article is also highly recommended reading, as any of Kenji's stuff.
Modern eggplants are bred to be less bitter, so if you are buying store bought eggplant you shouldn't have to worry about it. I've never noticed my eggplant being bitter, no matter how I prepared it. If you are using heirloom seeds (i.e. you are getting the eggplant from your grandmother who has been growing the same variety for decades), then you might want to go ahead and salt it.
OK, I am going to enter my thoughts here. The accepted answer is a pretty good one, and the article is a great way to help you prepare eggplant parm without it being to heavy with oil. But I don't think you can skip the salting.
As in the article I have always baked/roasted my eggplant prior to assembling it. I did not use the paper towels as at that heat the paper towel shouldn't need to absorb the moisture. It should be able to just evaporate. In fact I would guess that if the moisture was bitter adsorbing it into the paper towel that is sitting right on top the eggplant would be bad. Better just to let it evaporate away.
Now, as I have said I baked my eggplant. And I have to tell you that the first time that I made eggplant (not for eggplant parm, but I still baked it first) it was so bitter I couldn't eat it. That is when I discovered salting it. I think there is something different that happens here. Were the bitter molecules actually leech out when you salt it (osmosis) but they don't evaporate out with the water when it is baked. The key is it takes a long time depending on how bitter the eggplant was. At least 30 min possibly up to an hour.
I think the reason why it works for the author and others is what Sara D Gore was referring to in her answer. Most eggplant you find today are just not bitter to start with. And you really don't have to worry about that problem. If you don't want to take the time to salt the eggplant every time, then taste it first. Raw eggplant it perfectly edible. Or there are some signs it might be bitter like a thicker skin and a large number of fairly large seeds. Then if you think you need to generously salt it, let it drain in a colandar, rinse it, and dry it. Then proceed exactly like the article.
This of course is just my experience in my kitchen. And sometimes weird, strange, and wonderful things occur in a ones kitchen that can't be replicated outside it :) That is why if I think the eggplant will be bitter I salt it (or throw it away), and it seems to be working for me.
Yes, it is a way to remove the bitterness from eggplant, for larger more mature fruits with developed seeds. You may sprinkle the salt on both sides, let them sit for 15 - 20 minutes, and then rinse before baking. If you have younger or smaller eggplant, this is not necessary. I love to bake sliced eggplant, and serve it as a side dish, in a pasta dish, or on pizza. I usually slice 1/4 inch slices, and dress with olive oil, and a little tamari and any other seasoning desired, bake in a 350 - 375 degree oven, turning once till soft in the centers, brown and a little crispy on the surface, about 20 - 30 minutes.
To answer the last bit of your question; "Why do people do that?", I can only tell you why I do that.
I peel, slice (either length wise, or on the bias), salt and press eggplant for an hour prior to cooking; I raise Japanese eggplant, which is slender and long, so I slice them lengthwise.
I will slice the eggplant evenly, usually about 2 cm-3/4" thick, assemble each half of the sliced eggplant on the counter, salting each side of each slice as I go, with the widest slice down and the smallest on top. Then I figure out how to weight the individual stacks of sliced, salted eggplant. Very often I use a cutting board with some cans on top of it for weight.
After about an hour, I pull the weight, remove the eggplant slices to a bowl, clean up the water on the counter, rinse the eggplant slices very well and dry them. They are now ready to use in whatever recipe I am using at the time.
This process removes the greater portion of the free water and compresses the eggplant to a bit less than half the original thickness.
Up until I learned this technique, we considered eggplant as somewhere at the bottom of the desirable vegetable list. After learning and using it, my eggplant went from being chunks of mush to chunks with texture, improving the esteem in which it was held tremendously.
If you can find it in a restaurant, order eggplant parmigiana and ask your waiter if the eggplant is pressed. The difference between pressed (meaning salted and pressed) and unpressed eggplant in this dish is pronounced and very noticeable.
