A Request for Tomato Information

On page 184 of my well-loved copy of Jane Cooper's Woodstove Cookery: At Home on the Range (Storey Publishing, 1977), there is a recipe for tomato sauce which is simmered for two days.  Hmmmm . . . . perhaps I am being overly generous when I call it a recipe.  It includes no measurements for anything and merely says to pare a bunch of tomatoes; to put them in a heavy, non-aluminum pot; to simmer them for two days until you've reached the desired thickness; and to add whatever else you would like to flavor it at about halfway through.

This "recipe" has a certain element of romance and intrigue to it, doesn't it?  I mean, it seems to me like anything that has to simmer for two days ought to come to a savory climax that, upon one taste, transports a person to a completely different time period when people slaved for interminable hours over a wood fire creating all manor of delicacies which have been lost to the twenty-first century palate.  Furthermore, the recipe seems perfect for the wood cookstove because that is the only appliance for which I am willing to foot the bill for such a long cooking time.

I have tried this at various times over the last several years, and I have to say that I've never been transported anywhere but to a state of frustration.  First, I decided that what was happening was that I was adding sugar to the tomato sauce too early.  I theorized that the longer the sugar was cooking, the more caramelized it was becoming, and that was what was causing a slightly burned flavor to creep into the sauce.

Here's what has made me begin to wonder, though.  Recently, we have been making my sister-in-law's pizza sauce, which doesn't have any sugar in it, and I still managed to ruin it.  The first time we made it, we didn't simmer it all that long because we had used Roma tomatoes, so the fresh pulp from them was not too juicy.  The sauce turned out great.  This last batch was made with a combination of Roma and regular tomatoes, so the pulp was very runny.  I put nothing in the pulp and simmered it very gently for two days before putting the spices and lemon juice in it, but I could tell before I even added anything to it that it was going to have that old familiar slightly burned taste to it.

The long simmering time appears to be mainly intended to evaporate the excess moisture from the sauce.  Before finding the ketchup recipe that I shared here on the blog, I had tried this long simmering method to reduce the tomato pulp for ketchup, but always got similar results.  Now, however, I think I'm really partial to the "draining by hanging in a bag" method.

My question, readers, is this: can anyone explain to me why the burned flavor appears in the two-day simmered sauce?  I'm cooking it slowly in a stainless steel stockpot.  It has not scorched or even mildly stuck to the bottom of the pot.  I don't understand it, but I've not had the years and years of tomato cooking knowledge and experience that some of you have undoubtedly accumulated.  Does anyone else use this method for reducing tomato sauce but end up with a desirable end product?  Please be generous with your advice down in the comments.  I'm anxious to have this mystery solved.