(4) Lemon Explanation: Lemons, oranges, limes, and other citrus fruits naturally possess high concentrations of citric acid. The juice of the lemon is about 5% to 6% citric acid, which gives lemons a sow- taste. In fact, Citric acid was first isolated in 1784 by the chemist Carl Wil-helm Scheele, who crystallized it from lemon juice.