Hop freshness

A year-old ounce of Cascade does not bitter like a fresh one, and until this model existed every calculator pretended otherwise. Adjust for age on a hop row uses it, and this page is what it is doing.

The model

Garetz’s curve is first-order decay with three multipliers on the rate: the variety’s own tendency to lose alpha, the temperature you kept it at, and what it was packed in. The variety figure is the share of its alpha acids gone after six months at 20 °C in packaging that does nothing, a published property, quoted per variety by the people who sell hops. Temperature halves the rate for every 15 °C you take off. Packaging runs from no protection at all down to a professionally nitrogen-flushed pouch.

What it does not know

We should be blunt about this. It is kinetics fitted to a single six-month endpoint, not a mechanism. Oxidation is driven by oxygen getting in, and a bag’s seal is a step function rather than a coefficient, so one puncture and the packaging factor is fiction. The factor for a modern flushed foil pack is not Garetz’s number at all: it is a value later implementations added for packaging that did not exist in 1994, a twentyfold claim passed on rather than verified. And the variety figure is an average over crop years, regions and whatever the broker’s sample happened to be. It is not a figure for your bale. Where a variety has no published figure, we say so and use an average.

The largest caveat

Alpha acids decay, and so do the oils, on a different curve. Age a dry hop with this and you get a number that is arithmetically right and tells you nothing about whether the beer will smell of anything. We age the alpha only, and we would rather say nothing about the aroma than guess.

The beer these numbers come from

Reference pale ale — 20 L into the fermenter, a 60 minute boil.

Grist	Amount	Color
Crisp Maris Otter Pale Ale Malt	4.5 kg	2.8 °L
Thomas Fawcett Caramalt	0.35 kg	9.9 °L

Hops	Amount	Alpha	Addition
Magnum	15 g	13.5 %	60 min boil
Cascade	30 g	6.8 %	10 min boil
Citra	40 g	13 %	20 min stand at 80 °C

It comes out at
Original gravity	1.0507
Final gravity	1.0094
Alcohol	5.52 %
Bitterness	35.66 IBU
Color	5.14 SRM (10.12 EBC)
Into the fermenter	20 L

The numbers

Alpha acid after storage

6.28 %, from garetz-hop-storage.

future alpha = A × 1 / e^(k × TF × SF × days)
⇒ Alpha acid after storage = 6.28 %

Input	Value	Unit
Fresh alpha acid	6.8	%
Alpha lost six months	0.5	fraction
Months	8	months
Days	240	days
Storage temperature	-18	°C
Packaging	vacuum	—
Rate constant k	0.00385	per day
Temperature factor	0.1727	—
Storage factor	0.5	—
Fraction remaining	0.9233	fraction

What it assumes:

• Six months is 180 days here, which is the divisor in Garetz’s own rate constants.
• The storage factor for a professionally nitrogen-flushed pouch is not Garetz’s figure, and it is the single biggest lever in the model.
• Alpha acids decay; so do the oils, on a different curve. This says nothing about aroma, so ageing a dry hop with it gives a number that is arithmetically right and useless.
• Percent lost is a varietal average over crop years and regions, not this bale.

Sources:

• Hop Storage: How to Get — and Keep — Your Hops’ Optimum Value, Brewing Techniques 2(1) — Mark Garetz, 1994
• Hop storage factors for modern packaging — alchemyoverlord
• Hop storage index (absorbance ratio 275/325 nm in alkaline methanol) — Gail B. Nickerson & S. T. Likens