The term 'turn sour' can be defined as a process of becoming acidic, curdling, spoiling, rotting, or going bad. Acidify is a 7-letter word that means to make something acidic. CURDLE is a 6-letter word that means to cause a liquid to become thick and lumpy. SPOIL is a 5-letter word that means to ROT or decay. ROT is a 3-letter word that means to decay or decompose. Go bad is a 5-letter word that means to SPOIL or become inedible. All of these words refer to the process of turning sour, which is when something becomes acidic, curdles, spoils, rots, or goes bad.