I was hiking just north of Waimanu Valley on the Big Island of Hawaii (Laupahoehoe Nui) when we came upon hundreds and hundreds of banana plants that had been originally planted by the ancient Hawaiians that inhabited the area. Unlike apples or oranges, banana plants ripen their fruit year round so lots of fruit was hanging on the stalks in various stages of ripeness. We ate several big, beautiful yellow bananas right off the tree and they were the best bananas I’ve ever eaten.
Like the acorns mentioned above, buckeyes (aka Horse Chestnuts) are poisonous (even to horses) and most animals will avoid them, but apparently the Indians would leach them and make an edible paste or flour from them. I’ve tried searching but to no avail, what exactly is entailed in this “leaching process” that makes them edible? Is it a long boil, a chemical process or something?
Corn is edible fresh picked. Some varieties are better that way than others. Regardless of variety, the sooner after they are picked the better (I’m talking like within minutes).
Kiwis must be picked when they are still hard and sour. To ripen them you bring them inside where it is warm and they get soft and sweet. I imagine that in the wild they sit on the ground all winter and only ripen when it warms up again.
I think the big problem with most of the grains is that they are tough on the teeth when they are raw. Maybe they are softer at some point, but when they are harvested they are very tough (or at least that’s my experience with wheat).
Besides being bitter, I believe fresh raw olives are pretty tough (though still chewable).
In order to render horse chestnuts edible, they are pounded to a pulp, rinsed, boiled and rinsed again several times; this removes a good deal of the harmful substances from them, however they apparently still don’t taste good and are still not necesarily entirely safe to eat in large quantities this way.
[anecdote]
Once when I had spent a day biking I ran out of food. I was close to a friends house so I stopped in. They weren’t home so I raided their garden, filling myself with parsely and rhubarb stalks (peeled). I went right back to biking with no ill effects. The leaves are certainly poisonous. Its the only green plant that was never ravaged by our livestock.
[/anecdote]
From A Field Guide to Edible Wild Plants of Eastern and Central North America by Lee Allen Peterson (son of Roger Tory Peterson):
Jack-in-the-pulpit, Arrow arum (or Tuckahoe), and Skunk Cabbage all have edible roots, but must be thoroughly dried before being eaten, due to the presence of calcium oxalate crystals. Eating the raw plants causes an intense burning sensation in the mouth, which boiling does not remove. Only thorough drying can render these roots edible.