From hop ( Humulus lupulus L.) to beer: hop pellets as fungal vectors in dry-hopped craft beers

Dry hopping, a popular technique in modern craft brewing, introduces non-sterile hop material that may act as a source of microorganisms. Although beer is generally considered microbiologically stable, recent findings indicate that hops can harbour viable fungal and bacterial strains with potential effects on beer quality and hop creep enzymes. We investigated the fungal DNA reservoir of commercial hop pellets and its transfer into beer during dry hopping. Using ITS2 metabarcoding, we characterized fungal communities in hop pellets, pre-hopping beer, and dry-hopped beer, complemented by untargeted volatile profiling (HS-SPME-GCMS). Hop pellets contained diverse fungal assemblages dominated by common foliar endophytes. Several yeast genera of fermentative or spoilage relevance were also detected, including Saccharomyces, Wickerhamomyces, Rhodotorula, and Debaryomyces. While most taxa were found only in hops, four genera (Wickerhamomyces, Vishniacozyma, Bipolaris, Curvularia) were additionally found in dry-hopped beer but absent from pre-hopping samples, indicating transfer from hops. Metabolomic screening revealed that, besides enrichment of hop-derived metabolites, dry hopping induced shifts in volatile profiles through increases in ethyl esters, higher alcohols, and short-chain fatty acids. Our results demonstrate that commercial hop pellets carry diverse fungal assemblages and that their DNA is detectable in beer after dry hopping, together with aroma shifts consistent with microbial or enzymatic activity.