Hop Essential Oil Research Articles

Sequential Hybrid Three-Dimensional Gas Chromatography with Accurate Mass Spectrometry: A Novel Tool for High-Resolution Characterization of Multicomponent Samples

A novel sequential three-dimensional gas chromatography-high-resolution time-of-flight mass spectrometry (3D GC-accTOFMS) approach for profiling secondary metabolites in complex plant extracts is described. This integrated system incorporates a nonpolar first-dimension (1Dnp) separation step, prior to a microfluidic heart-cut (H/C) of a targeted region(s) to a cryogenic trapping device, directly followed by the rapid reinjection of a trapped solute into a polar second-dimension (2DPEG) column for multidimensional separation (GCnp-GCPEG). For additional separation, the effluent from 2DPEG can then be modulated according to a comprehensive 2D GC process (GC×GC), using an ionic liquid phase as a third-dimension (3DIL) column, to produce a sequential GCnp-GCPEG×GCIL separation. Thus, the unresolved or poorly resolved components, or regions that require further separation, can be precisely selected and rapidly transferred for additional separation on 2D or 3D columns, based on the greater separation realized by these steps. The described integrated system can be used in a number of modes, but one useful approach is to target specific classes of compounds for improved resolution. This is demonstrated through the separation and detection of the oxygenated sesquiterpenes in hop ( Humulus lupulus L.) essential oil and agarwood ( Aquilaria malaccensis) oleoresin. Improved resolution and peak capacity were illustrated through the progressive comparison of the tentatively identified components for GCnp-GCPEG and GCnp-GCPEG×GCIL methods. Relative standard deviations of intraday retentions (1 tR, 2 tR,, and 3 tR) and peak areas of ≤0.01, 0.07, 0.71, and 7.5% were achieved. This analytical approach comprising three GC column selectivities, hyphenated with high-resolution TOFMS detection, should be a valuable adjunct for the improved characterization of complex plant samples, particularly in the area of plant metabolomics.

Comparison of sensory and chemical evaluation of lager beer aroma by gas chromatography and gas chromatography/mass spectrometry.

Although the evaluation of beer is conducted by sensory experts, we cannot neglect the influence of human factors and subjectivity. This problem could be solved by the chemical analysis of the volatile part of beer aroma and, from this, we can build a database for the construction of a model that classifies samples in a comparable manner to sensory assessment. Twenty-two batches of the same beer brand were assessed by sensory evaluation and described chemically in terms of the contents of alcohols and esters (n = 9), hop essential oil compounds (n = 15) using gas chromatography (GC) and other aroma volatiles (n = 33) as analysed by head space solid-phase microextraction (SPME)-GC/mass spectrometry. The best match of 91% with respect to sample classification on the basis of chemical analyses to sensory scores was achieved with a dataset of results from headspace-SPME and analyses of higher alcohols and esters by regularized discriminant analysis. The results of the present study show that deviations in beer aroma are not a consequence of a permanent repeatable error in brewing process, nor are they influenced by raw materials, but, instead, they are a consequence of alcoholic fermentation. Sensory analysis could be replaced with chemical/statistical analysis on an appropriate data set and for a distinct beer brand. The good results achieved confirm our approach; however, for different beer brands or types, this method should be optimised. © 2017 Society of Chemical Industry.

Parallel comprehensive two-dimensional gas chromatography

We introduce an information rich analytical approach called parallel comprehensive two-dimensional gas chromatography (2GC×2GC). This parallel chromatography approach splits injected samples into two independent two-dimensional column ensembles and provides two GC×GC separations by using contra-directional thermal modulation. The first-dimension (1D) and second-dimension (2D) columns are connected using planar three-port microchannel devices, which are supplied with supplementary flow via two pressure controller modules. Precise carrier gas flow control at the junction of the 1D and 2D columns permits independent control of flow conditions in each separation column. The 2GC×2GC approach provides two entirely independent GC×GC separations for each injection. Analysis of hop (Humulus lupulus L.) essential oils is used to demonstrate the capability of the approach. The analytical performance of each GC×GC separation in the 2GC×2GC experiment is comparable to individual GC×GC separation with matching column configurations. The peak capacity of 2GC×2GC is about 2 times than that of single GC×GC system. The dual 2D chromatograms produced by this single detector system provide complementary separations and additional identification information by harnessing different selectivity provided by the four separation columns.

Chemotyping of new hop (Humulus lupulus L.) genotypes using comprehensive two-dimensional gas chromatography with quadrupole accurate mass time-of-flight mass spectrometry

Comprehensive two-dimensional gas chromatography with quadrupole accurate mass time-of-flight mass spectrometry (GC×GC-Q-TOFMS) is employed to profile Humulus lupulus L. (hop) essential oils. Comparison of characterised essential oils allows discrimination among chemotypes. Experimental and commercial hop genotypes displayed distinguishable chemotypic patterns among the volatile secondary metabolites making up their essential oils. In total, 210–306 unique compounds were detected (depending on specific genotype), with 99 of these compounds either positively or tentatively identified. Identified volatile secondary metabolites were grouped into esters, monoterpene hydrocarbons, oxygenated monoterpenes, sesquiterpene hydrocarbons, oxygenated sesquiterpenes and ketones. Terpenoids were the dominant chemical families across all hop genotypes analysed, representing between 67% and 90% of the total ion count. The multidimensional chromatographic profiles of hop essential oils are extremely information-rich, making GC×GC-Q-TOFMS useful for fast screening of new hybrid hop genotypes, and therefore informing breeding strategies to derive new commercial hop cultivars for the development of distinctive and desirable beers.

Heat-Induced Changes in the Composition of Varietal Hop Essential Oils via Wort Boiling on a Laboratory Scale

We previously demonstrated that boiling of hop essential oil in water transforms sesquiterpene hydrocarbons into oxidation products (oxygenated sesquiterpenoids), which impart typical spicy/woody scents associated with “kettle hop” aroma when added to beer. In this study, boiling experiments in water as well as in wort with increasing hop oil (cv. Saaz) concentrations and subsequent headspace solid-phase microextraction GC-MS analysis revealed a positive correlation between the initial added hop oil concentration and formation of oxygenated sesquiterpenoids. Intervarietal differences were investigated by comprehensive GC-MS fingerprinting of hop-derived volatiles in unboiled and boiled hop essential oils (cv. Hallertau Magnum, Hallertau Tradition, Hallertau Perle, and Saaz) in wort. Unboiled essential oils of European aroma hops were shown to contain significantly higher levels of α-humulene, β-caryophyllene, and sesquiterpene oxidation products compared with cv. Hallertau Magnum. However, changes in the volatile profile upon boiling in wort appeared largely variety independent (i.e., chemically identical α-humulene and β-caryophyllene derivatives were formed). GC–olfactometry further showed that some of these derivatives formed upon boiling (i.e., 6(5→4)-abeo-caryophyll-8(13)-en-5-al, caryophyllene oxide, humulene epoxide III, humulenol II, caryophylla-4(12),8(13)-diene-5α/β-ol, 3Z-caryophylla3,8(13)-diene-5α/β-ol, and 14-hydroxy-β-caryophyllene) eluted in flavoractive zones.

Flavor Activity of Sesquiterpene Oxidation Products, Formed upon Lab-Scale Boiling of a Hop Essential Oil–Derived Sesquiterpene Hydrocarbon Fraction (cv. Saaz)

The “kettle hop” aroma, a most complicated flavor characteristic of conventionally hopped lager beers, has been associated with hop essential oil–derived oxygenated sesquiterpenoids. In this study, an oxygenated sesquiterpenoid fraction, mainly consisting of sesquiterpene oxidation products, was obtained by lab-scale boiling of a varietal sesquiterpene hydrocarbon fraction (cv. Saaz), and its volatile fingerprint was characterized analytically via headspace solid-phase microextraction GC-MS. Addition of this fraction to nonaromatized iso-α-acid-bittered beer resulted in a shift of the flavor profile in favor of woody, spicy, and hoppy notes. GC-olfactometry analysis of the sensory-relevant oxygenated sesquiterpenoid fraction revealed two zones that were clearly flavor active, the first zone containing humulene epoxide III, humulenol II, and caryophylla-4(12),8(13)-diene-5-ol, and the second zone comprising (3Z)-caryophylla-3,8(13)-diene-5-ol (α and β) and 14-hydroxy-β-caryophyllene. These compounds are candidate key impact compounds for kettle hop aroma. Although the oxygenated sesquiterpenoid fraction was created offline, the fraction shows potential relevance to brewing practice.

Determination of Volatiles in Hops from Romania by Solid Phase Fiber Microextraction and Gas Chromatography–Mass Spectrometry

ABSTRACTThe main objective of this study was to evaluate the variability of the essential oils of hops harvested in two consecutive years using in-tube extraction with gas chromatography–mass spectrometry. The second objective was to assess the possibility of using the volatile profile to characterize and discriminate hop cultivars based on a chemometric approach. Three hop varieties from Transylvania, Romania were studied. Principal component analysis and cluster analysis were used to differentiate hop varieties and to identify discriminant varietal markers. Forty-nine volatiles were separated from hop cultivars, with forty-seven identified. The main components of hop essential oils included monoterpenes (β-myrcene) and sesquiterpenes (β-caryophyllene and α-humulene). The results indicated that even though the composition of hop essential oil may be influenced by environmental factors, volatile profiles may be employed to identify cultivars.

Changes in the hop-derived volatile profile upon lab scale boiling

Hop terpenes might be oxidized during kettle boiling into more water soluble compounds that could contribute to ‘hoppy’ aroma of kettle hopped lager beers. Our current research proves that the boiling process induces significant changes in the hop oil volatile profile. The discrimination between volatile profiles of unboiled and boiled hop essential oil was evaluated via principal component and cluster analysis (PCA and CA). HS–SPME–GC–MS analysis revealed quantitative changes (e.g. increases in the levels of oxygenated α-humulene and β-caryophyllene derivatives) as well as qualitative changes (i.e. detection of compounds, not found in unboiled hop essential oil) in the hop oil volatile profile upon boiling. Many of these compounds were previously found in lager beer and may therefore contribute to beer flavor. Interestingly, the analytical difference between unboiled and boiled hop essential oil proved to be more pronounced as the initial hop essential oil concentration used for boiling was increased. In addition, lager beers spiked with boiled hop oil were described as ‘hoppy/spicy’ during sensory evaluations. Therefore, the newly formed products and hop oil constituents that are characterized by an increased recovery after boiling, are candidate compounds for ‘hoppy’ aroma in real brewing practice.

Novel Desaturation Cell to Quantify Gushing Intensity: A Preliminary Study on Model Solutions

Beer gushing is an unwanted phenomenon that occurs when the beer foam spontaneously gushes out when a bottle or can is opened. Despite long-term research, gushing still has not been fully explained. To our knowledge, there is no simple, quick, and inexpensive method for reliably measuring gushing intensity. In this article, a desaturation cell enabling saturation, step-wise pressure drop, and image analysis of foam formation is presented. The gushing intensity was evaluated through foam formation rate and relative final foam volume. To study the physicochemical mechanism of gushing, the measurements were carried out in a simple gushing matrix made of water and bovine serum albumin (BSA), the aggregates or micelles of which serve as nucleation sites for bubble formation. The effect of hop bitter compounds (iso-α-acids and tetrahydro-iso-α-acids) and essential oils (linalool) in the BSA matrix was also tested. The hop bitter compounds increased the foam formation rate significantly while affecting the foam volume less. Linalool at low concentration caused an extremely high foam formation rate whereas, at higher concentration, did not affect the foaming of the BSA matrix.

Identification of Significant Amino Acids in Multiple Transmembrane Domains of Human Transient Receptor Potential Ankyrin 1 (TRPA1) for Activation by Eudesmol, an Oxygenized Sesquiterpene in Hop Essential Oil

Transient receptor potential ankyrin 1 (TRPA1) is a calcium-permeable non-selective cation channel that is activated by various noxious or irritant substances in nature, including spicy compounds. Many TRPA1 chemical activators have been reported; however, only limited information is available regarding the amino acid residues that contribute to the activation by non-electrophilic activators, whereas activation mechanisms by electrophilic ligands have been well characterized. We used intracellular Ca(2+) measurements and whole-cell patch clamp recordings to show that eudesmol, an oxygenated sesquiterpene present at high concentrations in the essential oil of hop cultivar Hallertau Hersbrucker, could activate human TRPA1. Gradual activation of inward currents with outward rectification by eudesmol was observed in human embryonic kidney-derived 293 cells expressing human TRPA1. This activation was completely blocked by a TRPA1-specific inhibitor, HC03-0031. We identified three critical amino acid residues in human TRPA1 in putative transmembrane domains 3, 4, and 5, namely threonine at 813, tyrosine at 840, and serine at 873, for activation by β-eudesmol in a systematic mutational study. Our results revealed a new TRPA1 activator in hop essential oil and provide a novel insight into mechanisms of human TRPA1 activation by non-electrophilic chemicals.

A powerful methodological approach combining headspace solid phase microextraction, mass spectrometry and multivariate analysis for profiling the volatile metabolomic pattern of beer starting raw materials

The volatile metabolomic patterns from different raw materials commonly used in beer production, namely barley, corn and hop-derived products – such as hop pellets, hop essential oil from Saaz variety and tetra-hydro isomerized hop extract (tetra hop), were established using a suitable analytical procedure based on dynamic headspace solid-phase microextraction (HS-SPME) followed by thermal desorption gas chromatography–quadrupole mass spectrometry detection (GC–qMS). Some SPME extraction parameters were optimized. The best results, in terms of maximum signal recorded and number of isolated metabolites, were obtained with a 50/30μm DVB/CAR/PDMS coating fiber at 40°C for 30min. A set of 152 volatile metabolites comprising ketones (27), sesquiterpenes (26), monoterpenes (19), aliphatic esters (19), higher alcohols (15), aldehydes (11), furan compounds (11), aliphatic fatty acids (9), aliphatic hydrocarbons (8), sulphur compounds (5) and nitrogen compounds (2) were positively identified. Each raw material showed a specific volatile metabolomic profile. Monoterpenes in hop essential oil and corn, sesquiterpenes in hop pellets, ketones in tetra hop and aldehydes and sulphur compounds in barley were the predominant chemical families in the targeted beer raw materials. β-Myrcene was the most dominant volatile metabolite in hop essential oil, hop pellets and corn samples while, in barley, the predominant volatile metabolites were dimethyl sulphide and 3-methylbutanal and, in tetra hop, 6-methyl-2-pentanone and 4-methyl-2-pentanone. Principal component analysis (PCA) showed natural sample grouping among beer raw materials.

Comparative Gas Chromatographic–Mass Spectrometric Evaluation of Hop (Humulus lupulus L.) Essential Oils and Extracts Obtained Using Different Sample Preparation Methods

The main aim of investigations was to identify chemotypes and determine differences between some domestic hop varieties and wild hops, which were collected from some regions of Lithuania and cultivated at the same edafoclimatic conditions in hops collection of Kaunas Botanical Garden of Vytautas Magnus University. One of objectives was to compare essential oils of hops (2 years harvest) by the evaluation of volatiles content. Among the main components of hop essential oils monoterpenes (β-myrcene) and sesquiterpenes (α-humulene and β-caryophyllene) were determined using gas chromatography coupled with mass spectrometry (GC-MS). Retention parameters (t R, calculated retention index, and Kovats retention index) and m/z value of molecular ion for selected compounds from hop essential oils were determined. Samples were prepared by applying solid phase microextraction (SPME), supercritical fluid extraction (SFE) and accelerated solvent extraction (ASE). The steam distillation was used to obtain hop essential oils. The chemometric comparison of domestic and wild hops based on GC-MS analysis data was carried out. The obtained statistical results allow us to classify the investigated wild forms and domestic varieties of hops according to the similarities of their chemotypes. The concentration of β-myrcene, α-humulene in hop essential oils obtained from cones 2 years harvests is much higher than other volatile organic compounds (15.2–23.7 % in total contribution). In analysed essential oils β-farnesene is a constituent in higher quantity of hop essential oils obtained from cones from second time harvest than from cones from first harvest. This can be explained by the year-to-year vegetation conditions difference.

The Dynamic Of Essential Oil Accumulation In Hop Cones During 2012 Year

Hop cones have lupulin glands which occur on the outer lower surface of the bracteoles and the entire surface of the perianth. The constituents of these glands include bitter acids and essential oil components (Auerbach et.al., 2000). Dry hop contains 0.5-2.0% of essential oil. Hop essential oils consist of a large number of different components and although their composition is influenced by the environment (Ĉerenak et.al., 2011). Aims: This study is a continuation of the research started in 2011 (Salanţă et.al., 2012), where the dynamic of essential oil accumulation in hop cones during 2011 year, were reported. In this paper, samples from 2012 year crop were subjected to hydrodistillation in order to content in essential oils. All reported results will be used in a future research, to determine the gas chromatographic composition of essential oils extracted.

TECHNOLOGY APPLIED TO THE DEVELOPMENT OF NEW FLAVOUR HOP VARIETIES IN AUSTRALIA

Hop production in Australia occupies a total of approximately 500 hectares, spread across two major farms and several smaller growers. Production is based in south-eastern Australia, spread roughly evenly between Victoria and Tasmania, latitudes 37-41° south respectively. Every hop grower in Australia relies upon varieties developed locally since 1950, in a breeding program, which continues today. Since 1994, collaborative work (predominantly with the University of Tasmania - UTAS) has seen the development of technologies such as in vitro colchicine induction to produce tetraploid plants, and flow cytometry, which have led lately to the availability of tetraploid male and female plants, and the ability to conduct tetraploid by tetraploid crosses. An extensive database of gas chromatography-mass spectrometry (GC-MS) of steam distilled hop essential oil profiles has been built up, containing information on international varieties, Australian commercial varieties, and selected material. Australian hop researchers have participated actively in the international collaboration to develop Diversity Arrays Technology (DArT) markers. Future work will include quantitative genetic analysis of large progeny trials grown in both Tasmania and Victoria, the establishment of a linkage mapping population replicated in both Tasmania and Victoria, and application and further development of DArT markers in specific roles with the new cultivar development process. Active collaboration provides hop breeding efforts in Australia with the tools necessary to create, identify, and develop novel varieties with diverse and exciting flavour potential.

Production of novel varietal hop aromas by supercritical fluid extraction of hop pellets. Part 1: Preparation of single variety total hop essential oils and polar hop essences

A new methodology for the selective isolation of varietal total hop essential oil from hop pellets and further chromatographic fractionation of total hop oil into terpeneless polar hop essence, is presented. The methodology is essentially based on supercritical fluid extraction (SFE) of total hop essential oil using carbon dioxide of appropriate density, followed by solid phase extraction (SPE) using octadecylsilica and ethanol/water mixtures for preparation of polar hop essence. Different SFE temperature–pressure combinations were tested for extraction of total hop essential oil from pellets. A carbon dioxide density of 0.50g/mL (50°C, 110atm), proved to be the best compromise in view of selective and quantitative extraction of total hop essential oils. Further fractionation of total hop essential oil by SPE in order to remove hydrocarbons, resulted in varietal polar hop essence, highly enriched in water-soluble, oxygenated hop oil constituents. All of the applied procedures of the proposed SFE/SPE methodology are in full accordance with the principles of clean-label technology. As a result, the novel hop aroma products are fully compatible with the beer matrix. When added to beer, the novel hop oil preparations impart a typical, varietal dependant pleasant hoppy character and increase beer bitterness and mouthfeel.

Headspace solid‐phase microextraction combined with mass spectrometry as a powerful analytical tool for profiling the terpenoid metabolomic pattern of hop‐essential oil derived from Saaz variety

Hop (Humulus lupulus L., Cannabaceae family) is prized for its essential oil contents, used in beer production and, more recently, in biological and pharmacological applications. In this work, a method involving headspace solid-phase microextraction and gas chromatography-mass spectrometry was developed and optimized to establish the terpenoid (monoterpenes and sesquiterpenes) metabolomic pattern of hop-essential oil derived from Saaz variety as a mean to explore this matrix as a powerful biological source for newer, more selective, biodegradable and naturally produced antimicrobial and antioxidant compounds. Different parameters affecting terpenoid metabolites extraction by headspace solid-phase microextraction were considered and optimized: type of fiber coatings, extraction temperature, extraction time, ionic strength, and sample agitation. In the optimized method, analytes were extracted for 30 min at 40°C in the sample headspace with a 50/30 μm divinylbenzene/carboxen/polydimethylsiloxane coating fiber. The methodology allowed the identification of a total of 27 terpenoid metabolites, representing 92.5% of the total Saaz hop-essential oil volatile terpenoid composition. The headspace composition was dominated by monoterpenes (56.1%, 13 compounds), sesquiterpenes (34.9%, 10), oxygenated monoterpenes (1.41%, 3), and hemiterpenes (0.04%, 1) some of which can probably contribute to the hop of Saaz variety aroma. Mass spectrometry analysis revealed that the main metabolites are the monoterpene β-myrcene (53.0 ± 1.1% of the total volatile fraction), and the cyclic sesquiterpenes, α-humulene (16.6 ± 0.8%), and β-caryophyllene (14.7 ± 0.4%), which together represent about 80% of the total volatile fraction from the hop-essential oil. These findings suggest that this matrix can be explored as a powerful biosource of terpenoid metabolites.

Production of novel varietal hop aromas by supercritical fluid extraction of hop pellets—Part 2: Preparation of single variety floral, citrus, and spicy hop oil essences by density programmed supercritical fluid extraction

In this study on the production of novel varietal hop aromas by supercritical fluid extraction of hop pellets we aim at fractionated extraction of hop oil essences by density programmed supercritical fluid extraction (SFE) of hop pellets T90 using carbon dioxide. For that purpose, SFE is performed in two steps. In the first step, the most volatile compounds from hop essential oil are isolated by extracting pellets T90 applying a carbon dioxide density of 0.29g/mL (90atm, 50°C). Subsequent fractionation of the crude extract from this first SFE step via solid phase extraction (SPE) using octadecylsilica and ethanol/water mixtures, results in flavour-active single variety hop oil essence, highly enriched in ‘floral’ compounds of total hop essential oil. When added to a non-aromatised pilot lager, such varietal essences impart pleasant and pronounced floral, hoppy and citrus scents to the beer flavour. In the second step of the SFE procedure, the hop residue from the first extraction step is further extracted using a carbon dioxide density of 0.50g/mL (110atm, 50°C). As a result of the typical construction of the collection vials and further fractionation of the crude SFE extract via SPE, single variety hop oil essence, highly enriched in oxygenated sesquiterpenoids (‘spicy’ compounds), is obtained. When added in appropriate amounts to a non-aromatised pilot lager, such ‘spicy’ essences impart revealing sensory impressions, i.e. pleasant herbal/hoppy aroma, and in particular, increased bitterness intensity, mouthfeel, and fullness.

Production of novel varietal hop aromas by supercritical fluid extraction of hop pellets—Part 1: Preparation of single variety total hop essential oils and polar hop essences

A new methodology for the selective isolation of varietal total hop essential oil from hop pellets and further chromatographic fractionation of total hop oil into terpeneless polar hop essence, is presented. The methodology is essentially based on supercritical fluid extraction (SFE) of total hop essential oil using carbon dioxide of appropriate density, followed by solid phase extraction (SPE) using octadecylsilica and ethanol/water mixtures for preparation of polar hop essence. Different SFE temperature–pressure combinations were tested for extraction of total hop essential oil from pellets. A carbon dioxide density of 0.50g/mL (50°C, 110atm), proved to be the best compromise in view of selective and quantitative extraction of total hop essential oils. Further fractionation of total hop essential oil by SPE in order to remove hydrocarbons, resulted in varietal polar hop essence, highly enriched in water-soluble, oxygenated hop oil constituents. All of the applied procedures of the proposed SFE/SPE methodology are in full accordance with the principles of clean-label technology. As a result, the novel hop aroma products are fully compatible with the beer matrix. When added to beer, the novel hop oil preparations impart a typical, varietal dependant pleasant hoppy character and increase beer bitterness and mouthfeel.

Determination of Volatile Compounds in Different Hop Varieties by Headspace-Trap GC/MS—In Comparison with Conventional Hop Essential Oil Analysis

A headspace (HS)-trap method in combination with gas chromatography (GC)-mass spectrometry (MS) was developed for the determination of volatile constituents in hops. The highly sensitive HS-trap system reduces the detection limit by using up to four trap enrichment cycles. Seventy hop samples of different varieties and cultivation regions, from the 2008 harvest, were examined using the HS-trap-GC-MS method and the established "European Brewery Convention" (EBC) method for hop essential oil analysis. Twenty-one different volatiles were quantified for each hop sample. For all compounds, except caryophyllene oxide, a strong correlation was found between the results of the HS-trap method and the EBC method. Experiments have revealed that the EBC method using steam distillation is not appropriate for thermolabile compounds, such as caryophyllene oxide, due to decomposition during boiling. The HS-trap method is fast and sensitive, requires small sample amounts and minimal sample preparation, and is easy to apply.

Biotransformations of hop-derived aroma compounds by Saccharomyces cerevisiae upon fermentation

Although hops are only added in relatively small amounts at particular stages of the brewing process, they have a high impact on the final beer flavor characteristics. The aroma of raw hops, caused by flavor-active constituents present in hop essential oil, clearly differs from the characteristic hoppy aroma of finished beer. Because of the very complex chemical composition of hop essential oil itself and insufficient knowledge on the many (bio)chemical conversions of hop oil components during the brewing process, the chemical background of hoppy aroma is still ill-defined. Since no reviews about the influence of yeast on hop oil-derived aroma compounds during the fermentation process – and thus the influence of fermentation on the hoppy aroma of final beer- have been spotted, this article was drawn up aiming at a clear overview of scientific research that has already been done in this field and of the current know-how, resulting thereof. Proposed flavor-active constituents for hoppy aroma (terpenes, sulfur compounds and oxygenated compounds) originating from hop essential oil and possible transformations of these compounds during fermentation will be discussed. In addition, the effect of yeast enzymes on hop glycosides will be reviewed, since several investigations point to their high potential as precursors of highly flavor-active aroma compounds that may contribute to the hoppy aroma of beer.