By Greig McGill
Hops are, for all intents and purposes, the mascot of the craft beer revolution. While there are plenty of beer aficionados who are not hop-heads, the loudest, proudest, and certainly most visible are hop hunters of the highest order. And why not? Hops are almost infinite in their flavour and aroma characteristics due to the interrelationship of the various essential oils which provide such sensory delight.
Oils are very fragile things though, and will degrade rapidly under all but the most carefully maintained environmental conditions. The three enemies of hop flavour and aroma are:
I’ll cover off each of these factors, addressing why they are a problem, their specific effects on the hops, and how to minimise exposure and thus maximise the bittering, flavour, and aroma compounds of your hops.
Temperature is not damaging in itself. Except at extremes, it causes no physical changes in the hop compounds or cell structures. What it does do is provide energy to speed up chemical reactions caused by light and oxygen. A rule of thumb is that a temperature increase of 10 degrees Celsius will double the speed of a chemical reaction. This is not absolute, and may vary depending on the molecular composition and density of the chemicals involved, but it’s good enough for our purposes in keeping our hops as fresh as possible. The colder you can store them, the better. Freezing hops will not damage them, due to high oil content, and it will preserve those amazing aromatics in almost “as-harvested” condition. Beware of keeping your hops in the same freezer as food though. They may pick up unwanted flavours and aromas of other foods stored in the same freezer. The process goes two ways too, and while you might enjoy your steak with a bit of a hop aroma, it’s probably not ideal!
Oxygen is an extremely reactive element, and hops contain so many different compounds for it to react with. The products of those oxidation-reduction (redox) reactions are seldom pleasant or positive for the brewing process.
Alpha acids react to produce isobutyric acid, which has a distinctive and undesirable rancid cheesy character. They also lose their ability to be isomerised during the boil, leading to lower than expected bittering levels. Beta acids oxidise to hulupulones, which can lead to a harsh and unpleasant bitterness in your beer. Whilst essential oils do not become rancid, oxidation does degrade their aromatic properties, leading to lifeless and dull or muted flavours and aromas in your beer.
Your best bet then is to keep your hops away from oxygen as much as possible. There are many methods of doing this. You could purchase a nitrogen cylinder and a vacuum sealer, and after using hops, flush them with nitrogen to remove as much air as possible before vacuum sealing them in plastic. If you have ready access to CO2 already, you can use this instead of nitrogen to flush. CO2 may break down leaving some oxygen, whereas nitrogen won’t, but either of these methods is preferable to just chucking the opened bag of hops back in the freezer! Far less effective, but still better than nothing would be to vacuum seal without flushing, and finally, the least effective method would be to just purge as much air as possible by squeezing the bag, and then resealing. The better you remove oxygen, the longer your hops will last. If, for example, you plan to brew again the following day, you won’t lose a heck of a lot of hop character by just re-sealing the hop bag and popping it back in the freezer. If it might be weeks or months before you next use your hops, you will want to be as “extreme” as possible in purging oxygen and storing air-tight and very cold.
UV light also accelerates degradation of volatile hop compounds in a similar fashion to heat, though without an obvious linear relationship. Particularly susceptible to the effects of UV light are the alpha acids, so hops stored in otherwise perfect conditions, but still exposed to daylight, would lose their bittering potential quite swiftly. Luckily, if you’ve solved the temperature problem by storing your hops in the freezer, it’s usually nice and dark in there, so that problem can be considered solved. Just be careful to minimise light exposure when your hops are temporarily out of storage. UV light based reactions can happen very swiftly.
That’s the holy trinity covered! If you’ve followed all these recommendations, you’re well on your way to “as fresh as can be” hops whenever you need them. However, there’s still another useful bit of information hop manufacturers provide which will assist in your quest for freshness.
The Hop Storage Index (HSI) is an industry standard method of measuring and communicating how rapidly hop compounds degrade in storage. The HSI measures the level of alpha and beta acids as well as oxidative compounds, and is expressed as the relationship between the two. An HSI of 0.3 (30%) or less is good, while any higher than 0.4 (40%) represents unstable hops which will degrade swiftly. The cultivar of hops, quality of processing, and time between harvest and packaging of the hops will all have an impact on the HSI. If the HSI is too high, even the most stringent of precautions will not save the hops from degrading fairly rapidly. Your hop retailer will have analysis sheets which will detail the HSI for the hops you are purchasing, so be sure to ask for it, as it’s usually “for professional brewers only”, but it’s a useful number to have for homebrewers also.
So there you go. If you make sure to minimise hop exposure to light, heat, and air when using them, and store them oxygen-free in a very cold and dark place, your hops will last for a very long time indeed, assuming they have a low enough HSI. Happy hopping!