Used to increase acidity. Also caled, sour salt. Use as needed.
Note: this also can be used as a great coffee maker cleaner. Dissolve 6 Tbsp in 8 cups of water and run throught the coffee maker, then repeat with the same soultion that you collected in the pot. Run a few cups of clean water through, repeat with clean water until all the build up the citric acid losened no longer shows up in the pot.