Early in my brewing career my IPAs sucked. Other styles were solid, hitting all the numbers for bitterness, flavor, dryness, etc, but my IPAs were inconsistent, often too sweet, and error prone. As I slowly checked off errors and made realizations on my brewing process and recipe formulation I decided IPAs are hard to brew right, but for a good reason. There are extra variables added when brewing IPAs and most of it centers on creating and maintaining hop flavor.
Hop flavor and aroma is a delicate undertaking. There are things that cover hops like malt and sweetness and things that degrade hops oils like oxygen and age. When you read guides that identify off flavors in beer, specifically oxidation, they often mention cardboard. For an oxidized Pilsner, yes, it will taste like cardboard when outdated. However, for an IPA, cardboard flavor rarely an indicator of oxidation, which led my brewing trouble-shooting astray for some time. To me, descriptors of oxidized IPAs are sweetness, caramel, and loss of delicate hop oils to a more general bitter sweet malt flavor. If you need an example, go to the grocery store and buy a stone IPA (or any IPA) that is more than a few months past its bottle on date. Oxidized IPAs are easy to find.
So, what makes IPAs oxidize and lose hop presence?
Exposure to Oxygen During Transfer
Reducing oxygen exposure is paramount to fresh and hoppy beer. The most vulnerable point of the beer in my process is during the transfer after fermentation.
To reduce the potential for error I ferment IPAs fully in the primary where I add some dry hops during fermentation, then transfer to a keg for a second round of hopping, followed by carbonation and serving. By using only one transfer, there is just one point where the beer is exposed. To reduce risk during the transfer, I add hops to the receiving keg, flush fully with CO2, flush the transfer tubing and siphon with the CO2 in the flushed keg, then siphon the beer from the primary to the keg using an auto-siphon. The siphon tubing is connected to a serving keg post connector, which connects to the serving post to further reduce oxygen exposure. In order to release the CO2 displaced from the liquid entering the keg I attach a gas side keg post connector to allow CO2 to escape from the keg.
This process works extremely well for me. The primary keys are preparing everything before you start, so no mistakes can be made, flushing everything with CO2, and only transferring from an open system (carboy or primary) to a closed system (kegs) once.
The next attribute to investigate is the dry hops. There three topics to discuss surrounding dry hops: storage practices and age, dry hopping process, and oil composition.
Hops should be stored in airtight containers, only opened when absolutely necessary, and kept in the freezer. During each hop harvest I purchase hops in one pound bags, as I need a variety, I open the bag then use a vacuum sealer to re-package in 3-4 oz quantities hops that are not used. I try to use hops within a year and will throw hops out if they are much older. If hops are old or stored improperly they will often make an IPA taste oxidized quickly after dry-hopping. Additionally, as hops age the total oil and acid content decreases, skewing bittering calculations and reducing flavor and aroma achieved from a constant dry hop or late boil hop quantity.
Dry hopping process and its importance to oxygen exposure is frequently discussed in forums and there are arguments for both sides. I choose to do a double dry hop method for most of my IPAs to ensure the minimum possible oxygen exposure and maximum oil extraction from the hops, however I have made great beers with a single hop in the primary, as well. Dry hopping at the tail end of fermentation allows the yeast to clean up any oxygen added to solution by tossing hops in the beer. Dry hopping in a keg pulls the beer off the original hops to ensure no over extraction occurs, eliminates oxygen exposure, and allows for multiple dry hop additions, which is all the rage in the IPA scene these days (DDH Double Dry Hopped IPAs). Check out the Dry Hopping Methods page for more details into this process.
The final dry hopping topic is each hop varietals oil composition. If you know you are allowing oxygen to infiltrate your beer during bottling or transfer to kegs and are unable to fully fix the issue, it may be of use to identify hops high in easily oxidized oils and avoid those. Of the four primary essential oil in hops Humulene, Caryophellene, and Farnesene oxidize the most readily. Thus, using a hop with a high oil content and a high percentage of these oils in the dry hop and somewhat in the late boil additions could cause your beer to have an oxidized flavor. Simcoe is a wonderful hop for IPAs, but also contains a high percentage of Caryophellene and Humulene. Thus, an IPA heavily dry-hopped with Simcoe has a higher risk of tasting oxidized.
Another equally important contributor to focus on and probably remove is Crystal Malts. Crystal malts take over as hoppy beers age. The beer may taste great out of the fermenter, but after transfer to the keg, two weeks of carbonation, and a month sitting the lively hop bomb you first experienced is replaced with a sweet and bitter caramel soda. The bitterness may still be there, but the hop ensemble has turned to one note of grapefruit or citrus. Instead of crystal malt, try a combination Munich or darker pale base malt if you are looking for malt flavor. Additionally, to maintain a balanced sweetness, use a lower attenuating yeast, such as WLP002 or Wyeast 1318, depending on your target beer clarity.