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Investigating the potential for re-using spent dry-hops in the brew kettle
D. G. Hauser, A. Stokholm and T. H. Shellhammer

With the rising popularity of hop-forward American ales, dry-hopping has emerged as a ubiquitous technique for achieving pronounced hop aroma and flavor in beer. Due to the low temperatures at which dry-hopping is carried out (relative to kettle additions), hop-derived bitterness precursors are neither isomerized nor transferred to beer and remain, in significant quantities, within the discarded waste material. Therefore, this tudy was carried out to assess whether spent dry-hops could potentially be re-used for bittering. Spent hops following dry-hopping were collected from a local brewery, in addition to samples of the same pelletized hops used to dry-hop that specific brand. Hop acid utilization rates were measured on a lab scale using 1.5 L of un-hopped wort dosed separately with spent hops or hop pellets and boiled for 60 minutes in 5 L round-bottom flasks, and also on two pilot scale (~ 160 L) brews. Lab scale utilization rates for the hop pellets and spent hops were not significantly different, both averaging 29 %. On the pilot scale, utilization rates differed, at 21 % and 27 % for the hop pellets and spent hops, respectively. Finished pilot scale trial beers were then statically dry-hopped with Cascade hops at a rate of 386 g/hL (1 lb/bbl) for 48 hr at 13 C (55 F) in 50 L treatments, resulting in four beers for sensory analysis. The dry-hopped beers were found to be significantly different via an unspecified tetrad test, and the higher bitterness of the spent hop beer was confirmed via a two-alternative forced choice test. However, consumer testing for overall liking and liking of the beers aroma and bitterness showed no significant differences between any pairwise comparisons of the four beers. These results demonstrate that from both an in-brewery utilization and organoleptic perspective, spent dry-hops could provide a feasible alternative to traditional kettle additions, while potentially saving brewers money and reducing environmental impact.

Descriptors: dry-hops, spent hops, sustainability, waste valorization, humulus lupulus, hop acid utilization

BrewingScience, 72 (May/June 2019), pp. 125-131