bittering agent

:  an ingredient added to a substance to give it a bitter flavor The Sumerians flavored their beer with honey, while Europeans flavored and preserved their beer with a bittering agent such as hops. — Bill Yenne, The American Brewery, 2003 Some pesticide makers thought putting a bittering agent into the poisons would make rats less likely to eat the products … — Robert McClure, MSNBC.com, 22 Dec. 2010 The vast majority of automotive antifreeze contains ethylene glycol, though some contains a bittering agent to eliminate the sweet taste. — Steven Cole Smith, Orlando Sentinel, 15 Apr. 2011