This week I take a look at how the use of high proportions of specialty malt can impact the fermentability of your wort, ultimately driving your attenuation, final gravity and alcohol percentage.
The Fermentability of Wort into Beer
First lets define a few terms to frame the discussion:
- Original Gravity: The original gravity of the wort before fermentation, typically measured in plato by pro brewers, or using the unitless specific gravity (1.050 for example) for home brewers. This is measured easily using a hydrometer or refractometer.
- Final Gravity: The final gravity after fermentation, measured again with a hydrometer or refractometer.
- Apparent Attenuation: Often just called the attenuation but it is the percent of sugars that appear to be fermented away – measured as (100 x (OG-FG)/(OG – 1.000). This is different from the real attenuation which is less often used takes into account that feremented alcohol has a gravity slightly less than water, so the real attenuation is slightly higher. However, brewers almost always use the apparent attenuation as its easier to calculate.
- Yeast Average Attenuation: The average attenuation for a given yeast strain, typically expressed as a range. Yeast is a major driver of fermentability, and the average attenuation numbers represent roughly how well a yeast strain might perform for a typical beer made appropriate to the style of yeast.
Specialty Malt and Its Impact on Attenuation
A number of factors will drive how well your wort is fermented into beer. These include the yeast strain selected, fermentation temperatures, health of the yeast, pitch rates, and many other environmental factors. However one thing many brewers ignore is that specialty malts can also drive fermentability, especially for beers made with a high percentage of specialty malts. Lower fermentability means a higher finishing gravity, lower attenuation and often an imbalanced beer.
The reason for this is that darker specialty malts tend to have fewer fermentable sugars and more unfermentable starch chains. So while adding a dark roasted malt will contribute to the original gravity and body of the beer, it won’t contribute much in the way of simple fermentable sugars like maltose. This can result in lower attenuation and a higher finishing gravity than expected if the specialty malt makes up a large portion of the malt bill.
How big is this effect? Results vary. One of the more authoritative studies, done by Castro, Affonso and Lehman, compared a 100% base malt against malts prepared with 20% specialty malts. It found that the measured fermentable sugars was lower in beers made with 20% specialty malts and generally dropped as the malts became darker. For example the baseline 2 row all malt wort had 98.7 g/L of fermentable sugars. Adding 20% Munich reduced it to 96.3 g/L of fermentables, Vienna was at 96.8 g/L, and Victory at the low of 84.6 g/L. Briess malts were used, so moving from 100% 2 row palt malt to 20% Victory (at 28 SRM color) resulted in a loss of 14 g/L of fermentable sugars in the wort which is a 14.3% loss of sugars.
Interestingly dark mashed Crystal malts also had a significant impact. Their experiment with 20% Crystal 60L, a fairly dark crystal malt, resulted in only 81.7 g/L of sugars, a drop of 17 g/L of sugars which is a 17.2% loss. Another widely quoted set of experiments posted by nilo on Homebrewtalk (post #108 here), gives slightly different results for Crystal malts. He found that mashed crystal malts were still pretty fermentable, as long as they remained in the 10-20% of the base malt range.
Though his measurements were not done in strict laboratory conditions like the earlier study above, he does show a drop in attenuation using a large portion of crystal malt (50%) as the crystal malts get darker. For example his 100% 2 row tests attenuated at an average of 80%, while those using half Crystal 10 dropped down to an average of 77%. Crystal 40 attenuated to an average of 69.5% and Crystal 120 attenuated to an average of 67%. While obviously using 50% crystal malt is extreme, it does show that the darker the malts, the lower your overall attenuation will be.
The above two studies give us some indication of what to expect for many kilned and crystal malts but does not tell us much about roasted malts. One would expect that roasting a malt at high temperature would substantially reduce the percentage of fermentable sugars, even after mashing. I did find one study by Marčiulionytė, Johnston and Maskell on using a 50% addition of dark roasted malt for distilling. In this case they roasted malts at a temperature of 140, 180 and 220 C for 30 minutes each and then used that in varying percentages up to 50% of the grist in a distilling wort. The drop off in fermentability was almost linear as you added more roasted malt. For the lighter roast malt, fermentability dropped only slightly from 82% to about 80% as the percentage of roast malt rose from 0-50%. The medium roast gave a drop from 82% to 76%, and the dark roast malt resulted in a significant drop down to 62%. From this we can conclude that clearly roasted malts do contain fermentable sugars, but the percentage will drop off significantly as the malts get darker.
Summary
The above studies were done using a 20% and two 50% specialty malt additions. While some beers may approach 20% specialty malt, most brewers should not be brewing worts with 50% specialty malt. However it does drive home the point that if you take a “everything but the kitchen sink” approach to your grain bill you will raise the final gravity and may not end up with the beer balance you were shooting for. Also clearly the darker malts have a bigger impact on fermentability than lighter ones.
As mentioned in my Principles of Good Beer Recipe Design post, you should always strive for simplicity in your beer design and only add the specialty malts that are really needed to achieve the flavor and body balance you want in a given beer. This means keeping the specialty malts to around 10-15% of the grain bill in most cases, and not trying to add too many malts that will just muddle the flavor of the beer.
How much of a loss in fermentability can I expect? The studies above show a drop of 1-10% in attenuation when using a high percentage of medium to dark specialty malts. What does this mean for a typical beer? Starting with a 1.050 (12.4 P) beer, a drop from 75% attenuation to 65% attenuation (10%) corresponds to a rise in final gravity from 1.012 (3 P) to 1.017 (4.3 P) which is a significant shift that would impact the beer balance. 10% more residual sugars would certainly create a sweeter beer than intended. However to go this far, you would need to be using at least 30% specialty malts, though the exact percentage would depend on how dark the specialty malts are.
I hope you enjoy this week’s article on specialty malt fermentability. Thanks for joining me on the BeerSmith Home Brewing Blog. If you want to take the guesswork out of brewing, please try my BeerSmith recipe software from BeerSmith.com. Be sure to sign up for my newsletter or my podcast (also on itune and youtube) for more great tips on homebrewing.