A month or two ago, James Spencer at Basic Brewing Radio and Chris Colby at Brew Your Own Magazine propositioned their second collaborative brewing experiment. Being both a beer geek and a science geek, not to mention a big fan of both BBR and BYO, I had to participate. I participated in their first experiment earlier this year, which concerned the effects of leaving beer on the primary yeast cake for an extended period of time. As a brewer, it was a very enlightening experience and made me reconsider my brewing process. It also got me a mention on an episode of BBR and an issue of BYO 
I’ll post those results anachronistically some time in the future.
This second experiment is intended to study the effects of yeast pitching rate on beer. We were to brew a recipe of our choosing and split the unfermented wort into 3 separate portions. I brewed 3 gallons an American Pale Ale of my own design, which ended up more like a Cream Ale since I had a completely different variety and amount of hops in the freezer than I thought when I went to brew. I split the wort into three 1-gallon jugs. In one jug I pitched the proper amount of dry yeast according to Mr. Malty’s Pitching Calculator, which is to say 2g of Safale US05. In the second jug I pitched a fourth of that – 0.5g. In the third, I pitched four times the recommended amount – 8g. I then let them ferment and took notes.
I’ve been crazy busy due to my wedding happening in two weeks (!!) so I couldn’t check on them as often as I would have liked, but I took photos and notes periodically…
12 hours after pitching:
0.5g: 4 seconds between airlock bubbles
2.0g: 2 seconds between airlock bubbles
8.0g: ~1 second between airlock bubbles
36 hours:
0.5g: 2 seconds
2.0g: 1 second
8.0g: 3 seconds
60 hours:
0.5g: ~1 second
2.0g: 4 seconds
8.0g: 10 seconds
84 hours:
I stopped keeping track of times at this point as all three were greater than 10 seconds between bubbles.
10 days after pitching, ready to be bottled…
All three appeared to be finished by this point. For all you non-brewers out there, that chunky stuff floating around and settling on the bottom is completely normal(!) although admittedly gross-looking. It’s break material – fats and proteins and other junk created from the brewing and fermentation process. That white creamy layer beneath it consists of yeast cells – something I would harvest to reuse if it were anything other than cheap dry yeast.
After a few days in the bottle…
On the left is a bottle of the 0.5g beer, with the 2g in the middle, and the 8g on the far right. Wow that 8g beer is brilliantly clear. My beers NEVER look that good when they’re that young! The one in the middle (the proper one, remember) is slightly hazy, but still looking good. The 0.5g beer is about as cloudy as you can get. It looks like a wheat beer – and it still does, over a week after bottling!
I should mention that I carbonated each bottle with a Cooper’s Carbonation Drop and conditioned them for about a week at 72F before drinking. There were 21 bottles total (but only 3 in Newcastle bottles for observation 
Gravities
I’ll post the recipe on my Recipes page soon, but here are some of the numbers involved:
Starting Gravity: 1.056
Finishing Gravity
0.5g: 1.005
2.0g: 1.007
8.0g: 1.009
Yes… very strange numbers indeed. So much so that I thought I had misread my refractometer. So, I cleaned it off, took additional samples, and checked each one again. Sure enough, that’s what they were. (btw I use Beersmith to calculate gravity of fermented wort.) These numbers seem the opposite of what I would expect as the 8g had the most yeast activity and the 0.5g had, by far, the least. Maybe I did something wrong or maybe I’m just misunderstanding something here.
Sensory Profile
Appearance: as I stated above, the 0.5g is incredibly cloudy, but the 2g and 8g beers are very clear, with the 8g being brilliantly clear.
Aroma: I had Brittany pour the beers for me with my eyes closed since I knew if I could see them I would be able to pick them out and I wanted to guess which was which. Wow the 8g smells like yeast. I’m sure if someone experienced in homebrewing were to smell this without being told what yeast was used, they would guess correctly. The 2g had practically no yeast aroma, very little hop aroma (a fault of the recipe/brewer), but it was overall pleasant. The 0.5g had almost no aroma at all. It somehow smelled weak and watery. It just didn’t smell like a beer.
Taste: ah, the important part! The 8g tasted very yeasty, as was expected from the aroma. It was more complex (since you could really taste the yeast), it had a slightly fuller body than the others, and more carbonation. The 2g tasted as I thought it should. I could taste the malts I used and actually taste the hops – unlike the others. Not a bad beer if I do say so myself 
The 0.5g was a perfect drainpour. It just tasted like a mistake. It was as if someone mixed bad craft beer with water 50/50. I didn’t even finish the bottle I opened.
Lessons Learned
The results were generally what I expected, but I didn’t expect them to take such extremes. Severely under-pitching is something I never want to do. It ruined that portion of the batch. It tasted worse than my earliest homebrewing attempts. The over-pitched beer turned out fantastically clear, which I’m assuming had something to do with all those extra busy yeasties. However, it made for an overly yeast-flavored beer that isn’t really appropriate for any style – although it’s still enjoyable. Pitching the proper amount definitely made the best beer. It was clean and balanced and most closely fit the idea of the beer I had in mind when I designed the recipe.
I tend to overpitch yeast a lot. I’m always harvesting too much or making too big of a starter or throwing in an extra dry packet or two, when I should really pay attention to the proper pitching rate. If anything, this experiment has taught me just how important that pitching rate really is… something I didn’t even think of before this.