The two main compounds that react during fermentation are:

* Sugar (usually glucose): This is the primary energy source for the microorganisms that carry out fermentation.

* Water: Water is necessary for the chemical reactions that occur during fermentation.

Fermentation is a metabolic process where microorganisms (like yeast or bacteria) break down sugars in the absence of oxygen, producing energy (ATP) and byproducts like ethanol, carbon dioxide, or lactic acid.