A few posts back I talked about yeast pitching rate, or the amount of yeast used to inoculate unfermented wort, and how critical this number is to brewing great beer. Pitching not enough yeast or too much yeast can create off-flavors and make a subpar beer. Moreover, the pitching rate itself is a number that’s not set in stone but rather subject to different variables such as the strength of the beer and the yeast strain selected. Brewers, both pro and amateur, change the pitching rate to suit their needs. A slightly higher pitch rate for an ale may produce a completely different beer. The most important thing to do is experiment with your system and determine the best pitch rate for a strain of yeast.
To this end, I did an experiment to find for myself the effects of over-pitching or under-pitching a beer on my brew system and the results are in this post. This experiment created its own thread over at Homebrewtalk so I wanted everyone reading to get these results. I want to point out, however, that I don’t feel the experiment was a success, although some information was gleaned from the experiment. Here are the reasons why it failed:
- Beer style. I chose a blonde ale thinking that off-flavors would be more apparent. Differences were actually very subtle.
- Yeast strain. This is probably the biggest mistake. I chose Wyeast 1056, or Chico yeast, to test. Unfortunately, this yeast is pretty clean and pitching rate changes might not change the yeast flavor profile that much.
- No oxygen or aeration. I did not oxygenate the wort prior to pitching. I thought the action of sprayed wort hitting the side of the fermentation vessel would introduce enough oxygen but I think I was wrong. Several brewers on homebrewtalk pointed this out and I think they are right. The control beer had a strange off-flavor that was hard to place.
- Fermentation vessels were not covered. Another big mistake. There was varying degrees of lightstruck, with the control pitch the most affected.
Having said all of this, here is the fermentation profile of the three beers:
I measured the gravity of each beer every 8 to 12 hours using a 200 ul pipette-man. As you can see, the fermentation profiles are exactly what I was hoping for. The over-pitched beer’s rate of fermentation was faster than the control, while the under-pitched beer was slower. One interesting thing to note was the under-pitch beer finished at a lower gravity than the control pitch (1.009 compared to 1.012). I’m not sure what this means and this bears repeating. Could under-pitching give slightly lower final gravities? One speculation is that the yeast are stressed to the point where attenuation is raised.
Once the beers finished bottle conditioning, I had my wife, Kim, set up the samples in a blind tasting for me. I could not pick out the control beer. Actually, I totally missed on guessing which beer was which. The next day, I took all the beers to a yeast class I was teaching at Brooklyn Homebrew yeast and the students could not pick out the beers either. This was my first time teaching a class on yeast for homebrewing, and I went over the 2 hour time limit. I was rushed, due to no fault but my own, and did not tally what people thought of the beers.
However, after knowing which beers were which (I know – not very controlled/blinded) I could taste subtle differences. Hopefully my ability to critique my beer is not subject to suggestion. Here is my take on the three beers, faults and all:
Control-pitch: The nose smelled of faintly oxidized hops and medicinal. Uh oh. Toasty malt background with agressive bitterness that approaches astringent. There is a phenolic, almost plastic-like taste in the backend. I don’t think this is due to contamination as the other beers seem fine. It’s possible that the lack of oxygenated wort and light produced this result. However, cutting through this reveals a very clean beer with the american two-row shining.
Over-pitch: No evidence of phenols. However, the is no evidence of anything. The beer is super clean with a bland malt profile. The beer is thin and almost water-like with very little body.
Under-pitch: The nose of the beer gives away some fruiter ester that lingers and disappears. The taste has an odd assortment of esters that clash with the malt and make the beer quite unpalatable. This beer had a thicker body (although slight) compared to the control. The ester profile, although not strong for Wyeast 1056, reminded me over-ripe peaches.
Conclusions: As a scientist, I would have to say these results are pretty much inconclusive since my control did not work. However, the over-pitch beer was cleaner and thinner while the under-pitched beer had some strange esters going on. This would seem to back what other people have seen about pitching rates: over-pitching produces a bland beer, while under-pitching produces a stressed beer with esters being produced from the yeast.
Future: I plan on teaching another class at Brooklyn Homebrew on February 26th and plan on repeating this experiment in the coming weeks. My presentation will be more streamlined and more time will be given to the students to sample and critique the beers – more data for the experiment. Also, a few posters on homebrewtalk have asked for the presentation slides. I will post them on February 26th after revising them for my next class. The next experiment will be a characterful English Bitter and I will use an expressive english ale yeast. I may also increase the over-pitch rate to exacerbate any results. I will also try assess head retention in each condition. Once the experiment commences I will post here and on homebrewtalk. Hopefully, this will be the last word (at least for me) on pitching rates.
Awww. That’s too bad. I was looking forward to seeing your results. For what it’s worth, I like to underpitch my belgians, especially saisons with their crazy yeast, to max out on the weird esters.
Maybe I’ll be able to get to your class in February!
I know tell me about it…
However, I have another opportunity in the next experiment. You know, I doubt that you need to go the class per se. I’m just showing basic lab technique and working with yeast and fermenting beer. I’m sure your pretty versed in this and can probably teach the class with me.
J
Well perhaps I could just sit in as an honorary assistant or something
I just got a few sour brews from a friend of mine and perhaps I’ll isolate something and share with people if I can make it.
I appreciate that you took the time to share this info. Even though it did nt turn out exactly as y ou would have liked, I think there is some value in the results. Looking forward to the next run.
I have a question regarding oxygenation. My understanding is that oxygenated wort is desired initially to allow the yeast to undergo a few multiplication cycles, thus increasing in number by an order or more, and once the O2 runs out they go into fermentation with numbers sufficient to tackle the sugars without stressing out or getting killed by the waste products before it’s done. However, if you pitch enough yeast, deliberate oxygenation is not all that important. Personally I seldom make really big beers that I have to bubble oxygen through and like to think that pouring the beer through the strainer oxygenates it enough. If it is a large beer then I just pitch more or use a cake from a small beer (like I did with my Anglo-Scandinatian Wheat-Gruit-IPA).
What’s your opinion on the issue?
So, in my opinion, oxygenation is definitely essential, and I even oxygenate for low gravity brews. Too bad I didn’t do it for this experiment.
The need for O2 goes far beyond more replication cycles. O2 is principally scavenged for production of unsaturated fatty acids (UFAs) and sterols. The more sterols you have, yeast membrane will be more fluid and pliable. This is important for adequate placement of transmembrane proteins that transport sugars across the membrane. However, the most important function in having a pliable membrane is cell division. More sterols allows the yeast to recover from an increased number of bud scars. If you don’t oxygenate, further generations of yeast will ne be able to grow and your 2nd or 3rd pitch will suffer. So even if you over-pitch, oxygenating your wort is critical. Pouring the wort through a strainer will not give enough oxygen for yeast, this has been shown before. You may get 6-8 ppm max if you shake, spray the hell out of your wort. Yeast really needs about 10-12 ppm. Pure oxygen is the only way to go.
About fermentation. Modern brewing yeast actually begin alcoholic (anaerobic) respiration as soon as they hit wort. Mutations and evolution has favored alcoholic fermentation over the Krebs cycle. Also, the crabtree effect ensures fermentation begins at high enough sugar concentration. I only say this because when you first pitch, fermentation begins immediately and O2 is used for membrane health.
Hmmm that’s really interesting. I just looked up some stuff and from a quick look it looks like yeast can’t do UFA synthesis anaerobically. You know, now it makes more sense. I’ll have to try a split batch with poured vs bubbled and see what happens. Like I said, I bubble pure O2 when I make big beers, but haven’t been following through with that recently and haven’t really notice any differences.
You might not see any difference initially, but without using pure O2 deleterious effects can be amplified in later batches. Off-flavors may be subtle initially. This is an interesting idea to test and something I plan on running an experiment on. Actually, I was thinking of doing this particular experiment for the next class, but want to repeat the pitching rate experiment.
Good post. I’m getting off flavors similar to what you had on your control, but I haven’t been able to pin down why I’m getting them. I figure I either have some sort of contamination issue, or my yeast health is to blame. Lately I think it’s the health of the yeast. “Agressive bitterness that approaches astringent” almost perfectly describes the issue I’m having.
There are a couple of possibilities to the problem you and I are having…
1) Like you mentioned, yeast health. In the experiment, I did not provide oxygen which would make yeast stressed during fermentation and produce off-flavors.
2) Contamination. Some organism, mostly likely wild yeast, has produced some spicy phenol that is coming across as astringent bitterness. I seriously doubt this is my problem.
3) Tannin extraction. This could come from the husk. If you sparge to hot, over-sparge, or crush your grain too finely, this could be a cause.
BTW, stay tuned for an upcoming post for my next pitching rate experiment.
Thanks for the reply! Looking forward to your next experiment.
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