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.