What role does oxygen play in yeast growth during fermentation?

Oxygen plays a critical role in the growth phase of yeast during fermentation. When yeast cells are provided with oxygen, they can undergo aerobic respiration, which is a more efficient process for energy production compared to anaerobic fermentation. During this aerobic phase, yeast can multiply rapidly, allowing the population to increase significantly before the brewing process transitions to anaerobic fermentation, where alcohol and carbon dioxide are produced.

In the initial stages of fermentation, the presence of oxygen encourages yeast to grow and reproduce, establishing a strong population that can later effectively carry out the fermentation process. This is why the correct answer emphasizes the importance of oxygen in promoting both growth and fermentation.

The other options reflect various aspects of yeast metabolism but misrepresent the role oxygen plays specifically during the growth phase. While it is true that alcohol is primarily produced during anaerobic fermentation, oxygen's role is more about promoting cell growth beforehand. Similarly, although oxygen can contribute to flavor compounds under certain conditions, it can also lead to oxidation, which is generally undesirable in finished beer. Lastly, while excessive oxygen exposure can indeed harm yeast viability, especially during fermentation, the correct answer focuses on the positive effect of oxygen on yeast growth before fermentation starts.