Malic acid

Lore & Background

In German it is named Äpfelsäure, while its salts are called Malate. The word 'malic' itself comes from Latin mālum, meaning 'apple', and the related mālus is used as the genus name Malus for apples and crabapples. In biochemistry, L-malic acid is the naturally occurring form, while a mixture of L- and D-malic acid is produced synthetically. Malate plays a key role in the citric acid cycle as an intermediate, formed by addition of an -OH group on the si face of fumarate. It is also involved in C4 carbon fixation, where it serves as a source of CO2 in the Calvin cycle, and can be formed from pyruvate via anaplerotic reactions. In plant guard cells, malate is synthesized by carboxylation of phosphoenolpyruvate and helps maintain electrical balance during solute uptake. Malic acid is the main acid in many fruits, including apricots, blackberries, blueberries, cherries, grapes, peaches, pears, plums, and quince, and contributes to the sourness of unripe apples. It is present in wines at concentrations sometimes as high as 5 g/L and confers a tart taste. The process of malolactic fermentation converts malic acid to milder lactic acid.

Reader's Guide

Malic acid is significant as a naturally occurring organic compound essential to life and widely used in food and industry. Its role as a metabolic intermediate in the citric acid cycle makes it fundamental to cellular respiration in all living organisms. In plants, malate is critical for C4 carbon fixation and stomatal function, influencing photosynthesis and water regulation. The compound's sour taste makes it a key flavor component in fruits and a common food additive (E296), used in sour sweets and artificial vinegar flavors. Its discovery by Carl Wilhelm Scheele and naming by Antoine Lavoisier highlight its historical importance in chemistry. The Walden inversion, involving malic acid, was pivotal in understanding stereochemistry. Malic acid's dual natural and synthetic forms, its role in malolactic fermentation in winemaking, and its use in soil treatment via molasses supplementation demonstrate its broad relevance across biochemistry, agriculture, and food science. Its energy content of 10 kJ per gram also makes it a minor caloric contributor in foods.

Did You Know?

• The word 'malic' is derived from Latin mālum, meaning 'apple'.
• Malic acid is the main acid in fruits such as apricots, cherries, grapes, and peaches.
• The process of malolactic fermentation converts malic acid to much milder lactic acid.