Citation: Rutnik, K.; Ocvirk, M.; Košir, I.J. The Stability of Hop (Humulus lupulus L.) Resins during Long-Period Storage. Plants 2023, 12, 936. https://doi.org/10.3390/ plants12040936 Academic Editor: Franklin Gregory Received: 20 January 2023 Revised: 16 February 2023 Accepted: 16 February 2023 Published: 18 February 2023 Copyright: © 2023 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/). plants Article The Stability of Hop (Humulus lupulus L.) Resins during Long-Period Storage Ksenija Rutnik 1 , Miha Ocvirk 1 and Iztok Jože Košir 1,2, * 1 Department for Agrochemistry and Brewing, Slovenian Institute of Hop Research and Brewing, 3310 Žalec, Slovenia 2 Biotechnical Faculty, University of Ljubljana, 1000 Ljubljana, Slovenia * Correspondence: iztok.kosir@ihps.si Abstract: The stability of alpha-acids, beta-acids and hop storage index (HSI) values under different conditions (aerobic/anaerobic, 4 ◦ C/room temperature) was studied in a two-year trial. Six different varieties (Celeia, Aurora, Bobek, Styrian Gold, Savinjski Golding and Styrian Wolf) were used in the form of cones and pellets. Alpha- and beta-acids were determined by HPLC and HSI by spectrophotometry. Anaerobic conditions at 4 ◦ C were best for alpha-acids, beta-acids and HSI values; however, 10–35% of the alpha-acids were still lost after two years. The decline was greater (63–99%) under aerobic conditions and at room temperature. Alpha-/beta-acid ratios increased in hop cones and decreased in hop pellets, whereas HSI values increased in all storage conditions. Overall, the performance was better for pellets than for hop cones. Storage conditions, storage form and hop variety had significant effects on the stability of hop resins. Keywords: alpha-acids; beta-acids; hop storage index; stability; hop storage 1. Introduction The female inflorescences of the hop plant (Humulus lupulus L.), a perennial climbing plant, play an indispensable role in the brewing industry. These inflorescences contain lupulin, a complex of resins and essential oil [1]. The resins are further divided into soft and hard resins according to their solubility in hexane. Soft resins contain alpha- and beta-acids, the most valuable brewing compounds, as well as essential oils [2,3]. Alpha- acids, also referred to as humulones, are mixtures of five analogues: co-, n-, ad-, pre- and post-humulone, with last two being less common. They are insoluble in their native form, but they undergo isomerisation during the process of wort boiling, resulting in six soluble isomers (cis- and trans- forms of co-, n- and ad- analogue) that are responsible for 90–95% of beer bitterness [4]. In addition to the alpha-acid distribution, the ratio between co-humulone and (n+ad)-humulone is also an important contributor to beer bitterness. In 1972, Rigsby revealed that hops with high co-humulone content imparted stronger, sharper and less pleasant bitterness compared to low co-humulone hops [5]. His findings attracted substantial attention and had significant consequences for the hop and brewing industries. Brewers demanded more low co-humulone hops, which then led to breeding the low co-humulone varieties. This prompted other researchers, such as Wackerbauer and Balzer [6], Hughes [7], Shellhammer [8], and Schönberger [9], to investigate the influence of co-humulone levels on bitterness quality. However, they found no evidence for co- humulone causing a harsher or more unpleasant bitterness than observed with the other homologues. Despite these findings, a belief remains among some brewers that high levels of co- humulone result in low-quality, bitter beers. In fact, hops with high co-humulone levels could contribute more bitterness, since the isomerisation yield is higher for co-humulone than for other homologues [9,10]. Besides undergoing isomerisation, alpha-acids also oxidise into humulinones, which are more polar than iso-alpha-acids due to the additional Plants 2023, 12, 936. https://doi.org/10.3390/plants12040936 https://www.mdpi.com/journal/plants