Graduate Thesis Or Dissertation
 

Investigating the Quality Dynamics of American Aroma Hops Intended for Dry-Hopping Beer

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  • Historically, brewers have used dry-hopping (a cold extraction of nonvolatile and volatile chemicals from hops into fermenting or finished beer) to increase the microbial stability and shelf life of their beer. As hoppy beer styles have gained in popularity over the last decade (2007-2017), the objective of dry-hopping has turned to imparting hop aroma and flavor to beer while minimizing bitterness extraction. To extract hop aroma into beer, brewers have been using extreme hopping rates (sometimes > 0.7 kg/hL, equivalent to more than 18 lb/US bbl), which are mostly driven by increases in dry-hopping rates. These addition rates may be unsustainable from an agronomic perspective, potentially wasteful due to beer losses, and suboptimal at efficiently extracting aroma from hops. Therefore, the extraction efficiencies of a number of key hop volatile and nonvolatile constituents related to hoppy beer aroma and flavor were investigated over a range of commercially relevant Cascade dry-hopping rates (0, 0.2, 0.4, 0.8, and 1.6 kg/hL). It was determined that adding more hops during dry-hopping did not simply lead to increased aroma intensity but also changes aroma quality in the finished beer. Dry-hopping rates >0.8 kg/hL had hop aromas that were more herbal/tea in quality than citrus. To maintain a more balanced hop aroma quality, the use of a static dry-hopping rate between 0.4 and 0.8 kg/hL was suggested. Also, using dry- hopping rates >0.8 kg/hL lead to diminishing returns in terms of increasing hop aroma and is an inefficient use of raw material. From 2007-2017, Cascade and Centennial hops were the most commercially important aroma varieties to the American hop and craft brewing industries. They were very popular with US (and global) brewers because of the unique aroma and flavor they impart to hop-forward beer styles, especially during dry-hopping. However, there is no scientifically-validated method to predict beer aroma intensity and quality during dry-hopping. Many brewers rely a hop’s total oil content as a measure of its aroma potential, but to date the connection between total oil content and a hop’s aromatic intensity has not been proven. Additionally, the variation that exists in the hop volatile profiles and dry-hop aroma potential within these important commercial hop varieties over a given harvest year is not documented. Over the 2014, 2015, and 2016 hop harvests a large sample of Cascade (n=51) and Centennial (n=33) hops were procured from farms throughout the Pacific Northwest (WA, ID and OR). Within each of these harvest years, significant differences were observed in the hop volatile chemical profiles and the aroma intensities/qualities that these hops attributed to beer. These results indicate that at the same static dry-hopping rate of 3.86 g/L, there were significant and measurable differences in the aroma intensity as well as the quality of aroma attributed to beer from different commercially available Cascade and Centennial samples from the same harvest year. In agreement with prior research, it was also determined that total oil content (mL oil/100g hop) did not serve as an effective predictor of dry-hop aroma performance in beer. Instead, the concentration (mg/ 100g hop) of specific hop volatiles in hydrodistilled hop oil (geraniol for Cascade and β-pinene for Centennial) served as superior indicators of dry-hop aroma performance. Strategies both on the farm and in the brewery were investigated as ways to promote or modify aroma quality and intensity during dry hopping. On the farm, the impact of harvest maturity on Cascade quality and dry-hop aroma potential was evaluated using a unique weekly sampling protocol, whereby, 5-6 samples were collected from the same location within a commercial hopyard over three consecutive harvest years. For this specific hopyard, hop aroma intensity (OHAI) and citrus quality attributed to the beer during dry-hopping increased as a function of harvest date. Total hop essential oil content and a number of different hop essential oil volatiles (notably geraniol) displayed a significant positive trend with harvest date. For the first time, concentrations of thiol precursors (mainly S-3-(hexan-1-ol)-l-cysteine) were observed to decrease over harvest, while the concentrations of free thiols (mainly 3-mercaptohexanol) increased. Taken together these findings suggests that for brewers to best utilize Cascade hops, early harvested hops might be better for bittering or kettle/whirlpool additions, while later harvested hops might be better for dry-hopping or aroma additions. In the brewery, a sensory directed study on beers dry-hopped with Cascade, Centennial, and Chinook was used to evaluate the qualitative changes in the aroma of dry-hopped beers when these hops were used individually and in different blended combinations for dry-hopping. Blending hops as opposed to dry-hopping with single varieties produced the most intense aromas. In addition, specific blends of hops were found to achieve similar aroma qualities to single varieties. Therefore, by utilizing hop blends brewers may be able to make substitutions when faced with shortages due to cost and/or quality while maintaining similar aroma profiles. Overall, the results from these studies provide hop breeders with aromatic quality and metabolite targets for creating new / replacement hop varieties that have similar aroma profiles to these important American varieties. Growers benefit by being able to fine tune growing and post-harvest processing conditions to promote the concentrations of these hop volatiles in these varieties. Finally, this research will help brewers maximize the efficiency of aroma extraction during dry-hopping and guide the development of more sustainable techniques to better utilize this raw ingredient, improve beer quality, and obtain consistent hoppy aroma in beer.
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