A beautiful sight (and a wonderfully aromatic one for the photographer): One hundred pounds pounds of whole-leaf hops sit in the hop-back at Heavy Seas Brewing (of Baltimore, Maryland), adding aromatics and flavor to its Loose Cannon Hop3 IPA.
What is a hop-back? According to The Oxford Companion to Beer:
[A] hop back is a straining vessel used in traditional brewhouses to separate leaf hops boiled in the kettle before the wort [unfermented beer] is cooled prior to fermentation. <...> Boiled wort is run into the hop back and after a short settling period a filter bed of hops is formed on the false bottom. The hot wort is then drained through the filter bed of hops before cooling. The hop bed also traps trub (coagulated protein), which is produced during the boil. The wort run off from the vessel should be clear, as the hops act as a good filter material.
The article notes that modern breweries, by and large, have eliminated the use of hop-backs to clarify wort, replacing them with more efficient kettle whirlpools. At the same time, however, many 'craft' breweries have re-installed hop-backs, not to clarify wort, but to add fruity hop aromatics, a character different than that of boiled hops or of 'dry-hops' added after fermentation.
To learn more about that, I talked with Joseph Marunowksi, the Director of Brewing Operations at Heavy Seas.
The hop-back is a wonderful item which takes advantage of some very simple principles.
When hops are added to the kettle during boil, there are compounds in the hops that are altered by the high heat to fix bitterness into the wort. This happens to different degrees based on the amount of time spent in the boil. Essential oils, however, tend to be boiled off because of their volatility.
The hop-back gently brews the hop flowers in much the same way that a fine cup of tea is made. You don't want boiling water necessarily because it may add astringency to the tea. [For a similar reason,] we wash hot wort over the hop flowers to collect these essential flavor components, and then immediately crash cool the wort in our heat exchanger to prevent the loss of those aromatics. By this method, we achieve our second "level" of hop character. It's what separates Loose Cannon Hop3 IPA from other, simply bitter, IPAs.
We use a proprietary blend of precious and difficult to obtain American hops to infuse our brew with a fresh, herbal and spicy goodness. We use almost a pound of hops per barrel in this fashion for a grand total of three pounds of hops per barrel, added in three ways (in the brew kettle, the hop-back, and our 'hop cannon' dry-hopping in ruh storage). You just cannot get that flavor in any other way.