The term 'become sour' can be solved by two crossword solutions: 'TURN' (4 letters) and 'gobad' (5 letters). 'TURN' is a verb that means to move in a circle around an axis or center, or to change direction or position. 'Gobad' is an informal phrase that means to become worse or to deteriorate. Therefore, the two crossword solutions together indicate that the term 'become sour' means to worsen or deteriorate in quality.