There are different types of acid in wine- tartaric, malic, succinic, lactic and citric. If problems occur, different acids, such as acetic acid (think vinegar) can pop up later. The primary fermentation refers to sugars turning to alcohol. The secondary fermentation is a misnomer. It isn’t actually a fermentation, it is a bacteria that we introduce into the wine that converts malic acid into a higher pH acid, lactic acid.
Malic comes from the word mal, in Latin, meaning bad, or sour. Malic acid is most notably found in green apples. It really makes you pucker.
Lactic acid is found in lactose products such as yogurt or milk and it is a much mellower acid. Red wines will generally be encouraged to go through malolactic fermentation while whites will generally not.
Diacetyl is a byproduct of the malolactic fermentation while the process is still in motion. It vanishes once the process is totally complete. It is the same chemical compound found in movie theatre popcorn butter and is responsible for all of those buttery Chardonnays that were stopped midway through the malolactic process on purpose.
The name of the game for us as we wind things down, is getting these wines through malolactic fermentation, racking and setting the barrels in a cold, clean room.