Yeasts live primarily on sugars, such as glucose ($$\ce{C6H12O6}$$). They convert glucose into carbon dioxide ($$\ce{CO2}$$) and ethanol ($$\ce{C2H5OH}$$) in a chemical transformation that is represented as follows:
$\ce{C6H12O6 → 2CO2(g) + 2C2H5OH(ℓ)}$