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Osmotolerance is basically the ability of micro organisms to tolerate stress. The stress comes from solutes dissolved in a solvent. Yeast stressors (or lack there of) are probably one of the most significant ways by which rum fermentations differ. In many naive cases, the stressors select the yeasts and in cases where you pick the yeasts (team Pombe!), you need to follow up with appropriate stressors (if you cannot work completely sterile).
Osmotolerance relates to the Delle stabilization concept I’ve talked about before. The concept uses the simplified rule of thumb where a wine becomes stable at 18% alcohol. Then according to professor Delle, each percentage point of sugar can offset an amount of ethanol and you can start to achieve stability at far lower alcohol levels. If you add carbonation on top of both, you can start to achieve stability even lower and that may be the secret of wines like Moscato d’Asti which are often stable at 5%. The point here is that all these variables function in a system of stressors and changing one variable, changes response to the others.
All these variables exist in rum fermentations and then some. Rum in most cases isn’t just one single yeast or bacteria but a mixed community. Changes to the stressor matrix shifts the ability of any member of the microbial community to grow at all or even to become dominant. Schizosaccharomyces Pombe yeasts are known for their osmotolerance relative to budding yeasts and in many cases, though they have a lower frequency of occurrence, they can become dominant in spontaneous ferments. Before anyone attempts to play around and go huge hoping for greatness, remember, it is easy to create a ferment so stressful near nothing beneficial grows but bacteria you don’t want and you end up with an unpredictable sluggish brew working so slowly your economy goes to hell, half the vats stick, and you quickly go out of business. Rum magic only happens when you know what you are doing so you can walk that magic line.
If common clean rum is being made stick to common clean and never allow things to drift in the direction of making flavoured rum in the pious hope that you may wake up some day to find that you have become famous by making flavoured rum where it was never been made before. You are much more likely to find an infuriated Busha awaiting to tell you that your services are no longer required on that estate.
Playing with osmotolerance is like playing with fire. Arroyo actually didn’t want to play the game and went in the other direction pioneering molasses pre-treatment and creating conditions where ferments could produce extraordinary aroma while fermenting to high concentrations of ethanol with great economy and in record time.
Arroyo went osmo-intolerant by heating to sterilize molasses just like a grain mash, but with modified pH and calibrated buffers to preserve aroma. He then somehow got a hold of an Alfa Laval pilot plant continuous centrifuge (in the late 1930’s!) and clarified the molasses. This changes the stressor matrix and it also sets up the ferment to be distilled in a continuous column still where scaling is much more of an issue than a batch still. Molasses pre-treatment became a rule of thumb to anyone using a continuous still. No longer related to osmotolerance, Arroyo also employed the same centrifuge again pre-distillation to remove the lees as well as dissolved gases. Unlike other spirits such as Cognac, Arroyo didn’t even distill heavy rums on their lees.
I recently contacted Alfa Laval and am trying to get more information on their continuous centrifuges and what exactly they sell that is pilot plant scale. They have models, seemingly small, but are for tasks like centrifuging bio diesel and not molasses. Alfa Laval sells to all the big Kentucky distilleries who centrifuge their stillage to remove water and prepare it to become animal feed. They also sell to very large breweries who centrifuge their beers to gain economy from the bottom of the vats. I have yet to find out conclusively, but I’m estimating a pilot plant continuous centrifuge for distillery tasks may cost about $30K. A barrel a day distillery would still have room to grow into their pilot plant scale equipment. That cost, only on the hunch that it is really beneficial, is very hard to swallow. They promised me more information so hopefully I can update this with something optimistic.
The big takeaway is that so many of the rums we know, love and are inspired by are the products of these very serious centrifuges. Small distilleries will have a lot of trouble going osmo-intolerant (my arbitrary term). If a small scale, low involvement distiller says they don’t like the effect of centrifuging or any molasses pre-treatment, they basically have no clue and just need to accept their limitations when being “small batch”. Another category of rums are naive rums, endearingly produced by people who do not know their options and some of these rums are the most extraordinary and tell the best stories. Distillation requires certain scale and the new arm of the industry is slow to accept that.
As I always say, there is nothing finer than rum as we make it and no category of rum ferment is superior to another. Osmo-intolerant is the direction rums are commonly taken when pure yeast cultures are used and when economy is a large consideration. These rums are more likely to be distilled continuously and they are more likely to be lower risk over all. Due to a few other really cool reasons I’ll get to eventually these spirits will also age much quicker.
There are very few spontaneous ferments these days, but due to techniques like back slopping of yeast, exotic starters, and the usage of bacteria infected dunder, some ferments can use osmotolerance to create a sort of chaotic timbre. Stressors will effect the growth kinetics of the varied microbial community that eventually develops. The pure yeast culture that kicks things off at the beginning of the season may eventually be supplanted by a wild yeast that rises to dominance under the conditions encountered. Big windows for chaos through which we glimpse terroir, are opened by producers both consciously and unconsciously. There is risk, chance, and irrational energy, the duende!, all over the place. This category is a place for both the naive and the truly masterful. The most masterful of wrangling glorious chaos these days is probably Hampden estates in Jamaica which is known to be very significant to the Smith & Cross blend. I’ve aspired to make a similar rum, but don’t think I can do it until I really explore and master all of the analytic techniques. I request 20 years.
Stressors reduced by Arroyo style molasses pre-treatment are mainly gums and ash. The pH is also adjusted to be optimized for the selected yeast. Total sugars are increased due to the decrease in volume of the precipitated and separated fractions. Because the yeast can now ferment to higher concentrations of sugar, they can also better take advantage of the nutrients so less need to be added though they are often carefully calibrated. Dunder, on the other hand, though it leads to an accumulation of gums and ash, also brings yeast nutrients. It probably also brings nefarious copper salts leached from a copper boiler under acidic conditions, but I don’t recall seeing research specifically tied to that yet.
So dunder itself brings brings stress and relief. Many dunders were and/or are ripened to accumulate aroma beneficial bacteria. The most desirable being Clostridium Sacharo Butyricum. The byproduct of bacteria’s metabolism is fatty acids and those can stress the yeasts by lowering the pH. They can also stress the bacteria themselves.
In the Arroyo method, the pH is carefully adjusted to remain constant, and far higher than you’d think which is possible because of the pure cultures he employed. As pH decreases, alkaline substances are carefully added to lock up the acids as salts. The acids that have the most affinity for salting also happen to be the most ordinary like acetic and formic. These ordinary (as opposed to extraordinary cough cough frequency of occurrence!) acids and their ethyl esters are typically in part separated during distillation. Having less to remove due to salting means the heads fraction can be smaller and the spirits will mature faster. Spirits going both ways with osmotolerance can benefit from the pH buffering / salting method but spirits produced using the Arroyo method are more likely to employ it.
If the pH were allowed to run away the accumulation of stressors would slow down or completely shut down various actors in the microbial community. This can be a feature or a flaw. Low pH ferments can produce lighter spirits because bacteria has less leeway to act.
These ideas were definitely not new to rum making and go back to Jamaica well into the nineteen century. The great Agricola, W.F. Whitehouse, (father of modern rum according to me) mentions how alkaline lime was introduced to Jamaica by Dr. Bryan Higgins. It became central to the operation of a muck hole. The contents would undergo putrefactive fermentation producing acids until the pH dropped too low maxing out the osmotolerance of the bacteria then lime would bring the pH back up and fermentation would restart. The process would go on and on. The locked up aromas would unlock when combined in a ferment with another acid, the most ignoble acids fortunately having an affinity for staying locked up.
But don’t forget, though salts buffer the pH, they are also a stressor and yeasts and bacteria will have different resistance to salt concentrations. Another big source of osmotic pressure is ethanol, and Arroyo reminds us we can’t just think of alcohol tolerance itself because it is always relative to temperature. Yeasts can resist the osmotic pressure of ethanol much better at low fermentation temperatures than high.
Arroyo re-imagined and re-applied all the concepts with more finesse and calculation. Not all fatty acids and not all esters are created equal and Arroyo more than anyone else at the time kept his eye on the prize of creating and selecting for the most extraordinary and suave (his favorite rum descriptor). The concept of osmotolerance is at the heart of coaxing all of it out.
I’d love to work on this more but I’m out of time