Ask Mr. Wizard

Terminal Gravity Woes


Paul Epp — Via email asks,

Can I add alpha amylase enzymes to my fermenter to help my beer attenuate? My original gravity (OG) was 1.079 and it has been two weeks. Now my beer is at 1.040 and is not showing any fermentation activity.


Alpha amylase can be added to the fermenter to aid in attenuation, but it has limited efficacy in converting dextrins and starch into fermentables. The primary role of the endoenzyme alpha amylase in the mash is as a debranching enzyme that chops up amylose and amylopectin into pieces from the interior of these molecules. Although alpha amylase does produce fermentable sugars, the yield of fermentables is better when beta amylase works alongside alpha. Before digging deeper into the enzyme rabbit-hole, let’s back up a few steps and talk about the factors that influence the final gravity of a batch of beer.

The OG of wort increases as “stuff” is dissolved in water during wort production. All-malt brewers dissolve this stuff from malt, and this stuff is primarily made up of fermentable sugars, unfermentable dextrins, starch bits that may not have been converted in mashing, and proteins. Brewers who use enzyme-free adjuncts, like rice and corn, also dissolve stuff, mainly carbohydrates, from these ingredients. And then there are ingredients in the “other” category that contribute fermentable sugars, unfermentable sugars, e.g., lactose, carbohydrate gums like beta-glucan, and myriad flavor compounds from the wide range of non-traditional ingredients used by brewers. Wort OG does not tell the brewer anything about how much stuff in the wort that drives OG is actually fermentable; OG is simply the density of wort before fermentation. Salt water’s density is greater than pure water, but is not fermentable. You get the point.

For the sake of discussion, let’s assume that a batch of wort was made with 60% non-enzymatic rice adjunct and 40% malted barley with a diastatic power (DP) of 135 °Lintner. Rice solids are approximately 90% starch, so most of the rice used in brewing can contribute fermentable extract. But enzymes are required for this to happen and in this example the enzyme package from the malted barley has been significantly diluted to ~54 °Lintner by the rice adjunct. The enzymatic threshold cited for functional conversion is about 60 °Lintner, depending on mash time and temperature, and this example describes a mash that is borderline enzymatically weak. This situation does not mean that the rice will not contribute extract, however, but it may result in decreased fermentability.

Given the details provided in most recipes, the FG can be approximate at best and, at times, not much more than a guesstimate . . .

Continuing down this path, how does a brewer know what to expect from wort in terms of FG (final gravity) when brewing a beer for the first or fiftieth time? Many homebrewers like brewing new beers on a regular basis and rely on a recipe for the expected FG. Given the details provided in most recipes, the FG can be approximate at best and, at times, not much more than a guesstimate added to a recipe that has never been brewed. However, if the recipe is something that is routinely brewed by a brewer, the FG value holds much more weight, especially if the ingredients used for the beer are consistent and if the brewer is not constantly adjusting the recipe. My point in relation to your problem is that you probably don’t know what the FG of your brew will be after fermentation is complete unless you have performed a forced fermentation test (aka an accelerated form of fermentation).

How can a recipe have a guesstimated FG if the recipe has never been brewed by the recipe’s author? As it turns out, most infusion-mashed worts have an apparent degree of fermentation (ADF) somewhere in the 70–75%, depending on the yeast(s) used for fermentation. For example, a 1.065 (16 °Plato) with an assumed ADF of 75% will finish at 1.016 (4 °Plato). This approximate value is useful when brewing a beer for the first time and also handy when trying to troubleshoot a problem. Your 1.079 (19 °Plato) wort should probably finish somewhere around 1.019 (4.75 °Plato). Assuming that you have produced a relatively “normal” wort in terms of ingredient selection and production method, your current gravity is quite a bit off the normal course.

The real question for you is “what’s the problem?” And the answer to this question likely lies on one of the following buckets: 1) yeast, 2) pre-fermentation preparation, and/or 3) wort properties. Here is a high-level view of what may be considered to help narrow things down:

• Did you pitch enough yeast? The rule of thumb for a 1.075 SG wort is somewhere in the 10–25 million cell/mL range. Higher gravity brews require more yeast than lower gravity brews.
• Was your yeast in good health when it was pitched?
• Is it possible that you are using a highly flocculent strain that dropped out early?
• Does your chosen strain do well in higher gravity fermentations?

Pre-Fermentation Preparation
• Was your wort properly aerated/oxygenated?
• Did you add yeast nutrients?
• Did you add a source of zinc?
• Did you cool the wort to the approximate fermentation temperature?
• Is the fermenter located in a stable environment with a relatively constant temperature?

Wort Properties
• Do you know what the actual FG should be?
• Is it possible that your wort is nutrient-poor?
• Did anything occur during mashing/wort preparation that makes you suspect that adding enzymes is going to fix the problem?

Unless you really diluted your malt enzyme package, used a boat-load of starchy adjuncts as steeping ingredients added to an extract wort, or mashed at a very high temperature (non-calibrated thermometer), I don’t think enzymes are your problem because your gravity is so off of the mark. If I had to place a bet on the root cause, I would place my wagers on yeast and wort zinc level. I am not going to address what you could have done differently before fermentation to address these issues because that is a separate question, so I will finish on what you may want to do to help your fermentation finish, and that is to kräusen your beer.

On the surface, kräusening is just a traditional method of carbonation. And in today’s world of brewing, traditional is oftentimes lumped into the bag of “boring tricks” that your grandfather used. But kräusening is much more than a carbonation method and is the perfect tool to dust off when fixing fermentation problems. The method is to make a “kräusen beer” that is about 15% of the volume of the beer being rescued. This kräusen beer can be different from the beer being kräusened; that’s OK if the blended recipe
is considered.

Kräusen beer is essentially a yeast starter that is added to beer after fermentation, or in mid-fermentation to stuck fermentations, for the purposes of carbonation and flavor maturation. Traditional lager brewers would add kräusen beer to the lagering tank, seal the tank up with a relief spunding/bunging valve, and allow the beer to carbonate and mature. Homebrewers can kräusen a batch without capturing the carbon dioxide if the goal is to simply help the fermentation finish and provide fresh, healthy yeast cells to clean up things at the end of the game.

If I were you, I would kräusen this batch before ever thinking about adding enzymes, unless I had a very good reason to suspect that the beer in my fermenter had a lower-than-usual wort ADF and that the enzyme treatment was more than a Hail Mary. By the way, granddad’s brewing toolbox had a lot of really cool and useful tools that the modern brewer should consider dusting off!

Response by Ashton Lewis.