Always make sure that you aren't using a reactive pot. Reactive cookware: a cast iron pot that's not enamel coated, copper cookware, or aluminum cookware. I always choose my trusty enamel-coated dutch oven when I make tomato soup! If your tomatoes were acidic, and it's not the cookware, add a little more baking soda (about 1/2 teaspoon) to neutralize more of the acid. Give a taste, and repeat if needed. You may need to season a little more once you're happy with the acidity.