Sitting down with Troels Prahl, Director of Innovation at White Labs Interview

Sitting down with Troels Prahl, Director of Innovation at White Labs Interview

Eric Abbott: Repitching yeast is an important tool for brewers to reduce costs, which is more and more important these days with the rising cost of, well, pretty much everything. But many brewers are intimidated by the process. What would you say to a brewer who wants to start repitching yeast in their brewery?

Troels Prahl: I want to say that some brewers tend to be overly cautious when it comes to repitching. You don’t need to have a lab or be a trained microbiologist. A yeast slurry is much less susceptible to contamination than a freshly aerated wort at the start of fermentation because there is some alcohol present, no fermentable sugars, and there are billions of yeast cells to compete with any contamination. If a brewer is comfortable with cleaning and sanitizing a fermenter, then they already know enough about brewery sanitation to repitch yeast.

Would you say that all brewing yeast is repitchable?

In principle, all brewing yeast is repitchable. As long as the fermentation is healthy and the yeast is viable, the yeast can be reused again and again. Good yeast handling practices are important though, and there are some cases where repitching is not recommended. For example, some beer styles such as high ABV, dry hopped IPAs, or sour beers make it difficult to harvest a good amount of highly viable yeast. There are also challenges related to carryover of color or flavor with the repitched yeast, for example yeast from a very dark or very bitter beer repitched into a lighter flavored beer. And fermentation performance and sensory may change over time due to genetic drift. But the repitchability does not depend on the strain itself.

Is there a limit to how many generations the yeast can be repitched?

Of course, this may vary on a case-by-case basis. I know some breweries that have repitched their culture for decades and hundreds of batches without issues. This is rare and specific to their yeast handling practices and brewing process. As a rule of thumb, a commercial brewer should be able to get to five generations, or up to 10 generations, with good yeast handling and favorable conditions. Beyond this, we see a significant drift in the culture that is independent of yeast handling practices, and additional cost savings are minimal.

That’s in-line with our own recommendations as well. What should a brewery look for in terms of yeast performance when determining whether to repitch, or to start again with fresh yeast?

The best strategy is to look at the fermentation data from the previous batch. There is so much good information in the kinetics of the fermentation graph. A longer lag phase, slower fermentation, or incomplete attenuation are all signs that you should pitch fresh yeast. Consistent fermentation graphs over consecutive batches are usually associated with consistent flavor characteristics of the final beer.

What causes a yeast culture to slow down after multiple generations?

For each yeast pitch, you need to make sure that majority of yeast cells are young, fresh daughter cells from the previous fermentation. One of the most common issues with repitching is overpitching, leading to less cell growth and therefore an older and less optimal population to carry on. This will cause issues in the second or third or fourth generation of repitching because there are older soldiers in the mix. The result is a slower fermentation or incomplete attenuation.

What are some things a brewer can do to improve their repitching practices?

It is important to pitch an adequate amount of viable yeast, so use a microscope to count cells and measure viability. If you don’t have a lab, you can get a crude measurement of yeast quality by measuring the total amount of biomass present at the end of fermentation. More biomass suggests more cell division, more young and fresh daughter cells, and higher viability. After collecting yeast for repitching, use a flow meter, or simply measure the number of buckets of slurry dumped from the fermenter. The total biomass is usually consistent, so any changes will give you information about optimizing aeration rates, pitching rates, or nutrition.

White Labs now has dry versions of some strains available. While liquid yeast is pitched by cell count, dry yeast is generally pitched by weight. Has this led to any confusion with customers?

It’s quite important to remember that harvested yeast slurry has undergone an alcoholic fermentation, so the physiological condition of a culture after a beer fermentation is quite different from the culture coming out of a lab. Since the yeast physiology is different, they will perform differently, and this is true for both liquid and dry yeast. The brewer can compensate by customizing the pitch rate and fermentation conditions for the first-generation pitch to achieve more consistent fermentation performance and flavor. For the most consistent results, follow the manufacturers’ recommendation about pitching rates for each strain.

Do you have any tips for achieving greater flavor consistency from one batch to the next?

Experienced brewers know that first generation yeast, from any source, performs slightly different compared to subsequent generations. For both liquid and dry yeast, there is a signature flavor in the first generation, and the second and third generations are generally producing better beer. Flavor differences can be minimized by blending the first generation with other batches from repitched yeast. If you can’t blend the first generation beer out, optimize your pitch rate and fermentation conditions to achieve consistent fermentation kinetics and biomass yield to minimize some of those flavor differences.

Do you have any nutrient recommendations with regards to repitching?

Repitching success is very much related to the cell growth, which depends on the nutritional state of the wort and the nutritional conditions of the media that the yeast was propagated in. It depends a lot on the malt quality and the malt supplier, and whether you are using adjuncts. All-malt wort has almost everything that a yeast cell needs to be happy. Zinc is the most important element that can be deficient in wort to varying degrees depending on the harvest year and growing conditions. Zinc enriched inactivated yeast such as Servomyces™ provides great benefit for propagating and repitching yeast. I recommend doing comparative fermentations with and without nutrients and then looking at the fermentation profile. If there’s a difference, then it’s probably a sign that you need to add that nutrient, and zinc would be the first nutrient type I would recommend.

How long can harvested yeast be stored before repitching, and under what conditions?

It’s a very good question, and it’s very strain dependent. It’s important that the brewer gets to know their yeast strain well since some are very robust and some are more sensitive and need to be used more quickly. Yeast should be used ASAP and stored for no longer than two weeks. At the end of fermentation, the yeast builds up glycogen levels inside the cell, which acts as a carbohydrate source to keep the yeast alive over time. Glycogen is very quickly depleted in the presence of oxygen, so it is important to harvest yeast as oxygen-free as possible. Keep the yeast as cold as possible (2-4°C) to slow the yeast metabolism. Yeast is a very good insulator which can allow for hot spots to develop due to metabolic activity in the yeast slurry. The slurry can be mixed to ensure the temperature is homogenous. The storage vessel should be vented to allow CO2 to escape and avoid pressurizing the culture, which would cause yeast stress. Don’t store yeast in a normal brewery keg, it must be modified to allow CO2 to be released and prevent dangerous levels of pressure to build up. At White Labs, we developed the FlexBrink®, which is essentially a heavy duty, sterile plastic bag that allows you to harvest yeast without having any ingress of oxygen, store easily, and pitch in line via peristaltic pump.

Do you have any advice for how to harvest yeast from a NEIPA?

NEIPA is often dry-hopped during active fermentation, so generally it is best to avoid harvesting yeast from these beers to avoid carryover of hop material. However, we have seen many breweries that are able to successfully repitch for many generations from hazy beers by finding ways to crop the yeast before they dry hop, even if that means harvesting before the fermentation is complete. This goes against the normal best practices of yeast harvesting, but with some experience you can bend the rules with these types of beer with good results.

Can you speak a bit about your ongoing R&D programs related to repitching?

We have 28 years of knowledge about how to best reuse the liquid cultures in our collection. We are working now to build up some of that same data for our dry yeast strains to understand how they repitch over time. We should be ready to share these results early 2024. We’re extremely excited about this new dry format for the strains that we’ve known for so many years.