Hop-induced formation of ethyl esters in dry-hopped beer

Abstract

Fruity smelling esters play an important role for the aroma of hops and beer and they have been characterized as key aroma compounds in different hop varieties. Studies on the transfer of hop-derived compounds into beer during dry-hopping showed calculated transfer rates of different ethyl esters far above 100%, leading to the assumption that these esters must be newly formed. To investigate this formation, dry-hopping was imitated in water to eliminate the influence of the beer matrix on the formation of these odorants. Thereby, the formation of ethyl esters of 2-methylbutanoic acid, 3-methylbutanoic acid, and methylpropanoic acid, induced by the addition of hops, was shown. Different approaches inhibiting enzyme activities and experiments with different hop extracts might lead to the assumption that enzymes are involved in the formation of these esters, beside possible transesterification.

Studies on the odorant concentrations and their time dependencies during dry‐hopping of alcohol‐free beer

“…After days 3‐4, an additional formation of ethyl and methyl esters started (Figure 2A‐D). This phenomenon of a hop‐induced ester formation has been recently described 11 . As the reaction type has not yet been completely clarified, it cannot finally be explained, which external factors caused the differences between the two replicates.…”

“…Nevertheless, it was observed that a higher formation rate of methyl esters seems to correlate with a lower rate of ethyl esters (except for propyl 2‐methylbutanoate with a maximal transfer rate of 60% within the first 24 hours, followed by a constant decrease of its concentration; Figure 2E). In a very recent study, it has been reported that the propyl ester is not affected by the hop‐induced ester formation 11 …”

“…Thus, also for the esters, a too high amount must be avoided, but is normally not reached by common practices. Since the major amount of these esters in alcohol‐free beer is caused by the hop‐induced ester formation 11 and not by the transfer during dry‐hopping, a higher hop dosage does not necessarily lead to an increase in the odour intensity.…”

“…While, among others, 3‐methyl‐1‐butanol, 2‐phenylethanol, 2‐methoxy‐4‐vinylphenol, and 2‐ and 3‐methylbutanoic acid were identified as important key odorants in different beer types, 5,6 hop key odorants like linalool, geraniol, or 4‐mercapto‐4‐methylpentan‐2‐one were shown as highly important for the typical aroma of dry‐hopped beers 4,7‐9 . Also, different esters like ethyl 3‐methylbutanoate and ethyl methylpropanoate were demonstrated to be present in concentrations above their odour thresholds in regular and alcohol‐free dry‐hopped beers 7,10,11 …”

Beer as a Source of Hop Prenylated Flavonoids, Compounds With Antioxidant, Chemoprotective and Phytoestrogen Activity

“…IBU (International Bitterness Units) are units by which the content of bitter substances in mg/L of beer is expressed by a spectrophotometric determination of the isohumulone concentration. The process should not exceed 120 minutes because beer could acquire an old taste and the color of the wort could increase too much (Brendel et al, 2020). Novotný (2019) states that there are also so-called unconventional hop methods, e. g. mashing hops and cold hops.…”