I was reading up on tomato sauce, and it seems important to simmer the sauce for at least a few hours. The “Frankies Spuntino” recipe is about as simple as it can get, it doesn't even contain onions. It's said to produce a “thick and rich sauce, with the flavor of the sweetest summer tomatoes.” The key points to the recipe seem to be to use canned San Marzano tomatoes and to keep the sauce at a simmer for four hours. It's clear that the quality of the tomatoes plays a role in the sweetness of the sauce, but why the long simmer? What exactly happens to the sauce during this? This snippet of advice from allrecipes.com suggests the sweetness is due to caramelization:
Cooking time can range from two hours to all day, depending on how thick and caramelized you like your sauce.
Is that correct? I thought that the sauce, being liquid and kept at a gentle simmer, wouldn't reach the necessary temperature for caramelization. If not, what exactly does happen? And does the process really need to take so long, is there a way to speed it up a bit?