The BeerSmith Hop Age Tool and the Hop Stability Index

by Brad Smith on July 10, 2019 · 2 comments

This week I take a look at how to use the Hop Age Tool in BeerSmith, as well as address the hop stability index, proper hop storage and how your hops age over time.

How Hops Age

Hops, like any organic ingredient, have a finite shelf life, and their aroma, bitterness and flavor will degrade over time. How quickly your hops degrade is a function of their form, packaging, temperature and time.

The enemies of hops are oxygen, heat, light and time. Hops exposed to oxygen will oxidize, which is why most modern hops are packaged in oxygen barrier packages, often vacuum or gas-purged made of foil or mylar. Once you open a package of hops, the oxygen will start the process of oxidizing it.

Light and heat are also enemies of hops, so proper packaging is opaque to prevent light from reaching the hops, and your hops should be stored in a freezer at or below freezing to maximize shelf life.

The form of the hops also plays a minor role. Compressed hop pellets and plugs which minimize the exposed surface area will degrade more slowly when exposed to oxygen than whole leaf hops, for example.

As hops age, the effectiveness of the hops will also degrade. Part of this degradation includes the alpha acid percentage which is usually printed on the label for the hops. As hops get older, less of the alpha acids will remain in a state where they can be isomerized during the boil to provide bitterness for the beer.

When the remaining alpha acids for a hop reach roughly 50% of their original fresh level, most sources consider the hops spoiled, and they should be discarded. Keep in mind that flavor, aroma oils and other compounds are also degrading as the hops age.

The Hop Stability Index

In addition to heat, light temperature and time, each hop variety ages at a slightly different rate. There is a measurement called the Hop Stability Index (HSI) that is used to determine how quickly a given hop degrades.

The HSI is expressed as a percentage of the hop alpha acids that are lost in 6 months at standard temperature of 68 F (20 C). So a hop variety that has a HSI of 25% means that it would lose 25% of its alpha acid content in 6 months if stored at 68F (20 C).

So if the original alpha acid percentage of the hops was 10%, it would lose 25% of that in 6 months, leaving an effective alpha percentage of only 7.5%.

The BeerSmith Hop Age Tool

Obviously few of us store our hops at 68F (20 C) and also with proper pelletization and oxygen barrier packaging the hops can last much longer. This is where the BeerSmith Hop Age Tool is useful as it takes into account the packaging, temperature and HSI to estimate the total aging effects for your hops.

Open the tool from Tools->Hop Age Tool on BeerSmith desktop. You can either enter your hop name, starting alpha and HSI at the top or select a hop variety by clicking on the Choose Hops button.

After you have entered the hop information at the top, you can enter the age of the hops, storage temperature and type of packaging in the Storage Conditions section.

The main output, the Adjusted Alpha is shown at the bottom of the tool window. A good rule of thumb is that you should discard hops when they reach approximately half their original alpha acid. So if the starting alpha acid was 10%, then you would want to discard the hops if their adjusted alpha drops below 5%.

The Adjusted Alpha number is the one you should use in your beer recipe as that percentage reflects the aged state of your hops. For properly stored, vacuum sealed hops, the change will be small over the first year, but for older or improperly stored hops the adjusted alpha can be significantly lower.

Keep in mind, also, that fragile hop oils may degrade more quickly than alpha acid, so you probably don’t want to use 3 year old hops as whirlpool/aroma/dry hops even if they still retain enough alpha acid. Use some hops from this year’s crops instead for your whirlpool/dry hop stages and save the older hops to use in the boil.

Thank you for joining me this week on the BeerSmith blog – please subscribe to the newsletter or listen to my video podcast for more great material on homebrewing.

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{ 2 comments… read them below or add one }

Jay Waldner January 3, 2022 at 12:22 pm

I’ve seen HSI listed as a number ie. “0.319” instead of a percentage. Would this be represented as 31.9% in your software? That seems very high but not sure how to convert this otherwise. Thanks

Brad Smith January 3, 2022 at 12:30 pm

Yes that is correct – 31.9% is not an outrageous number for some varieties.

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