The First 48 hours of Fermentation

A highly simplified summary of the first stages of fermentation:

• During the first 3-15 hours after pitching the yeast, the aerobic phase, oxygen (added to the fermentor) is consumed by the yeast.

• When the O2 is exhausted, the anaerobic fermentation phase begins, and the yeast digests sugars to produce CO2 and alcohol.

The wort was poured into the fermentor and oxygen added from an external tank after lunch on the 6th. In the graph below we see the aerobic phase starting with an unexplained spike followed by a quiet period until the next afternoon where the transition to the anaerobic phase begins with the yeast rapidly consuming sugars and producing increasing levels of alcohol as detected by the TVOC sensor data shown in the green trace, and CO2 in blue.

TVOC Green; CO2 Blue.

This example shows data collected inside a closed single-car garage with a high ceiling and subject to wind-driven air infiltration. Approximately five gallons of wort was added to the fermentor with an external chiller for temperature regulation. The sensor module is located approximately 6 feet from the fermentor/chiller system.

For reference, the OSHA standard for the CO2 level a worker can be exposed to over an 8-hour day cannot exceed 5,000ppm (average). A short-term exposure limit is set at 15,000ppm, and at 30,000ppm alarms should be blaring and the building evacuated.