The term 'turns sour' can be described by two crossword solutions: 'rots' (4 letters) and 'acidifies' (9 letters). 'Rots' is a verb that means to decay or decompose, usually due to the action of bacteria or fungi. 'Acidifies' is a verb that means to make something acidic or sour in taste. Both words describe the process of something turning sour, either through decomposition or through the addition of an acid.