The term 'MAKES BITTER' can be solved by two crossword solutions: SOURS (5 letters) and ACERBATES (9 letters). To sour something means to make it taste bitter or acidic, usually by adding an acid such as vinegar or lemon juice. Acerbates is a verb meaning to make something bitter or sharp in taste or character. Both terms are used to describe the process of making something more bitter than it was before.