Monday, June 21, 2010

Fermentation Cogitation

(Editor's Note: This post originally appeared on on June 21, 2010)

The two beers which I have fermenting right now are the Light and the Dark. Each of these was brewed with the same yeast strain, White Labs American Ale Yeast WPL060. They are being stored in the same location, and so have been kept at the same temperature. However,the Light has seemed much more aggressive in its fermentation. I had to switch out my standard three-piece air lock for my makeshift vinyl-tubing-submerged-in-a-bucket-of-water air lock.

It makes soothing aquarium type noises when the CO2 bubbles out. Very relaxing.

There are a lot of things which can affect the fermentation rate, one of which is temperature, which I already mentioned is the same for each of these beers. Another is the sugar content of the wort. In general, the higher the sugar content, the higher the fermentation rate. Sugar is yeast food, and so the more food, the faster the yeast can eat and reproduce. (Up to a point. The actual rate of growth of the yeast population follows a logistic equation. See this or take my differential equations class this fall.)

Now, the sugar content of the wort is indicated by measuring the specific gravity of the wort before the yeast is added. I took these measurements for the Light and the Dark. (For comparison, water has a specific gravity of 1.000.)


Higher specific gravity indicates more sugar content, so it should be the Dark which has a higher fermentation rate. So, why isn't the Dark spewing yeasty foam all over my beer closet? The answer is actually simple. I'm using a 5 gallon glass carboy for the primary fermentation of the Light, and a 6 gallon plastic bucket for the primary fermentation of the Dark. There is just more room in the Dark's primary fermenter for the yeast to expand. (You can see this in the picture above. The Dark is in the white plastic bucket on the right.) It probably is experiencing a higher fermentation rate than the Light, it's just not erupting since it has plenty of room to expand in.

In the future, I'll need to keep this in mind if I want to have more than one beer in a primary fermenter at a time. If I do, I'm better off getting another big bucket.


  1. Did you oxygenate them both the same? This strikes me as the most likely culprit. Also, I used to make the mistake of aerating wort when it was still hot/warm. This doesn't help, as all that oxygen just up and leaves. Anyway, during the first 12 hours or so, yeast that has been dormant needs plenty of oxygen to go to work vigorously.

  2. Also, I like the runoff system :) Undesirable compounds come in two forms. Most are the kind that sink to the bottom - and so you siphon to leave them there. But a smaller portion are the type that rise to the top. Using a blowoff tube pushes that junk out. Or so I like to believe.

    I've never used a bucket, mainly because I'm afraid of contamination deep in plastic crevices. But blowing off oils and proteins sounds good too.

  3. Apparently I can't stop leaving comments here.

    You might not have the space for it, but here's a suggestion. Invest in another carboy. But instead of a 5-gallon, buy a 7.5-gallon. Use your math/engineering/Die Hard: with a Vengeance skills to mark where 5 gallons is with some tape.

    My dad does this, because he shares my aversion to buckets, but doesn't have to use a blowoff tube this way. Then he racks directly into a 5-gallon. I do this myself when something solid goes into my primary, like fruit, veggies, or dry hops. Perhaps this is not of interest, if you're cool with a bucket. But unlike plastic, there's no microscopic crevices for funk to hide in with glass. Eventually your stirring spoon and cleaning attempts are going to scrape all kinds of little caves into the sides and bottom of any bucket.

  4. I may have given one a more vigorous shaking than the other after pitching the yeast. I think you hit the target.

    I hear you on the buckets. Eventually, I'll stop using them for anything other than priming before bottling. But for now, I'll brew with what equipment I have available to me.

    Did I hear right that you've started brewing all grain recipes? How is that going? What do you use for a mash tun?

  5. I'm partially upgraded to all-grain. I still don't have the proper equipment, but you should check out this guy's page:

    You can easily make 3 gallons of all grain at a time, at the slight expense of a little more grain than 3 gallons should take. Without proper sparging equipment, it's what needs to happen.

    I have used this technique three times now, and I still am doing something wrong. The abv potential is always 3 to 4%, which is just fine and it's good beer. But this guy claims it should be closer to normal. It also might have to do with the fact that I've been using grain that sat in storage for 4 years after the Yamhill brewery closed. But hey, I found it on Craigslist free page, so the price was right.

  6. That's pretty cool. It might be your old grain. It might also be the temperature. The given temperature might be favoring the unfermentable sugars at the given temperature. have you tried waiting more than an hour? Maybe an extra half hour is all it would take to get more fermentable sugars.

    I'll give this a shot before I invest in a mash/lauter tun and after I move to L.A.