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Calculating Hop Bitterness

How much beer hops is enough? It is important to understand the quantity of hops you need to properly balance your home brewed beer. Knowing exactly how much to use for your target equipment and beer style can save you a lot of money and enhance the quality of your finished beer.

Home Bitterness Units

My first book as a homebrewer was Charles Papazian’s excellent work The Complete Joy of Homebrewing. In his book he introduces the Home Bitterness Unit (HBU) defined simply as the number of ounces of hops times the alpha content for that hops. Fuggles hops have an alpha acid content of about 4.5%, so 2 ounces of fuggles hops would be 9 HBUs.

While HBUs are easy to calculate for beginners, they are not very accurate. An accurate estimate of bitterness depends on important factors like the size of the batch, size of the boil, original gravity, and boils time for the hops. HBUs just don’t do it!

A much more accurate method to determine beer bitterness is the International Bitterness Unit (IBU). One IBU is measured directly using a formula with a spectrophotometer and solvent extraction. Professionals and advanced brewers use IBUs estimates exclusively to help them design world class beers.

Beer Style guides such as the BJCP 2008 Style Guide list the bitterness range in IBUs for dozens of beer styles. This provides an excellent guide for anyone who wants to know how much hops to add for a particular beer style. You can adjust your bitterness in IBUs to be within the beer style range.

Estimating IBUs

Measuring the actual IBU content of a beer requires a laboratory. As a practical substitute for an elaborate lab, home brewers use equations to estimate the IBU content of their beer. A simplified equation from Ray Daniel’s Book Designing Great Beers for IBUs is:

  • IBUs = U% * (ALPHA% * W_OZ * 7489) / (V_Gal)

Where U% is the hop utilization in percent, ALPHA% is the percent alpha for the hop variety, W_OZ is the hop weight in ounces, and V_GAL is the volume of hops in gallons. Note that the hop utilization U% varies by boil time, wort gravity, and a number of other factors. Also, this gives the IBUs for a single hop addition. If you have multiple hop additions, you need to add up the IBUs from each.

In this equation, the utilization percentage is the one factor that varies depending on equipment used, brewing methods used, boil time, boil size, and boil gravity. The variations between different hop estimation equations basically come down to different ways of estimating the utilization.

Hop Utilization: Practical Application

If your eyes glaze over looking at IBU equations – here’s a practical guide. Hop utilization increases with boil time, so the longer you boil your hops the more bitterness and IBUs you will add. Late addition hops (boiled for 5-10 minutes) add very little bitterness, and are used primarily for aroma. Bittering hops are usually added for the full boil time (60-90 minutes).

Hop utilization also increases as you lower the gravity of your boil. If you are brewing a high gravity beer, or an extract brewer using a partial batch boil (small pot) you will get much lower utilization. This is why extract brewing requires more hops (in general) than all-grain brewing. Since different brewers use different equipment, it is important to take into account your own boil size and boil gravity when estimating the bitterness of your beer.

IBU Estimation

Three equations to estimate utilization and IBUs are most often used: Rager, Tinseth and Garetz. The equations differ in the way that they estimate the utilization percentage described earlier. Rager is most often associated with extract and partial mash brewers. The Rager equation takes original gravity of the boil into account, and tends to produce IBU estimates that are on the high side of the three equations.

Tinseth is often associated with all-grain brewers, or brewers that do full batch boils. It generally produces lower IBU estimates than Rager, but is considered very accurate. BeerSmith uses Tinseth by default, though you can change this from the Options dialog. The Garetz equation is less popular than the other two methods, but generally provides estimates somewhere between Rager and Tinseth.

I won’t go into the details of calculating each equation, though you can visit this site if you want to build your own spreadsheet or calculator. A quick google search on any method will yield a number of online calculators and spreadsheets. In addition, all major brewing software programs including our own offer the ability to estimate IBUs directly from the recipe. I recommend using a program or spreadsheet, as the complexity of multiple hop additions and late extract additions make it tedious to calculate by hand.

It’s important to understand that the three estimation methods will provide widely varying results in some cases. Each provides only an estimate of bitterness, and none are perfect or all encompassing. I would not spend too much time worrying the differences. Choose a single estimation method and stick with it.

How Much Hops is Enough?

Now that we understand the basics of calculating IBUs, we come back to the original question of how much is enough? The answer varies by the style of beer we are targeting. The bitterness needed for an Imperial Stout is dramatically higher than a simple Pilsner. In general, beers with higher gravity need more bitterness to offset the maltiness of the beer. Similarly styles such as India Pale Ale where bitterness is a significant flavor component require more hops.

Fortunately, a number of beer style guides offer IBU ranges needed to achieve a particular beer style. The most widely used US style guide is maintained by theBeer Judge Certification Program (BJCP). The 2008 BJCP Style Guide provides detailed IBU ranges for dozens of popular beer styles from around the world. Other countries also have beer style guides that provide similar information. Programs like BeerSmith have the style guide built in for easy reference when designing a recipe.

Lets look at a BJCP example: From the standard, an American Amber Ale should have between 20 and 40 IBUs. If we target the middle of the range, 30 IBUs is about right. Using a spreadsheet or brewing software, its easy to adjust your hops quantity and boil times to reach the target.

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Brad Smith BeerSmith.com Follow BeerSmith on Twitter and Facebook

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