Filip Van Opstaele

Characterization of Novel Varietal Floral Hop Aromas by Headspace Solid Phase Microextraction and Gas Chromatography–Mass Spectrometry/Olfactometry

In this study, headspace solid phase microextraction (HS-SPME) and gas chromatography-mass spectrometry (GC-MS) were optimized and implemented to investigate the volatile composition of novel floral hop essences prepared from four German aroma hop varieties. In total, 91 different constituents were assigned, which were further grouped into monoterpene hydrocarbons, esters, ketones, aldehydes, furans, and oxygenated and nonoxygenated sesquiterpenes. Most volatiles belong to the ester group, whereas the monoterpene hydrocarbon β-myrcene appears to be the predominant compound in all hop oil preparations investigated. Furthermore, as demonstrated by principal component analysis, varietal floral hop essences are clearly discriminated on the basis of their characteristic volatile composition. Via GC-olfactometry on the floral essence variety Spalter Select, β-myrcene and 2-undecanone were identified as the most potent odorants. Several hop oil constituents were reported for the first time as impact odorants of hop aroma.

Hopping technology in relation to beer bitterness consistency and flavor stability

The fate of α-acids, iso-α-acids, and their chemically modified variants was monitored in pilot brews as a function of hopping regime and beer aging. HPLC analysis indicates that α-acids, iso-α-acids, and dihydroiso-α-acids in beer are not stable during forced aging. This is reflected further in the sensory performance of these beers. Beer exclusively bittered with tetrahydroiso-α-acids was completely stable, in terms of hop components, under the experimental conditions employed. In addition, overall flavor stability was significantly improved. These results provide further evidence that hop-derived bitter acids, including the light-stable dihydroiso-α-acids, could play an important role in beer flavor deterioration during storage.

Hopping Technology in Relation to α-Acids Isomerization Yield, Final Utilization, and Stability of Beer Bitterness

A detailed study of α-acids isomerization kinetics was undertaken by performing pilot-scale brewing experiments with T90 hop pellets, nonisomerized hop extract, and nonisomerized hop extract plus hop residue. In addition, a brew was prepared by adding preisomerized hop extract at the onset of wort boiling. Compared with the use of nonisomerized hop extract, significantly higher isomerization yields were obtained when vegetative material was included in the hopping. However, when vegetative material was present final α-acids utilization was compromised by large losses of iso-α-acids postboiling. Evidence of reverse isomerization of iso-α-acids during wort boiling was not found when preisomerized hop extract was applied. The results obtained using quantitative HPLC profiling confirmed that both α-acids and iso-α-acids were not stable upon beer aging. In particular, trans-iso-α-acids underwent rapid degradation in finished beer. Therefore, beers prepared with preisomerized hop extract containing relatively less-sensitive trans-isomers, and thus more cis-isomers, show improved bitterness stability upon aging compared with conventionally hopped beers.

Characterization of novel single-variety oxygenated sesquiterpenoid hop oil fractions via headspace solid-phase microextraction and gas chromatography-mass spectrometry/olfactometry.

The volatile composition of novel varietal oxygenated sesquiterpenoid hop oil fractions (spicy essences) was characterized by headspace solid-phase microextraction in combination with gas chromatography-mass spectrometry. Oxygenated sesquiterpenes represent the major chemical compound class, accounting for at least 65% of the total volatile fraction. In addition to oxygenated sesquiterpenes, spicy hop essences consist of several ketones, sesquiterpene and monoterpene hydrocarbons, and a relatively high number of unidentified compounds. On the basis of their relative composition, spicy hop essences can be fully differentiated according to their varietal origin. Multidimensional gas chromatography in combination with time-of-flight mass spectrometry on spicy hop essence cv. Spalter Select further demonstrated the enormous complexity of this particular hop oil fraction. The aromagram obtained via gas chromatography-olfactometry comprised nine odor-active regions described in terms of citrus, green, haylike, earthy, woody, and spicy. 2-Undecanone, 2-tridecanone, γ-cadinene, α-calacorene, calarene, humuladienone, caryolan-1-ol, caryophyllene oxide enantiomers, and humulene epoxide II are tentatively identified in the odor-active zones.

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.

Fermentation assays reveal differences in sugar and (off-) flavor metabolism across different Brettanomyces bruxellensis strains

Abstract Brettanomyces (Dekkera) bruxellensis is an ascomycetous yeast of major importance in the food, beverage and biofuel industry. It has been isolated from various man‐made ecological niches that are typically characterized by harsh environmental conditions such as wine, beer, soft drink, etc. Recent comparative genomics studies revealed an immense intraspecific diversity, but it is still unclear whether this genetic diversity also leads to systematic differences in fermentation performance and (off‐)flavor production, and to what extent strains have evolved to match their ecological niche. Here, we present an evaluation of the fermentation properties of eight genetically diverse B. bruxellensis strains originating from beer, wine and soft drinks. We show that sugar consumption and aroma production during fermentation are determined by both the yeast strain and composition of the medium. Furthermore, our results indicate a strong niche adaptation of B. bruxellensis, most clearly for wine strains. For example, only strains originally isolated from wine were able to thrive well and produce the typical Brettanomyces‐related phenolic off‐flavors 4‐ethylguaiacol and 4‐ethylphenol when inoculated in red wine. Sulfite tolerance was found as a key factor explaining the observed differences in fermentation performance and off‐flavor production. Sequence analysis of genes related to phenolic off‐flavor production, however, revealed only marginal differences between the isolates tested, especially at the amino acid level. Altogether, our study provides novel insights in the Brettanomyces metabolism of flavor production, and is highly relevant for both the wine and beer industry.