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Hop flavoromics for distinctive beer
Abstract
Hop is the spice of beer, as it provides unique flavour chemicals imparting taste (bitterness) and aroma. The continued release of novel hop cultivars with uniquely identifiable organoleptic characters is evidence that the flavour potential of hop has not been fully explored. Similarly, the genetic basis of hop organoleptic chemistry is relatively poorly understood. A mapping cross containing 166 genotypes (and the male and female parental plants) has been established into two replicated field trials (Tasmania and Victoria), separated by ~5° latitude. To date we have collected two years of data on survival, plant structure, flowering time, sexual phenotype, yield, and hop bitter acid chemistry contents on the male and female plants. Such data allow analysis of genetic variation and genotype-by-location interactions. However, the ultimate aim for the study is to obtain an extended chemical phenotype, including headspace solid phase micro extraction (SPME) of hop flavour volatiles, polyphenols, glycosides, laboratory scale beer volatile SPME profiles, and integrate this with organoleptic data. Successful completion of this hop flavoromics study will allow the identification of quantitative trait loci (QTL) in hop directly associated with beer organoleptic traits. © 2019 International Society for Horticultural Science. All rights reserved.
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The advantages of using molecular markers in modern genebanks are well documented. They are commonly used to understand the distribution of genetic diversity in populations and among species which is crucial for efficient management and effective utilization of germplasm collections. We describe the development of two types of DArT molecular marker platforms for the new oilseed crop lesquerella (Physaria spp.), a member of the Brassicaceae family, to characterize a collection in the National Plant Germplasm System (NPGS) with relatively little known in regards to the genetic diversity and traits. The two types of platforms were developed using a subset of the germplasm conserved ex situ consisting of 87 Physaria and 2 Paysonia accessions. The microarray DArT revealed a total of 2,833 polymorphic markers with an average genotype call rate of 98.4% and a scoring reproducibility of 99.7%. On the other hand, the DArTseq platform developed for SNP and DArT markers from short sequence reads showed a total of 27,748 high quality markers. Cluster analysis and principal coordinate analysis indicated that the different accessions were successfully classified by both systems based on species, by geographical source, and breeding status. In the germplasm set analyzed, which represented more than 80% of the P. fendleri collection, we observed that a substantial amount of variation exists in the species collection. These markers will be valuable in germplasm management studies and lesquerella breeding, and augment the microsatellite markers previously developed on the taxa.
Hop-derived thiols are believed to contribute to beer hop aroma. Here, we examined this contribution, and detected extremely strong fruity, muscat-like aromas in beers hopped with some American cultivars. The main contributor to the fruity aroma was identified as 4-mercapto-4-methylpentan-2-one (4MMP). In hop pellets, 4MMP concentrations were affected by the cultivar and copper ion content. The 4MMP content was highest in 'Simcoe' cultivars followed by 'Summit', 'Apollo', 'Topaz', 'Cascade' pellets. 4MMP content also differed between crop years. 4MMP was detected only in American, Australian and New Zealand cultivars, but not European ones, which are treated with Bordeaux mixture and therefore have a high content of copper ions. The field trial to spray Bordeaux mixture was conducted in the hop field of USA. The results indicated that the accumulation of the copper ion in the soil in the past years affect the higher content of copper ion in the hop cones. In the brewing process, 4MMP was found to rapidly decrease during boiling, while 3-mercaptohexan-1-ol (3MH) increased. During fermentation, both 4MMP and 3MH content increased, and 3-mercaptohexyl acetate (3MHA) was newly formed from 3MH.
A method for analysis of hop-derived polyfunctional thiols such as 4-sulfanyl-4-methylpentan-2-one (4S4M2Pone), 3-sulfanylhexan-1-ol (3SHol), and 3-sulfanylhexyl acetate (3SHA) in beer, hop water extract, and wort at ng/L levels was developed. The method employed stir bar sorptive extraction with in-situ derivatization (der-SBSE) using ethyl propiolate (ETP) followed by thermal desorption and gas chromatography-tandem mass spectrometry (TD-GC-MS/MS) with selected reaction monitoring (SRM) mode. A prior step involved structural identification of the ETP derivatives of the thiols by TD-GC-quadrupole-time-of-flight mass spectrometry with parallel sulfur chemiluminescence detection (Q-TOF-MS/SCD) after similar der-SBSE. The der-SBSE conditions of ETP concentration, buffer concentration, salt addition, and extraction time profiles were investigated and the performance of the method was demonstrated with spiked beer samples. The LODs (0.19-27 ng/L) are below the odor threshold levels of all analytes. The apparent recoveries at 10-100 ng/L (99-101 %) and the repeatabilities (RSD 1.3-7.2 %, n=6) are also good. The method was successfully applied to the determination of target thiols at ng/L levels in 3 kinds of beer samples (hopped with Cascade, Citra, and Nelson Sauvin) and the corresponding hop water extracts and wort samples. There was a clear correlation between the determined values and the characteristics of citrus hop aroma for each sample.
Application of a comparative aroma extract dilution analysis on the volatiles isolated from five different hops (Hallertau
Perle, Hallertau Hersbrucker Spät, Slowenian Golding, Hallertau Smaragd, US Cascade) revealed linalool and myrcene with the
highest Flavour Dilution (FD)-factors in all varieties, followed by 2-isopropyl-3-methoxypyrazine, 3-methylbutanoic acid and
geraniol. Some odourants, however, showed high FD-factors only in certain varieties, for example, (5Z)-octa-1,5-dien-3-one and germacrene B in Hersbrucker Spät, (3E,5Z)-undeca-1,3,5-triene in Hersbrucker Spät and Cascade and nonanal in Cascade. The overall odour profile of the Cascade sample
clearly differed from the other varietes, and was dominated by a black currant like odour note. The identification experiments
revealed 4-methyl-4-sulfanylpentan-2-one, so far unknown as hop constituent, as key contributor to this odour. In addition,
an odour-active undecatetraene was present, in particular, in Perle and Cascade. Synthesis and structural assignment of the
four stereoisomers of (3E)-undeca-1,3,5,9-tetraene allowed the identification of the fresh, pineapple-like smelling compound as (3E,5Z,9E)-undeca-1,3,5,9-tetraene. Among the four isomers synthesised, this compound showed by far the lowest odour threshold of 0.01ng/L
in air.
Tomahawk hop (Humulus lupulus) is a recently developed Super Alpha cultivar (14-18% α-acids w/w), already widely used by brewers to impart bitterness and a citrus-like aroma to beer. By comparison with two bitter varieties (Nelson Sauvin and Nugget) and two aromatic ones (Cascade and Saaz), the Tomahawk cultivar showed a very particular terpenoid profile, rich in both α- and β-selinenes (>600 mg/kg IST equiv in total), methyl geranate (>40 mg/kg IST equiv), and geraniol (>200 mg/kg). Tomahawk also proved to contain a wide variety of odorant polyfunctional thiols. The major β-sulfanyl acetate, 3-sulfanyl-2-ethylpropyl acetate, newly identified here, was found at similar levels in the famous Sauvignon-like Nelson Sauvin and Tomahawk varieties (15-44 μg/kg IST equiv). On the other hand, lower levels of total β-sulfanyl alcohols were measured in Tomahawk, although 3-sulfanylhexan-1-ol was found at a similar level and the 3-sulfanyl-4-methylpentan-1-ol previously claimed to be specific to the Nelson Sauvin variety was also evidenced in the Super Alpha cultivar (9-13 μg/kg IST equiv). As revealed by boiling and fermentation, Tomahawk hop also contains very interesting bound polyfunctional thiols that should be investigated for better use by brewers.
Application of aroma extract dilution analysis on the volatiles isolated from a Bavarian Pilsner-type beer revealed 40 odor-active constituents in the flavor dilution (FD) factor range of 16-2048, among which ethyl octanoate, (E)-beta-damascenone, 2- and 3-methylbutanoic acid, and 4-hydroxy-2,5-dimethyl-3(2H)-furanone showed the highest FD factor of 2048. After quantitation of the 26 odorants showing FD factors > or =128 by stable isotope dilution analysis and determination of their odor thresholds in water, odor acitivity values (OAVs) were calculated. The results indicated ethanol, (E)-beta-damascenone, (R)-linalool, acetaldehyde, and ethyl butanoate with the highest OAVs, followed by ethyl 2-methylpropanoate and ethyl 4-methylpentanoate, which was previously unknown in beer. Finally, the overall aroma of the beer could be mimicked for the first time by recombining 22 reference odorants in the same concentrations as they occurred in the beer using ethanol/water as the matrix.
The "spicy" character of hops is considered to be a desirable attribute in beer, associated with "noble hop aroma". However, the compounds responsible have yet to be adequately identified. Odorants in four samples of the spicy fraction of hop essential oil were characterized using gas chromatography-olfactometry (GC-O) and CharmAnalysis. Four hop varieties were compared, namely, Target, Saaz, Hallertauer Hersbrucker, and Cascade. Odor-active compounds were tentatively identified using comprehensive two-dimensional gas chromatography (GCxGC) combined with time-of-flight mass spectrometry (TOFMS). An intense "woody, cedarwood" odor was determined to be the most potent odorant in three of the four spicy fraction samples. This odor coincided with a complex region where between 8 and 13 compounds were coeluting in each of the four spicy fractions. The peak responsible was determined by (i) correlating peak areas with Charm values in eight hop samples and (ii) heart-cut multidimensional gas chromatography-olfactometry (MDGC-O). The compound responsible was tentatively identified as 14-hydroxy-beta-caryophyllene. Other important odorants identified were geraniol, linalool, beta-ionone, and eugenol.
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