Comprehensive Two-Dimensional Gas Chromatography-Mass Spectrometry as a Tool for the Untargeted Study of Hop and Their Metabolites  
1 Australian Centre for Research on Separation Science, School of Chemistry, Monash University, Wellington Road, Clayton, VIC 3800, Australia; [EMAIL_ADDRESS] (G.A.P.R. ); [EMAIL_ADDRESS] (M.S.S.A.) 2 Chemistry Department, Universidade Federal de Minas Gerais, Belo Horizonte 31270-901, Brazil; [EMAIL_ADDRESS]
*Correspondence: [EMAIL_ADDRESS]  
1 Australian Centre for Research on Separation Science, School of Chemistry, Monash University, Wellington Road, Clayton, VIC 3800, Australia; [EMAIL_ADDRESS] (G.A.P.R. ); [EMAIL_ADDRESS] (M.S.S.A.) 2 Chemistry Department, Universidade Federal de Minas Gerais, Belo Horizonte 31270-901, Brazil; [EMAIL_ADDRESS]
*Correspondence: [EMAIL_ADDRESS]  
[1]M.S. terpenoids, and thiols. Alpha acids are responsible for the sensation of bitterness and foam stability, representing a mass fraction of 2 to 20% of the hop composition [5; 6]. Beta acids influence the aromatic profile of the beer and are present in lower concentrations, due to their biochemical transformation during processing of the beverage. Polyphenols, such as tannins and flavonoids, act as stabilising agents in beer, influencing turbidity and flavour [6]. Finally, hydrocarbon and oxygenated terpenoids, as well as thiols, are some of the classes of components found in hop essential oil, which reportedly include more than 300 aroma compounds [4; 7]. Essential oil compounds undergo biotransformation by yeast fermentation, such as geraniol generating beta-citronellol and, finally, the esterification product citronethyl acetate [8]. Numerous hop varieties are currently available in the market, with more than 100 cultivars being patented in the last decade and new products being continually developed and reported [9]. There are 363 results for _Humulus lupulus_ species-metabolite relationships in the KNApSAcK Core System database [10], which aids in searching metabolomics and analytical studies. Metabolomics is a defined research field that has developed over the last 25 years, which involves the comprehensive analysis of small molecules (metabolites) present in complex biological matrices. Within this broad field, there are a number of different sub-disciplines that study the behaviour of matrices such as hop proteomics [11], genomics [12], beeromics [13], hopomics [14], and volatilomics [15]. Regarding the analysis of volatile organic compounds (VOCs) and volatilomics, gas chromatography (GC) is a standardised technique that has been applied to the determination of plant metabolites for more than 50 years [16]. Multiple analytical studies have targeted key aroma compounds in hop essential oil, such as myrcene (related to fresh odour), and linaloal and geraniol (both related to floral notes) [17]. GC has also been applied in numerous analytical approaches for hop studies, such as chemical profiling [18; 19; 20], comparison of different types of sample preparation [21; 22; 23], and the study of the effect of hop in the brewing process [24; 25]. [PERSON] et al. [26] reviewed research associated with different sample preparation methods, such as steam distillation [17; 27], simultaneous distillation extraction [25], dynamic [18] and static [19] headspace, direct thermal desorption [21], direct microwave desorption [28], liquid-liquid extraction (LLE) [29], stir bar sorptive extraction (SBSE) [23], and headspace solid-phase microextraction (HS-SPME) applied to hop [22; 30]. This last method attracts attention as being a \"greener\" approach since it is a solventless technique using sorptive polymeric fibres, is simple, versatile, and has a wide applications base, with SPME reported for samples in the environmental industry, food, drugs, and other areas, although the quantitative application of SPME is rather complex [22; 31]. According to the level of complexity of the hop matrix, the full resolution and identification of compounds can be difficult to achieve.