Hop Science - October 2025

Review on beer flavor stability

The most important recognition if we talk about flavor stability is the acceptance that we are actually talking about flavour instability. This extensive review should be in every brewers diary. The review covers all raw materials and every process step, but here let’s talk about the role of hops for flavor instability. One of the most notable transformations associated with hops during beer ageing involves the degradation of iso-α-acids, the primary bittering compounds formed from the thermal isomerisation of α-acids during wort boiling. These compounds are inherently unstable and undergo gradual degradation over time, influenced by oxidative, photochemical, and thermal conditions. Among their isomers, trans-iso-α-acids degrade more rapidly than their cis-counterparts, contributing to bitterness loss during shelf life. Although some early studies proposed that these pathways might contribute to the formation of volatile aldehydes, more recent investigations have observed similar increases in aldehydes such as 2-methylpropanal, 2-methylbutanal, and 3-methylbutanal across beers regardless of the type or presence of iso-α-acids. Nevertheless, hops do play a protective role against oxidation, largely due to their polyphenolic content and α-acids, both of which possess antioxidant properties. Hop polyphenols have been shown to enhance radical scavenging activity in beer, potentially delaying flavor deterioration. Additionally, α-acids exhibit antioxidative behaviour by reacting with radical species and chelating metal ions. Beyond kettle additions, hops are often introduced during fermentation or maturation in dry hopping, aimed at enhancing aroma without additional bitterness. However, dry hopping introduces certain risks to flavor stability. Oxygen in-gress during hop additions can promote oxidative reactions, while transition metal ions (leached from hop material) can catalyse the formation of reactive oxygen species. Despite these risks, recent studies suggest that dry hopping may still provide net oxidative benefits. Furthermore, the post-fermentation addition of hops in a beer-like system, such as kombucha (a non-alcoholic fermented beverage produced by a symbiotic culture of bacteria and yeast) was found to decrease the rate of radical formation 

Ditrych, M., Aerts, G. and Andersen, M.L. (2025) “Technological strategies for controlling aldehyde formation in beer: a review of brewing-related flavour instability”, BrewingScience, 78(7/8), pp. 77–102. doi: 10.23763/BrSc25-10ditrych

Smells like hops… or hemp?

This article reads like a scientific study on hop volatiles, but it is about hemp. The results of this study represent the first application of GC-O (chromatography-olfactometry) in combination with AEDA (aroma extract dilution analysis ) on dried hemp flowers from different cultivars. The result revealed a total of 52 odor-active compounds from various chemical classes. Although many of the compounds identified in this study have been reported before in other hemp and nonhemp materials their occurrence as odor-active constituents in dried hemp flowers was reported here for the first time. Beyond terpenes, terpenoids and thiols, compounds such as 2- and 3- methylbutanoic acid, methyl anthranilate, eugenol and 3- hydroxy-4,5-dimethylfuran-2(5H)-one (sotolon) were detected in all cultivars, albeit with varying FD (flavour dilution) factors. Their occurrence suggests their integral role to the overall odorant profiles of hemp flowers. We do know all these compounds by heart from a lot of different hop varieties, so these similarities might be due to the degree of relationship between hops and hemp.   

Thi Khanh Linh Tran, Tatiana Avellaneda, Amandine André, Elodie Gillich, Martin Steinhaus, Dániel Árpád Carrera, Leron Katsir, and Irene Chetschik, The Plant of Many Scents: Unraveling the Odorant Composition of Selected CBD Hemp Cultivars Journal of Agricultural and Food Chemistry 2025 73 (38), 24314-24325 DOI: 10.1021/acs.jafc.5c07208 

My pleasure to introduce to you Phyllobilins in Hops

Hops is investigated for hundreds of years already, still every month we learn new things about hops. To discover new compounds in hops is very exciting and does not happen very often. Hops is increasingly valued as medicinal plant. Its complex phytochemistry includes phenolic compounds and bitter prenylated polyketides, but also phyllobilins. Phyllobilins are a group of chlorophyll-derived bilin-type linear tetrapyrroles, generated in the process of chlorophyll breakdown. Since the first phyllobilin was isolated and characterized in 1991, more and more structures of these bioactive chlorophyll catabolites were identified alongside the biochemical players involved in chlorophyll breakdown. In the meantime, phyllobilins are known to occur in a large natural structural variety, and new modifications are still being discovered. Phyllobilins have been regarded as products of chlorophyll detoxification for a very long time, hence they have been completely overlooked as a natural product class in terms of their biological role or pharmacological activity.  In this work, several dioxobilin-type phylloleucobilins (DPleBs) and phylloxanthobilins (DPxBs) were identified in yellowish leaves of common hops (Humulus lupulus). Isolation from 107 g of leaves yielded 0.24 mg of Hl-DPleB-28 and 0.80 mg of Hl-DPxB-31. Structural elucidation confirmed those as new phyllobilins, featuring an unusual hydroxylation motif, indicating an uncharacterized metabolic pathway. Hl-DPxB-31 possessed high antioxidative activity, comparable to quercetin. A virtual tool predicted over 60% bitterness probability. These findings expand the phytochemical profile of hops and highlight potential for upcycling leaf waste. There is definitely the need to look into this discovery more deeply.  

Christian Nadegger, Patricia Frei, Christian A. Elvert, Cornelia A. Karg, Johanna M. Gostner, Jonathan S. Lindsey, Christoph R. Kreutz, Stefan Schwaiger, Thomas Müller, and Simone Moser: Characterization of Phyllobilins in Hops: Antioxidant and Potentially Bitter Senescence-Related Metabolites Journal of Agricultural and Food Chemistry 2025 73 (28), 17637-17645 DOI: 10.1021/acs.jafc.5c03549