Yeast Harvesting Q&A
This is a recap (paraphrased)of the yeast harvesting Q&A from the EHG Discord channel on April 20th 2020
Q Is it horrible to just spoon it out of a fermentation bucket directly into your wort if you've prepared to time your brew day in that manner?
A Not horrible, but definitely not the most ideal way. There are a couple of things that you'll sacrifice by simply reusing yeast from the bottom of a previous fermentation, but this probably won't matter at the homebrew level. First is sanitation. The microbes you're picking up by transporting yeast through the open air into the next fermenter is an issue, but it's not like commercial open fermenter breweries aren't doing the same thing. Granted, they have special negative pressure rooms with filtered air, so it's not quite the same as your dank basement. The real concern is repetition. A lot of the microbes that are going to contaminate your yeast are going to be outcompeted, but will start to gain a noticeable foothold after repeated fermentations. As long as you're only doing this once, infection shouldn't be a concern.
The other issue you'll need to consider is the amount of dead yeast cells and trub that you'll be carrying with you. A rule of thumb is that the lighter coloured yeast is the stuff that has the most live cells. You'll still be picking up a significant amount of trub that will oxidise almost imediately, though the effects won't be seen for a week or so. Again, repeatedly transferring yeast this way is what will do you in as autolysis and lipid oxidation turns your beer more and more quickly into cardboard.
Q When collecting yeast, do you fill the container all the way to the top or leave headspace?
A Since oxygen is the "wake up" signal for yeast, you want to leave as little oxygen in the container as possible. Obviously, the container should be non-porous and able to be sterilised, such as glass, but you also want to be able to seal it. Less headspace is always preferable. At a brewery setting, if they're collecting yeast for reuse, but need to store it, it's always under inert gas, such as CO2 or N2.
Q What's the process for washing yeast?
A In brief, don't. In a brewery setting, it's considered a process of last resort - as in, "I was planning on dumping this yeast, but my pitching culture isn't ready and I've got something in the boil kettle right now". Washing is stressful for the yeast and provides no protection against wild/mutant yeasts. The theory is that it lowers the pH to a level that yeast can tolerate, but other microbes can't. You also need to do this as close to 0C/32F as possible, stirring constantly and for between 2-4 hours. If you aren't set up to be able to do that, don't bother, just repitch and hope for the best. Plan better next time.
Q I spun up a crop from a Belgian Wit that was a year old and got no off flavours. What did I do right?
A You used a starter and you pitched a PoF+ strain. Generally when brewers talk about mutations, we talk about three areas: flocculation characteristics, carbohydrate consumption and respiratory function. Flocculation is obvious, there are multiple FLOC1 genes and losing or gaining copies is a common mutation. Carbohydrate consumption is usually in relation to the ability to break down melibiose, usually happening with the var diasticus strains and then respiratory function tends to happen over successive generations where yeast can't use oxygen and so build up mass very slowly. Production of "off flavours" doesn't really show up in a single mutation and just keeping yeast in the fridge dramatically slows reproduction. Some strains are simply better able to withstand long periods of cold and using a starter builds up the viable portion. Now if you had pitched it directly, you would have likely seen a very different result.
Q Is harvesting from an 8% abv beer worth saving?
A Probably not. The yeast will have been under a significant amount of stress. The portion you collect will already be less viable and storing decreases that viability even further. That's not to say it won't work, but it's probably not best to plan a brew day around using that yeast.
Q Can you talk about Imperial or Escarpment's 200 billion cells packs?
A It's basically double the cell count of Wyeast's smack packs. The real number to be concerned about is the cells/mL/°P formula. You generally want 0.75 million to be the cells number for an estery ale, 1 million for a high gravity or clean ale, double that or more for a lager. Wyeast says it delievers 6 million cells/mL for a standard 19L batch, which is correct on the day they make it. Divide that by 10°P, or 1.039 wort, and you end up with only 0.6 million cells/mL/°P, less than you actually need on the lowest end. Obviously, the 200 billion cell packs will be double that, at 1.2 million cells/mL/°P, but even then, that assumes you're brewing at 1.039. Move it closer to an IPA, for example, and that number drops to a cell count per mL akin to an estery ale. In short, always make a starter.
Q Top cropping, yah or nah?
A Yes. Some of the best breweries in the world still top crop and have preserved their specific strain for generations. Now, it's doubtful that the strain is mutation free, but thus far, the yeast that have colonised the beer brewed at these locations has been successfully domesticated. With top cropping, the same rules apply - chuck the first skimming as it contains trub and hop debris, collect the second skimming. At that point, these breweries are propagating it either directly into the wort for their next batch or building it up. At the homebrew level, you'd have to brew every day in order to make this work, so you'll be skimming it for reuse later.
Q How much wort should you use to bottle condition?
A It depends on gravity and carbonation desired. 20% is great for a moderate gravity hefeweizen. Half or even a quarter of that for a British style ale. There are plenty of krausening calculators on the internet.
Q If you're overbuilding a starter to keep for propagation, how do you divide it?
A I'll do a cell count to calculate cells per mL and then divide based on volume. This works best if it's on the stir plate as you're brewing so you have an even division.
Q With starter cropping, how many times can you reuse?
A Theoretically, indefinitely. At the brewery level, it's called a shunt. It's not considered best practice because the moment you do that, you lose the ability to say you've grown it from the Master Culture, which is the preserved slant you took a single colony from and confirmed was genetically identical. Building a pitchable culture from a single colony takes 2 weeks however, so it's a lot less time to do a shunt from a brink. The thing is, it's one of those procedures that every brewer "knows" you shouldn't do because you could potentially stray from the Master Culture, but there's been zero research on how quickly mutations arise doing it. Based on what we "know", it's theoretically much better to do a shunt than to reuse yeast from a fermenter, which is standard operating procedure in most breweries.
Q It sounds like people are cold crashing starters and decanting the liquid off, but isn't changing the temperature stressful for the yeast?
A Not really. Yeast are well-equipped to flocculate in response to low temperatures and it's part of their cycle. It would be more stressful to keep them at room temperature and "awake" as they'd burn through their reserves and lose protection against the alcohol in the starter. on the other hand, decanting the liquid to remove esters or other yeast metabolites isn't really necessary. If your culture is done a day or so early, put it in the fridge. If not, keep it spinning.
Q How long can a jar of yeast last in the fridge?
A It's hard to answer that question because as soon as someone puts a date on it, someone else will have a story about how they went past that date with no problems. I've had yeast that have died after 3 months. I've had yeast that have spun up after a year. It's species depended as well as how well prepared the yeast were when they went into the stationary phase. All anyone can say for sure is that the longer you leave yeast in the fridge, the lower the viability.
Q If the starter starts foaming over, should I kill the stir plate?
A Get a bigger flask.
Q Are there yeast strains that are easier to harvest from a beginner point of view?
A Wheat strains seem to be the easiest to collect from the top, British strains from the bottom. As long as you harvest soon after fermentation, you should be fine. The longer you wait, the lower the viability and the more chance you'll develop off flavours
Q Can you repitch directly onto the yeast cake in secondary?
A Can you? Yes. Should you? Probably not, but this depends on how quickly you're drinking your beer. If you pitch directly on top of the yeast cake, you're keeping all of the cold break and hop debris that was in your last beer. It'll decompose and give you that goaty flavour - especially since you're oxidising it by taking the old beer off first.
Q Is it time for a beer?