Effects of storage conditions on alpha acid degradation of Indiana Grown Hops (Humulus lupulus)

Hop (Humulus lupulus L) belongs to the Cannabaceae family and is well-known to be a dioecious, perennial climbing plant. The common hop is native to temperate climates, but due to its widespread commercial use in the brewing industry, hop plants are grown worldwide. The bittering components in hops, especially the a-acids, provide a pleasant bitterness and characteristic flavors to beer. On their own, a-acids do not contribute much to beer bitterness but do so after they are converted to iso-a-acids during the kettle boil step in brewing. b-acids in hops are not as important as a-acids, since they only supply about a third of the bittering power as a-acids but are mostly responsible for hops’ antimicrobial properties. The goal of this study was to investigate the effects of storage conditions on the degradation rate of hop constituents. Pelletized hops from two varieties grown in Knightstown, IN were studied: Cascade (typically used as aroma hops) and Chinook (mostly used as bittering hops). The impact of storage conditions on hop chemical constituents, hop storage index (HSI), and color parameters was evaluated at three storage temperatures (4℃, 25℃, 35^oC) with hop pellets exposed to two different gases in headspace (nitrogen or air) for a duration of up to 168 days (4℃ and 25℃) and 70 days (35℃). Hop acids content was determined by UV/Vis spectrophotometry and HPLC-DAD. Results showed that increased storage temperature decreased the a-acid concentration in hops. At 4°C and 25°C the loss in a-acids was (14.5 – 23.4%) whereas at 35°C there was a greater loss in a-acids. The a-acid loss between nitrogen flushed and air exposed pellets was not significantly different (p>0.05) except for Chinook pellets stored at 35°C. b-acids remained mostly stable for the duration of the study; however, at 35℃ the loss of b-acids over time became significant for both varieties. HIS values increased over time for both varieties at 4 and 25^oC, however, those values remained below the recommended 0.40 value. At 35^oC, the HSI values indicated unsuitable hops for brewing at the end of the study for both varieties. The color parameters L* and b* remained constant after 168 days at 4 and 25^oC, while the a* value and the hue angle showed a significant decrease with time and increasing temperature. At the conclusion of the study it was demonstrated that the loss in a-acids from Cascade pellets at 35℃ was lower than the loss in Chinook, suggesting that at high temperature the a-acids in cascade were more stable than those in Chinook.