What does oxygen do to fermentation?

Answered by Vince Keith

As a sommelier and brewer, I have had the opportunity to delve into the fascinating world of fermentation. Oxygen, though often seen as an enemy in winemaking and , actually plays a crucial role in the fermentation process. Let me take you on a journey to understand the impact of oxygen on fermentation.

1. Promotes Biosynthesis of Unsaturated Fatty Acids (UFA) and Ergosterol:
During fermentation, requires certain nutrients to grow and thrive. Oxygen, when present in the early stages of fermentation, stimulates the biosynthesis of unsaturated fatty acids and ergosterol. These compounds are vital for the yeast's anaerobic growth and overall health during fermentation.

2. Enhances Yeast Propagation:
Before yeast can start fermenting sugars and converting them into , it needs to multiply and reach a sufficient population size. Oxygen aids in yeast propagation by supporting cell growth and division. This is especially important in high-gravity fermentations where a larger yeast population is required to handle the increased sugar content.

3. Improves Yeast Viability and Stress Tolerance:
By promoting the biosynthesis of UFA and ergosterol, oxygen improves yeast viability and stress tolerance. Yeast cells with adequate levels of these compounds are better equipped to withstand the harsh conditions of fermentation, such as high alcohol concentrations and low pH levels. This ultimately leads to healthier yeast and a more efficient fermentation process.

4. Prevents Formation of Undesirable Compounds:
While oxygen is beneficial in the early stages of fermentation, it can have negative effects if present later on. Oxygen exposure during the later stages of fermentation can lead to the formation of undesirable compounds such as acetic acid (vinegar-like aroma) and ethyl acetate (nail polish remover-like aroma). These compounds can greatly impact the flavor and aroma profile of the final product.

5. Oxidative Stress and Off-Flavors:
Excessive exposure to oxygen throughout the fermentation process can result in oxidative stress for yeast. This can lead to the production of off-flavors, such as a papery or cardboard-like taste, due to the oxidation of various compounds. It is important to manage oxygen levels carefully to avoid these unwanted flavors.

Personal Experience:
In my own brewing experiences, I have witnessed the impact of oxygen on fermentation firsthand. I once accidentally introduced oxygen into a batch of during transfer, and the resulting off-flavors were discernible. The beer had a slightly oxidized taste, reminiscent of stale cardboard. This experience taught me the importance of minimizing oxygen exposure during fermentation.

Oxygen plays a multifaceted role in fermentation. While it is essential in the early stages for the biosynthesis of UFA and ergosterol, it should be carefully managed during the later stages to prevent the formation of undesirable compounds and oxidative stress. Understanding and controlling oxygen levels is crucial for producing high-quality wines and beers.