The process or quality of becoming sour or acidic; the state of turning into vinegar or acquiring an acidic taste.
From Latin 'acescere' (to turn sour, to become acid), derived from 'acere' (to be sour). The suffix '-ence' nominates the quality, making it the abstract noun for the process of souring.
Acescence is why your apple juice turns into apple cider vinegar if left exposed—bacteria are actively converting the sugars into acids, which is exactly how humans discovered how to make vinegar thousands of years ago!
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