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.Graphical abstract
Highlights
Previous studies on dry-hopping have evaluated the transfer of, among other odorants, different ethyl esters from hops into beer
The added monocarboxylic acids in model A did not influence the ester formation since their concentrations (0.29–1.8 μg/L) did not change significantly without the addition of the acids. These results indicated that monocarboxylic acids derived from hop humulones and lupulones may not be the precursors for the esterification
In conclusion, the present study demonstrated that hops induce the formation of ethyl esters of 2- and 3methylbutanoic acid and methylpropanoic acid during dry-hopping
Summary
Previous studies on dry-hopping have evaluated the transfer of, among other odorants, different ethyl esters from hops into beer. It is already known that hops can be the source of monocarboxylic acids, that are needed for the formation of these esters, namely, 2- and 3-methylbutanoic acid and methylpropanoic acid, formed by the degradation of humulones (Rettberg et al 2014; Williams and Wagner 1979). Different formation pathways of ethyl esters have already been suggested, e.g., a non-enzymatic reaction during beer storage (Vanderhaegen et al 2003, 2007; Williams and Wagner 1978, 1979). A further possibility is the enzymatic esterification during yeast fermentation by acetylCoA:ethanol O-acetyltransferases (AEATs) (Dank et al 2018; Rettberg et al 2014; Saerens et al 2008). Isoleucine, leucine, and valine are known as precursors for 2-methylbutyl and 2-methylbutanoate esters, 3methylbutyl and 3-methylbutanoate esters, and methylpropyl and methylpropanoate esters (Matich and Rowan 2007; Rowan et al 1996)
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