Very true. I can use the same tomatoes (San Marzano) purchased at different times and some will be more acidic than others. Depends on how and when they were grown. I even see the same problem with prepared marinara.
I wrote a column on this very subject.
Tomatoes are naturally acidic.
The kind of tomato is important. Sugar, baking soda, or grated carrot offer enough sweetness to neutralize the acidity. Start out with a pinch and go from there. Remember, as you cook the sauce you evaporate liquid making the sauce more intense, Use caution when adding the sweetness.
Marcella Hazan's tomato sauce with butter and onions is delicious. When I put up fresh tomatoes I roast them on a sheet pan with bulbs of garlic then remove the skins and run through a food mill adding garlic as I go. then I simmer the water out of them, bag and freeze flat. The liquid and fond, or rather (tomato essence) that is left in the pan gets bagged and frozen as well. Our chickens get the skins - and boy do they go nuts.
Roasting seems to take out a lot of acidity - IMHO - I have no data to back that up, but everyone love my sauce and no one complains.
Was talking with a family member, said she slices onions and adds directly in the peeled tomato.
She does not sauté them, she lets it simmer slow, adds salt basil olive oil pepper. Then through the mill, back on the stove adjust for consistency that is it anyone tried this ?