- The popularity of hop-forward American craft beers has had a profound impact on the use of hops as an ingredient; where hops were once thought of as the “spice” of beer, they may now be considered the primary driver of flavor in beer styles such as American India Pale Ales. Dry-hopping, which is the cold, aqueous extraction of hops into beer or partially fermented wort, has become a key tool for brewers in imparting beer with intense hoppy aroma. The widespread adoption of this technique has led to brewers using increasingly large quantities of hops for dry-hopping, with the consequence that the full potential of the hop is often not realized. In addition to flavor, hops can make a substantial contribution to the environmental impact of highly hopped beers and are an expensive raw ingredient. Therefore, the research presented herein was undertaken in order to better understand the efficiency of dry-hopping, in addition to ways hops may be better utilized, or potentially re-used.
The retention of valuable volatile and non-volatile hop components within spent hops, as well as their extraction into to beer, on both a pilot and industrial scale was investigated. On the pilot scale (80 L), an un-hopped pale ale was statically dry-hopped with differing lots of whole cone Amarillo, Centennial, and Cascade at a rate of 386 g/hL (1 lb/bbl) for 24 hours at 13 °C (55°F). Spent hop material was also collected from a local brewery that dry-hopped ~60 hL (50 bbl) of beer at a rate of 1592 g/hL (4.125 lb/bbl). Approximately one-third of the dry-matter composition of hops was lost during dry-hopping regardless of hop variety; however, there was high retention of both α-acids (77% pilot scale, 52% industrial) and hop essential oil (51% pilot scale, 32% industrial). The oil remaining in the spent hops was enriched in hydrocarbon compounds and depleted in oxygenated compounds. These results indicate that spent dry-hops contain considerable brewing value and have the potential for re-use.
Given their high retention of bitterness precursors, spent hops were explored as a source of beer bitterness. Spent hops from 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 21.2% and 27.2% 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 386g/hL (1lb/bbl) for 48 hr at 13°C (55°F) in 50L treatments, resulting in four beers for sensory analysis.
The dry-hopped beers were found to be significantly different via an unspecified tetrad test, and this outcome was likely due to differing bitterness intensities of the two treatments. The higher bitterness of the spent hop beer was confirmed via a two-alternative forced choice test. Nevertheless, 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 hops from dry-hopping could provide a feasible alternative to traditional kettle additions, while potentially saving brewers money and reducing environmental impact.
A simple technique widely used in industry to increase overall hop aroma intensity of dry-hops was also studied, whereby brewers add hops to beer in multiple lower doses, instead of a single, high-dose hop charge. The actual efficacy of this technique has not been tested or published, and the goal of this third study was therefore to compare the compositional and organoleptic properties of beers alternately produced by single or two-stage dry-hopping at the same cumulative rate on both the pilot and commercial scale. On the pilot scale (80-100 L), a filtered, un-hopped base beer was subjected alternately to single- or two-stage dry-hop additions at rates of 386, 772, and 1544 g/hL; commercial beers (~350 hL batch size) from a regional brewery were dry-hopped at a cumulative rate of 733 g/hL in single or two-stage additions. Pilot scale dry-hopped beers showed increases in residual extract, pH, bitterness units, humulinones, and total polyphenols, accompanied by a decrease in iso-α-acids, with increasing dose. Changes in bitterness units, humulinone, and iso-α-acids all appeared to be more pronounced in the two-stage dry-hopped beers. In contrast, single- and two-stage dry-hopped commercial beers were nearly identical in terms of chemical composition. A trained panel was used to scale six aroma attributes among the pilot scale dry-hopped beers, and significant increases in all six of the aroma attributes were observed with two-stage dry-hopping relative to single additions at the same cumulative hopping rates. Slight increases in aroma potential were observed for the commercial beer made with two-stage additions, although these differences were not statistically significant. Taken together these results illustrate the impact multi-stage hopping additions have on dry-hop aroma potential and provide direction to the industry to consider whether multi-stage dry-hopping may yield desirable results in their breweries, given the potential to achieve similar or higher aroma yield using less hop material.
This research has demonstrated the inherent inefficiency of dry-hopping and proposed two techniques by which hops can potentially be better utilized. The results of these studies will help provide direction to and incite further discussion among brewers and researchers towards the more sustainable use of hops in beer production.