From this article:
Everywhere but at home, American milk chocolate — specifically Hershey’s — is known for its tangy or sour flavor, produced by the use of milk that Mr. Landuyt refers to as “acidified.” Although Hershey’s process has never been made public (and a spokeswoman declined to comment on its techniques), experts speculate that Hershey’s puts its milk through controlled lipolysis, a process by which the fatty acids in the milk begin to break down.
This produces butyric acid, also found in Parmesan cheese and the spit-up of babies; other chocolate manufacturers now simply add butyric acid to their milk chocolates. It has a distinctive tang that Americans have grown accustomed to and now expect in chocolate. “I can’t think of any other reason why people would like it,” said Mr. Whinney, of Theo Chocolate.