Hitting Final Gravity

Q. My last couple imperial stouts stopped fermenting around 1.040. Original gravity was 1.100, which includes about 2⁄3 liquid malt extract and 1⁄3 grain (about 50% 2-row pale malt plus specialty grains). I use a single step infusion mash at 152 °F (67 °C), standard sparge and boil, before running through a plate chiller into the fermenter, including inline oxygenation. Last batch, I used LalBrew Nottingham. After speaking with the nice folks at Lallemand (and against their advice to pitch multiple packs), I made a 2-step starter that should have produced about 700 billion cells to pitch at 65 °F (18 °C). I’ve held it at 62–65 °F (17–18 °C). After five days the beer was at 1.041, and after 14 days had stopped at 1.038. According to your video on high-gravity brewing, unfermentable sugars should limit final gravity to about 1.022. What am I missing?
Gary Asher
Chapel Hill, North Carolina

Mr. Wizard say…

A. I agree that you should expect a lower final gravity for this style. Although there is not an obvious problem based on what you describe, several factors can cause high finishing gravities in big beers. Let’s go through them in search of solutions!

Yeast pitching rate often ranks as the most important consideration when brewing big beers. Without question, pitch rate is vital for all fermentations, especially with styles that subject yeast to high osmotic stress and the high alcohol that follows. In your case, it appears you covered the bases and have pitched ample yeast.

Wort oxygenation is another key consideration when brewing high-gravity beers, but only when liquid yeast is used. Because I have already addressed this topic in an earlier question, I will not spend much time on it other than to point out that this is the time to use pure oxygen rather than air. If you did not aerate your propagation, it is possible that the cells you pitched needed more oxygen for growth.

Another consideration when brewing high-gravity styles is yeast strain. LalBrew Nottingham reportedly has an alcohol tolerance up to 14% and its attenuation range is 78 to 83 percent. That would take a wort starting at 1.100 to 1.017–1.022, which is much lower than what you see. 

Wort composition is another important factor. There are three sugar classes relevant to non-diastatic yeast: Monosaccharides such as glucose and fructose, disaccharides such as maltose and sucrose, and maltotriose. Yeast preferentially brings glucose and fructose into the cell because these are easiest to transport. Although sucrose is a disaccharide, it is split outside the cell by invertase, associated with the cell wall, into glucose and fructose and is therefore similar to those sugars in terms of uptake. Maltose, unlike glucose and fructose, requires the expenditure of energy to transport across the cell wall. That is one reason maltose is used after the simple sugars are consumed. Maltotriose is important to brewers because most lager strains and many ale strains ferment it, while others do not.

Maltotriose utilization is not typically shown on yeast specification sheets, but it can be inferred. If the upper end of the attenuation range is around 80 percent or higher, the strain ferments maltotriose. If the upper end is less than 75 percent, the strain ferments little if any maltotriose. 

The next carbohydrate class to consider are dextrins. Dextrins are not fermented by brewing yeast, but diastatic yeast secrete glucoamylase that breaks down dextrins into glucose that can then be fermented. Examples include certain saison and farmhouse strains and many Brettanomyces species. Yeast suppliers clearly identify which strains are diastatic.

Apologies for mixing fermentable carbohydrates with yeast specifics, but the topics are related and both matter. Nottingham is a non-diastatic strain that ferments maltotriose. That provides insight into what is possible, but it does not reveal wort composition details. Earlier, I used the phrase “typical wort” and now want to clarify. Yeast suppliers often use some type of standard wort, usually 12 °Plato (1.048 SG) Pilsner wort, for the trials used to determine fermentation profiles. This allows brewers to compare a supplier’s strains without questioning wort composition. When brewing styles with very different original gravities, assumptions about typical behavior change.

In your case, two-thirds of the wort extract came from liquid malt extract, one-sixth from Pilsner malt, and one-sixth from specialty malts. While you controlled the mash for the malts you used, you likely know little about how the liquid malt extract was produced because those details are rarely shared. Your case is a common example illustrating why it is difficult to predict how a wort will compare to the standard wort used by suppliers unless you have brewed something similar before. The best way to gauge fermentability is to run an accelerated fermentation trial by over-pitching about 500 mL of wort, fermenting warm, preferably on a stir plate, to determine the approximate final gravity. This eliminates guessing whether your fermentation is complete.

Lastly, let’s discuss yeast nutrients. The most common deficiency in wort is zinc and I am a firm believer in adding zinc because it consistently supports faster and more complete fermentations. When using high levels of brewing sugars such as sucrose or dextrose, wort can become nitrogen deficient. 

If my hunch is correct, start with a forced fermentation test to determine where the true finish line lies. That piece of information may provide some closure to this conundrum!