Finishing A Fermentation Strong

Whenever there seems to be an attenuation problem with a batch, I think it always helps to go through a mental check list of the basics. This checklist of mine includes the following:

• Mashing issues?
• Yeast nutrients and/or zinc added to the wort?
• Yeast into wort cooled to the correct temperature?
• Wort aerated normally?
• Sufficient yeast added to the fermenter?
• Anything unusual that may have caused an early end to fermentation?
• Yeast strain known for lagging fermentation?
• Final gravity known?

I am going to run this list to illustrate a point about mental checklists, so bear with me while I talk to myself!

Mash OK? It looks normal. 151 °F (66 °C) for an hour puts the mash right in the middle of the peaks for beta and alpha amylase. All malts had good enzyme levels (nothing really high kilned) and the adjunct ratio looks OK.

Nutrients added? Hmm, I typically don’t worry too much about FAN (free amino nitrogen) or phosphorous with all-malt worts, so did not add nutrients to this batch. Should consider this topic more for the next brew. I did add zinc to the wort (0.20 mg/liter). I am not thinking that nutrients are the issue.

Wort temperature OK? Wort was cooled to 61 °F (16 °C), pitched with yeast from my propagation at 65 °F (18 °C), and the fermentation was allowed to free-rise up to whatever temperature was in the fermenters with a room temperature of 65 °F (18 °C). Not sure what the actual fermenter temperature was, but probably not too cool or too warm.

Wort aeration? This batch was aerated normally, but I really don’t know much about how much oxygen is in my wort. I used oxygen and a stone, and gave each fermenter a 90-second blast after filling. Should probably spend some time thinking about this a bit more.

Yeast pitching rate? A good rule is to multiply the propagation volume by 8–15 to determine how much wort can be pitched. Lagers are toward the 8x end of the range and ales towards the 15x end. Three liters x 15 = 45 liters of wort. This batch was 50 liters; maybe a bit on the low side but probably not too low. No microscope . . . how can I ball park the pitch rate? I have read that propagated yeast that is aerated through a cotton plug in the top of a stirred flask (magnetic bar and stir plate method) normally provides 100 million cells/mL (1 x 10⁸ cells per mL or 1 x 10¹¹ cells/liter). This works out to 3 liters x 1 x 10¹¹ cells/liter ÷ 50 liters = 6 x 10⁹ cells/liter = 6 x 10⁶ cells/mL = 6 million cells/mL. Pitching rate looks good. Note to self: Brush up on using scientific notation.

Fermentation normal? Nothing odd. Room temperature constant during fermentation, and no big weather swings.

Strain particulars? White Labs website indicates this strain works best between 68–73 °F (20–23 °C) range and usually attenuates beer in the 70–75% range. The original gravity (OG) was 1.061, so the expected FG is in the 1.015 to 1.018 range based on attenuation range from the web. Could lower fermentation temperature be causing issues?

Known final gravity (FG)? According to what I just calculated the FG should be in the 1.015–1.018 range and the recipe is telling me 1.012–1.018. That’s a little funky.

I don’t know how most people go about troubleshooting, but the linear approach above is how I go about thinking through brewing problems. These items from above stand out: Enzyme dilution, wort aeration, fermentation temperature, and the target final gravity.

When adjuncts like flaked oats are used, it is important to think about enzyme dilution. Most New England IPAs contain about 30% adjunct (calculated as portion of total wort extract), use North American 2-row base malt, sometimes contain malted wheat, and may contain a splash of higher kilned pale malt as a flavor boost. Calculating blended diastatic power (DP) and dextrinizing units (DU) is easy if malt analyses are available. This question does not have enough information to calculate a weighted average for DP and DU, but a good rule of thumb for North American malt is that adjunct ratios up to about 25% are no problem at all. If a good dose of wheat malt in the 10–20% range is used, bumping the adjunct ratio up to 30% rarely causes a problem because wheat malt is usually more enzymatic than barley malt. One problem that flaked adjuncts can cause is starch extraction after mash-off because not all flakes easily yield their extract. For this reason, a cautious method is to skip mash-off and delay kettle heating until the kettle is about 80% full. Both of these steps extend the brew day, but starchy wort is one cause of reduced fermentability.

Wort aeration is a topic that interests me as a practical brewer because it has a very real influence on yeast growth, fermentation rate, and beer flavor, but is rarely measured by the majority of brewers because dissolved oxygen meters (DO) are relatively expensive and measuring DO in wort is not required if aeration or oxygenation is consistently performed. This is pretty easy to do when a controlled volume of tiny air or oxygen bubbles is added to cool wort. And the easiest way to measure gas flow rate is with a rotameter. Although these gadgets are easy to find and are not expensive, few homebrewers measure air or oxygen flow and, instead, rely on time. Flow rate through a stone can easily change as a stone becomes fouled with protein and hop resins, making control by time approximate. High finish gravities can definitely be related to wort aeration.

Fermentation temperature is a red herring for most fermentations because cool fermentation temperatures typically do not result in high final gravities, they just slow the rate of fermentation. Most ale strains have no problem fermenting in the 61–64 °F (16–18 °C) range, especially when a healthy pitch rate is added to well-aerated wort.

Target final gravity is the last item on my list of possible problems, and is really the problem I most suspect. Recipes and yeast profiles provide little more than an educated guess about the final gravity of the batch of beer a brewer brews using another brewer’s recipe with a yeast purchased from any number of reputable sources because the final gravity is a function of wort carbohydrate spectrum coupled with yeast strain. Even when the same beer is brewed in the same system, variations in the FG are not uncommon. Wort carbohydrate spectrum is influenced by grist bill and the specific characteristics of the grains used, malt milling, brewing water chemistry, mash time and temperature, sparge temperature, and wort temperature in the kettle during wort collection. There are definitely lots of things that can influence wort fermentability from a single recipe.

The only way to know the true target FG is to perform a forced-fermentation test where an excess of the fermenting strain of yeast is added to a sample of wort from the brew in question, and the fermentation accelerated in a stirred flask. The American Society of Brewing Chemists (ASBC) forced fermentation method specifies adding 1 gram of compressed yeast to 250 mL of wort and allowing the fermentation to complete, as determined by consistent gravity readings measured over a 3-hour (minimum) time period. Many commercial breweries, especially those that bottle condition, perform forced fermentations on every single batch.

You suggested in your question that adding more yeast may help. This is a very helpful method when you know for certain that your fermentation has not finished. I think kraüsening is the best way to add yeast to a stuck fermentation because adding yeast at high kraüsen helps ensure that the yeast will quickly finish the fermentation, mop up acetaldehyde and diacetyl, and be done. Simply adding more liquid yeast that is not metabolically up and running will not have the same effect as adding yeast in high kräusen.

Diagnosing problems on paper is difficult, but my bet is that you have a combination of factors leading to a higher-than-expected FG, coupled with perhaps inconsistent wort aeration, that has resulted in three fermenters of wort from the same batch ending at different gravities. If that is the case, kräusening should be considered.

Response by Ashton Lewis.