Harvesting and Re-Pitching Slurry

At some point in a homebrewers life he or she starts thinking about re-pitching yeast from prior batches to save money, for which there are countless tutorials online. You can opt to wash yeast slurry left behind after a previous batch. Or the more recent, and very popular, technique documented on brulosophy.com where Marshall harvests from a larger starter. I've used both of these methods in the past with varying levels of success but each method poses hurdles that were either concerning to me or simply more work than I wanted to take on. 

For the last two plus years I have been simply re-pitching unwashed harvested slurry batch to batch similar to how a commercial brewery would harvest and repitch. I realize this is not a revolutionary technique, there are plenty of homebrewers who do this as well, but it's a technique I think that is being overlooked by a lot of homebrewers. It's a very simple process and actually has helped improve my beers and my knowledge of how a yeast pitch can evolve from generation to generation. Before I get into this keep in mind that I am not a microbiologist, so there may be some things that folks disagree with here but this practice has been tried and true over 40+ batches of my own beer, as well as in a commercial setting. 

A look down the carboy neck at some fairly clean slurry.

There are a two "prerequisites" of sorts that makes this process work for me, the first being that I plan my beers out to use a particular yeast strain in successive brews. I brew at least once a month which is frequently enough that I can actually keep two strains relatively fresh and healthy, usually a Saison strain and one for Hoppy beers. The second being you keep the trub amount going into the fermentor as low as possible. Not because it might affect the flavor of our beer, it likely does not, but because it will be easier to estimate the amount of yeast you're harvesting/re-pitching with less kettle trub in the way (try as you might, there will always be some). I use a Stainless Hop Spider, and a good whirlpool with my pump, to achieve this. Don't worry about the proteins and other debris from the boil and fermentation that is in the slurry, some of it can actually beneficial to the remaining yeast cells during storage.

With those two points in mind all the only equipment you really need is a vessel to store your slurry in. Mason jars work fine but I opt for something more fit for yeast storage in these Media Bottles or Polypropylene Jars. I like the media bottles best because both the lid and bottle are autoclavable, with the lid being polypropylene and the borosilicate glass container, so I can ensure the thing is super sterile by boiling or better yet using the pressure cooker. Not to mention this is a lab grade product and should last forever, assuming you don't drop it! The Polypropylene jars are great as well but I notice the rubber gasket can get gunked up over time, I tend to use those more for storing strains of Brettanomyces and mixed cultures where they spend more time in the jar being fed and will reside in for long stretches.

This was, believe it or not, a pretty hoppy beer. Look how "clean" the slurry is.

Obviously sterilization and sanitation is key here so be as thorough as possible to ensure your culture stays as clean as you can. Prior to racking I will use the pressure cooker to sterilize the media bottle, boiling should work fine its just not totally sterile. Once the bottle cools a bit use star-san to sanitize everything, and give the bottle a little co2 flush for good measure. Rack your beer off of the yeast cake and simply pour your slurry into your sterilized, sanitized, co2 purged yeast storage vessel. One thing I do to try keep as sterile an environment as possible is to light an alcohol lamp (not pictured because I'm a shitty photog) to get an updraft and keep things from falling into the jar while I am pouring, this isn't a lab so it won't be perfect. Throw the lid on ,but don't tighten it fully as there might be some off-gassing, then throw it in the fridge. Come back the next day and tighten the lid down. Make sure to mark the date of when you harvested the slurry so you can determine viability once you're ready to re-pitch, also keep track of what generation the pitch is.

The Alcohol lamp is there, i swear.

Leading up to the next batch you plan to brew using your harvested culture take a look at the date it was harvested to calculate the viability of the yeast remaining. Depending on how long it's been you may or may not have enough cells to complete a healthy fermentation based on your batch size and gravity. I use Mr. Malty to roughly calculate the viability and how much slurry I need. Based on my experience I think the the calculator assumes a drop in viability quicker than it does in reality. Steve at woodlandbrew.com has done a bunch of tests on viability of washed and unwashed harvested slurry and found that the viability decreses at a significantly slower rate than Mr. Malty calculates. If it's within 2 weeks of harvesting you should be good to pitch the slurry only into well aerated wort of a reasonable gravity (~1.050). If you're concerned just wake it up with 250-500ml of starter wort and you'll see activity super quickly. Some people might cringe at this since it's a very rough assessment of how many cells and being pitched. But by using the calculator and ensuring the slurry is fresh and healthy you will get a healthy fermentation via close approximation on your pitching rate. Lets face it, most of our home starters are rough assessments anyway given that we use a generic pitching calculator across all strains, each of whom likely have different growth charts. 

Once settled this was about 800ml of dense slurry.

Aside from the simplicity of this process I do genuinely think this has improved my beers to an extent. In my experience the strains tend to get comfortable in the environment and the manner I use them making for quicker ferments and more repeatable results. This has especially been the case for The Yeast Bay's Wallonian Farmhouse and WLP565, still my all time favorite, both of which I have been able to push to 8 generations with exceedingly great results with every passing generation. This is something that has been reiterated by Neva Parker from White Labs just recently during her Reddit AMA. Below is her response to the question regarding the benefits of harvesting from batches of beer.

There is absolutely a benefit (see answer at the top of the thread):
In general, yeast from a lab takes 2-3 generations before they are optimal condition for actual fermentations, so if yeast can be harvested well, you'll get some great yeast out of it.
It can take a few turns for the yeast to be completely acclimated to the fermentation environment, but once they are, performance is optimal around generation 3.
With a starter, its not that you're necessarily losing these benefits. While the yeast is not getting used to environments without oxygen (fermentation), you're still building up yeast metabolism and yeast activity so you'll get a faster, stronger start with a shorter lag.

I already had this post mostly finished when I saw Neva say this and I am glad I waited to post it because this is useful information from someone smarter than I. Her point about the 2nd or 3rd generations being the most optimal is totally in line with my experience, but I would have said generation 4 was the sweet spot. It's nice to have some reassurance from a leader in the industry.

Call me a romantic but I love seeing how the culture performs as those generations pass, sometimes the changes are good but there is a point of diminishing returns. You'll know when that time comes, it's been more of a gradual shift than an abrupt one for me. If you notice off-flavors (fusel alcohols, acetaldehyde, diacetyl etc) or under attenuation you should dump and get a new pitch. In the case of the 8 generation Wallonian pitch I had recently, I would have pitched it further but it got older than I wanted and decided to add it to a mixed culture that needed some extra Saccharomyces help. I think Wallonian was released in early 2014, and I've only bought two pitches in that time. With it being a Saison strain I'm not as afraid of some contaminations of LAB, Brett, or wild yeast as I would be my preferred hoppy beer strain Wyeast 1318.

As with any home yeast procedures it's inevitable that you will get some sort of contamination in your slurry, per Jamil Zainasheff on the Brewing Network most professional breweries do! There are some more advanced techniques including acid washing to clean up your culture if you want to go that route, I don't however I may try it just for the learning experience. Normally after 4-8 generations I am fine spending another $7-10 for a new pitch. I know it might seem a bit scary to do this, but give it a try at least once and see how you make out. One things for sure, you'll see active fermentation as fast as you've ever seen.