Citation: Rutnik, K.; Ocvirk, M.; Košir, I.J. Impact of Hop Freshness on Dry Hopped Beer Quality. Foods 2022, 11, 1310. https://doi.org/10.3390/ foods11091310 Academic Editors: Antonietta Baiano and Pasquale Massimiliano Falcone Received: 28 March 2022 Accepted: 27 April 2022 Published: 30 April 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). foods Article Impact of Hop Freshness on Dry Hopped Beer Quality Ksenija Rutnik , Miha Ocvirk and Iztok Jože Košir * Department for Agrochemistry and Brewing, Slovenian Institute of Hop Research and Brewing, 3310 Žalec, Slovenia; ksenija.rutnik@ihps.si (K.R.); miha.ocvirk@ihps.si (M.O.) * Correspondence: iztok.kosir@ihps.si Abstract: The hop plant is seasonal, but beer production continues throughout the whole year. The quality of hops begins to decrease immediately after harvesting; therefore, maintaining the highest possible quality is important. A good indicator of hop freshness is the hop storage index (HSI). In this study, three different varieties of hops with five different HSI values, from 0.3 to 0.7, were used for brewing with the dry hopping technique. The main goal was to evaluate the impact of the HSI value on beer quality in terms of hop aroma and bitterness. Alpha acids, iso-alpha acids, humulinones, bitterness units and hop aroma compounds were chemically analysed. Sensorial analysis was also conducted on all samples. Decreases in the intensity and quality of hop aroma were detected with increasing HSI. The quality of bitterness was also reduced. High HSI also led to undesirable gushing. Beers brewed with hops with HSI values greater than 0.4 had deviations in aroma and bitterness when compared with beers brewed with fresh hops. Keywords: aged hops; beer quality; hop storage index 1. Introduction Dry hopping has become an indispensable technique in beer production, especially in craft breweries. In this process, hops are added to cooled down wort, during fermentation or maturation, mainly to impart the aroma and flavour of beer [1]. Technically, dry hopping is the cold extraction of hop volatiles and non-volatiles. When traditional kettle hopping is applied, most of the volatiles evaporate during boiling and the contribution to beer aroma is limited. In dry hopping, there is no boiling, so the direct transfer of hop constituents is allowed and, consequentially, higher amounts are present in beer. Therefore, dry hopped beers have a more intense aroma, while bitterness is reduced compared to kettle hopped beers [2]. However, there are numerous variables that have to be taken into account when dry hopping is applied. Its efficiency is dependent on the number of hop additions, contact time, temperature, the regime (static or dynamic) and, last but not least, hop parameters such as alpha acid content, essential oils, form of hops, polyphenols, etc. [3]. For a long time, dry hopping was not considered to contribute to bitterness, since no isomerisation of alpha acids occurs. However, this idea has been revisited recently by researchers who have investigated the influence of dry hopping on beer bitterness [4–6]. For example, Parkin and Shellhammer [4] reported that bitterness in dry hopped beers is also derived from humulinones and polyphenols. However, the majority of the contribution comes from the former, since polyphenols have a tenfold lower influence on bitterness units (BUs) and perceived bitterness. Humulinones are oxygenated products of alpha acids, and their levels increase during hop storage. They are approximately 66% as bitter as iso-alpha acids; therefore, they should be desirable components in hops to impart bitterness during dry hopping [7–9]. However, during the period when the levels of humulinones are increasing, numerous other changes are also occurring in the chemical composition of hops. One is that the levels of hop essential oil decrease and its composition changes significantly during storage. The oxygenated fraction of hop oil increases at the expense of hydrocarbons, and various compounds are Foods 2022, 11, 1310. https://doi.org/10.3390/foods11091310 https://www.mdpi.com/journal/foods