Enzymes are proteins that act as catalysts. Catalysts is where these proteins speeds up the chemical reaction. So enzymes help change 1 stuff into another stuff. Example, your stomach has enzymes that helps turn the food you eat into food for your body to eat.
Enzymes that’s used in beer brewing is alpha and beta amylase. Alpha-amylase breaks down large complex insoluble starch molecules into smaller soluble molecules for the beta-amylase. Beta-amylase make starch turn to soluble sugars.
Without these enzymes, the grain’s starch will not turn into sugar, and zest has no sugar to eat to turn into alcohol.