A few days ago, on the Homebrewer’s Association forum, I came across a link to this Peter Wolfe’s thesis on dry hopping. As with most papers, there useful information could be found in both the discussion of existing knowledge and the experiments themselves. Here are a few things I took away from it:
1.3.1 Hop Essential Oils: Hops produce the monoterpene mycrene, one of the major hop aroma compounds, immediately in the young cones. Mycrene is the largest essential oil fraction (up to 70%). “As the cone ages oxygenated terpenes are formed followed by the synthesis of sequiterpenes. Mumulene and Caryophyllene are the doninat sequiterpens and are also the send and third largest constituent of the overall oil”. – This supports the idea that hop cones need to “ripen” in order to develop their full aroma potential. An aspect that is important to those of us who go our own hops.
1.3.3 Hop alpha and beta acids: Bitterness contribution through isomerized alpha acids does not exist in dry hopping due to the lack of isomerization. In their unizomerized form alpha acids are not expected to add to the perceived bitterness based on work that has been done. My experience has been that hop pellets to taste bitter but that may also be due to the extremely high density of alpha acids compared to beer. Later it is mentioned that polyphenols extracted from the vegetal matter may add to perceived bitterness by themselves and synergistically with iso-alpha acids.
1.3.5 Glycosides: these are the combination of hop oils with a sugar molecule. Glycosides are less volatile than the hop oils themselves and thus provide the hop plant with a means of transport and storage. In brewing glycosides are extracted during boiling and survive that boil better than the essential oils. Hydrolysis of the glycosidic bond is know to occur in beer and most likely the low pH environment is the cause. Once the clycosdic bond is broken the essential hop oil is freed and able to contribute to hop aroma. In my brewing I commonly observe more hop aroma in the finished beer than what was present in the wort.
1.3.6 Biotransformed hop compounds: It has been shown that yeast activity is able to change hop aroma compounds. This changes the dry hopping character depending on when the hops are added to the fermenter.
1.6.1 Packaging and its potential ability to scalp dry-hop flavor: “The hydrophobic nature of hop aroma compounds makes them vulnerable to adsorption and absorption by hydrophobic polymers”. The most common occurrence of this is in cap liners. The extent depends on the type of polymer. In one study mycrene and humulene were found to have completely migrated into the cap liners of examined retail beers. This is an interesting aspect to those of us who bottle beers and don’t pay much attention to the type of bottle cap we are using.
2.3.3 Long Term Dry Hop Aroma Extraction: In long term experiments a model solution of 6% ethanol in water buffered to a pH of 4.2 was dry hopped and the linanol and mycrene content was measured on day 1, 4 and 7. The solution was not agitated. The amount of these compounds dissolved did not increase over time. It actually decreased which suggests that the maxium extraction is reached after one day of dry hopping.
2.3.4 Short Term Dry Hop Aroma Extraction experiments were performed in shaken flaks and showed that the maxium concentration of mycrene and linanol is already reached after 240 minutes. This supports the idea that aroma extraction in dry hopping happens much faster than brewers think. Although it is mentioned that temperature can play an important role and that extraction at lower temperatures is expected to be slower. In general more hop oils were extracted from pellets compared to whole flowers of the same batch of hops. This is contributed to the fact that in pellet processing the hop material, including lupulin glands, is crushed.
Some of the aroma compounds were stable while others showed a decline even during the exanimated extraction times.
Brewery trials were performed in conicals equipped with a pump that circulates the beer. All yeast was removed before dry hopping.
The trials showed that beer agitation (pumping) significantly improved hop aroma extraction. This was observed in the overall hop aroma of the beers and the measured hop oil levels. Again, pellets lead to more hop aroma extraction compared to whole cones. When whole hop cones were used extraction was also slower and reached its max not until around day 6. Polyphenol extraction was also higher with hop pellets compared to whole cone hops.