Stuck Fermentation, Yeast Biology and Attenuation

The maple syrup fermentation is going a little funky. About 24 hours into the fermentation, activity has slowed dramatically, which is several days ahead of schedule.

“Stuck fermentation” is essentially when the yeast stops fermenting before it should, or at least before I think it should. Most wine with normal wine yeast can ferment to dryness, which means that the gravity gets down to 1.000 and there is very little residual sugar left in the wine. Beer is another story. Beer always has a certain amount of residual sugar which is left unfermented, usually because it's not actually sugar, but starches that were not fully converted into sugar during the brewing process. But sometimes sugar does remain in the beer and the yeast go inactive for some other reason.

When you’re buying yeast, you’ll see something called the yeast’s “attenuation” rating, which is essentially the percentage of sugar which is consumed by the yeast. When the yeast gets to the end of the fermentation process, they begin to flocculate, which means they settle to the bottom and go dormant. Different yeasts have different attenuation ratings, which mean they can be expected to begin flocculating and going dormant with different levels of residual sugars/starches left in the beer. Where you end up in the provided range is usually dependent on how difficult a beer you are trying to ferment. For example, higher gravity beers will be more difficult to ferment and will have a lower attenuation rates. These usually require the more aggressive yeasts, such as wine yeasts. In some cases, brewers start off with an agressive beer yeast and then finish with an agressive wine yeast. This gives you the best of both worlds; beer yeast which adds distinctive flavors to the beer and wine yeast which adds no flavor but can handle difficult conditions like high alchohol levels and difficult to ferment sugars.

Usually you don’t need to worry too much about attenuation, because your beer will ferment cleanly and normally. But sometimes the yeast stops when there is still a good amount of sugar left in the beer. This may go well with certain styles of beer, but may be undesirable in others. Unfortunatly your hydrometer measures suspended solids, which could be sugars or starches that were not converted in the brewing process. So essentially you could have stuck fermentation with yeast that shut down before eating the sugar or you could have unfermentable starches, and you wouldn't be able to tell the difference. Why does this happen? My understanding is that it's either a weak boil (so the enzymes couldn't work their conversion magic) or a lot of unique adjunct grains which add lots of stuff other than sugar. Interestingly enough, Beano (yes the stuff for gas) can convert the starches to sugar, but it can be a slow process and isn't really practical for brewing.

Most yeasts have an attenuation of between 65%-85%. This means that you can expect the yeast to ferment between 65%-85% of the available sugar. With a 1.050 beer, you’d get down to 1.010 to almost 1.000. However it should be noted that there are many extrinsic factors which control how far your fermentation progresses aside from the yeast choice, for example unfermentable starches. In my example, you’d never get down to 1.000 because the yeast would never be able to ferment to dryness. You’d only approach 85% attenuation if you started with a higher gravity beer, which may have its own fermentation problems. The relationship is somewhat like a parabola.

With the IPA and Maple IPA I brewed last weekend, I started with gravities of 1.046 and 1.055 respectively. Adjusted for the temperature (as gravity readings are set to 60 degrees, so you need to calculate for other temperatures), the actual gravities are 1.049 and 1.058 respectively. Twenty-four hours later the gravity of both brews is at 1.020 (or 1.021 adjusted for the temperature). That’s an attenuation of 45.5% (ABV of 3.69%) for the IPA and 50.7% (ABV of 4.89%) for the maple IPA. Check out this great calculator to do your own calculations. These numbers are very low and further, the gravity is still too high with a good amount of available sugar left. Though it’s interesting that they started at different gravities and both stuck at the exact same gravity only 24 hours later. So we may actually be looking at problem of unfermentable starches.

The generic yeast I used has “medium attention” and “low flocculation”. Higher quality liquid yeasts would probably give me more specific parameters, but essentially this means that the yeast is in the middle of the road when it comes to attenuation (approx 75%?). Low flocculation means that the yeast doesn’t drop to the bottom of the fermenter quickly. This means that my beer is probably still fermenting at a low level, as some yeast is still in suspension and still working. This can be tested by checking the gravity over a few days to see if it's still dropping, but the activity is likely so low that it won't make a difference. Yeasts with low flocculation are also harder to clarify, because the yeast isn’t as prone to settling out of the beer. In any event the fermentation is clearly winding down, as I can also tell by the fact that yeast cake on top of the beer has fallen (or flocculated) to the bottom of the fermenter. As with all ales, an active fermentation is evidenced by a thick, goey, bubbling layer of yeast on top of the beer- also called krausen. Lagers, as everyone knows, ferment from the bottom.

So what to do? Given the choice between a sweeter beer with lower alcohol and a dryer beer with more alcohol, I’ll choose the latter anytime. There are different theories about rousing the yeast to continue fermenting, but the best solution, and the least likely to oxidize the beer through unnecessary mixing, is the inoculation of a more aggressive yeast. In this case, I’m going to try using some champagne yeast to get the beer going again. Aside from the fact that I happen to have some champagne yeast lying around, it is also very aggressive and will ferment almost anything. Given the fact that there is partially fermented wort, it's almost mandatory to do a big yeast starter so the yeast is raring to go when it hits the beer. Dumping in dry yeast, typically not the best practice, is even less of a good idea, as the yeast will likely go dormant.

So is this a stuck fermentation or simply a lot of unfermetable starches? I didn't go crazy with the adjunct grains on this one and I had a solid boil, so I don't have a theory on why I would end up with a high final gravity due to unconverted starches. Stay tuned to see how successful I am with this rescue operation.