I'm still pondering how that happens, actually. What causes a poofy skin to form on a uniform corn mixture? Why is the surface of the polenta all crispy, enrobing a poof of air and soft, creamy (but still hot) polenta filling? It's like gruyeres. Cheese puffs.
In terms of physics, I know that when the temperature increases, volume increases. Therefore, when you stick polenta in the oven, the pockets of air within the polenta, which get closed off by the crispy surface of the polenta, expand. The expansion causes the poofiness. However, I don't understand what causes the surface of the polenta to stick to form a crisp. I initially thought, hm, gluten. But, gluten isn't present in polenta, is it? In corn? so what causes the surfaces to stick? Water? Water soluble proteins forming a matrix? The Maillard reaction? Of course, the outside is exposed to a higher temperature, meaning that it has a higher chance of burning. I suppose this means that there is less moisture on the surface of the polenta. Because the temperature is so high (425 deg. F), there isn't enough time for the water within the polenta to diffuse down its concentration gradient to distribute itself to the surface of the polenta. With a low temperature, the polenta would cook more evenly. With high temperatures, the outside is crispy, and the inside is moist (not as cooked). I could be wrong, but this is what I've gathered from science. I suppose it would be cooler to be a food scientist rather than a molecular biologist.
It's very delicious... Next time I ought to make thinner slices (though, these were already hard to slice). 425 F for 15 minutes on one side, 15 minutes on the other. Bottom rack. It's like a chip... but not really. No oil, either. Just slice polenta from a tube (I know, I should make polenta from scratch.) Delicious!