You really don't want ripe eggplants, at least of most varieties. As they ripen, the seeds develop more, and the fruits harden, and turn bitter, usually turning yellow - this is what you have to do when saving seeds. Some small eggplants, such as Thai and Indian eggplants, develop the seeds early, and don't get bitter, even with the large seeds, until they turn yellow.
I have grown at least 42 varieties of eggplants, with at least 2 more coming this year! I don't like the larger eggplants as much as the smaller, Asian style varieties varieties. They are more spongy, and they are the ones that gave EP the bad rap, for being bitter, and we were always instructed to salt, and drain the juices from, in old books. That must be the type you are talking about that should be fresh and "light" when you buy it. When most of those varieties are unripe, they should be very dark, and when you cut in, seeds should be almost invisible. When they start ripening, the skin gets lighter in color, and the seeds start turning brown, and many start turning bitter. Even a favorite Asian variety of mine (Ichiban, always my earliest) gets that bitterness, so I have to pick them when close to black. Good thing is, this has been bred out of many types, though much of the flavor has been, too! Every white variety I have tried has been relatively flavorless. The lighter purple varieties are some of my favorites, with good flavor, and not bitter, even when picked a little too late (easy to loose them in large plants!). Most are firm varieties, that don't turn mushy as fast as the larger, spongy types.
Then there's the Thai bitter eggplant, that you use for the bitterness! Just about 1/2" , and you let them turn yellowish. This is a different species, which I only grew twice - they become large trees, and I got years worth of 1/2 cup portions, to add to Thai curries (keeps forever vacuum sealed). Asian markets now carry them occasionally.
