Sulfuration or black spots on the inside of cans can be caused by the breakdown of certain proteins that release sulfur ions. These can react with the metallic components of the container, such as iron and tin, producing dark-colored sulfides. This phenomenon can also occur due to the use of certain sulfur additives in the early stages of the packaging process to whiten or preserve the product, which are then not adequately removed.
It is important to note that the appearance of these sulfidation stains does not represent a sanitary problem, since the sulfides formed are completely harmless. However, they can cause a significant commercial problem, as they affect the presentation of the product, especially if it is found with dark spots.
As for corrosion in the cans, this may be lower in cans that are less attacked and with liquid loss. Cans that retain the liquid inside, as well as the pressure, do not show signs of corrosion on the bottom of the cans. However, in the most affected cans there may be several suspicious corrosion spots, which can be observed by inspecting the inside of the can.
In any case, it is always advisable to inspect the cans before use and avoid those that show signs of damage or discoloration.