Hop (Humulus lupulus L.) is known as an indispensable raw material for beer brewing and is usually added in the form of dried cones. As a result, drying has become an important postharvest practice for hops, with kiln or belt drying being the most common practices to achieve mass production in industrial scale. However, there has been a significant increase in the number of microbreweries and home growers in recent years, suggesting the needs for effective drying techniques for small-scale hop handlers. This study aimed for selecting an efficient small-scale drying method to appropriately preserve aroma and quality of hops from oven, dehydrator and freeze-drying approaches.

Fresh Cascade and Chinook hops that harvested from different regions in Virginia were dried to 8.5 to 10% moisture content (wet basis) by oven, dehydrator and freeze-drying techniques, respectively. Aroma profiles of dried hops were characterized by headspace solid phase microextraction-gas chromatography-mass spectrometry-olfaction (HS-SPME-GC-MS-O). Stable isotope dilution analysis (SIDA) and standard addition method (SAM) were applied to quantify predominant aroma-active compounds. Principal component analysis (PCA) and hierarchical cluster analysis (HCA) were employed to indicate the volatile variation among hops obtained from different drying techniques. Total essential oil content and bittering acids panel were analyzed following ASBC standard methods 13 and 14, respectively. Color quality of dried hop cones was measured using colorimeter with L, a^*, b^* scales.

A total of 36 aroma-active compounds were identified in all dried hop samples, with β-myrcene and α-humulene dominating the profiles. In general, the contents of both individual aroma compound and total essential oil were always the highest in dehydrator-dried hops especially when comparing to oven-dried hops. PCA and HCA results revealed the distinctive aroma profiles for hops from different drying approaches. Additionally, the α-acid and β-acid contents in hop cones were not significantly affected by different drying methods in most of the hop samples. Freeze-drying approach provided the most desirable color for all hop samples. In conclusion, dehydrator drying was a realistic and effective drying practice for small-scale hop handling due to the low equipment investment and the advantages on aroma and quality retention. The findings of this study will help improve the quality and value for local hop and brewing products.

Yixiang Xu

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