This section is from the book "A Treatise On Beverages or The Complete Practical Bottler", by Charles Herman Sulz. Also available from Amazon: A Treatise On Beverages.
Citric acid occurs in a free state in the juice of the whole genus of plants citrous, and many other fruits. The lemon, lime, bergamot, sour oranges, are the fruits from which it is extracted. It has also been manufactured from unripe gooseberries, and several other berries. Good lemons yield about five and one-half per cent, of crystallized citric acid. Citric acid has a strong, but pleasant acid taste. It dissolves in three-fourths of its weight of cold, and in half its weight in boiling water, the hot saturated solution readily depositing crystals of the acid on cooling. 80 per cent, alcohol dissolves 1.15 parts. Aqueous solutions of citric acid readily turn mouldy, and for that reason should be prepared only as used. When a solution is made by dissolving forty grains of the crystals in one ounce of water, it resembles lemon juice in strength, and, like lemon juice, undergoes decomposition, acetic acid being among the products. Citric acid crystals are-permanent in dry air, and become damp and gradually deliquesce in a damp atmosphere.
The manufacture of citric acid requires chemical knowledge, a great deal of care and expensive apparatus, and is therefore manufactured only wholesale. Citric acid converts cane-sugar into invert-sugar. It is sometimes substituted by tartaric acid, but for finer beverages citric acid only should be used, as its taste is more pleasant. Always buy citric acid in crystals in preference to pulverized. Any adulteration in the former is more easily detected than in the latter.