Ask Mr. Wizard

Mystery Fermentation


Bob Hines — Chapman, Kansas asks,

I brewed A Founder’s Breakfast Stout clone from BYO, added some extra grain and my OG was 1.090. It was my first beer with coffee, chocolate and cocoa nibs. I pitched half of the batch with some S-05 yeast harvested from a recent batch of porter and pitched the other half with fresh S-04 as an experiment. I brewed the stout on Sunday, and on Monday when I got home there was only a bit of foam (it was oily looking) on top of the beer but no real kräusen and there were no bubbles coming out unless I shook the carboy. After about three days of shaking, I couldn’t get any more bubbles to come out from shaking. Thinking I did something to kill my yeast, I added extra yeast (S-05 to both). I kept watching and saw no signs of fermentation. Thinking that maybe the chocolate or coffee did something to prevent the yeast from fermenting, I gave up on the beer but decided to hold off on dumping it. Six weeks after brewing the gravity reading was 1.030. How and when did this ferment? Did an infection consume the sugar?


True mysteries are rarely encountered in a brewery when the facts related to a particular problem are at hand. The problem lies in obtaining the facts and this is particularly true when homebrewing. In a commercial brewing operation it is common for brewers to be in the brewery most of the time and in some of the larger breweries instrumentation and data collection is used to gather and track data related to production. Production staff and historical data are pretty handy when it comes to tracking down the ghosts that seem to haunt the brewery, but in cases like yours there is not much information to evaluate.

The beer you brewed had a fairly high original gravity and also contained a high proportion of specialty malts. Based on this information I would not predict a very low finishing gravity. Couple that with the single step infusion mash at 155 °F (68 °F) for one hour specified in the recipe and I suggest that 70% is a reasonable guess for the apparent degree of fermentation (how much extract is consumed during fermentation as measured using a hydrometer). Thirty percent of 1.090 is 1.027 and your final gravity was 1.030. This tells me that your OG and FG numbers are believable. When I solve problems I first begin by questioning the validity of the information at hand — because that is what Wizards do!

The other fact at hand is that you added an ingredient that contains fat to your wort; baking chocolate contains cocoa butter. You also added ground coffee beans and coffee beans contain oils. Fats and oils are the same general class of components that are deposited on the rim of a beer glass if you happen to be noshing on greasy finger foods while enjoying a frothy pint . . . well, perhaps previously frothy pint. Fats and oils are well-known anti-foaming compounds because they have a higher affinity for the surface of liquids than do foam-positive compounds from beer. This means that proteins and hop compounds normally involved in beer foam are prevented from stabilizing foam bubbles when these strongly hydrophobic molecules are present. In simple terms you may have had carbon dioxide escaping from your fermenting batch of Breakfast Stout with little to no commotion at the surface of the carboy. Some brewers intentionally add anti-foams to fermenters to suppress foam during fermentation and increase the capacity of their fermentation vessels.

I can understand everything so far, but now I am faced with the piece of evidence that I cannot believe and that is the seeming lack of carbon dioxide gas. When beer ferments, nearly half of the sugar (by weight) is converted to carbon dioxide gas and nearly all of this gas leaves the fermenter. Your beer did ferment, therefore carbon dioxide gas left the fermenter. Until physicists at CERN in Geneva present data showing that C6H12O6 does not release CO2 when fermented by yeast I will continue believing in fermentation as we know it. I think what happened is that you had a faulty carbon dioxide gas detector . . . I mean a leak in your airlock. Hmmm, I wonder if that is how they clocked those neutrinos moving a few nano-seconds faster than photons of light in their accelerator?

A leaky airlock would explain why your airlock wasn’t gurgling during fermentation and a slow fermentation would explain why shaking your carboy did not seem to generate much activity. Six weeks is a long time to ferment 60 gravity points and I think what happened is that you had a long and slow fermentation that never appeared to be doing much of anything. The only negative consequence of fermentations that drag out is the potential for contamination associated with slow pH reduction in the early hours of fermentation and off-flavors associated with under-pitching, often the cause of slow fermentation. Also, I don’t think contamination explains your observations. All microbes that consume carbohydrates for energy give off gas.

Response by Ashton Lewis.