Citric acid is used in cheesemaking when acidifying milk to the target pH for curd coagulation.
When to use Citric Acid when making cheese: Usually added in place of a starter culture. Recipe will advise proper temperature to heat milk to before adding the Citric Acid.
Where do I store Citric Acid? In a dark, cool environment, Refrigeration is not necessary.
Does Citric Acid go bad? Technically it does but takes either a very long time or neglecting the storage instructions. You have nothing to worry about if you keep it out of direct sunlight.
What if I don’t have citric acid? You can replace citric acid with white vinegar or lemon juice. Use around 1/3rd of a cup of white vinegar, or around 1/3rd of a cup of lemon juice. You will get a mild to moderate flavor of the substitute acidifying agent.