Zymolysis is the breaking down of substances by enzymes during fermentation — from the Greek zyme, leaven, and lysis, loosening. When yeast converts sugar to alcohol, or lactic acid bacteria convert milk sugar to lactic acid, the enzymatic transformations at work are zymolysis. The older synonym zymosis means the same. The term describes the core event of every fermentation: complex molecules taken apart and rebuilt by microbial enzymes into simpler products plus the flavours, acids, gases, and alcohol that define fermented foods. Understanding zymolysis lets fermenters control it — adjusting temperature, pH, and nutrients to favour desired pathways and suppress off-flavours. It is digestion performed by microbes on our behalf.