What is to cause to become sour; to cause to turn from sweet to sour; as, exposure to the air sours many substances. So the sun's heat, with different powers, Ripens the grape, the liquor sours. Swift called?

sour — To cause to become sour; to cause to turn from sweet to sour; as, exposure to the air sours many substances. So the sun's heat, with different powers, Ripens the grape, the liquor sours. Swift. (Webster's 1913.)