So What Is the Balloon Doing There?
During fermentation, microorganisms such as yeast can consume sugars and produce carbon dioxide gas.
That gas needs somewhere to go.
A balloon fitted over the bottle can expand as gas is produced, making it a very simple visual indicator that gas is being generated. Similar balloon setups are commonly used to demonstrate fermentation and gas production.
The balloon can also act as a basic barrier, helping prevent outside air and contaminants from freely entering the container while gas escapes.
