[Since writing this I’ve done a ton of experimentation and learned quite a few things. A muck pit can be approached with different objectives and possibly you should have multiple, each pursuing an end. One version of a muck pit may maintain a very sensitive aroma-beneficial bacterial culture that is in danger of getting displaced by unproductive bacteria. This culture is a “closer” and can be added at the end of fermentation. Another version of the muck pit may be to peptonize yeast and create a nutrition source for future yeast growth. The next objective may be scavenging aroma beneficial acids to boost esters. Each objective may require different amounts of maintenance in terms of feeding and adjustment as well as opportunities for processing steps to make them more effective. As far as bacteria goes, aroma-beneficial bacteria can be displaced easily and we need to learn how to both prevent contamination and regenerate it. Muck may seem dirty but it may need to be approached with the same sanitary precautions a clean ferments sees.]
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This muck processing post is going to leave a lot to be desired because I cannot say much yet about productively using muck. I’m also not truly sure how each distillery currently processes and uses it, but I’ll share my ideas. Even if you are not actively using muck, some of these ideas may be relevant because elements of muck are still part of your ferment, especially if you are leaving fermentor bottoms.
Muck is the spent lees from ferments and possibly even settled dunder, but most anecdotal evidence I have is that muck is the product of yeast bottoms from ferments instead of yeast cooked during distillation. This undergoes a putrefactive “fermentation” most likely with lactic acid bacteria (LAB) as the dominant organism which peptonizes the dead yeast.
At the same time Jamaica was developing this technology in the lathe 19th century, so was E. Bauer in Europe to make yeast extracts for the brewing industry. The three main products of peptonization are insoluble yeast ghosts or husks, soluble peptone which is a nutrient & nitrogen source, and finally long chain fatty acids which were some times called Bauer oil. A fourth wild card product, I have yet to observe, it conversion by bacterial action of amino acids into short chain fatty acids like butyric and that may be a goal of introducing other bacteria than LAB to a muck pit.
I have observed muck that can smell distinctly fruity like peach. My muck has always been much more pleasant in aroma than my dunder. Some producers mix muck and dunder, but so far mine ends up being fairly pure because of centrifugation. [Something I should add is that as my ferments have grown progressively heavier and featured a variety of volatile acids, I’ve started to notice the aroma of butyric and/or propionic acid in my muck.]
After ample time and peptonization, under the microscope all cells should stain blue as dead and muck can be centrifuged to separate the ghosts from the peptone broth. After the first centrifuging, these can be rinsed and centrifuged again to remove all nitrogen products. A formol titration can easily prove there is no significant nitrogen in the ghosts. The wine industry currently uses ghosts as an additive to reduce stress in high gravity musts. Ghosts contribute sterols and lipids to the fermenting yeast as well as absorb fatty acids which may be their chief mode of reducing stress. They appear to be added at a rate of 0.25-.0.5 g/L (dry basis), but no doubt a ferment intending to be distilled may have different and likely higher scaling. However, there is warning from the wine industry of off aromas at higher usage levels.
If fermentor bottoms are left to accumulate after subsequent ferments, a percentage no doubt become ghosts with minimal nitrogen contribution and/or viable yeasts. These also accumulate at a rate that may be higher than the wine industries scaling. My understanding is yard sticks are used and fermentor bottoms are only allowed to be a certain depth. Distilleries that use this practice also likely do not centrifuge their molasses so the bottoms also likely contain a lot of molasses junk.
If we are creating research scale ferments to evaluate the volatile acid tolerance of a yeast, such as will be used at full scale up, usage of ghosts may help approximate production conditions where bottoms will be left for the next ferment.
Liquid centrifuged from the ghosts may have a considerable nitrogen content and can be used as a yeast nutrient. A formol titration can be used to calculate the YAN for scaling purposes when it is used. Kervegant did however warn that using peptone or yeast extract can increase fusel oil, so this part of muck should only be used with a below average fusel oil producer such as a fission yeast. Peptone may also have its Bauer oil acids salted out or removed by distillation because they can be antagonistic.
The highest value part of muck may be the Bauer oil and there is a lot of potential modernizing that could be done here. Ashby does not use the explicit term Bauer oil, but explains that lime was gradually added to the muck pit to salt out these acids. These insoluble salts were then transferred to the vinegar cistern to create flavour where acetic acid would liberate Bauer oil acids from lime salts. This flavour was then eventually added to a ferment, but it is not clear whether it was at the beginning or end, prior to distillation. My understanding is that no one currently uses the process described by Ashby at the beginning of the 20th century.
Productively employing Bauer oil gets tricky. We know high value esters can ultimately be created from these acids, but we also know these acids can have an inhibitory effect on both yeast and bacteria that is synergistic with acetic acid. If too much flavour was added at the beginning of fermentation, it could conceivably halt everything. If it was added just before distillation, it may not benefit from bio transformation to an ester and would rely on brute force esterification in the still which is not as powerful as you would think. If added just right, in conjunction with acetic acid, there would be selective inhibition of all but a fission yeast as well as maximized bio transformation to ester.
Something to consider is what volumes a production would be producing and what the product ratios will become. Experiments will also have to be conducted to determine how much Bauer oil is produced, at what scaling it can be added, and when. Salting with lime is also a delicate process and I’ve found that until you get empirical data to draw from, it is best stretched over a multi day period. Muck processing may not be something worth attempting until you can add analysis to each component using microscopy, acid titration and formol titration.
As the products of muck become clear, there are many modern ways to update this process. Instead of LAB peptonization which produces a lot of fixed acidity to deal with, Kervegant described a process where rum distillers pressure cooked their lees in the presence of a volume of sulfuric acid that would be used for their next ferment. Sulfuric acid is not desirable with fission yeasts and a modern alternative may be using concentrated acetic acid proportional to what may be added to a subsequent ferment.
If an LAB peptonization process is used, a modern update to be explored is vacuum distillation to collect Bauer oil acids so that they may be more precisely recycled to subsequent ferments. Volumes may be small enough and products high value enough to justify a rotovap.
Muck processing may be increasingly important if ferments are centrifuged before distillation, as favored by Rafael Arroyo, to reduce the incidence of tufo and facilitate distillation at a lower ABV.
Long duration ferments may also be different than short in terms of how the distiller extracts aroma from yeasts. In a short duration ferment, the aroma is likely bound to the yeast and possibly liberated by distillation. In a long duration ferment, much yeast has already died and had their aroma bled into the ferment. This would be continued by leaving a fermenter bottom. There would be less to be gained by distillation on the lees. Effective muck processing and centrifugation before distillation may allow distillation at lower ABV, capturing more high value aroma, because aroma-negative tail products are less of a liability. However this may need to happen in conjunction with a fission yeast which produces less fusel oil than a budding yeast.
These ideas will evolve as we gain more experience. I have already started examining the volatile acids in my muck with the RD80 Cash still and hopefully I can gain some insights on timing and magnitude of Bauer oil liberation during LAB peptonization. I have also successfully used the birectifier to analyze muck.