The Riddle of Bitterness

Scientists long ago figured out the mechanism through which hops turn beer bitter: the alpha acids in the lupulin glands become isomerized over the boil--a process that allows the bittering compounds to become soluble. There's a mathematical curve that demonstrates the process, and the amount of bitterness is a direct function of alpha acids plus boiling time. This all beer geeks know.

But now we come to the riddle. American brewers have developed a new way of brewing that exposes very few hops to the long boiling process. Most of the hops are added at the very end of the boil, are used after the heat is turned off so they steep like tea leaves in the wort, or are added to cold, fermented beer as dry hops. Accordingly, these beers contain very few iso-alpha acids. And yet, amazingly, they still taste bitter. How can this be?

In one of the most remarkable moments in our interview with hops researcher Tom Shellhammer a couple of weeks ago, Patrick and I learned why. This strikes me as one of the most important findings in recent research, and something we're going to have to learn a lot more about as a result of our new practice of using vastly larger quantities of hops in our American IPAs. Here's Tom (with a transcript slightly edited for clarity):

"There are some beers, like Kona Big Wave, a beautiful beer, but there are homeopathic levels of hops in the kettle. When it comes out of the brewhouse, it's got like 7 BUs. Or less! And then they just load it full of hops and they dry-hop it. But it's not 7 BUs. The BU level creeps up. BUs--and the sensory bitterness--is creeping up in dry-hopping or late-kettle hopping or whirlpool hopping for a number of reasons.

"Probably the most significant reason is that there are oxidation products in hops that prevent the hops from isomerizing. If that isomerization process doesn't occur then the bitterness isn't there. But it turns out that the oxidation products of the alpha acids themselves are bitter. And so, just like isos [iso alpha acids] are more soluble in beer than the alpha acids are, the oxidation products are more soluble as well. And so as brewers add hops that have some degree of oxidation--and all hops will have some degree of oxidation ... they are extracting the fairly water-soluble hop oxidation products.

"Right now in our lab we're trying to hunt bitterness and figure out what things are driving bitterness in really hoppy beers. It turns out that if you track isos like you would in lager beer you're about 90% of the way there. Maybe even 95%. And with really hop-forward beers, if you just track isos in some cases you may only be about 40% of the way there. There's other stuff that's adding to bitterness. And we've found in the last year or two that these oxidation products, these humulinones, in some beers are there in higher amounts than the isos.

"So if you think of a typical beer ten or twenty years ago the hopping--even if you think about Jever [a German pilsner], which is a hoppy beer--the hopping level, which is just a front-end kettle hop addition, compared to a beer that's kettle hopped, whirlpool hopped, and dry hopped, you might have twenty to forty times more hops being used in those beers than a traditional lager beer. So even though you may have a small amount of oxidation products, the fact that you're using forty times as many hops means that you just can't avoid it. These things impart bitterness in beer."

"Polyphenols as well to some degree. Like when you make tea at home, you can get bitterness from that. You could think of it like a tannin. As these things polymerize they can create some degree of bitterness. When they're really small they can taste a bit more astringent, as they isomerize or polymerize large enough they drop out of solution. So these polyphenols can contribute to bitterness as well, but we found that that contribution is much, much lower than the contribution coming from the oxidation products. And the BU [measurement] will capture that, but not entirely."

This may seem like a weird little cul-de-sac in the world of hop bittering, but for brewers it's massive. We've been making beer with hops for a thousand years, but no one has ever used them the way Americans (and now everyone else) now do in their IPAs. The amount of bitterness that did not come through the boiling process was so insignificant that, even during the last hundred years when we understood brewing chemistry, nobody bothered to consider other sources.

So when brewers started shifting away from kettle hops to late-addition, post-kettle, and dry hopping in IPAs, they expected them to be low in bitterness. That's what the manuals said they'd be, and when you plug in your hop schedule to a formula that calculates BUs, they come out very low. But brewers found that it was wrong--their beers were bitter, sometimes pretty sharply so. When I spoke to Ben Edmunds about hops for my (soon to be released!) homebrew book, he told me:

 “I think all the formula out there are nonsense. It’s simply not true. We really need to retool what we use as our utilization for whirlpool and late kettle additions across the board for all the beers. The thing it opened our eyes to was that from a balance point of view, we were way higher in BUs than we wanted. So we started peeling away, peeling away. And the beers all got better. So the way we calculate it is that the 60 minute addition should add no more than 5-10 BUs.”

And so brewers are adding less kettle hops, but not because more iso-alpha acids are being absorbed into the beer than we thought, but because these formerly-marginal sources of bitterness were now being added in such volume that they became the majority source. Soon we will literally be re-writing the books.

As always, if you missed our interviews with Tom, track them down on Soundcloud. These quotes come from our second interview, which has several moments of similarly enlightening wisdom. Give it a listen for sure.