When Hops Fight Back: Understanding Hop Burn
It was one of those hot and humid Toronto summer days where heat radiates out of the pavement, hitting you from all sides with no relief. My wardrobe choices weren’t exactly helping the situation either, but finally I arrived at my destination, a semi shaded patio. The friend I was meeting had also just arrived and we eagerly ordered some cool and refreshing beers. He chose a generic lager and I went for a hazy IPA. When the drinks arrived, I raised the glass and poured the cold liquid into my anticipating mouth. Something immediately seemed wrong and upon swallowing I felt a series of reactions akin to swallowing a belt sander. My throat started to close, I could not swallow, it was very uncomfortable to say the least. Had I developed a beer allergy in the last 24 hours? Was it a glass of liquid bees? Semi panicked, I reached across and grabbed my rather confused friend’s glass and took a hefty swig. Never in my life had I been so happy to see an industrial lager.
It’s called hop burn, and it’s absolutely awful.
Hop burn is usually the result of an overly aggressive dry hopping program, and is a phenomenon mostly reserved for hazy IPAs. Essentially what happens is, with a heavy dry hopping programme, the level of hop derived polyphenols are too high to drop out of suspension.These polyphenols remain in the finished beer, and then bind to the proline rich proteins in saliva, essentially reducing its lubricating quality, and leaving the mucosa in the mouth unprotected, and subject to irritation. The various hop oils can now freely interact with the trigeminal nerves and send elevated negative signals to the brain. This is the triple threat in action. The mouth feels rough and dry, the brain is receiving elevated burning signals, and on top of it all, the microparticulate acts as a kind of grit and provides some additional mechanical stimulation. The brain assembles all this information into the “hop burn”
The Polyphenols
Let’s look at the polyphenols first. A phenol is an organic compound consisting of an aromatic ring and a hydroxyl group (-OH). A polyphenol, you might have guessed, is a compound with more than one hydroxyl on more than one aromatic ring. Tannins, which most will be familiar with as the astringent agent in wine and black tea, are a type of polyphenol. The particular ones we are focused on here are catechin and epicatechin, the proanthocyanidins, and xanthohumol. Catechin and epicatechin are small, light flavanols that can stay easily in suspension, and effortlessly bind to salivary proteins. The proanthocyanidins are polymers of catechins and epicatechins. They are larger and bind to the salivary proteins even more readily. Proanthocyanidins are an important contributor to haze but also cause very strong astringency and roughness in the mouth. Xanthomumol is a prenylated chalconoid that isomerizes into isoxanthohumol in the boil. It is far less abundant than the other polyphenols, but is highly hydrophobic and can irritate the mucosa, adding to the burn.
What happens when these polyphenols enter the mouth is either the hydroxyl group on the polyphenol binds with the carbonyl and amide groups on the salivary proteins via hydrogen bonding, the hydrophobic regions on the proline residues link with the aromatic rings, or, what is called π-π (pi to pi) stacking. This is when a noncovalent bond between two pi bonds of two aromatic rings stacks with proline sidechains. That’s a little sciency, but basically, any of these reactions will serve to strip saliva of its lubricating qualities and leave the nerves exposed, all the while making the mouth feel dry and rough.
The Oils
There are many volatile oils, and many types of volatile oils in hops. The hydrocarbons like myrcene and humulene, along with oxygenated terpenes like linalool and geraniol are soluble in beer and can cause irritation of the mucosa if present in high levels. These oils stimulate the TRP receptors in the trigeminal system. TRPV1 is the receptor responsible for heat, which also reacts to capsaicin (hot chili). TRPA1 reacts to irritants like wasabi or mustard, as well as polyphenols. The hop oils stimulate both of these receptors, sending signals to the brain of both burning and irritation.
The Particulates
Some of the particulates in hazy beers, consisting of hop particles and yeast cell walls, can also cause some mechanical irritation. It’s microscopic to be certain, but it’s there nonetheless. This is likely the least significant contributor, but in the presence of the other two, is just a little twist of the knife.
So these three factors get integrated by the brain into this very unpleasant sensation. Luckily, it can usually be avoided, and the key is of course to reduce suspended particulates, and specifically, to control residual polyphenols. Cold crashing, filtering, finning, or centrifuging, are obvious strategies, but these often aren’t feasible for hazy IPAs. Longer conditioning time will allow more polyphenols to bind with proteins and drop out of suspension, and for yeast to floculate and settle out. It will also allow some harsher volatile oils to dissipate.
The third piece of the puzzle, and critical to styles where haze is a requisite, is controlling the dry hopping. Later additions will reduce contact time and therefore reduce polyphenol extraction into the beer, while allowing for the most of the desired aroma contributions. Dry hopping too early can push the beer’s PH up as well. This is a double whammy. Higher PH increases polyphenol solubility, as well as increasing the sensitivity of tissue in the mouth, and suppressing malt perception, making the beer seem more bitter, and possibly harsh. Lowering the hop charge when feasible will clearly reduce the level of polyphenols available for extraction, as will choosing hops with lower polyphenol content. In this instance, the brewer is looking for hops where the ratio of leaf to lupulin is lower. Some classic varieties like the Cascade, Centennial, or Tomahawk, tend to be higher than some newer varieties like Citra, Mosaic or Nelson Sauvin. Note that the form of hops has great influence as well. The high lupilin products, like cryo-hops, will be much lower in vegetal matter while the pellets will have retained significant amounts. T-90 pellets will, of course, have higher polyphenol potential than t-45. Whole cones will have the most risk. More harshness comes from older hops as well, so the fresher the better. And, since hops are not an industrial product, growing conditions and other factors can affect the ratio.
Hop burn is most common in hazy IPAs, but any heavily dry hopped beer can be found guilty, especially if rushed to packaging. Fortunately, it seems to have become more rare, but if you are unlucky enough to experience it, at least now you know what’s going on.