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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PricklyCider.com 
There are unfortunately some incorrect information in this article. To summarize, yeast does not grow/multiply during the lag phase and there is virtually no respiration going on during a normal fermentation.
The second point depends partly on the Crabtree effect; if enough sugar is present (above 0,2 g/l roughly) S. cerevisiae (and other so called Crabtree positive yeasts) choses fermentation instead of respiration even if oxygen is present. And even for Crabtree negative yeasts, respiration would still be negligible due to the low oxygen amounts; oxygen is measured in ppm while sugar is measured in per mille.
The lag phase is about the yeast (or bacteria for that matter) adopting to the new nutrient rich environment and preparing for budding new cells. Any oxygen is used up in this phase in the synthesis of sterols and unsaturated fatty acids, both important compounds for a healthy cell membrane. Growing requires a lot of energy, and fermentation is where the yeast get that energy. The yeast does not grow so that it can ferment, it ferments so that it can grow!
This is a good article on the subject (even though it incorrectly states that Brettanomyces is Crabtree negative):
https://www.morebeer.com/category/wort-oxygenation-aeration.html/#Biochemistry%20of%20yeast
Any good textbook on some advanced level, like “Yeast” by White & Zanaisheff is also a good source in these matters. Unfortunately, a lot of basic homebrewing texts spread misinformation, and this seems also to be the case for cider (both Lea and Joliceur are guilty of this).
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Frederik,
First, thank you for sharing. I always appreciate thoughtful and insightful comments. I made some adjustments based on your feedback. My Mālus Trivium articles are intended to be a short summary of a topic and to be visual when possible. To that end, I purposely avoided the Crabtree Effect. That is another article currently in draft. Thanks again for your feedback. I don’t always have an expert editor to help me avoid errors or confusion.
As for books, I agree, Yeast by White and Zainasheff is a good starting point to learn about yeast. I also recommend Molecular Wine Microbiology and Wine Science. For cider lovers, a great book is by V.K. Joshi called the Science and Technology of Fruit Wines, which goes beyond yeast into many different topics.
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As always, things are complicated … we recently had a cider go bad that had seemingly fermented to dry and was stored in a stainless tank with no oxygen. Apparently the yeast wasn’t quite done; and instead of protecting the cider, this anaerobic environment caused the yeast to produce horrible sulfides and mercaptan. I assume there was also some nutrient deficiency involved – but unfortunately it’s not as simple as avoiding oxygen exposure after the initial ferment.
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Thank you for reading and sharing p. With fermentation, I don’t think there ever is simple. It seems everything is dependent on something else. There are many potential causes for hydrogen sulfide. Yeast are where I would look first. Some are just more prone to it’s production over others and if you have it, it can turn to disulfides, which are even harder to remove. This can occur in an anaerobic condition with low sugar if the hydrogen sulfide is already present. Yeast that are prone to making hydrogen sulfide will generally produce more when nutrient deprived. Though, I have read that too much can also result in hydrogen sulfide. Aging on lees and autolysis can also generate hydrogen sulfide in an anaerobic environment without sugar. Some non-Saccharoymces strains can help naturally stabilize the cider. While not usually associated specifically with hydrogen sulfide, Brettanomyces can also work in an anaerobic environment with low sugar and create some very funky aromas. What type of yeast did you use? Did you add any nutrients? What was your gravity level? What was the turbidity level of the cider when you racked it for aging? I would love to learn more about your process. It’s helps all of us make better cider.
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