Groom of the Stool – A Cautionary Tale

Content warning: contains bad puns, even worse "humor", and a light sprinkling of mild profanity

As my regular readers know, I like to brew things using wild yeast found in odd places. Whether its the guts of bees, pulled from the air, or from random objects, I tend to find yeast in unusual places. I’ve also been known to enjoy the odd pun, a touch of sarcasm, and occasionally allowing myself the mental maturity of a three year old.

So imagine my joy when I conceptualized a new brewing adventure. One that would harvest yeast from a very unusual place, make people squirm, was completely childish, and would make a beer with one hell of a backstory. And imagine my sorrow when it failed miserably. And failed in a way which 10 minutes of reading could have avoided.

So join me in my quest to brew a beer from shit.

The Backstory

gif of a caterpillar going into a crysalis
Henry the 8th going into a chrysalis. Boudica watches.

I have a young son in whom I am trying to foster a love of science and nature. Which means that we usually have a few home experiments running at any particular time. One of this summer’s projects was raising Monarch Butterflies. Watching the pinhead-sized caterpillars grow to 5 cm worms, turn into a chrysalis, and hatch into butterflies, was something I’ve wanted to do my whole life. If anything, my son was even more excited and would spend hours watching the caterpillars do their thing in their little terrariums.

While I knew they would grow quickly, I didn’t appreciate that this quick growth would be accompanied by the prodigious production of frass. Seriously – a monarch caterpillar easily generates it’s own weight in crap every 12 hours. We constantly had to clean out the bottom of our terrariums to keep things presentable.

wild yeast free frass in a terrarium
A large pile of crap…and the caterpillar that made it

One evening, a few beers into the night, I found myself cleaning caterpillar dookie for what had to be the ten-thousandth time. To help pass the time I was watching a documentary that happened to be on the Tudors. And their court. With a focus on some of the worst jobs in history, this documentary informed me of a job I did not know existed. Apparently, an important position in the medieval English Court is the Groom of the Stool – a trusted confidant of the king. . .who wipes the kings ass (and I’m not talking about the kings donkey).

This is something I learned, not unironically, as I too was wiping my Monarch’s proverbial buttocks. The alcohol and the irony were more than my poor brain could handle, and in lieu of a stroke, my brain instead made the best (or worst) connection possible. Insects have intestinal tracts full of wild yeasts, the Monarch Butterfly is the royalty of the insect kingdom, and I – a hunter of wild yeasts – found myself grooming the butterflies stool.

So we had wild yeast, more puns than I can shake a stick at, and a yeast source not for those of a delicate disposition. How could I resist?

Where Be’ith The Wild Yeast?

Nero, out for a walk.

The next step was obvious. I prepared a wort from dry malt extract. 1.040 specific gravity, pre-acidified to a pH of 4.2 to help suppress pathogens. I set up a dozen test tubes, 5 ml of wort in each, and into each added the turds from each of our caterpillars. Caterpillars we had dutifully named after some of the worst Monarchs in history. I had high hopes for Nero’s fecal donation, while my wife was rooting for George the third.

2 days later…no fermentation

5 days later…no fermentation

25 days later we released the butterflies…but still no fermentation.

Inspections under the microscope found a few desultory bacteria, but not much else. So we gathered up a dozen more caterpillars and attempted round 2. Same setup as last time, and the same result – nary a yeast cell to be found. Even pooling the 24 samples in a lower-gravity and non-acidfied wort led to nothing but a bit of mould.

24 caterpillars – each named after one dubious royal, mountains of butterfly dung, two months of work, and no yeast.

Where are the Wild Yeast?

Nero, whom while pretty, is a pretty shitty source of yeast.

To figure out what went wrong, I did what any competent microbiologist would have done before starting the experiment. I read the scientific literature. Not that there is scientific literature on the use of caterpillar dung to produce beer. But there is scientific literature on the caterpillar microbiome.

Turns out, a microbiome is a thing that they don’t really have.

That prodigious growth rate that caterpillars experience comes along with an enormous appetite. Which the caterpillar satiates by eating several times its volume in plant material a day. Which means that material moves so quickly through a caterpillars gut that there isn’t an opportunity for yeast – or anything else – to grab hold.

Yep, caterpillars are literally turd cannons that fire dookie through their gastric track so quickly that they wipe themselves clean. Neither toilet paper, nor grooms of the stool, are required.

The Moral of this Story

Does this story have a moral? I’m not so sure it does. I did learn that I know far fewer synonyms for fecal matter than I thought I knew. I also learned that microbiomes are, perhaps, not as universal as I had thought. And maybe – just maybe – that formulating scientific (or, at least, science-adjacent) experiments while several beers into the evening is not the best of ideas.

And, as I always tell my students, “research your experiment first”. Looks like I should take my own advice.

2 thoughts on “Groom of the Stool – A Cautionary Tale

  • November 2, 2021 at 11:53 PM

    A couple of months in the laboratory can frequently save a couple of hours in the library 😉

  • October 23, 2021 at 7:55 AM

    Excellent reading!
    I enjoyed this contribution immensely.


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