## 1 Introduction  
Hop oil is among the most complex plant oils and accounts for up to 4% of the dried hop weight [1]. It can be obtained by hydrodistillation (essential oil) or extraction from hop (_Humulus lupulus_ L.) cones and then is analysed for its volatile and non-volatile components [2]. Most of the volatile components of hop oil belong to the three chemical groups (hydrocarbons, oxygenated, and sulphur-containing compounds), and the non-volatile ones include carboxylic acids, hop resins, carbohydrates, amino acids, and polyphenols [3]. Gas chromatography (GC), implemented with increasingly more precise detection systems, has been the main analytical tool used to study the volatile components of hop oil since the early days of this technique, and nowadays their number is estimated to be higher than 1000 [4]. An extensive and informative review on the composition of hop oil and the chemical and biological properties of individual oil components is given in reference [5]. This review mentions the curative properties of hop oil (sedative, anti-cancer, analgesic, and anti-inflammatory) [6; 7] and also harmful ones (allergic reactions, neurotoxicity, and abortion when used at higher doses) [8]. Hops and hop-derived products have also been tested for their antioxidant [9; 10] and antibacterial [11; 12] properties. The volatile and non-volatile components of hop oil are largely responsible for the taste, flavour, and aroma of beers [13], although this effect is not straightforward and depends upon numerous reactions among different volatile and non-volatile components. These complex and still largely unknown processes can be classified as synergistic, antagonistic, additive, and masking [14]. Structural transformation of compounds belonging to the non-volatile fraction occurs at the word boiling step, but the most important transformation is the isomerisation of hop \(\alpha\)-acids to iso-\(\alpha\)-acids, which are responsible for the bitter taste of beers [15]. The contents of volatile and non-volatile hop oil components depend on several factors, such as the genetic characteristics of a given hop variety [16; 17], the plant rootstock age, geographical location of the hop plantation, climatic and soil conditions, and harvest time [18; 19]. The traditional beer brewing processes are kettle, late, and dry hopping, and these processes differ regarding the stage at which hops are introduced to the beer brewing system. A drawback of kettle hopping is that up to 85% of the components of the most volatile fraction evaporate from the system due to thermal exposure [20; 21]. This drawback can be mitigated by applying late hopping, which results in strongly aromatic yet moderately bitter beers [21; 22]. This compromise can be circumvented by dry hopping (i.e., adding hops to the beer fermentation vessel, or cold lagering beer), which helps to achieve a kind of balance among beer flavour, aroma, and bittermess [23]. Hops available on the global market can be divided into two groups: bittering hops (characterized by high contents of bitter acids) [14] and finishing hops (rich in volatile aroma components) [24; 25; 26; 11]. Summing up, the steering of traditional beer production needs outstanding expertise and is far from an easy task [16; 27]. Seasoning of post-fermentation beers with commercially available hop oils hydrodistilled from different hop varieties is regarded as an emerging alternative to dry hopping and manufacturers of commercial hop oils carefully see to it that hop cones are harvested during the best phase of the hop vegetation season [28]. This approach is a relatively new option of considerable economic and environmental importance, as it saves energy and limits brewery waste. Reference [29] reports on the GC-flame ionization detection (FID) analysis of hop oils hydrodistilled in-home from ten different hop cultivars of Belgian, Dutch, German, and North American types, and a comparison is made of their chemical composition.