All hops degrade with time, especially if in contact with air. The HSI (Hop Stability Index) is a measure of the rate of oxidation of the alpha and beta acids with time. The value quoted relates to the extent of oxidation of these bittering acids when in the presence of air at 20 °C over a period of six months.

Why is this important?

Whilst the bittering potential of an open package of a high HSI hop will reduce significantly over time, it is not the economics which matter most to a brewer who only only brews infrequently or at a small scale. The issue which should concern you is a change in the nature of the bitterness which is imparted. As the bittering acids oxidise their flavour changes, and rather than the typical neutral bitterness you want, you are likely to get a vegetal like bitterness; like that you would get with overcooked cabbage or sprouts. This is not pleasant.

How do I avoid this problem?

All the time your hop pellets are in sealed, oxygen free, bags they cannot oxidise. It is open bags, which are now in contact with oxygen where you will have a problem. For more information, see this article. It is good practice to chill all of your hops, but this is even more important once the bag has been opened. If you want to store an open bag of hop pellets for more that a few weeks you are best advised to seal the bag as best you can and keep in a freezer. Larger breweries will use whole bags of pellets / whole tins of extract either within a batch, or within the week and this overcomes the issue, but for smaller producers, like brew pubs, or new smaller businesses this is not going to be an option.

The easiest and most practical option to manage hop acid oxidation is to choose your hop varieties carefully. Look up the characteristics of several hop varieties which give you the properties / flavour profile you are looking for, and choose only hops where the HSI is < 30%. To exemplify this consider Columbus, this is a very high alpha acid hop, seemingly ideal as a bittering hop. However the HSI is 45% and thus is not a variety I’d recommend to smaller breweries who will not use a whole bag within a fortnight. Similarly Centennial is a sublime aroma hop for lovely, well retained citrus / grapefruit flavours (see my article concerning hops whose flavour with the best retention into the final beer) but again it has a high HSI and will start to give you unpleasant flavours if the bag has been open for more than 2-3 weeks.

Good hop choices

Fortunately we are now blessed with such a broad range of hop varieties that, with access to the right knowledge one can choose a hop which has the flavours you want and is practical for your scale of operation. Here are some examples to get you started…