What are the 'acidity controllers' used in canned whole tomatoes? I was looking at the label on a can of Italian roma tomatoes (similar to San Marzano) and I noticed an ingredient called an 'acidity controller'. 

This makes perfect sense to me, but it doesn't go as far as to say what that ingredient consists of. It seems like it might fall under the same ambiguity as 'herbs and spices' as far as requirements to list ingredients. Baking soda-ish things immediately come to mind, but does anyone have a more factual grasp of what's commonly used to control acidity in canned tomatoes?

Most varieties are actually quite tart out of the tin, which is why I was a little surprised to see that they use anything at all. Is it just something they use to lower the acidity enough in the event that the can is dented and the barrier stopping the tomatoes from actually interacting with the metal damaged?
It is normally just a mild organic acid like citric acid, they tend to act as buffers. But the reason to use a word like "acidity controller" is exactly to be able to use something else (say malic acid) so they don't have to reprint the label if they change the recipe. 

The reasons for acidity controllers is both safety (not enough acidity will make the can unsafe) and taste - too much or too little acidity will result in a taste difference between charges, because each batch of tomatoes comes with different acidity. 

Basic buffers like baking soda cannot be used, because this would make the contents practically uncannable, and very unpleasant to eat to boot.