It is common to talk about fermentation in stages and often there are three stages identified. If you are talking about a wild or natural fermentation, those stages might be how initially non-Saccharomyces strains start the fermentation. This is followed by Saccharomyces yeast that complete fermentation. The final stage is maturation where Brettanomyces strains often influence the final aroma and flavor profile. However, the fermentation stages for this article focus on how any given yeast cell will go about the process that converts juice into hard cider.
You can find these stages or phases called different things. In fact, you can even find different definitions of how many stages or phases there are. I’ve seen three phases and four phases defined. In their book, Yeast, Chris White and Jamil Zainasheff called the three phases the following(1).
- Exponential Growth
Personally, I like the name of the second phase, Exponential Growth, because it gives a good description of the main thing that is happening. Some call this the Log phase but, Log doesn’t really help me understand what is happening. This is similar to the first phase. What does Lag really mean? The last phase also doesn’t make a lot of sense to me. The yeast isn’t really stationary so should we call it that? Others list four phases by adding a fermentation phase after the Exponential Growth phase. Part of the challenge with defining phases is that none of these phases are absolute. You can have elements of different phases occurring at the same time. For example, you might have both aerobic and anaerobic activity occurring together. This is because you literally have millions of yeast cells in your cider. It’s also important to remember that each cell is trying to survive and reproduce.
Additionally, it is good to remember that for the most part commercial yeast strains are no different from wild or natural yeast when it comes to fermentation. All yeast started as wild strains that have been isolated and cultured. All yeast have the same goal and that is to survive and reproduce. That means they need energy, known as ATP, which is generated during both respiration and fermentation. While some yeasts are better at converting sugar to ethanol, or creating certain esters, commercial yeast or wild yeast found on processing equipment are fundamentally the same. They are both natural yeast. The commercial strain has just been selected and grown (e.g. cultured) to make it pure. Commercial strains were originally found in nature, isolated, and grown. They are part of some wild fermentation somewhere. Ultimately, whether you have one or many, the process the yeast follow will be the same. I’m sticking with three phases or stages but I’m naming mine from the perspective of the yeast and what they are doing. In parentheses, I note the impact to the fermentation process that cider makers desire.
Nutrient Absorption (Respiration):
When yeast are added or find themselves in nutrient rich medium, the first thing they do is assess their current capacity to reproduce. They are often lacking some nutrients and they immediately start absorbing these and preparing themselves. The main compound they consume is oxygen. Yeast begin the respiration process because it’s the most efficient way to make energy and store compounds needed to reproduce. We generally don’t see anything during this process. The yeast consume the oxygen suspended in the juice. They don’t really make any aromas at this stage but they are making precursors that can be turned into aromas and compounds in the future. Once the yeast have consumed the oxygen and absorbed the nutrients they lacked, they will begin to reproduce. This process usually takes hours versus days but is temperature dependent.
After the yeast have absorbed the nutrients it needs and processed as much of the oxygen it can, it will start to reproduce. This is fermentation, where sugar is turned into ethanol, CO2, and ATP. This process is anaerobic and not as efficient as respiration but, it’s and example of yeast using different pathways to achieve their goal of reproduction. Besides creating ethanol, CO2, and ATP, yeast also create other compounds like esters and higher alcohols. These are the types of compounds that most impact the aroma and flavor of your cider. As the sugar is consumed and the alcohol increases, the yeast begin to sense an environment not conducive to its goal of reproducing. This process is usually measured in days or weeks. Depending on the temperature, it might even take months.
Dormancy (Clean Up):
Yeast will stop fermenting and reproducing if all the sugar is consumed, the alcohol level increases, or a nutrient runs out and can’t be made. As these conditions occur, the yeast cells begin preparing for dormancy. They can sense the changes in their environment and start preparing to become dormant. Yeast want to survive, autophagy is an example of the lengths yeast will go to survive. Like a bear preparing for the harsh winter, yeast will begin absorbing compounds that will help them survive the coming harsh season. No more fermentation takes place but something important for cider making does. As yeast absorb compounds in preparation for dormancy, they can also perform another vital service to the cider. They clean it up. Yeast will often reabsorb diacetyl and acetaldehyde(1) that were produced earlier. They will also often start to flocculate together and drop out of suspension. This is all done to help the yeast survive and be prepared to react when the next opportunity to reproduce presents itself. Like nutrient absorption, this phase is usually measured in hours or days. Some yeast might take weeks or months to flocculate and precipitate out completely but the clean up aspect is usually completed with in hours or days after the environment is no longer capable of sustaining reproduction.
While we might like to think about the process in terms of these predefined phases or even in terms of fermentation, the reality is that yeast don’t care whether they are using oxygen and absorbing nutrients or creating ethanol and CO2. They are simply trying to survive and reproduce. They will use whatever is most efficient and available to them to realize that goal. Just like they aren’t interested in whether your cider aroma is what you wanted, they will simply look to absorb or create whatever compounds they need to survive.
(1) C. White and J. Zainasheff, Yeast The Practical Guide to Beer Fermentation, Brewers Association, 2010
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