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Microscopic Details On Yeast - With Maggie Campbell From Privateer Rum

Updated: Sep 6, 2019

Can you please give a short summary of who you are, what you do and how you acquired your extensive knowledge?

I'm the President and Head Distiller of Privateer Rum. I also am the elected Vice-President of the American Craft Spirits Association (ACSA) and serving my second elected term on the Board of Directors there. I serve as the Alumni Advisory Board for the Wine and Spirits Education Trust (WSET) representing all diploma holders from the USA.


I am a current Masters of Wine student. Distilling is my first career and I began to study wine and spirits in formal programs directly after getting my BA in Philosophy. I began with an interest in the whiskey world as I was based in Denver at the time and got to meet and befriend Todd Leopold (Leopold Bros) and Jake Norris (at the time the founding distiller at Stranahan's) who are great folks and were nice enough to encourage me in my career and answer my questions.


I accepted the position of Assistant Distiller at Germain-Robin and made brandy in the Cognac inspired method as well as some apple and pear brandy. Then I took a brief year and a half to work for a boutique wine and spirits distributor to learn that side of the business at the urging of Todd Leopold, which taught me an invaluable amount about how products live out in the market. Then Hubert Germain-Robin met Andrew Cabot the founder of Privateer and connected us back in 2012.


Maggie Campbell - President and Head Distiller of Privateer Rum

For people new to the topic of alcohol production, what is yeast and what is its function?

In simple terms yeast are the microscopic fungus that consume sugar (from grain as malt syrup, fruit, cane, etc) and convert that sugar molecule into alcohol, and Co2 (plus other components important for final flavor). The fungal elements of a microbiome are referred to as mycobiota and this includes the fungus yeast. In general in distilling we start with a sugary liquid called wash that yeast ferments into a dry (no sugar left) alcoholic liquid. We know yeast have finished a fermentation when we measure the sugar content and all sugar is gone and the reading is zero (or very close as there may be some entwined sugars that are hard for the yeast to access or a small amount of un-fermentables). This means sugar is now converted to alcohol thanks to the yeast. At this point a distiller will put the fermented wash into the still to coax out and capture the alcohol vapor.


There is yeast all around us. What could some of the reasons be for a distiller to use yeast that’s naturally present vs other types of yeast?

I love the alchemy of wild yeast and sense of place that it can bring. Wild yeast is a generally accepted industry term for the natural yeast that has not been actively added to a fermentation, but grew naturally. It can be picked up from the air, be introduced from the surface of equipment, or come in on the base ingredient. Much of the yeast that would cause a wild fermentation is in fact from the species we call Saccharomyces Cerevisiae, the same yeast species many distillers inoculate with (added yeast rather than wild). The type of Saccharomyces Cerevisiae would just happen to be the locally occurring strains randomly selected from the environment instead of purposefully selected strain(s) added on purpose.


Of course other local microbiome will also participate in a wild fermentation and the complexity of that natural microbiome will create a unique flavor to the fermentation distinct to that place. Other species of yeasts may be present, but Saccharomyces Cerevisiae is latin for 'sugar eater' and is fact well and naturally suited to convert sugars to alcohol (while also imparting generally pleasing 'esters' which we can think of as flavor molecules). These Saccharomyces Cerevisiae will out compete a lot of other yeasts in a fermentation and take over. In this they are responsible for much of the alcoholic fermentation throughout human history. There is a massive variety of unique and special Saccharomyces Cerevisiae - it is not simply to be thought of as one type of yeast but thousands of yeast strains.


Traditional wood fermentation vessels at Croft in Oporto, Portugal. You can see the open wood slat roof allows fresh and yeast filled air into the fermentation. (Photo: Maggie Campbell)

Wild Yeast strains that are non Saccharomyces Cerevisiae, such as Brettanomyaces (Brett) for example, can often create unique and distinctive flavors (such as 'horse blanket' in Bordeaux wines) when they participate in fermentation, but they are often very slow fermenting compared to Saccharomyces Cerevisiae. This means that most fermentation taking place in the short window common to spirits (a window from 24 hours to 2 weeks) will not express the character of Brett as Saccharomyces Cerevisiae will quickly do most of the work in this period, though of course some nuance will come through. In Calvados you will get notes of leather and possibly lacquer from the Brett influence as much longer fermentation and holding of fermented juice is common. Brett is also good at converting tiny amounts of residual sugar that Saccharomyces Cerevisiae may leave behind. So fruit, notorious for not fermenting completely dry easily, is more likely to show some of those notes if the fermented juice is left to sit as long storage of fermented must (aka juice) is common.


Of course there are more species than Brett, like Scizosaccharomyces Pombe, but this is another topic. Most of these other non-Saccharomyces Cerevisiae yeast strains are introduced through wild or wild-cultivated methods (traditional knowledge of adding dragonfruit to muck in the case of encouraging Pombe) and are not commonly inoculated through additional yeast pitching.


How does a distiller choose a particular yeast and why? How many types are there?

The selection of yeast speaks to the goal of a distiller and the final product. A large scale, globally produced brand looking for high consistency may select a very specific yeast to bring standard flavor. They will pitch this yeast strain in a large dose to out-compete any natural yeast. I would say this, certainly by volume, is the most common in the Western hemisphere distilling world. It is extremely rare for a distillery to sterilize their 'wash' or fermentation liquid to kill natural yeast. Therefore natural yeast are still present and often play a small role depending how long the fermentation is. Though, you do see this sterilizing of the sugary wash liquid and sterile fermentation with strictly inoculated yeast fermentation in some traditions such as Puerto Rico rum where a palate with direct and precise flavors is the goal. All in all it is about the intended goal and desired character.


Notably rum is very special, in that wild yeast for fermentation is somewhat common, especially in well known and widely available Jamaica rum where the cultivation of dunder, cane vinegar, and muck means a beautiful diversity of microbiome, creating a truly unique character. In other spirits categories, especially in the Western Hemisphere, this is considered more rare as a quick and predictable, controllable, and reliable fermentation is often the goal - though traditional Jamaican producers are so skilled from experience at this process their fermentations are very consistent and predictable.


In Jamaica the cultivation of these natural yeasts for different marks and the complexity of spirits produced from them adds nuance in the blending process (these fermentations are paired with distillation technique to sculpt the flavors from specific fermentation). They have gone on to further protect this tradition by limiting their GI to only support the natural yeast fermentation tradition with the addition of traditional Saccharomyces Cerevisiae species when pitching additional yeast which as discussed are common, historic, and naturally well suited to alcohol fermentation and offer a massive variety of nuance due to the plentiful varieties of strains. I really applaud the steps the Jamaican GI have taken to protect these traditional and natural yeast sources and special flavor profiles unique to the community and culture there. As with any GI, anyone wanting to use GMO yeast, or any other species, for any reason may do so while they calling it any name except 'Jamaican Rum' as it does not reflect the flavor, character, or legally established practices of Jamaican rum.


As of now there is over 5,400 types of just the Saccharomyces Cerevisiae species of yeast identified! But of course I am sure there are more out there floating in the air yet to be identified. I've gotten experience with different yeast in my career and trusted peers will often give tips on their favorite strains. Of course trials are needed for thoughtful selection in your own production. We use a number of POF+ (a certain class of Saccharomyces Cerevisiae yeast) because of the characteristics they impart. Where some POF+ strains give tropical pineapple, apple blossom, and even guava notes you have to be careful because others can give off varnish and sickly artificial banana so quality and thoughtful selection is important.


Open top wood fermenters at Four Roses. They pitch yeast as well as let wild yeast play a role. (Photo: Maggie Campbell)

Is there yeast that works for one spirit, but not for the other?

I would say that it is all about the goal of the producer again. There are certainly traditional yeasts identified for certain spirits categories. In making brandy in the Cognac tradition methods of Germain-Robin I became familiar with yeasts that are commonly used and identified with the flavor, culture, and community that has been established in that tradition - it reflects a sense of place in the glass. Those flavors are very recognizable and intertwined in the essence of Cognac and ring very true to the founding distiller's Cognac roots. I was very proud to learn those techniques authentic to his heritage and practice them. Of course we did not call our products Cognac because we were not producing in that region, Germain-Robin is in California so all our products were labeled brandy.


This same approach is also true for Haiti - the artists, farmers, and scientists who have been making rum there for centuries know what reflects their community and identity via flavors in the glass insofar as wild yeast cultivation and use of selected yeast. If I were to say that for one to make the best choice would be to strive to be like Cognac, rather than recognizing the personhood and lived experience of those in Haiti, well, that would be very Eurocentric and self-referential of me and my experience. I truly think each community has their own lived experience and certainly I would not venture to believe I know better than those who live and breath it. I could offer perspective from my vantage (if asked for it) in a kind and cooperative conversation with friends from other regions, but I'm here to share a love of yeast, not control or police others in their own culture and prescribe what is best. I'm often a guest in many parts of our distilling community, especially as a newcomer of only 15 years experience, and I want to act accordingly.


Distilling in my region of North America is unique as we do not have a currently well established industry and lack a continuous distillation history here. I do not use dunder or mimic other rum methods distinct to the Caribbean as I feel it would be inauthentic to mimic their culture outside their community and country with no rooted connection of my own, instead we are finding our own way. We are pulling on practices I feel are best for our house style, raw material, and mesoclimate. (The macro and mesoclimate affect flavor development from many of the elements yeast create in the fermentation but that is a very long story indeed.)


I certainly have a yeast strain I particularly love for aged spirits in a pot distilled expression because of the flavor profile it creates, but I would not use it in a fermentation for an unaged spirit or a column distilled spirit because it offers more savory and earthy esters over fresh and bright esters that are a house style of Privateer unaged and column stilled rums. I use blends of yeast, and different blends for different expressions we distill (which of course you have to account for how well the yeasts all get along and what flavors they create when they interact with one another). We also allow wild yeast to play a role in all our fermentation as we do not sterilize our fermenters or wash liquid, adding to the complexity. Our 6 day, cool fermentation allows a bit more local microbiome influence than some styles of hot and fast fermentation, as some conversions take time. At the same time we do not go long enough for other certain conversions to take place. We hit our own sweet spot for flavor cultivation in fermentation on our own timeline through our own process.


When a distillery states they have their own unique yeast, how is this cultivated?

There are a number of ways this is done. They may grow their own sample from the ambient collection (leaving out sugary water for yeast to propagate in) and continually feed and grow that sample as they draw from it. This may look like a slurry - imagine a small tank with a mixer with a milky brown frothy liquid in it. Essentially they would pull off a few gallons/liters of the slurry (which would contain a certain load of yeast) and put this slurry liquid in a sugary wash liquid to kick off fermentation. Then they would re-top off the slurry tank with sugary water to signal the yeast to reproduce again.


The key here is yeast looks around, they see a certain amount of sugar, they reproduce enough to create a population to eat all the sugar, and then go nuts eating the sugar until it is gone. So topping off the small vessel with sugary water means the yeast would keep re growing to a certain concentration - but like sourdough bread this means keeping it active.


Alternatively, they may have wood fermenters like in Haiti, Mexico, Jamaica and beyond, or clay fermenters like Qvevri in the country of Georgia, or underground earthenware vessels such as those on Amami in Japan where they ferment cane with koji (another fermentation story altogether). These types of vessels have surfaces that hold a bloom of natural yeast on the hospitable and textured surface, this is a simple and elegant option for cultivating a local microbiome population. This means that an established distillery has used a fermenter enough times that their own yeast load is thriving in the wood and once the wash liquid is pumped in the high yeast population in the wood would kick off a steady fermentation unique to their environment. This is something I love about Jamaica's famed fermenters.


I would also consider that this has implications in the GI protection I mentioned above in regards to adding specifically traditional Saccharomyces Cerevisiae yeast to wild yeast already present. Meaning, that if you start pitching all kinds of different species of yeast into these historic and legendary fermentation vessels you would forever be altering their DNA and it would be incredibly difficult to recover these vessels from such a drastic change in microbiome. You would have to sterilize the vessel, a serious task, and start over building up and breaking in the microbiome all over again. The previously established, one of a kind, complex microbiome would be forever altered after the introduction of a new species and fully wiped out in sterilization - the traditional flavor of that fermenter would likely never be the same again. Imagine one used and cultured for say 60-80 years, the complexity of character built up over that lifetime would be wiped out - think of how precious the complexity of a long lived sourdough starter is to a baker and you start to get an idea. As you can see this new species introduction would most certainly be completely altering these incredible wood vessels that have been around and ebbing and flowing with these community practices for generations. These vessels reflect the microbiome present through their existence capturing a certain kind of legacy all their own.


Todd Leopold, known for his whiskey, who has implemented wood fermenters in his own facility, he mentioned that it takes years, if not decades, for new wood fermenters to truly blossom into a unique and one of a kind microbiome holding a powerful sense of place, reflecting local biome history, and giving a flavor signature unique unto itself. I know he, I, and many others hold a deep respect and awe for the fermenters of Jamaica. Just getting to look at them in real life gave me shivers, you feel their presence and heft of their legacy and contribution to the identity of an entire community. A familiarity with the signature of a rum makes you sort of feel like you already know the fermenter in a way when you see them. Like seeing a a famed piece of art from a books and posters for the first time in real life.


Another way they may have their unique yeast strain is from trub additions or acid washing. Trub is the brown-milky looking sludge in the bottom of a fermenter when it is emptied after fermentation is complete. It is full of dormant yeast. You could simply add this to fresh wash liquid. Of course if you have a negative bloom of bacteria this could potentially spread or infect your fermentation as it will have had time to get a good foothold in the material. Acid washing is where you would take some of this trub and 'rinse' it with a citric acid (vitamin c) water solution to drop the pH, kill any potential bacteria, and 'clean' the yeast to be re-pitched. Yeast will adapt over time and over these re-pitches so the flavor may drift if this is done continuously.


Simply, a distillery could also isolate a yeast from their environment and propagate it (grow it) on demand to keep genetic drift low. The main sample would remain controlled. Yeast labs are pretty cool.


Can you explain the process of “adding yeast”, how is it done and why?

Adding, or pitching yeast means taking a selected yeast and putting a certain dose of it into a wash (the sugary liquid you want fermented). By doing this you are giving the fermentation a high load of the yeast and ensuring that it will do most of the work to ferment the wash.


This might look like adding a liquid slurry you can order or adding freeze dried yeast from a packet. The best practice in doing this is to add it to luke warm water with a small amount of sugary wash to revive the yeast before mixing it into the larger fermenter which might prove to be a bit of a sugar shock to the sleeping yeast. Storing these yeast well is also key.

This is done because the producer wants to encourage a certain aspect produced by that yeast. For some producers that is efficiency, predictability or low price. For us our goal in yeast selection is flavor cultivation, and we often pay a bit more for specific strains we want to work with, rather than use more generic powerhouse yeasts that wont give us as meaningful, concentrated, and complex flavors. As I mentioned we will let natural yeast play a role, but we also pitch a blend of yeast (all strains of Saccharomyces Cerevisiae of course). We do this because we are looking for certain esters to be produced and cultivate flavors in the fermentation while getting some local nuance.


Open top stainless fermenters at Buffalo Trace. They pitch yeast as well as let wild yeast play a role. (Photo: Maggie Campbell)

Another factor is that we live near many apple orchards. Yeast strains that thrive in apple orchards often produce a hint of sulfur note that can be pleasing in a small dose to apple aromas, but when combined with a base of molasses can become unpleasant. We are looking to capture and propagate specific wild yeasts in our area. Of particular interest to me is the natural yeasts that bloom on roses. We have some lovely heirloom roses on our farm and rose yeast bloom is known to create some lovely esters in fermentation. Have you ever seen a rose bush at the end of a row of grape vines? It serves many purposes, including being a health indicator, but one of their traits is encouraging of their yeast bloom. Rum producers often talk of wanting a hint of rose on the nose and it is seen as a sign of quality so that has motivated our interest in the project to a new level lately.


There is an ongoing discussion concerning the Jamaican GI and the proposed Barbados GI. Part of this discussion is the restriction on using Saccharomyces Cerevisiae only, as it could result in less diversity, according to some. Can you tell us why this should or shouldn’t be an issue?

When this became a hot topic my phone nearly exploded with messages. I was of course honored that people feel they can turn to me with their geeky yeast questions. Yeast is a specific area of study and passion of mine. It's very heart warming when people recognize the love and effort I've put in there. In fact, I am really honored that I have been asked to be a Keynote Speaker in Montreal at a yeast physiology seminar in September. It really means a lot to be recognized as capable in my yeast knowledge, it's not often the first thing people think.


When folks reached out, especially journalists, I was clear to say I am not the one to quote on these issues and that cultivating relationships and adding contacts in the Caribbean to one's professional network is important, not brand people, actual locals running equipment or working in a lab. I personally treasure these peer relationships as we can share our experiences, knowledge and support each other. They also have a perspective that is authentic when questions like this pop up. I'm surprised how often I read an article that does not bother to contact and quote these folks - worse, writers can go on to project or assume how someone might feel rather than asking them.


So, I knew what my academic side was telling me when this first piqued people's interest but wanted to tread thoughtfully and defer to the experts in those regions, like Joy Spence, as I try to be careful not to assume I know all the details of their situation and local concerns, even if I might know a bit about the science informing the buzz I was hearing. Joy is a total badass, skilled chemist (she holds multiple degrees and taught chemistry for goodness sake), and has lived experience few distillers can dream of - so I knew she would be a key person to speak intelligently and holistically on it.


It was extra reassuring in getting a chance to ask Joy further about this and confirm that my gut and professional training were correct and it was as clear as it seemed - what started this buzz was a dump of misinformation. The fact is that every Jamaican rum producer, back in 2016, cooperatively and proudly agreed on the GI, which of course told me that there was no way this was banning wild yeast.


When it comes to the pro GI voting producers I simply thought - we know these people, we know their passions, we know their actions align with their words, and this information on banning wild yeast was not coherent with everything I have ever seen them practice or preach. The thought they would ban all natural and traditional yeast felt like a giant leap as far as the simple principals of fermentation in the reality of their production technique and seemed completely the opposite of reality, which was disorienting and frustrating - personally, in the US, we're a bit exhausted and frayed by fake news. I was surprised to see some folks entertaining the misunderstanding, but it served as an opportunity in highlighting how much basic understanding is still lacking in spirits production.


It is wise to remember this is the life's work and lived experience of these GI supporting Jamaican producers, they are not unwise or foolish, rather they are guardians of their culture and community. No one needs to benevolently jump in and fight to save Jamaican rum from itself, they are protecting it just fine themselves and the GI laws are set up to do just that.



Back to the question more directly, I know I am rambling a bit, the simple fact is that wild and natural yeast were in no way ever banned. In fact, not only were they not banned but they are being actively protected. In so many ways this GI protects the local wild yeast biome, but for a basic example remember what I said about the wood fermenters. The historical practices of wild yeast were never to be eliminated. With basic understanding one can see that to eliminate wild yeast this would have meant that all molasses wash would have to be pasteurized and sterilized and then fermentation would have to only take place in sterilize, sealed steel fermenters with all sterilized pumps, hoses, etc. This is clearly not what has been happening since the implementation of the GI.


The GI only served to capture the traditional, cultural, and iconic flavors that come from the use of Saccharomyces Cerevisiae when purchased and pitched yeast are added to a fermentation. All natural occurring species, in all their variety and glory, are free to thrive as always. As mentioned, Saccharomyces Cerevisiae has thousands of strains, each producing their own unique character. This yeast tradition has been common for a very long time in Jamaica, it is what we all know to communicate the sense of place in Jamaica when you take a sip of fine Jamaican rum.


Barbados, understandably, is aiming to do the same in protecting their iconic flavors and hard earned name of quality under the title of 'Barbados Rum' and, in so far as I know from my own direct relationships Mount Gay, Saint Nicholas Abbey, and Foursquare are all in full cooperation and seeking the same goal for protections for the quality and heritage in creating the Barbados Rum GI standards.


To be even clearer, Jamaica and Barbados GI are not banning the use of any yeasts in any way. If one wants to use GMO yeast, or any other species to inoculate and pitch into their fermentation they can have at it, no one is stopping them. There are no limits like there are in say Austria where the use of certain yeast is banned nationally for wine production to protect quality and reputation from those seeking high yields at the expense of flavor. Rather, the GI is simply saying that if you want to innovate and express character that is newly distinct and not what is iconic to the essence understood as Jamaican rum, then go ahead and innovate a new style name for your expression. If you do not want to participate in the community standards and cultural practices then you do not also get to demand instant access to leverage that community's and culture's hard won reputation for excellence. You culture a yeast, and culture can also means community expression after all.


But again, I'm clear to share facts as my thoughts are not the important ones to center when it comes to Jamaica and Barbados, I'm happy to give visibility to and help champion the perspective I have been given by the cooperative community of distillers living there. It's been important to me not to get my information from only one producer or brand, it is more informing and exciting to see the community expression of support when you connect with a number of the producers and especially the distillery staff who live it every day.


Joy has said the spark of these conversations was a misunderstanding and I deeply admire her grace. Others, understandably, are more upset as misinformation could serve to destabilize and bring the GI into question based on something that is easily shown to be untrue. Sadly, some of the damage has been done as far as the misinformation on this topic and some confusion has taken hold. I've seen it in conversations recently so I am happy to share what I do know of it even if I'm rambling at this point.


My two biggest take away facts from the experience of this discussion have been that credibility is important, and I learned a lot about looking at power dynamics underlying a discussion.

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