Managing Dryness: Malt enzymes and yeast choice
Do you have a dryness target for the next beer you brew? Many factors play roles as you make choices for a new or modified beer recipe. Your goal for ethanol content (ABV) is probably among them. Likely you also consider the color of your eventual beer. Prominent for many brewers these days are the twin factors of hop bitterness and hop aroma. With specifically hazy beers among those currently popular, you may be considering deliberate haziness (or, conversely, bright clarity) as well. Overall flavor, aroma, and mouthfeel will be top of mind. But today we will specifically address dryness. That is, how low will you finish your final gravity and how will it relate to your starting gravity?
Homebrewers do, of course, consider whether a new recipe is intended to be dry and crisp, malty and rich, or something in between. Many of us consult the Beer Judge Certification Program (BJCP) Style Guidelines for reference when we set out to brew a new beer. There we find key parameters like IBUs for bitterness, SRM for color, ABV for the target alcohol level and, related to this column, both original gravity (OG) and final gravity (FG) ranges for the style we plan to brew.
When assessing numerical values for final gravity, we are comparing the density of the beer at the end of fermentation to the density of pure water, 1.000, expressed as a ratio. The density exceeds 1.000 by an amount that is loosely ascribed to residual maltiness or unfermented carbohydrates that include dextrins and sugars.
In reality, it is an empirical measure that includes any dissolved materials heavier than water, minus an offsetting figure that represents the extent to which the alcohol in the beer is lighter than water. Ethanol’s specific gravity is 0.79, so the overall FG is water plus dissolved solids minus the density reduction from ethanol. Beers can have actual final gravity readings that range from below zero for a very dry American light lager to perhaps as high as 1.040 for a wee heavy. The former would be perceived as “very dry” due to the very low level of residual sugars or other carbohydrates. A wee heavy on the other hand might be described as “sweet” or “malty.”
Some consumers find overt sweetness indicative of under-attenuation — failure to complete the fermentation of the particular beer wort. Maltiness, though, might be found to represent carbohydrates beyond simple sugars in the beer. From the measured OG and FG of any beer we can calculate a measure of fermentation completeness: Apparent attenuation (sometimes referred to as apparent degree of fermentation, or ADF). Expressed as a percent, apparent attenuation = (OG – FG)/(OG – 1) x 100.
Apparent attenuation is the measure I will discuss today. There is another measure known as real attenuation (or real degree of fermentation), but for our purposes it is not needed in this context. Yeast producers will typically use apparent attenuation to express a yeast’s fermentation efficacy.
There are additions you can make to a wort to help “dry out” your beer. Simple sugars are 100% fermentable, so an addition of corn sugar (glucose) or table sugar (sucrose) will add gravity (and alcohol) with no addition to residual maltiness. Many adjuncts have similar influence, with rice and corn typically used to make a lighter beer (see Mr. Wizard’s explanation why).
Even with an all-malt wort, there are exogenous enzyme products derived from fungal cultures that can help you make a beer with reduced residual carbohydrates. (For more on exogenous enzymes in brewing, see the October 2020 issue or online at https://byo.com/article/explore-the-world-of-exogenous-enzymes/.) On the other hand, brewers can make additions to create a “bigger” tasting beer. Using a high percentage of specialty malts or sugar additions like lactose or maltodextrin will typically raise the final gravity of the beer.
Managing Mash Enzymes
But with a given grain bill there are two natural methods for managing dryness in an all-malt beer: Manipulating mash enzymes through temperature control and selection of a yeast strain based on its attenuation capability.
There are two primary enzymes that break down the barley starches, beginning at the malting stage. When barley kernels are moistened, the embryo begins to grow, just as though the plant were growing in a field. That germination sets in motion the production of α-amylase enzymes, joining the other enzyme group, ß-amylases, in the barley kernel.
Both enzymes degrade starch into fermentable sugars and sometimes non-fermentable dextrins. As the embryo begins to utilize the sugars, starch breaks down and enzyme concentrations rise. Since the maltster is selling starch to the brewer, a time comes to cut off the embryo’s access. The grain bed is heated to inactivate the embryo but leave the enzyme population, which is relatively heat-stable, in place. The malt may be further toasted or roasted for color or flavor, but the mild heating of pale malt makes it possible to brew.
When the brewer crushes barley malt and then mashes it, the enzymes are reactivated and go to work breaking down starches without further participation by the now-inactive barley plant. Although α- and ß-amylases are at work in the mash, there is enough difference in their biochemistry that this is a point to influence fermentability and ultimately dryness. The mash needs to be hot enough to solubilize the starch, but not so hot as to inactivate the enzymes.
The enzymes have different temperature sensitivities and they act differently on the starch substrate. ß-amylase attacks from one end of a starch molecule and systematically cuts off maltose units. It has particular affinity for large molecules and rapidly begins breaking down chains of amylose or amylopectin. The process can continue to completion with amylose, but the enzyme is less effective and eventually gives up entirely as it approaches the branched part of amylopectin. It is often called the saccharifying enzyme as it releases simple sugars that contribute directly to fermentability. α-amylase, however, can attack any α-1-4 link in the starch molecule, but acts randomly. That means the initial activity of that enzyme produces many different dextrins and only tends to produce high levels of fermentable sugars later in the mash when breaking random bonds in small chains that will result in sugars.
Beers can have actual final gravity readings that range from below zero for a very dry American light lager to perhaps as high as 1.040 for a wee heavy.
As a brewer, you can manipulate these enzymes using temperature control for desired wort fermentability. Both are heat tolerant, but α-amylase is more so, surviving in temperatures as high as 160 °F (~70 °C). ß-amylase, on the other hand, works best around 130 to 140 °F (~55 to 60 °C). I brew primarily with single infusion mashing. The well-modified American and European malts available to me yield good extract levels even at a fairly low mash temperature like 145 °F (63 °C).
When I want a highly fermentable wort to produce a dry beer, I will use an infusion mash temperature like that, activating the ß-amylases to chop off regular chunks of fermentable sugars. On the other end, if I am working on a very malty beer recipe like a strong Scotch ale or milk stout, I will go to a much higher infusion temperature, possibly up to 160 °F (71 °C). Mashed in that range, the α-amylases are actively producing random starch fragments, some of which are unfermentable dextrins.
The less active ß-amylases, meanwhile, do not break down those fragments. The resulting beer still presents a satisfactory OG, but with a higher FG, even fully fermented. If you use other than single infusion mashing, the principles still apply. Low mash temperature supports fermentability and high mash temperature suppresses it.
There are a few other considerations when coming up with a mash schedule. Not only are the rest temperatures important but also the duration of each rest and possible time between the rests if using a step mash or when mashing out. A half-hour mash will yield a less fermentable mash, even if all the starch is converted, compared to a one-hour mash at the same temperature.
While the starch is chopped up in the shorter mash and showing negative in an iodine test, the extra time will allow for continued action by the amylase enzymes to more finely dice the dextrins. Also, if you’re running a 1500W recirculating infusion mash system (RIMS) for a 12-gallon (45-L) batch, ramping up to mash out will take a while and that time needs to be taken into consideration.
Yeast Attenuation
Once a wort of given fermentability is on hand, a next influence on dryness is your choice of yeast. If you really want to perfect a single recipe, you could split a batch or make multiple batches the same way with different yeast strains. Most of us, though, are going to look at the yeast producer’s product literature and decide from there. In determining the published figures, the producer probably presents the candidate yeast with fairly standard, straightforward conditions.
The wort will likely be all-malt and the gravity will be in the low-to-medium range to avoid creating especially difficult fermentation conditions. The simple calculation of apparent attenuation is what will appear in the guidelines. Those published values are often long established, subject to periodic verification as part of ongoing quality control.
If instead of standard conditions, you want to try an extreme brew with unusually high gravity or non-standard fermentation temperatures, do not necessarily expect textbook attenuation. That said, you can certainly look to the published figures for yeast strain comparisons. The ranges for different strains are different enough that you can easily get a different beer from the same wort, based on yeast selection alone.
While most attenuation percentages are in the 70s, they can drop into the upper 60s, particularly for some British ale yeasts. That is where you want to look if you are designing a rich, malty, full-bodied traditional-style ale as these strains are unable to ferment maltotriose. At the other end of the scale, the high end of attenuation can rise above 80% for some strains. You might be seeking something like that for a light or amber lager where you want a crisp, clean finish. If you go further afield and get into strains for farmhouse ales that may include non-Saccharomyces yeasts, you may find figures that rise well beyond the standard range for beer.
One yeast strain of noteworthiness when discussing attenuation is S. cerevisiae var. diastaticus. Most longtime brewers would know this strain as it’s common among saisons and Belgian yeasts, among others. You must note that that these strains emit an enzyme called amyloglucosidase, which can continue the work of mash enzymes and break down dextrins into simple sugars. If you are looking for a way to really dry out a beer, then choosing one of these yeast strains is one way to achieve that goal.
Amyloglucosidase can be found elsewhere as well. As many unfortunate breweries have found, these enzymes are also found in hops and when a beer is dry hopped, can work to re-ignite fermentation. Amyloglucosidase can be purchased separately as well.
So when choosing a yeast to match your goals, look to the published yeast strain attenuation figures. Keep in mind that you have a lot of control over the eventual dryness of your beer beyond wort fermentability and yeast choice. You also need adequate oxygenation, good nutrition, a sufficient pitch, and steady temperature control. The yeast choice can nudge your dryness one way or the other, but you are the primary guide. If the dryness you get the first time doesn’t exactly match your expectations, keep in mind something someone once told me: “The yeast don’t know what the numbers are.”
