All food acids lower the pH of a dish, so you can use the substitute of your choice without losing the effect the acid creates, such as the coagulation of milk when making cheese or the activation of pectin when making jam. Substitute 1 tablespoon of lemon juice or white distilled vinegar for every 1/2 teaspoon of citric acid called for.
Making the Trade
Choose a food acid to substitute according to the dish you're making. For example, if you're making cheese, you can sub vinegar or lemon juice, as the acid serves to coagulate the curd and doesn't impart flavor. Jam, on the other hand, fares best with a lemon-juice substitution, unless you're experimenting with new flavor combinations, in which case you might be on to something interesting, depending on the fruit. Add the lemon juice to the dish at the time you would normally add the citric acid.