This experiment was my second attempt at asking the question, “what effect will different yeast pitching rates have on my homebrew?”. In my first try, I brewed a Blonde ale with Wyeast 1056 and three different pitching rates, an over-pitch, under-pitch, and a control pitch rate. I made some mistakes and the beer was generally not good, which confounded the results. The control did not serve as a basis of comparison or a baseline.
The second attempt went much smoother and this post is about the results. I obtained some interesting results and some great comments from Garrett Oliver, Brewmaster at Brooklyn Brewery. To recap, over-pitching yeast for fermentation supposedly produces a thinner and cleaner beer, while an under-pitched fermentation stresses the yeast to the point where unwanted off-flavors are present. There is a sweet spot that brewers, both commercial and amateur, aim for.
The beer was a simple Belgian Pale ale brewed with WYeast 3522, Belgian Ardennes strain. I chose this strain because, at least in my hands, I have seen different flavor profiles at different fermentation temperatures and thought this may translate to pitching rates. There were three conditions and each brewed exactly the same and fermented in one gallon carboys:
- Control pitch rate: 0.75 million cells / ml wort / Plato
- Under-pitch rate: 0.1 million cells / ml wort / Plato
- Over-pitch rate: 4.0 million cells / ml wort / Plato
Fermentation lasted two weeks and the beers were bottle conditioned for another three. I only sampled one beer of each, while the rest went to my yeast class that I was teaching at Brooklyn Hombrew, my local homebrew supply store. Each student sampled the beer and recorded different flavors they detected so I could plot some data. As before, my wife set up a blind tasting for me so I try to guess which beer was which.
Results:
I decided not to monitor fermentation kinetics this time around since I was away for four days during fermentation. However, fermentation occurred similar to last time; The over-pitch sample never had a lag time (i.e. krausen in an hour!), while the under pitch took 2 days to get a visible krausen. The control pitch had a krausen by 12 hours.
After I bottle conditioned the beers, I tried them in a blind sampling. I could only pick out the over-pitch but the missed the other two. In my personal notes, I noted that the over-pitch was a tad thin and one-dimensional. The control had some unique phenolics that I pegged as the under-pitched beer. As I let the beers warm up however, differences became more apparent. The under-pitched was fuller on the palate and contained more fruity esters that reminded me apples, pears, and some peach. The over-pitch beer was clearly thinner bodied and scrubbed of many flavors. It just seemed duller. The control brew had some spicy phenolics going on and the pilsner malt really shined through. All beers were good, but the control seemed that much more approachable. I liked the over-pitched beer over the under-pitched.
I also examined the poured beers for any differences. There were none. The beers had the same color and same head retention throughout the pour. In the photo below you can see an example. Unfortunately, it was very difficult to pour three beer exactly the same hence the slightly larger volume in the over-pitch. Despite this, the level of foam stayed the same with each sample.
This past Sunday I taught a class on using yeast for homebrewing and part of the lecture was about pitching rates and their effect on fermentation. The class blind tasted each sample and, in avery basic way, scored different flavors that they found. Below is a graph of the data. There were 12 students.
After everyone sampled the brews I asked them which beers they thought were which. It is pretty clear that my judgment, and perhaps my palete, is not as refined the students since most easily picked out the over-pitch versus under-pitch! The experiment beer was definitely warmer than when I sampled them which allowed for more flavors to come through. The data are clear, and follows expectations from different pitching rates; the under-pitch had more esters and was a fuller beer, while the over-pitch was thin in body and low in fruity esters. The control beer seemed to hit every category.
There were some unexpected results, which may be a property of the yeast that I chose, Wyeast 3522 (Belgian Ardennes). The over-pitched beer was consistently bitter compared to the others. Just as I detected, the control sample had some phenolics that the class described as smokey; this was reduced in the over and under-pitched. Fusel alcohols were diminished in the over-pitched samples but were present in the other two beers. These characteristics may be distinct for the Belgian Ardennes strain in particular, and may be different for other yeast strains. One interesting thing to do would be to make a “flavor profile” library of every strain with different pitching rates. Something that I have no time for…
One curious result to the experiment was the finishing gravity. Both the over-pitch and under-pitched finished at a dramatically lower final gravity than the control beer:
- Control-pitch FG: 1.013
- Under-pitch FG: 1.009
- Over-pitch FG: 1.009
I was surprised to see, despite the lag time of two days, the under-pitch beer cranking away while the control pitch slowed down. My first assumption was that both the over and under-pitches created a stressful situation that pushed the yeast to ferment to a higher level of attenuation. I decided to contact Garrett Oliver, brewmaster at the Brooklyn Brewery, to get his feedback. Garrett and I have been working on a wild yeast project together (a future post!) and we’ve been in contact over my results. His suggestions:
“Jason,
It is my suspicion that the overpitch achieves a lower gravity, as expected, because of the high numbers of cells available to do the work. The underpitch, however, eventually has a higher percentage of daughters vs older cells. The daughters have near 100% viability, high sterol levels and are raring to go, while older cells will have various viabilities, membrane condition, etc. Many of them may not be as ready for active fermentation as the daughters. I’d expect, therefore, for the underpitch to have a slower start than the overpitch or the control, but then to catch up pretty suddenly and attenuate well. Of course, all assumes relatively healthy yeasts, etc.”
I then asked if this is something they see in the brewery:
“I’d say yes, but only to a point. I think there is a curve to it. My gut feeling is that for a wort of average strength, you could cut the pitch rate from 30 – 50% and still achieve or exceed final attenuation vs a control. You’d have higher esters, but a revitalized (younger) yeast set afterwards. We will sometimes slightly underpitch when we want to put some more vigor back in a culture that otherwise seems healthy. Conversely, overpitching ages the culture – fewer daughters, over time, leaves you with a lot of battle-weary scarred cells with inflexible membranes that are no longer at their best. And not as many young, scar-free new cells.”
Conclusions:
I think Garrett Oliver’s advice makes perfect sense and is something to keep in mind when playing around with different pitching rates. If you want a beer with very healthy cells, slightly higher attenuation, then under-pitching the beer to some degree might be applicable. However, expect to get more esters which is something that has to be optimized. I think over-pitching a beer has its place too. If one wanted to brew a very clean, thin beer and achieve maximal attenuation, then over-pitching may be the way to go. However, don’t expect to use the yeast for successive generations. It is important to note that despite the drastic differences in pitching rates, all the samples were not bad beers, and the yeast class students thought so too. Hopefully, this post gives one more weapon in the arsenal to brew great beer, it has for me.
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