Hops (Humulus lupulus L.) are plants valued for their flavor and aroma compounds, as the cones impart bitterness, aromas and foam stability to beers. They are most commonly dried convectively to reduce water activity in order to maintain chemical and microbial stability before their use in brewing. However, the high temperatures used in the convective drying process results in the loss of compounds such as alpha and beta acids, mono- and sesquiterpenes, and thiols. Vacuum drying can be used as an alternative to convective drying to preserve flavor and aroma compounds. Due to the low pressures used in vacuum drying, hops can be dried at a lower temperature and thus potentially preserve these valued compounds. Hop cones (~70% moisture content) were dried on a bench scale under various temperatures and pressures (60oC/1 atm; 60oC/0.2 atm; 50oC/0.2 atm; 40oC/0.2 atm). Alpha and beta acids were extracted and analyzed using a standardized high performance liquid chromatography method. Thiols were extracted and analyzed using liquid chromatography mass spectrometry. Aroma compounds were characterized by solid phase microextraction gas chromatography mass spectrometry. Hops dried under vacuum at lower temperatures contained significantly (p &lt; 0.05) more alpha and beta acids than hops dried at atmospheric pressure. Hops dried under vacuum contained less of the oxidation product caryophyllene oxide compared to hops dried at atmospheric pressure. Ongoing thiol analysis are providing additional insight into the effects of drying hops at lower temperatures. The retention of flavor and aroma compounds is of great interest for hop growers, as current trends in brewing favor beers that are aromatic as well as bitter. Results from this study will give better insights into how low temperatures and pressures preserve hop quality, which can lead to a market of higher quality or premium dried hops.

Effect of vacuum drying on flavor and aroma compound retention and stability in hops (Humulus lupulus L)

ACS Fall 2021

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