Problems Arising From Reusing Yeast

A Troubleshooting issues with harvested and washed yeast is difficult without having more information, but you have provided lots of good discussion points. In your case, you are taking yeast from NEIPA fermentations, rinsing/washing your yeast presumably with water, and re-pitching. And your major problems are increased attenuation and less haze, but you are also thinking you may have some oxidized beer flavors.

Let’s start with the increased attenuation levels. Assuming you are brewing similar beers over time, which seems to be your case, the most common cause of this issue is either wild yeast or enzyme contamination. So-called diastatic yeast strains secrete the enzyme amyloglucosidase (AMG), also known as glucoamylase, resulting in the digestion of dextrins in beer into glucose, followed by the fermentation of this glucose by yeast. When diastatic yeasts are present in your pitching yeast, the result is abnormally dry beer. The same sort of thing can occur if yeast is harvested from fermentations where AMG has been added from exogenous sources (i.e., from a bottle). Although dry, light, and brut styles are the beers that come to mind when thinking about AMG additions, high dry-hopping rates are also associated with the problem that has become known as hop creep. If yeast is cropped from dry-hopped beers, AMG enzymes from hops can be carried with the pitching yeast and potentially lead to issues in subsequent batches.

I am speculating about the cause of your increased attenuation with yeast generation, but it is certainly not uncommon for one of the above problems to occur. In the case of diastatic yeast strains, phenolic off-flavors are almost always present. It is more common to see the terms “diastatic” and “POF+” (phenolic-off flavor positive) on yeast data sheets these days because brewers have become increasingly aware of special handling required for these strains. To be clear, many great brewing strains, such as all saison strains, are diastatic and POF+, so there is nothing inherently bad about these strains. As it turns out, many wild yeasts, such as Brettanomyces, are also diastatic and POF+. Depending on how you are handling your yeast and what background microflora you have in your brewery, you may be experiencing low-level contamination from wild yeast. If forced to choose a cause of the increased attenuation, wild yeast would be my pick.

Your next issue is the loss of haze over time. As an old-school brewer, it is really weird to be discussing improvements in clarity with subsequent generations as a problem. Back when I was a kid, we learned that brewing yeast, particularly lager strains, can lose their flocculation properties over time. And setting a generation limit is a common way to prevent the loss of flocculation over time. It also seems, in the case of hazies, that reusing yeast can also lead to overly clear beer!

Not sure what is up with your question about oxidation, but have heard that many brewers who recycle yeast from NEIPA brews are experiencing problems related to yeast health. And beer freshness is something that suffers when unhealthy yeast (i.e., low viability and low vitality) is cropped and re-pitched. Although the problems encountered by many a NEIPA brewer cropping yeast from dry-hopped beers is not related to wort oxygenation, high wort oxygen levels can result in oxidative damage to pitching yeast, and this cellular damage takes multiple yeast generations to manifest itself with poor yeast performance. Don’t know if that is what you are experiencing, but that is the main argument against high oxygen levels achieved using pure oxygen to oxygenate wort, as opposed to using air for aeration with a lower oxygen solubility.

OK, let’s pull some take-home messages from this meandering discussion:

1. Based on the general description of your observations, I suggest limiting the number of times you re-pitch yeast. Generation count may not be the direct cause of your issues, but there is something that seems to be changing over time. Removing reuse will help you narrow down the root cause of your problems.
2. Consider reducing your oxygenation rate and using a smaller hose so that you get smaller bubbles (better gas diffusion). I am assuming you are doing 20-L (5.3-gal.) batches, so you can adjust my math if this assumption is wrong, but 2.5 L (2.6 qts.) of oxygen contains about 3.6 grams (22.4 L per mole and the molecular weight of oxygen is 32) of oxygen. If only 10% of this gas dissolves in the wort, that is about 18 mg/L of oxygen, which is a bit on the high side. Most textbook references cite 8 mg/L as the norm (probably because that coincides with the oxygen solubility in cool, 1.048 standard gravity/12 °Plato wort from air).
3. Review your sanitation practices since you questioned that as a possible issue. Bleach is a great “bomb” if you want to just give your whole kit a severe hit (avoid on stainless steel). Thoroughly rinse, drain, dry, then clean and sanitize as normal before your next brew. Low-levels of Brettanomyces can be challenging to kill and bleach is a good way to deal with hard-to-kill microbes.

Response by Ashton Lewis.