Carbon dioxide (CO2) is a natural byproduct of
alcoholic fermentation, a process where yeast consumes sugars and converts them into alcohol and CO2. When fermenting grains, particularly for processes like brewing beer or producing ethanol, this CO2 is released.
Here's how it generally works with grains:

Prepare the Wort/Mash: Grains (like barley in beer making, or corn in ethanol production) are typically milled or crushed and mixed with water to create a liquid mixture called wort or mash.
Add Yeast: Yeast (a type of fungi) is added to the wort/mash.
Fermentation: The yeast then consumes the starches and sugars present in the grains. This process of breaking down the sugars is called fermentation.
CO2 Production: As a result of this metabolic process, the yeast produces alcohol and carbon dioxide gas as waste products.

https://www.researchgate.net/figure/Course-of-CO2-release-throughout-the-fermentations-at-different-temperatures-The-strain_fig2_332982465