Fruity and floral aromas are in high demand in the brewing industry at the moment, with a focus on improving the aroma of beer by intensifying or diversifying the flavour profile.
Yeast and hops are the main source of fruity and floral aroma compounds in beer. While flavour compound biosynthesis has now been investigated in considerable detail in yeast, the corresponding pathways in hops are much less known. By studying the available research on hops, and other species such as wine grapes, we can piece together the biogenesis of key compounds that contribute to hop flavour in beer. This enhances our understanding of the accumulation of precursor secondary metabolites in hops.
The key compounds reach their maximum accumulation in a very specific sequence as the crop matures. While each variety may differ in the proportion of various metabolites, and the point in the accumulation sequence at which they are harvested from a commercial standpoint, the sequence of accumulation remains relatively fixed:
1. Prenyl-flavonoids (ie. xanthohumol)
2. Beta acids
3. Alpha acids
4. Sesquiterpenes (humulene, caryophyllene, farnesene)
5. Monoterpenes (myrcene, linalool, geraniol)
6. Free/Bound Thiol (3S4MP)
Hop maturity is analysed as part of the hop production process. The measures that are typically used to determine maturity are either physical (dry matter, cone weight, oil content) or analytical (alpha or beta acid content). If we can understand the sequence of accumulation of key compounds, we can then use these measurements of maturity to make inferences about how the harvest window might influence the accumulation of the desired secondary metabolites.
The measurements of hop maturity over time during an Australian harvest.
Factors such as location, acreage, harvest capacity and harvest window all influence the analytical hop quality and their impact in beer. Similarly, the standard batch-based measures of analytical hop quality can tell a story from harvest and processing to storage. At this point, it becomes important for the brewer to understand the context of a particular hop cultivar. If the brewer is aware of the normal range of variability for each variety they use, then the associated batch-based data may be effectively interpreted.
The analytical variability in hop quality outcomes.
AUSTRALIAN RESEARCH COUNCIL
Some of the research discussed herein was supported under the Australian Research Council’s Linkage Projects funding scheme (LP140100160).
UNIVERSITY OF TASMANIA
Anthony Koutoulis, Aina Price, Lewis Rands and Ollie Hayes
AUSTRALIAN CENTRE FOR RESEARCH ON SEPARATION SCIENCE (ACROSS)
CENTRE FOR ADVANCED SENSORY SCIENCE (CASS)
Dandan Yan and Rob Shellie