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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?

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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.

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Why would it have to just be concentration? Couldn't starches be breaking down into sugars, or sugars being made more available? (If it's just concentration, you could get the same effect by boiling faster.) –  Jefromi Jun 2 '12 at 3:07
    
Also, if it were just concentration, tomato paste would taste really sweet, right? –  Jefromi Jun 2 '12 at 3:12
    
@Jefromi: Well, tomatoes contain negligible starch. I still think it's sugars being made more noticeable than "available." I'm not saying it's only concentration, but I think it's primarily concentration. The reason boiling a sauce faster doesn't work is that it usually imparts a bitter, overcooked flavor that counteracts the natural sweetness. Boiling just the water from the tomatoes doesn't have this effect. And tomato paste is a concentration of all the tomato flavors, including sweetness. –  Callithumpian Jun 2 '12 at 14:09
    
But isn't a slow-cooked sauce also a concentration of all the flavors? Why does it taste so much sweeter than tomato paste? (What happens to the acidity?) Also there is some starch - even if it's only a third as much as there is sugar, I could imagine it having an effect on flavor if it does break down. I'm not saying you're necessarily wrong, but I'm not entirely convinced you've given a full explanation. –  Jefromi Jun 2 '12 at 14:55
    
Good quality tomato paste is quite sweet. I just taste-tested a bit of Amoré and confirmed this for myself. Your link shows dietary fiber as being a third of the sugar in a tomato, but if you scroll down, the starch is listed as 0.0g. I can't back up my theory scientifically, and I'm sure there's more going on than I know, but I feel my experience backs up the main points of my answer. –  Callithumpian Jun 3 '12 at 23:10
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