Hop Freshness Calculator
This page provides a calculator for estimating the hop freshness factor, which is the opposite of alpha-acid decay. The formula for alpha-acid decay comes from Mark Garetz. The mapping between the Hop Storage Index (HSI) and freshness comes from Gail Nickerson and Sam Likens (1979). Note that some sources confuse the HSI with the loss of alpha acids after six months at room temperature; the HSI is a measure of the current freshness of a sample of hops.
Default values are shown in grayish-red. These defaults are based on other inputs, such as the hop variety, and may change when other inputs change. If you don't know the value of a parameter, the default is probably fine. You can change the value of a default parameter by simply entering a new value, at which point the font color will change to black. To get back to a default value, enter 'd'.
Inputs:
Input Units: metric or U.S. customary (e.g. Fahrenheit)
Hops:
| Hop Variety: | |
| Alpha Acids at Harvest (%): | |
| Beta Acids at Harvest (%): | |
| Loss After Six Months at Room Temp. (%): | |
| Decay Rate Constant (k): |
Storage Conditions:
| Packaging: | |
| Storage Temperature (X): | |
| Storage Duration (months): |
Hop Storage Index:
| HSI: |
Outputs:
| Freshness Factor: | 0.0 |
| Percent Lost: | 0.0% |
| Current Alpha Acids: | 0.0% |
| Oxidized Alpha Acids: | 0.0% |
| Current Beta Acids: | 0.0% |
| Oxidized Beta Acids: | 0.0% |
For packaging, I have taken Garetz's terms and corresponding storage-factor values. I have added a "professionally nitrogen flushed" category because I have found that such packaging produces far less hop deterioration than the (DIY) inert-gas and vacuum-sealing values specified by Garetz in the 1990's.
I would like to stress the importance of knowing the packaging and storage conditions of your hops! Just because hops at the store come in a vacuum-sealed foil pouch doesn't mean that they have been professionally flushed with nitrogen. Just because they are in a refrigerator at the time of purchase doesn't mean that they have always been kept at refrigeration temperatures. In some cases I have bought hops under the impression that they were flushed with nitrogen, only to find that the alpha acids had degraded far below the value written on the package. I currently prefer to buy hops direct soon after harvest in one-pound bags and store them in my freezer.
While beta acids do not usually survive into finished beer, oxidized beta acids produced during storage can be present when using both kettle hops and dry hops. I've found a distinctly herbal quality in beer made with hops high in oxidized beta acids, and so I try to limit their presence in my beer.
The best way to really know the freshness factor is to have a sample of the hops analyzed for the HSI and then use the Nickerson formula: freshnessFactor = 1.0 − (1.10 × log10(HSI × 4.0)). The freshness factor value and the default estimate of the HSI on this page are approximations only, and the real values will probably be at least somewhat different.