Changes

Takoi et al. (2014) continued their research into monoterpene biotransformations. They determined that some varieties of hops have higher concentrations of geraniol (floral flavor) than others, which when used in beer, can lead to higher citronellol levels (citrus flavor) in beer that were not present in the hops or wort. They found that while traditional German hops such as Saaz and New Zealand hops contain very little geraniol, American hops such as Bravo, Citra, Cascade, Mt. Hood, Mosaic, Chinook, Apollo, Amarillo, and others contain relatively large amounts of geraniol, with significant variations from different crop years. In this study, they measured the amount of linalool, geraniol, and citranellol in beers that were dry hopped at different time points: before yeast was added, 3 days after yeast was added, and at the end of fermentation. For each of these timings, they tested three different hops that contain high levels of geraniol: Cascade, Bravo, and Mosaic. Overall, the amount of linalool wasn't effected by the timing of the dry hop. The amount of citronellol was also not effected by the timing of the dry hop except for the Bravo hops where the post-fermentation hopping resulted in about half the amount of citronollel than it did for the pre-fermentation and 3-day fermentation dry hop timings(see the table below). The timing of the dry hop had the largest effect on geraniol: the earlier the dry hop, the less geraniol was present in the finished beer for all three hop varieties, with hops added pre-fermentation producing the lowest amount of geraniol and hops added post-fermentation producing the most geraniol in the finished beers. As in their previous study, citronellol increased during the first three days of fermentation, remained relatively stable for the rest of fermentation, and then increased again during storage. Geraniol dropped significantly during the first three days of fermentation in the case of the pre-yeast and 3-day dry hop timing, and increased slightly during storage. This data indicates that while earlier dry hopping reduces geraniol, only certain varieties of hops have an increase in citronellol depending on the dry hop timing. It's been suggested that the transformation of geraniol to citronellol involves unknown mechanisms that are relatively complex, particularly because the rate of the disappearance of geraniol does not map onto the rate of increase in citronellol, and when post-fermentation dry hopping there is a high amount of free geraniol but not a corresponding increase in citronellol during storage <ref>[https://www.researchgate.net/publication/261475199_Screening_of_Geraniol-rich_Flavor_Hop_and_Interesting_Behavior_of_beta-Citronellol_During_Fermentation_under_Various_Hop-Addition_Timings Screening of Geraniol-rich Flavor Hop and Interesting Behavior of beta-Citronellol During Fermentation under Various Hop-Addition Timings. Takoi, Kiyoshi & Itoga, Yutaka & Takayanagi, Junji & Kosugi, Takayuki & Shioi, Toru & Nakamura, Takeshi & Watari, Junji. 2014. DOI: 10.1094/ASBCJ-2014-0116-01.]</ref>.