Traditionally pomodoro sauces were cooked 2 to 3 hours and disputed greatly among Italians themselves. One of the reasons for this was the folk belief that the longer tomatoes cook, the less acidic they became. However, cooks added sugar to counteract this and so to the tongue, it seemed less acidic. But you cannot fool the stomach.
Fresh tomatoes fall into the 4.3 – 5.0 range when it comes to acidity and pH is the most widely used scientific method for ranking acidity, and it goes from 0 – 14, with low numbers being the most acidic and high numbers being the least acidic.
What I believe is; one cannot ‘de-acidify’ a product by cooking it longer. In fact, long cooking reduces the mass by ‘cooking off’ the liquid present thus concentrating the acid already present in the product. This results in the same acidity started with but in a mass that is now reduced by most likely a fourth or a third.
It is said that heat reduces the acidity but I have no scientific proof for this. The use of baking soda reacts with the tomatoes to neutralize acidity but I never use it in my preparation.
I always recommend making a sauce with no more than 8/9 minutes of cooking at most.