Simply put, this is a thing that causes other things to taste sour. If you’ve seen videos on the internet involving unexpectedly sour foods, then there is a good chance that this is one of the ingredients. Chemically speaking, the molecular formula  is C₄H₆O₅. As you might expect from such a formula, malic acid is an organic compound.

This compound is produced by all living creatures. If you would like to know, or if it would help in any studies that you might have, salts and or esters containing malic acid are referred to as malates. More often than not, these compounds come up during the citric acid cycle of a given organism. Specifically, the malate anion pops up, but more time would need to be spent on discussing the citric acid cycle in order to properly go down that specific rabbit hole. Malic acid is also a dicarboxylic acid, in case you wanted to know.

Now, where was I? Yes, malic acid is the compound that provides fruits with a sometimes sour flavor. For that very reason, it is also used as an additive in foods like lemon heads or other sour candies. It also has pharmaceutical applications as an acidulant (compound causing tart/sour taste). If you’re wondering about how the heck that makes any sense, keep in mind that that sour flavor can mask more bitter and unappealing flavors. 

Malic acid was first discovered when it was first extracted isolated from unripe apples. The person to perform this feat was a man named Carl Wilhelm Scheele, and he did so in 1785. Though it falls outside the scope of this page here, I highly recommend looking up modern methods for extraction and synthesis of malic acid. Specifically, there has been research into the use of coproducts from the production of various biofuels, such as crude glycerol from the production of biodiesel, to help aid with the synthesis of malic acid.