HanNing Tung (Brewing Researcher)
Oxygen is an essential element for human. Our cells uptake oxygen to make energy that supports cell survival. Oxygen helps cell to break down the nutrition in food. With oxygen, cell can completely degrade glucose to produce maximum energy in the Mitochondria, which are small double-membrane organelles in the cytoplasmic. In general, there are numerous Mitochondria in most of human’s cell. In human liver cells, there are about 1000–2000 mitochondria per cell, making up 1/5 of the cell volume. This fact shows liver cells need a lot of energy to present normal functions and they always require more oxygen supply. What about yeast? Do they need to uptake oxygen to survive? Do they need oxygen to ferment? Today we are going to talk about how does oxygen work in the process of yeast fermentation.
When glucose or other sugar molecules enter yeast cell, yeast can break down them and got a little energy in cytosol. 1 glucose molecule will be degraded to 2 pyruvate molecules. If there is oxygen, pyruvate will go to yeast Mitochondria to make more energy. Once pyruvate goes to Mitochondria, yeast only produces carbon dioxide and energy instead of alcohol. On the other hand, without oxygen, pyruvate will be kept out of Mitochondria. In this situation, yeast will convert pyruvate to carbon dioxide and alcohol in its cytosol. In other words, yeast has two modes to break down sugars and oxygen is a switch that determine which mode the yeast cell should be set. The aerobic mode also called respiration, because yeast uptakes oxygen and produces carbon dioxide. The anaerobic mode is called ethanol fermentation or alcoholic fermentation. In this mode, yeast produces carbon dioxide and alcohol. Here we got a question: Do we need oxygen to make yeast produce alcohol for us?
Despite no oxygen requiring for alcoholic fermentation, it is essential to add oxygen for yeast growth. Yeast cell need oxygen to make new cell membrane that builds next generation yeast cell. In fact, brewers inject pure oxygen directly into the wort before they pitch yeast and start to ferment beer. Oxygen can stimulate yeast growth, and it is important that yeast grows quickly and rapidly dominates the environment in the early stage of fermentation. Therefore, oxygenation is indispensable for large volume fermentation in breweries because the optimum dissolved oxygen concentration for yeast growth is 10 ppm, which would not be reached by vigorously shaking or even aquarium pump. Fortunately, 8 ppm dissolved oxygen that can be reached by stirring is acceptable for small batch homebrewing. If you use ALCHEMA to make cider or mead, do remember to stir the juice as much as possible before you pitch yeast. Enjoy home brewing!