Phenols are a class of chemical compounds commonly found in wine (and chili peppers, cannabis, raspberries and wintergreen to name just a few). In wine they can bestow pleasant aromas like vanilla, wood, cloves, carnations or animal smells such as horse.
Now you may enjoy a little horse leather in your glass, but probably not horse manure. 4-ethyl-phenol is the compound responsible for this horseplay and there’s 4 times more it found in red wines than in white. It shows up more often if the grapes have been macerated without any oxygen before the fermentation process. It’s formed when an acid (para-hydroxycinnamic) is broken down by bacteria. Some say the compound forms during malolactic fermentation (where malic acid changes to lactic acid; think acid in citrus fruit to acid in milk), while some say it develops fairly early on in the bottle aging process in wines that have not undergone any malolactic fermenation. 2 mg/l will give elegant leather, while over 4 mg/l will give manure.
It’s hard to control the development of this molecule and it’s thought now that it arises from the growth of lactobacilli (bacteria in the lactic acid group – same as in our GI tracts). To eliminate the risk of the manure smell, the wine would have to be sterile filtered, which would then rule out the possibility of the elegant leather aromas if they hadn’t already developed during malolactic fermentation.