Priming with flavorful sugars, ALDC, aerating dry yeast, & low FGs

Q. I want to add dark sugar flavors to an ale and am thinking about priming with muscovado sugar instead of corn sugar. This got me wondering, when using other types of sugar like muscovado, brown sugar, or maple syrup, is the addition amount always similar to corn sugar (I usually use ¾ cup of corn sugar for a 5-gallon/19-L batch). 
James Eaton
Via email

A. Using flavorful sugars and syrups for bottle conditioning is definitely a way to add flavor to beer. One helpful way to assess the contribution of these ingredients is to add a dose to chlorine-free water — distilled or reverse osmosis works well — at the same rate you would use for conditioning. In this sort of trial, mentally zero out the sweetness and focus on other flavors, aromas, and color, because those are the components that remain after the sugar is converted to carbon dioxide and alcohol.

The dosage rate of priming sugars, whether solid or liquid, is best measured by weight. And for accurate priming, beer volume is essential. Although it is possible to prime individual bottles using bottle volume in the calculation, this method is tedious and best reserved for small bottling runs unless you simply love the process. Whether you use a bottling bucket, keg, fermenter, or carboy for batch dosing, you must know the beer volume. While the exact carbon dioxide content of beer post-fermentation is also required, an estimate is sufficient for establishing a dosing range.

The first step is determining how much carbon dioxide you need to add. Most beers contain about 4.4–6.0 grams of carbon dioxide per liter of beer, which corresponds to roughly 2.2–3.0 volumes, but priming calculations are simplest in g/L, so we’ll stick with that. Most beers fermented atmospherically, i.e., without a spunding valve, contain 2–3 g/L of CO₂. This means we need to add between 1.4–4.0 g/L. For the examples that follow, I’ll use 3 g/L.

The second step is calculating the dry weight of sugar required to carbonate the beer volume. You must measure volume for anything more precise than an educated guess. Calibrated sight strips on translucent or clear vessels, dipsticks, or net weight (with known specific gravity) for stainless tanks are common methods. For this discussion, let’s assume 19 liters of beer are ready to bottle. The required dry weight of priming sugar is 19 liters × 3.0 g/L = 57 grams.

Here’s where things get approximate: You must know the moisture content of the sugar or syrup used
for priming. Cane sugar typically contains about 5% moisture, or 95% solids. To achieve 57 grams of sugar
on a dry basis, divide by 0.95. The calculation yields 60 grams of sugar from the bag.

Things get trickier with syrups because most do not list moisture content, so you need either a product specification sheet or online reference data. Even with spec sheets, you’ll find ranges. For example, Candico amber candi syrup from Belgium ranges from 78.0–80.5 °Brix. Using that range and the 57-gram target, you’ll need 71–57 grams of syrup. That narrow range contrasts with honey, which ranges from 70–88 °Brix — and therefore requires 65–81 grams to supply the same 57 grams of dry sugar.

Hope this information proves handy!

Q. I recently watched your video on controlling diacetyl. Would it be a good idea to just use ALDC in your cooled down wort as a “normal” everyday ingredient when you add your yeast? 
Rick Bray
Omaha, Nebraska 

A. I’ll start with a summary for those who aren’t familiar with alpha-acetolactate decarboxylase (ALDC). This enzyme is added to wort with yeast for one purpose: Converting the diacetyl precursor, alpha-acetolactate, into acetoin during fermentation. It’s also commonly added to beer a second time during heavy dry-hop additions because hop creep often triggers another fermentation phase in which diacetyl spikes. These ALDC additions essentially eliminate the need for a diacetyl rest and the worry of buttery beers.

The short answer to your question is EZ-PZ: If you want a hard stop on the possibility of yeast-related diacetyl in your finished beer, ALDC is the answer. But there is a “but.” Some brewers worry that hitting the Easy Button on diacetyl bypasses processes that slower, more traditional aging naturally resolves. What follows is an expansion of this answer, starting from the beginning.

During normal fermentation, yeast cells process wort components to synthesize essential compounds required for cell division, energy production, and all the intercellular goo and glue of life. One compound synthesized by yeast is alpha-acetolactate. This biochemical intermediate — typically used for valine synthesis — is secreted when metabolic processes back up. Outside the cell, alpha-acetolactate is chemically oxidized, carbon dioxide splits off, and diacetyl remains. And this is why some beers smell like butter.

During traditional aging, diacetyl is absorbed by hungry yeast cells as a source of energy. Biochemically, diacetyl acts as a hydrogen acceptor and a metabolic restorer of NAD+, which is vital to the continuation of many enzymatic reactions. In the process, diacetyl is transformed into acetoin with a much higher flavor threshold (odorless unless the concentration is very high), and acetoin is excreted from the cell. Whether we’re talking about lagering, kräusening, or cask conditioning as benchmarks for traditional aging, all are relatively slow. That slowness differentiates them from the accelerated approach many brewers take when using ALDC.

But there is much more to beer aging than diacetyl reduction. Acetaldehyde — another intermediate expelled when biochemical traffic backs up — is also reduced by yeast and converted to ethanol. When left in beer, acetaldehyde contributes green apple, pumpkin/squash, and latex-like aromas. For what it’s worth, I am better able to detect acetaldehyde when thinking about the smell of latex paint because there is rarely a beer where I associate acetaldehyde with green apple or pumpkin/squash.

Yeast and haze-particle sedimentation, flavor integration, and natural carbonation are additional changes that occur during aging. While ALDC can shorten total aging time, there are benefits to longer conditioning, and this is why some brewers view ALDC as something that may solve one problem while potentially opening the door to others.

This answer could have gone in a completely different direction if your question had been about bioengineered yeast strains. There are yeast strains available today that have had a gene deleted, preventing them from excreting α-acetolactate. These are known as DKO, or diacetyl knock-out, strains. Other strains have been modified to synthesize and excrete ALDC.

In practical terms, these options highlight the importance of diacetyl-free beer to commercial breweries, since consumers generally don’t like buttery beers, and brewers know that diacetyl is filling and reduces drinkability.

Q. Is there a downside to aerating my wort with oxygen after pitching dry yeast? I feel like I’m being obsessive, but I don’t perceive any off-flavors from poor yeast health when I do it.
Mike Seward
Barrington, Rhode Island 

A. This is a topic I’ve softened on over the past nine years. Before being exposed to the gory details of the day-to-day practices of small craft breweries on a near-daily basis, I held a strict belief that pure oxygen should only be used when dissolved oxygen or oxygen mass flow into wort could be measured. Too much oxygen leads to oxidative stress in yeast cell membranes. Over time, with repeated oxygenation, fermentation, and harvest cycles, oxidative stress reduces yeast vitality and viability, decreases fermentation performance, and is associated with increases in an array of compounds that negatively affect beer aroma.

My view has softened because many small-scale brewers, both home and commercial, do not re-pitch yeast enough generations for oxidative stress to become a meaningful issue. Furthermore, research suggests that oxidative damage isn’t exclusively tied to high wort oxygen levels. Membrane damage aside, wort oxygen levels unquestionably affect beer aroma, and aeration/oxygenation control is something to consider as a flavor control point.

OK, enough theory. Let’s talk practical brewing. Ideally, dried brewing yeast are grown in precisely controlled environments that maximize biomass production, minimize ethanol production, and result in yeast cells rich in sterols, unsaturated fatty acids, and glycogen. Sterols and unsaturated fatty acids are oxygen-containing lipids yeast use to synthesize cell walls, and glycogen is the yeast equivalent of starch — used early in fermentation as a readily available energy source before sugars from the surrounding media are transported into the cell.

The bottom line is that high-quality dried brewing yeasts do not require oxygen from wort because they contain ample sterols and unsaturated fatty acids for cell growth. In fact, dried yeast contains enough of these oxygenated lipids to go without wort oxygen for more than one use, though most brewers do aerate when yeast is harvested from a dry pitch and used for a second fermentation. That’s not what you’re doing, and you do not need to aerate.

Are you causing problems with your current practice? No. There is no harm in aerating wort and pitching dried yeast. However, because wort oxygen levels do influence beer aroma — sometimes more profoundly with certain strains used for aromatic styles like hefeweizen than with more neutral strains — you may want to rethink using bottled oxygen compared to using air. More on this later.

For those using liquid yeast, wort aeration is not something to skip because liquid cultures typically do not contain the high concentrations of oxygenated lipids found in dried yeast. In my practical brewing experience, wort aeration — along with pitching the right amount of healthy yeast, cooling wort to a defined temperature, and adding some form of zinc — is one of the critical steps to get right before fermentation.

I’ve used the terms aeration and oxygenation intentionally. While often used interchangeably, I use aeration when air is the oxygen source and oxygenation when pure oxygen is used. The two differ because oxygen solubility depends on gas composition, temperature, pressure, and the presence of solutes in the liquid. These variables are reflected in Henry’s constants for different gases and solutions. In short, oxygen solubility from air is about five times lower than oxygen solubility from pure oxygen because air is only 21% oxygen. Most traditional beer styles do just fine when air is the oxygen source. Pure oxygen is a great tool for brewing bigger beers where high wort gravity reduces oxygen solubility. These beers also tend to benefit from additional oxygen because greater yeast growth supports complete fermentation.

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

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!