Deterioration of hops over time

I regularly find dead useful stuff when searching for something else completely. For example, this paragraph about hop deterioration. Extremely useful for someone writing recipes where the original beer had hops of varying ages.

What I was looking for was information on the new hop variety OP21, which I'd come across in wartime William Younger brewing record. Brewer's Friend, if you were wondering.

Weirdly, the bit that really interested me also had a William Younger connection.

It will be noticed that in the majority of cases the preservative value (P.V.) of the hops used, both “experimental” and control, was not ascertained. In the writers opinion, no really satisfactory comparative test of varieties of hops can be made in the absence of this knowledge. In this connection we should like to mention that Mr. G. T. Peard, head brewer of Marston, Thompson & Evershed, Ltd., Burton-on-Trent, has made the suggestion that the merchant should be asked to provide an analysis with the hops he supplies, so that the brewer is enabled to select the most suitable hops when buying. Further, we would point out that if this were done, it would be possible, by assessing the loss in P.V. of such hops during storage, for those brewers who desired to know the P.V. of the hops at the time of brewing to have this information. In this way hops could be used at the proper hop-rate and with the maximum of economy. On the question of the practicability of this assessment, we have consulted Mr. J. S. Ford, of Wm. Younger & Co., Ltd., who kindly furnished us with the following very valuable information: “We have a large mass of data concerning the fall in a resin content during storage. Our cold store 1° C. shows a loss of 0.07 per cent, a per month during the first 10-12 months and about 0.10 per cent, per month during the second 12 months. In an ordinary store we find 0.13 per cent, per month during the first 10-12 months; during the second year it is very variable and may go as high as 0.5 per cent, per month. Indeed, the loss is so variable and great, that after one year in ordinary store, many hops should go below the copper, not into it — I mean to heat the worts not to hop them. As regards the new varieties and higher resin content hops we have not so many data but we have found that when properly packed, the cold store figures we have quoted for ordinary hops apply to the loss in the new varieties."
Journal of the Institute of Brewing, Mar - April, 1944, page 84.

The "Preservative Value" mentioned is calculated from both the alpha and beta resins. (PV=10(alpha + beta/3)). So it isn't exactly the same as the alpha acid content, but I think it's a fairly good guide to the decline in resin content.

I've made a few calculations based on the figures provided by Younger:

In cold store
variety	Fresh	6 months	12 months	18 months	24 months
Goldings	5%	4.98%	4.96%	4.93%	4.87%
Cluster	7%	6.97%	6.94%	6.90%	6.82%

Not that much deterioration at all. Ut looks much worse for those not in a cold store:

Not in cold store
variety	Fresh	6 months	12 months	18 months	24 months
Goldings	5%	4.96%	4.92%	4.77%	4.47%
Cluster	7%	6.95%	6.89%	6.68%	6.26%

Looking at one if the few sets of analyses I have, the alpha acid content declines more rapidly than the PV:

Analyses of Fuggle's hops during storage
	cold store			warehouse
storage period	alpha resin	beta resin	preservative value	alpha resin	beta resin	preservative value
	6.28	8.6	91.5	6.67	9.26	97.6
5 months	6.22	8.2	89.5	5.83	9.17	88.8
9 months	5.72	8.25	84.7	4.72	9.34	78.5
14 months	5.84	8.54	86.9	3.48	8.64	63.6
19 months	5.15	8.92	81.2	3.21	9.9	55.1
Source:
"Brewing Science & Practice" H. Lloyd Hind, 1943, page 349

The PV is falling by around 0.6% per month in cold store, while the alphas acid content is falling by around 1% per month. So It seems that while the bitterness declines by quite a bit, even after 19 months the PV hasn't been that badly affected.

The rule of thumb I'm going to use in future, for cold stored hops is 1% per month for the first year, 2% after that. Non cold-stored hops 3% per month for the first year, 6% after that.