Ask Mr. Wizard

Fine-Tuning Bitterness Post-Fermentation


Mike Figuray — Woodbridge, Illinois asks,

I’ve often wondered if there is an easy way to adjust bitterness post fermentation. I’ve read about a few techniques (Isomerized hop extracts, boiling hops in a small volume of water/starter wort, adjusting water profile, blending, etc.) and I’ve also read all the reasons why some of these may not work.

I’m mostly interested in adding something to the keg (Hop extracts or boiling a small volume of water/wort and adding it). If my beer did not hit the bitterness, I’m most likely just going to roll with it rather than brew another batch and go through the trouble of blending. I also use a sulfate-heavy water profile for my hoppy beers already so adding more probably won’t get me what I am looking for. I’d love to hear your thoughts on the best ways to go about this.


At times it is helpful to review why some brewing problems are not so easy to fix and discuss how to prevent these problems in the future. Under-hopped beers are an example of a problem that is not so easy to correct after a certain point in the process. You have noted several possible fixes, including the two primary ways to go about adding bitterness post fermentation, so let’s start with these possible solutions and then my suggestions.

Adding a pre-isomerized hop extract is really “the way” to boost beer bitterness after fermentation. While this process is pretty simple, there are a couple of drawbacks. The first is that measuring these extracts is not the easiest thing to do because the addition rate is rather low. For example, it only takes 200 mg of iso-alpha-acids (IAA) to boost the bitterness of 20 L (5.3 gallons) of beer by 10 IBU. Even if the extract is dilute and contains 33% iso-alpha-acids (IAA) by weight, only 600 mg of extract is needed for this addition. Attempting to weigh out this volume of extract is not practical, so the most common method of measuring these additions is by volume. But that is also a challenge because extracts tend to be thick and sticky. Using more dilute solutions makes this process more manageable; this is also true of benchtop trials to determine the addition rate as well as making the addition to your batch.

Another thing about post-fermentation adjustments to bitterness is that you will not be adding much hoppiness to your beer. If you have never adjusted bitterness with an IAA extract you may be surprised by the one-dimensional nature of the change. You will not get any hop aroma or mouthfeel characters from hop polyphenols that often accompany kettle additions. Adding IAA extract simply adds pure and clean bitterness. If that’s what you are after, then this method will certainly work for you.
Your other suggestion about making a hop extract is certainly another option. Although hop acids are partially soluble in aqueous solutions, the solubility of IAA increases as pH decreases (Rudin, A.D. Solubility of Iso-Compounds in Water and Their State in Solution. Journal of the Institute of Brewing, 66 (1960): 18-22.), where the solubility is approximately 600 mg/L versus 900 mg/L in wort. In neither case is the solubility high enough to produce a highly concentrated extract. Increasing the bitterness level of 20 L (5.3 gal.) of beer by 10 IBU (10 mg of iso-alpha-acids per liter) requires 200 mg of IAA; assuming the stock concentration is 900 mg/L, this boost of 10 IBU requires 227 mL of solution. As the desired boost in bitterness increases, so does the volume of dilute hop extract. And if wort is being added to fermented beer, fermentation is required. If the fermented beer to which this solution is added is not cold and has a decent cell count of healthy yeast in suspension, this sort of addition should ferment out without any major problem.

There are quite a few new products in the market for brewers, including less viscous hop extracts, that can be used in multiple ways, from adding bitterness, to boosting flavor and aroma. Photo by Jake Parrish/Yakima Valley Hops

Blending is definitely a viable option if you happen to have some overly bitter beer hanging about that would benefit from mixing with a batch that is low in bitterness. While blending is common in commercial brewing, it is usually used to improve the consistency of already consistent batches. The type of blending where two extremes are blended to yield something in balance is more of a Hail Mary play and the result is not something that is easily brewed again. Similarly, deriving significant bitterness from water salts falls outside of normal practices.

To be honest, if this were my beer I would either add some IAA extract if the bitterness is really low or simply roll with what you have and call it good. Making an extract from water or wort introduces a step that could end up making a marginal situation worse. It’s like making a batch of chili that is too spicy, fixing the problem by diluting, and ending up with a really big batch of mediocre chili! But this is a great opportunity to learn from the problem.

Adding a pre-isomerized hop extract is really “the way” to boost beer bitterness after fermentation.

Beer bitterness is one of those things that has bugged me for a long time because too many brewers and beer consumers talk about IBUs like the numbers really mean something definitive. “Man, I had a beer yesterday that was 60 IBUs and it was like totally not bitter! I don’t know what beer calculator the brewer was using, but the beer was NOT 60 IBUs.” Many of us have probably heard or read this sort of thing. I usually do an eye roll and turn my back to this sort of nonsense because it’s simply not a debate I enjoy having. The problem with these blanket statements is that the sensation of bitterness is not just about the concentration of iso-alpha-acids in beer. Hop bitterness is akin to a spice and is balanced by malt flavors (toast, caramel, chocolate, coffee, etc.), residual extract (sweetness), alcohol, carbonation, and other beer components that factor into complexity. More hop bitterness is generally required to provide balance as the bigness of a beer increases. This is why I have a thing against brewers using IBU levels as an indicator of sensory bitterness. 60 IBUs in a luscious, high-gravity stout is not likely to be perceived as overtly bitter. Put the same hop additions in a Pilsner and the story changes. OK, off of the hop bale.
There are several take-home messages from your current problem. Here are four to consider in the future:

  1. Aiming for a bitterness target requires knowing the alpha-acid content of your hops. If a recipe calls for 20 grams (0.7 oz.) of 10% alpha Magnum hops, and you have 8.5% alpha Magnums, more hops are required to obtain the same bitterness.
  2. Hop acids degrade with aging. This is a complex topic unto itself because oxidized alpha acids are bitter, but they are not as bitter as IAAs. Hop age, oxygen exposure, and storage temperature all factor into the brewing value of hops. Just because a bag of Magnum hops contained 10% alpha when they were harvested does not mean that the hops in the bag haven’t changed since packaging. Be mindful of the age of your hops and how they were stored.
  3. Be consistent with hop calculations and calibrate your brewery’s bitterness scale with itself. It’s more important for you to know that a 30-IBU pale ale brewed on your system has a certain level of bitterness than it does to actually know how your beer compares to someone else’s 30-IBU pale ale. Since few homebrewers ever have their beers analyzed for bitterness, it’s more important to be able to adjust your perceived bitterness using calculations that you use as a guide. The biggest unknown when calculating bitterness is hop utilization. Using the same reference guide and adjusting for gravity is a great way to be internally consistent with your methods.
  4. If you are just starting to formulate your own recipes, keep in mind that the IBU target changes with the type of beer you are brewing. Big, malty beers, like doppelbocks, are often much higher in IBUs than they taste.
Response by Ashton Lewis.