This post was motivated by a letter read by Cade at the beginning of episode 65 of The Brü Lab. The letter was from a brewer who was harvesting their spent yeast from ferments, and then pitching that into the boil as a yeast nutrient. The letter was rather long, but the main points the letter writer made were:
- Dead yeast should contain the ideal mixture of nutrients needed by yeast.
- Boiling will sterilize and release the nutrients.
- Boiling will break the proteins in the yeast down into amino acids.
But are these claims true? Can yeast simply be boiled to make a complete nutrient? I’ll tackle each of these claims, below.
Ideal Yeast Nutrient?
The first claim is that yeast contain an ideal balance of nutrients for other yeast. Is this claim true?
There are three issues with this claim:
Issue 1: Nutrients can be converted into other molecules. Its not enough that a nutrient be present; it also needs to be in a form that can be taken up and used by the yeast. But nutrient are often metabolized into something else, and thus are no longer present in the usable form in spent yeast. The main example of this is oxygen, which is why its critical to oxygenate even the best-fed yeast. There are a number of other examples where this is an issue (e.g. iron). Normal practices (aeration) and other nutrient sources (malt) render this is a minor issue.
Issue 2: Not all nutrients are bioavailable. Even if a nutrient is present, it may be in a form which is not accessible. As an example, yeast can easily take up amino acids (the building blocks of proteins), but cannot take up intact proteins (made of strings of amino acids). Yeast also lack the ability to break down extracellular proteins into amino acids. In practical terms, this is not an issue for beer brewers as wort is rife with amino acids. This is an issue with wine, cider, and mead.
Issue 3: Nutrients are consumed. This is the biggest issue with this claim. A lot of nutrients yeast need are consumed by the yeast. This means that the amount of the nutrient required for yeast growth exceeds what remains in the yeast after it is consumed. As an example, yeast retain only a few percent of the thiamine they consume – most is converted into DNA and other cellular components.
Sterilizing and Release by Boiling
The second claim is that boiling the yeast will sterilize the yeast and release the nutrients. But is this claim true?
Answer: Yes, mostly
Boiling the yeast will not sterilize them, but it will sanitize the added yeast to a point where the risk of contamination is minimal. And, of course, maintaining sanitary practices when harvesting and using the yeast will reduce risk further.
As for releasing the nutrients, this is where the “mostly” comes into play. A lot of the nutrients in yeast are water-soluble, meaning that breaking open the cell (which boiling will do with aplomb) is enough to release them. But there are many nutrients that are poorly water-soluble and thus will remain in the yeast “skeleton”. In addition, some larger water-soluble molecules will not be freed simply by boiling as they become trapped in the cell wall or in the interior of the cell.
Protein Hydrolysis by Boiling
The letter writers final claim is that boiling yeast will break proteins down into amino acids, allowing them to be absorbed.
But is this claim true?
Answer: no, No, and NO!
This is the largest flaw in the letter authors claims (and the claim which sparked this post). Breaking proteins down into amino acids is a process called “hydrolysis”. Hydrolysis can be driven in a number of ways – enzymes (proteases) can catalyze the breakdown of proteins, as can acidic conditions. However, heating along is not enough. And its a good thing too, as if heating broke down proteins to any meaningful extent, cooking food would cause the food to disintegrate. Your BBQ’ steak would be a pile of goo instead of a (hopefully) succulent piece of meat.
Can these issues be overcome?
So can spent yeast be used as a nutrient? Of course it can – simply boiling some yeast as the letter writer suggested will provide some useful nutrients for the yeast. It won’t be sufficient to support the fermentation of something challenging like a traditional mead or sugar wash – but it will provide more than a simple nutrient like DAP.
There is a way to get more nutrients out of spent yeast – autolysis. This is how nutrients like Fermaid O (and Marmite/Vegimite, for that matter) are made. The concept here is easy – yeast contain the enzymes needed to break down nutrients that are in a biounavailable form (e.g. proteins, DNA) into forms that are bioavailable. The challenge is activating those enzymes to release these nutrients.
By “challenge” I mean “easy as pushing a button”. All you need is a instapot, souse-vide, or other device that can hold the temperature at 50C (122 F) for 24 hours. Yeast will die quite quickly at this temperature, but their enzymes will not. This releases the normal control a yeast would keep on these enzymes, allowing them to break down proteins, lipids, DNA, and other molecules not normally bioavailable in boiled yeast into a form which is bioavailable. This also breaks apart the yeast cell sufficiently to free all the nutrients. I’ve been playing with this at home, and so long as you don’t need a large amount of nutrient (e.g. like you need for a mead), it works very well*.
* Because yeast bind strongly to hop acids, spent yeast from beer are often quite bitter. Without debittering, this carry-over is detectable in meads. Debittering is less-easy than autolysis and I’m still working on a home-friendly (and safe) way of doing it.
Making Yeast Extract
Step 1 – Harvest yeast: Yeast from any beer can be used, but younger yeast will have more nutrients. I avoid yeast from heavily hopped beers for the reason stated above.
Step 2 – Wash the yeast: This is optional, but I’ve found that resuspending a yeast cake in 2-3 times its volume of clean water reduces flavour and colour carry-over.
Step 4 – Dilute the slurry into a thick liquid: Once your yeast is washed and settled, remove the liquid portion (I use a small siphon). This should leave you with a thick paste, similar in consistency to ketchup. Add clean water until it is liquid enough that you can pour it, but keep it as thick as possible. This step can be skipped if the yeast slurry is already pourable.
Step 5 – Heat it: Pour the yeast into an instapot, or seal in a jar an immerse in a water bath heated by your souse vide, and hold at 50C/112F for a full day. It will change colour and become runnier as autolysis progresses. In 24 hours the extract will be done.
Step 6 – Freeze or dehydrate: If you’re going to use the extract right away, add 1-2 tablespoons of the liquid per 20L/5 gal of beer, cider or mead. Bacterial will love to grow in this extract, so care needs to be taken if you want to store it. I’ve found two ways that work well: To freeze, place a sil-pad or parchment paper on a cookie tray,and spread the extract evenly across that. Freeze, then break into pieces and store, bagged, until needed. To dehydrate use a food dehydrator on its lowest temperature setting. Once dried, this is roughly equivalent to Fermaid-O and should be used at similar doseage rates.
Credit Where Credit is Due
I’ve been working on making yeast nutrient at home for about 6 months now, and I’d love to claim I came up with this approach on my own…I didn’t. Adam Ragusea did a video on making extract for food purposes back in late 2021, which is what kicked off my own experiments. Check out his video: