Dry hopping is the process of adding hops to beer whilst it is in the fermentation vessel - sometimes while the beer is still actively fermenting, but more often just after. It’s done to give the beer a bright and pronounced hop character. This technique, and these characteristics, are completely synonymous with modern hoppy beers; any hoppy beer you encounter these days is almost certainly dry hopped. But dry hopping does create some issues for the brewer, and is the source of some potential complications worth considering.
Extracting the aroma and flavour-giving oils from hops is more easily done at warmer temperatures, that’s why hops are traditionally added before fermentation when the wort (pre-fermentation beer) is being boiled. However, at these temperatures precious aromas are driven away with evaporation, and some other more delicate flavour notes are masked or altered, giving a more ‘cooked’ hop flavour. If you want a beer to have fresh hop character, you simply need to be inefficient and add the hops when the beer is much colder. There is some debate around what temperature is best to dry hop, and breweries vary quite a bit in this regard. I’ve heard of temperatures from 4°C to 20°C, so it’s really at the behest of the brewer depending on what they are trying to achieve. Colder temperatures will aid processing but will come at the expense of aromatic efficiency. The accepted wisdom is that colder temperatures also mute the fruit flavours that come from hops, and will be less prickly in character, whereas a warmer dry hopping temperature will be more exaggerated.
How do breweries physically add the hops? More often than not it’s as simple as manually chucking some hop pellets into the top of the fermentation vessel near the end of fermentation. The downside to this is the plant matter of hop pellets soaks up precious beer. With this in mind, there are more sophisticated ways to add hops now, namely using dosing tanks. These essentially act as a giant tea strainer, allowing the beer to run through the hops, without allowing the larger particles to hang around in the vessel and absorb too much beer. Over exposure of hops in beer can give a ‘vegetable’ character that is undesirable, so brewers aim to limit hop contact time. Whilst these hop dosers can be expensive, they promise an increased efficiency that in the long term can save breweries money, or give a boost to hop character, depending on the aim of said brewery. The circulating nature of these vessels means that the brewer can mimic many hours of contact time between hops and beer in a fraction of the time. Again, this can be used to speed up processing time (and therefore save money), or can be used to extract more from the hops in the same amount of time.
Another consideration is that the time generally considered to be optimal to add dry hops to a tank is annoyingly just before the time you’d normally harvest some yeast from the fermentation vessel for the next batch. The oils present in hops coat yeast cells and affect their ability to perform fermentation smoothly, so it’s generally agreed that you can’t reuse yeast that has had dry hops added to it. Thus you essentially end up with a tug of war between yeast harvesting and optimal dry hop timings. You want to dry hop as early as possible (most of the time), but want to harvest your yeast late on in the process. Most breweries’ dry hopping regimes balance these two competing parts of brewing.
At this point it’s appropriate for me to point you towards Scott Janish’s incredible book The New IPA, which has a wealth of information for any brewer/beer geek to fawn over.