Investigations into the Transfer Rate of Volatile Compounds in Dry Hopping Using an Octanol-Water Partition Coefficient Model

Dry hopping is a powerful practice for imparting a multitude of flavors into beer. In this study, the influence of ethanol content, temperature, dosage, and hop variety on the transfer of essential oil during dry hopping was examined on a laboratory scale. The dry hopping was performed with nonalcoholic beer and beer containing 5.0 and 8.1% ethanol at 1 and 20°C using the hop varieties Tettnanger, Cascade, Hallertau Blanc, and Eureka. The results showed that the basic beer, hop variety, and dry hopping regime influence the composition of hop essential oil constituents in dry-hopped beer. The increase of the basic beer ethanol content, and especially the rise in temperature, led to a significant increase in the proportion of monoterpenes such as β-myrcene among hop volatiles in dry-hopped beers. Increasing hop dosage led to higher proportions of alcoholic compounds (linalool). Furthermore, the transfer rates of particular volatile hop-derived substances correlated with their octanol-water partition coefficients (log K_OW), which is a measure of the hydrophobicity of a compound, regardless of tested factors in dry hopping. Therefore, it is proposed that the log K_OW could be a useful model for the prediction of transfer rates of hop oil flavor components in dry hopping. However, the transfer rates of the alcohols linalool, geraniol, α-terpineol, and 1-octen-3-ol were higher than the expected levels from the log K_OW values. These compounds are reported present in bound form in hops and released during dry hopping.