Citation: Rutnik, K.; Ocvirk, M.; Košir, I.J. Changes in Hop (Humulus lupulus L.) Oil Content and Composition during Long-Term Storage under Different Conditions. Foods 2022, 11, 3089. https:// doi.org/10.3390/foods11193089 Academic Editors: Carlos A. Blanco and Isabel Caballero Received: 3 August 2022 Accepted: 3 October 2022 Published: 5 October 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 Changes in Hop (Humulus lupulus L.) Oil Content and Composition during Long-Term Storage under Different Conditions 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: Hop essential oil, in addition to alpha-acids, is one of the most valuable parameters for brewers, since it is responsible for beer aroma. The hop oil content and hop oil chemical composition deliver various aromas to beer. During storage, the hop chemical composition undergoes many physical and chemical changes that impact its quality. The main purpose of our study was to evaluate the changes occurring in hop oil content and its chemical composition during two years of storage under four different conditions (anaerobic, aerobic, cold room (4 ◦ C), and room temperature) in the form of cones and pellets, supplied by local suppliers in Slovenia. Hop oil content and composition were determined by steam distillation and GC-FID. The greatest decrease in hop essential oil content occurred when the hops were stored under aerobic conditions and at room temperature. Monitoring of eight hop oil compounds under various conditions revealed different behaviors of the changes. The best storage conditions were anaerobic at low temperatures. Oxygen resistance was lower for pellets than for hop cones, whereas high temperature was more devastating for hop cones. In addition to the storage conditions, the hop variety and form were important factors regulating the extent of changes in hop oil content and chemical composition. Keywords: hops; storage stability; hop essential oil; hop oil composition 1. Introduction Beer aroma plays a key role in our perception of beer quality and is a product of a synergistic effect between compounds found in hops, barley malt, and yeast. Barley malt gives beer its body, while higher alcohols, esters, and vicinal diketones produced by yeast are compounds that also have a great impact on the final aroma of beer [1,2]. A third factor influencing beer aroma arises from the components found in hop essential oil [3]. Based on their concentrations and ratios, they can impart various aromas, such as citrussy, herbal, floral, fruity, or typical hoppy aromas. When hops are harvested and processed, their levels of essential oil start to decrease, and the chemical composition of the oil starts to change. These changes can be so extensive that the aroma they deliver to beer is of a different type [4]. The changes can be slowed down with proper storage; however, despite the fact that awareness of the importance of proper storage is growing, it is still not adequate. Some hop growers still do not have cold rooms, and their hops—if they are not delivered to the breweries immediately—are definitely not of the best quality. In addition, in 2019 and 2020, the supply of hops exceeded the demand by the brewing industry, resulting in a hop surplus [5]. Part of the delivered supplies was used one or even two years after harvesting, and some of it is still in storage. Despite the awareness that the quality of those supplies is not as high as that of fresh hops, little is known regarding the changes in hop oil chemical composition over two years of storage. Even less information is available regarding the influence of different storage conditions. Previous research investigating hop Foods 2022, 11, 3089. https://doi.org/10.3390/foods11193089 https://www.mdpi.com/journal/foods