What's the source of the sweetness in tomato sauce that's simmered for at least four hours? 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?
I think caramelization might be the wrong word there. I would go for concentration. While some caramelization does occur around the rim of a pot of tomato sauce as the sauce reduces (and it's good to stir this residue into your sauce), the main increase in sweetness is due to a concentration of all things tomato-y as the water in the sauce evaporates.

Speeding up this process is not only possible, but, in my opinion, highly desirable. A long- simmered tomato sauce can often have an overcooked taste to it. One method to speed things up is to strain your canned tomatoes through a fine-mesh strainer, or something equivalent. Use what remains to start your sauce in the normal way and keep it at a simmer. The liquid, however, you can heat more rapidly at a low boil since there are very little tomato solids left in it to get that overcooked flavor. Once your liquid has been reduced by 60-75%, you can add this back to your simmering tomatoes.

If you start your sauce with fresh tomatoes, the quickest way to accomplish this is to freeze them. Once thawed, all their liquid will pour off easily. Boil down this liquid as described, quickly sauté your remaining solids, and then blend them into your reduced liquid with an immersion blender. This results in a very rich and flavorful thickened sauce that still has the brightness of fresh tomatoes and can be done in a fraction of the time mentioned in your question.