Why does the addition of butter make risotto shiny? Is there a chemical explanation? I was taught to initially saute onions and garlic in olive oil, then risotto Arborio rice with warmed broth until the dish is almost complete, when the rice is still quite al dente. At this point, I add butter, which invariably makes the risotto beautifully shiny. Without the addition of the butter, the risotto does not develop this sheen. I would like to know if anyone knows why? I am also interested in knowing if there is a specific chemical reaction associated with this process?
There is no special chemical reaction. Melted fats are glossy, look at any oil in a bottle.
When you add the butter, it coats the rice, and this coat of fat is glossy. That's all there is to it.
I don't know the exact explanation of why all oils are glossy, but it probably involves quantum physics. You could ask it on Physics or Chemistry SE, it is beyond our scope here.
Without digging into any deep chemical detail, the use of butter has a very specific purpose. It's needed to make risotto creamy with a sheen effect and creates a kind of "glue effect" among rice grains in order to give the typical aspect to risotto.
Parmigiano Reggiano cheese (that is not Parmesan!!!) and Grana Padano Cheese can be used instead of butter and give to Risotto more intense cheese Flavor.