Malic acid is an organic dibasic acid, (HOOCCH(OH)CH₂COOH). It has several important roles in the body's metabolism, notably in the citric acid cycle (Kreb's cycle), which is concerned with the production of energy during respiration. It is found in the juice of unripe apples and some other fruit. Melting point 113°C.

 

Fumaric acid

Fumaric acid is a strong organic dibasic acid, also known as trans-butenedioic acid, which isomeric with malic acid. It occurs in many plants, especially in Corydalis and Fumitory, and, like malic acid, plays an important role in the citric acid cycle.