Dry hopping does weirder things to beer than we thought. While conventional logic—and all existing software models for calculating theoretical IBUs in beer—say that IBUs can only be generated on the hot side of brewing (since alpha acids can only be isomerized by heat), brewers have long suspected that dry hopping can, indeed, make an impact on the perception of bitterness.

Last year, Stan Hieronymus wrote in the August-September 2017 issue of Craft Beer & Brewing Magazine® about evolving research into IBUs and the strange way that extreme dry-hopping regimens can actually reduce iso-alpha acids in beers, ostensibly by causing more of those acids to precipitate out with the dry hops material.

In 2017, Jason Perkins of Allagash Brewing presented at the Craft Brewers Conference on a study they conducted with Oregon State University to test the impact of dry hopping on beer attenuation and found that by adding dry hops to a fully attenuated beer (their sample was Coors Banquet), they could cause significant additional attenuation in the beer. Over 40 days, those dry hops were able to drop the finished Coors Banquet from about 1.014 SG to 1.007, taking the beer from 4.9 percent ABV to 6.2 percent by creating enzymatic activity that broke down nonfermentable dextrins in the beer.

Applying that same question to dry hopping in hazy New England–style IPAs, New Belgium’s Ross Koenigs recently presented (at the Big Beers, Belgians, and Barleywines festival in Breckenridge) the findings of a study he did with a test batch that used no kettle hops and four different levels of dry hopping. While he’s continuing to refine his results with further testing, the preliminary findings could be very useful for brewers whose perceptions of the beer they brew don’t always match the calculations produced by their brewing software.

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One » Dry hopping definitely adds more actual IBUs than previously calculated. “We took an un-kettle-hopped base beer and split it out into four fermentations,” says Koenigs. “We did an entirely non-hopped control and did three different dry-hops iterations. We did 500 g/hl, 1 kg/hl, and 1.5 kg/hl.”

The IBUs, measured with New Belgium’s in-house spectrophotometer, were fascinating. The control batch tested for the expected marginal IBU level (2.3 IBU), but despite the complete lack of kettle hops, IBUs then increased dramatically and scaled consistently, with the 500 g/hl dry-hop batch testing at 44.9 IBU, the 1 kg/hl batch testing at 58.5 IBU, and so on.

Two » Higher dry hopping raises the pH of beer. The control batch in Koenigs’s test registered 4.46 pH, while the 1.5 kg/hl batch registered a 5.05. The pH moved on a relatively linear scale, increasing with the amount of dry hops.

“There’s definitely a pH rise as you increase hops material,” says Koenigs. “The cool part about that, too, is that as you alter your pH, it also alters your perception of bitterness. As you decrease pH, to a point, you’ll get a decreased perception of bitterness, and it’ll just feel more juicy until you get really low.”

While unrelated to New England–style IPAs, this is one reason more acidic dry-hopped beers, such as dry-hopped mixed-culture farmhouse ales or wild ales, present hops in such a fruit-forward juicy manner at those lower pH levels.

Three » Greater dry-hopping levels do increase attenuation of the beer. “ABV from the control batch to the highest is almost a full percentage point ABV off,” says Koenigs. “Hops material does have glycosidic enzymes—a combination of amyloglucosidase, beta-amylase, a little alpha-amylase—very, very small. If you look at it in terms of diastatic power, a base malt will be 150 DP, and this is a 0.2 DP, but it’s enough. Especially as you start going up in concentration of hops material, it will actually start to attenuate.”

For brewers, this is one of the larger take-aways. If your goal is bigger mouthfeel from a higher finishing gravity, heavy dry hopping will knock that down, so consider that when making decisions about elements such as mash temperature or dry-hops timing.

“The New England style is generally highly underattenuated,” says Koenigs. “When we ran lab tests of [fellow Colorado brewers and seminar participants] Outer Range Brewing and Cerebral Brewing, they had finishing gravities around 5.5 Plato (1.021 SG). The lowest we tested was Weldwerks Juicy Bits, around 3.8 Plato (1.015 SG). So brewers are gearing it toward full mouthfeel perception.”

Four » IBU is, still, an insufficient way to describe perceived bitterness in dry-hopped beers. Echoing the findings of others, Koenigs found that the trained sensory panel at New Belgium Brewing pegged the blind samples at much lower levels of bitterness than their measured IBUs would suggest. As we drank a sample of the 1.5 kg/hl beer together in the taproom one afternoon, Koenigs said, “The beer you’re tasting right there, analytically, that’s a 62 IBU beer. But it doesn’t taste like it. Not even close. With our sensory panel, we do hedonic scaling—one to ten in bitterness perception. This beer ranked about a three. So what we’re seeing analytically about what we should have isn’t backed up by sensory.”

Part of this, Koenigs suggests, is an indictment of how the measurement is done. A spectrophotometer aggregates the bittering compounds and applies a number to them, but it cannot account for other sensory inputs that brewers use to manipulate the perception of those compounds. A big step will be developing useful correction factors that software calculators can use to account for the impacts of dry hopping, but that’s still a ways off.

As with most research in brewing science, more work remains to be done to tease out the full impact of modern techniques and new hops varieties on dry-hopped beer, but if you found your beer dropped in gravity after dry hopping or that the bitterness didn’t correlate at all with the calculation of your software, there’s a good reason for that.