Non-Saccharomyces Yeast: Inoculating for Control

This is the third article in my series on non-Saccharomyces yeast. Initially, I reviewed the concept that the yeast commonly used for wine and beer, Saccharomyces cerevisiae, is not ideal for cider and could be considered a spoilage yeast. I proposed that most Saccharomyces cerevisiae yeast lack the properties critical for creating the essence of quality hard cider. In my second article, I explored this concept by discussing the importance of creating complex aromas and natural sweetness. I reviewed how non-Saccharomyces yeast is an ideal tool to achieve these two critical elements and something Saccharomyces struggles to achieve. This has led me to the next question: how can I control the fermentation process using non-Saccharomyces yeast. As the title indicates, that means inoculation. It also means leaving behind some cider dogma or at least challenging it while embracing others.

If you believe cider should be made by sterilizing your juice and adding Saccharomyces cerevisiae yeast and nutrients, I must ask you why? If you are a cider maker that presses juice and just lets nature do the rest, I must ask you how you can be certain your product turns out the way you desire? How do you know that the right yeasts are the ones dominating the fermentation process. Again, I never said you couldn’t make good cider using these processes. That is what makes cider enjoyable. There really are infinite ways and means to make great cider. I want to potentially challenge some of your ideas about cider and ask you to consider new facts and information. I propose that using non-Saccharomyces yeast will offer you greater aroma complexity and the potential for residual sweetness. I also propose the way to best control the process is through inoculation and not another methodology, sterilization. Let’s explore the concept of sterilizing juice before we discuss how inoculating is our true path for control.

Sterilizing Juice

Racking Juice

The concept of sterile juice is admirable. The idea is that you eliminate the various microorganisms in the juice so they won’t negatively impact the development of the cider. There are different means this can be accomplished. The juice industry usually uses heat pasteurization. This makes the juice shelf stable by killing all the yeast and bacteria that might ferment or react with the compounds found in the juice. It is truly sterile. It’s also changed. Heat, the way most juice is heat pasteurized, changes the flavors and compounds found in the juice. This might initially seem like a good thing or at least not that bad. If your goal is to create juice that can sit in a bottle for months or years without fermenting, it is good. But, we are making hard cider and not juice. Sure, we can easily add yeast and even some lactic acid bacteria but, that can be expensive and ultimately, not as effective. We usually want cider to evolve. We often desire malolactic fermentation, polymerization of phenolic compounds, and the creation of various volatile compounds. Complete sterilization limits these positive reactions or makes them harder to achieve. We also need to consider the chemical changes caused by heat. This leads to the more common approach for wine makers, chemical sterilization.

Why is potassium metabisulfite (Campden or sulfite) often included in wine recipes. The idea is that it kills or inhibits organisms. But, what organisms is it killing or inhibiting? There is a belief that it eliminates the bad organisms. What are bad organisms? Can you name them and why eliminating them at this stage is critical? It’s important to remember that sulfites are most effective against bacteria and not yeast. For this discussion, I am ignoring the use of sulfites for treatment of oxidation or diseased fruit like gray mold. The focus is on the concept of whether you need or benefit from the addition of sulfites to your juice as it relates to fermentation. Yeast can be slowed or stunned by sulfites but, you aren’t killing them. Why is it often recommended? It’s recommended because you are supposed to add yeast after 24 hours and it’s expected that this yeast is healthy and vigorous. You are trying to ensure that the yeast you add will dominate. Note, this assumes you measured your pH and didn’t over sulfite your juice. All the other yeasts are stunned so the yeast you add is allowed to be the first to get going. Note, you are most likely adding Saccharomyces cerevisiae yeast strains, which have evolved to be aggressive fermenters. The likelihood that Saccharomyces strains need the help to dominate is minuscule. It’s like a professional football club playing a grade school team. Do we need to make the grade schooler play barefoot to ensure the pros win?

What about if you added just a little sulfite to knock out some of the wild organisms? Again, I ask what bacteria or yeast are you trying to remove and why? Do you want to eliminate any Pichia genus? Even if you could, why? Do you not like fruity flavors? If the yeast are so weak that sulfite eliminates them, what is the likelihood that they will outcompete any strong fermenting yeasts in the juice? Adding sulfites is a feel-good concept that isn’t really supported by any of my research on ensuring a fermentation isn’t spoiled by some wild yeast. They can have a use but, sterilizing the juice to ensure a good fermentation isn’t one of them. You might be thinking, sulfites are Generally Regarded as Safe or GRAS, so why not use them just to be safe. In research, sulfites are often used as a belt and suspenders approach to ensuring that what is being measured is not an outlier. This is why you often find them along with heat sterilization and control samples being employed. Researchers want a repeatable experiment so they can make changes and learn. They aren’t trying to make the most flavorful or healthiest cider. What is important to cider makers is a level of control so our process is repeatable. However, I don’t want control at the cost of quality or healthfulness. If we fail to question and understand, we can miss the opportunity to create better and healthier ciders. While sulfites are deemed GRAS, I have the belief that fewer preservatives added to my food is better. What I am finding is that the true mechanism for controlling the fermentation process isn’t sterilizing the juice but inoculating it with yeast.

Inoculating for Control

Inoculating with Saccharomyces will create a condition where the yeast dominates and usually ferments vigorously as it works to process all the available sugars and nutrients. As I noted in my second article in this series, the rate and vigor will usually create a dry, one-dimensional cider. The trade-off is to not add any additional yeast and allow a wild ferment. This usually results in a complex cider but, can also result in an undesirable cider. That is because you aren’t controlling which yeast will come to dominate the process. There are a multitude of yeasts competing for nutrients and sugars. While this can help create a wide number of aromas and usually keeps the process less vigorous, it doesn’t ensure the outcome is desirable or even what you wanted. There are a number of yeasts that create aromas of wet, horsey, soil, or worse. The goal is to control the yeasts that will dominate so you define the predominant flavors.

Most research indicates that non-Saccharomyces yeast won’t finish the ferment process and that wild fermentations are finished by Saccharomyces strains(1).  The generally accepted belief is that wild ferments start with non-Saccharomyces strains, which stall or are killed by Saccharomyces strains.  This is because the juice has both non-Saccharomyces and Saccharomyces strains in it.  The non-Saccharomyces strains come from the fruit and orchard.  They are on the peels or skin and get transferred to the juice during processing.  These are the true cider yeasts.  Saccharomyces strains are not typically found in nature or on fruit.  These come from your environment: the press cloths, the mill, the bucket, and even the air in the building where you process your apples into juice.  What this means is that wild ferments are really not wild or natural.  They are defined by the orchard where you pick your fruit and the building and equipment you use.  The orchard defines the non-Saccharomyces yeasts and your equipment defines the Saccharomyces yeasts that will finish your cider.  Change one of these aspects and you change the yeasts you are using.  You can see how even wild ferments are really inoculated ferments, though the yeasts are still unknown to us.  

We can assume that while we may not know the actual strains, the colony sizes will be similar at the start.  Sure, maybe an orchard has more Pichia kluyveri in it or your press cloth has high concentrations of Saccharomyces cerevisiae strain but, in general, the inoculation levels are relatively small.  You are not inoculating with millions of yeast cells like you do when you add a packet of dried yeast.  This means all the various colonies of yeast are actively competing for the same available resources.  Temperature, nutrients, and even sugar type will play into which strains win.  As the process progresses, alcohol, compounds, and susceptibility to killer factors will start impacting the process and which strains dominate. This is why Saccharomyces strains tend to be the dominant strain at the at the end of wild ferments.  They have evolved to not only survive but even thrive in the harsh final stages of fermentation.  Most non-Saccharomyces strains are not as alcohol tolerant so survival of the fittest starts to come into play.  While Saccharomyces strains tend to finish the fermentation, it’s incorrect to assume that non-Saccharomyces yeast aren’t present at the end.  Research shows that many non-Saccharomyces yeast are capable of surviving and even flourishing to the end of fermentation(2).  

Only recently has the scientific community started really exploring non-Saccharomyces yeast.  For the longest time, these yeast have been labeled spoilage yeasts.  The dogma of fermentation has become the overuse of sulfites and Saccharomyces yeast.  We have now even started looking at how to feed our Saccharomyces fermentations to address the lack of complexity.  This science may work but, it is based on the fundamental belief that Saccharomyces yeasts are what should be used to ferment cider.  I feel that belief is flawed.  Non-Saccharomyces yeasts are what we should be using to ferment cider.  While temperature, nutrients, and sugar type can impact which yeast will dominate, colony size has the biggest impact.  If you want a yeast to dominate, the best method is to inoculate it into the juice. It doesn’t matter what genus the yeast is, inoculating with millions of yeast cells will give it the advantage to dominate.

This makes inoculation our best tool for controlling and ensuring dominance by non-Saccharomyces yeast.  Commercial yeast suppliers are not making this job easier for cider makers because they offer limited non-Saccharomyces products. But, this is not an impossible task.  I’m a simple home cider maker and have figured out how to do it.  By obtaining pure samples from culture centers (USDA, universities, or other sources) or by isolating and propagating yeast from wild fermentations, we have access to pure cultures of non-Saccharomyces yeasts.  It takes some work but, the rewards are worth the efforts.  You can select a single strain or mix several together to create your own special “wild” blend. If you are not confident or don’t have the space or time, I’m here to help. I created the Cider Yeast Shop where I am offering non-Saccharomyces yeast for direct inoculation into your cider. My goal is simply to give other cider makers the opportunity to explore the world of non-Saccharomyces yeast with me.  Whether you try some of the strains I have or isolate and propagate your own, I just hope you will explore the real cider yeast: non-Saccharomyces strains.

(1) W.F. Morrissey and associates, The role of indigenous yeasts in traditional Irish cider fermentations, Journal of Applied Microbiology, 97, 647–655. 2004

(2) D. Rossouw and F. Bauer, Exploring the phenotypic space of non- Saccharomyces wine yeast biodiversity, Food Microbiology 55, 32-46, 2016

Understanding how yeast create great cider will help you make better cider. Knowledge and sharing it is why I wrote my book, launched this website, and provide products and recommendations on The Shop page. It is why I started offering non-Saccharomyces yeast strains in the Cider Yeast section of the shop. If you are interested in supporting, check out the shop. As with everything, my goal even with the shop is to help you make better cider.

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