These oils are full of acidic compounds that give coffee its famous bitter bite.
Champagne, which is also acidic, offers a nice complement to anything from tuna tartare to beef bourguignon.
As we were taught in high school Chem, any pH below 7.4 is acidic.
Any of a class of compounds that form hydrogen ions when dissolved in water, and whose aqueous solutions react with bases and certain metals to form salts. Acids turn blue litmus paper red and have a pH of less than 7. Their aqueous solutions have a sour taste. Compare base.