Have you ever heard that oxygen is bad and to avoid oxygen exposure when fermenting hard cider? It is or at least it can be. Yes, this is another one of those questions where the answer is, it depends. Generally speaking, yeast have two main ways to process carbohydrates, usually sugar like glucose and fructose, into energy that it uses to survive. Part of that survival is to bud or reproduce. This occurs in a process often called respiration. Respiration, because it uses oxygen. It is also the most efficient way for yeast to create energy or ATP. In fact the respiration process is usually 15 times more efficient than fermentation. However, as the name alludes, respiration requires oxygen. For hard cider makers, we care about respiration and fermentation because respiration is how the yeast we add to our juice reproduces and creates the biomass needed to process the juice into hard cider. In the first phase of the fermentation process, often called the lag phase, yeast use oxygen and nutrients to create energy in preparation for reproduction. Basically, the yeast is using the most efficient process available to it to make energy and that energy will go into reproducing. However, there becomes a point when the oxygen available to it is used up. This aerobic process that used the sugar and available oxygen to breakdown compounds through oxidation to create energy (ATP) stops. The yeast needs another way to use the sugar available to it to create energy. That is the anaerobic process called fermentation.
Yeast generally move to the fermentation process once the available oxygen to it is gone. Lacking oxygen as a tool to easily oxidize compounds and create energy, the yeast begin using the cider maker’s favorite process, fermentation. Fermentation is an anaerobic process, meaning it doesn’t use oxygen. Instead it utilizes enzymes to break down compounds and convert sugar to ATP. In that process, the yeast create two wonderful by-products: ethanol and carbon dioxide (CO2). The fermentation process is not as efficient as the respiration process. Where respiration might yield over 30 ATP molecules, fermentation might yield two. However, yeast are very resourceful organisms and they have numerous process or pathways they can use to support reproduction. They need various nutrients, enzymes and other compounds and what they didn’t store up during respiration, they begin to make through these pathways. They also use these pathways to build up more reserves in order to sustain themselves in severely deprived conditions.
So, if you are wondering whether oxygen is a good thing, it depends. It is good to help get your yeast bulked up so it can ferment your juice into cider. However, it’s not a good thing once the fermentation process starts and completes. At that point, oxygen can induce other aerobic process like acetic acid bacteria converting all your beautiful ethanol into acetic acid. So, shake your carboy or come up with other interesting ways to add oxygen to your juice before you pitch your yeast, but protect your cider once your fermentation process starts.
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