The main acid of lemons and limes, as a white powder: sourness with no water, no flavour and no cloudiness — for adjusting juices and making 'citrus' for clear drinks.
Food grade, from home-brewing, preserving or baking suppliers. A 500g bag lasts ages.
Worth knowing
Lemon and lime juice are both about 5–6% acid: lemon's nearly all citric, lime's roughly two parts citric to one malic. To bring orange juice up to lime strength, add about 32g citric and 20g malic per litre. A 6% citric solution in water is a 'lemon' that never goes off, for clarified and carbonated drinks.