- Hop aroma in beer is related to the unique compositional chemistry of the hops
used in the brewing process. While the range of these compositions is quite diverse
and primarily dependent on hop cultivar¹, other studies have also shown that
cultivation, seasonality, harvesting², processing³⁴, and storage practices⁵⁶ contribute
to differences in hop composition. However, it should be noted that the aroma and
composition of fresh and processed hops⁷ is different than the subsequent finished
beer. This irreconcilable difference that exists between hops and the finished product
has been a confounding variable for brewing scientists, in large part due to the
complexity and diversity of the compounds that are transferred from hops to beer, but
also due to an incomplete understanding of the interactions between these compounds
and the aromas they elicit. Of the many compounds found in hops, those belonging to
the class known as monoterpene alcohols have consistently been useful indicators of
changes in hop aroma due to different brewing practices.
Notable differences exist between American and European hops in terms of the
types of flavor they contribute to beer. Brewers tend to describe the former as
contributing citrusy, fruity and in some instances floral aromas to beer, while the latter
are often described as contributing herbal, tobacco, woody, and spicy notes. Single-hop brewing trials were carried out using either American hops (Cascade, Chinook, Centennial, Citra, or Simcoe) or European hops (East Kent Goldings, Hallertau Mittlefrueh HHA or Saaz) to identify hop-derived volatiles that contribute to American hop aroma in beer. The eight resultant beers were evaluated using both sensory and instrumental analyses. The sensory analysis identified Centennial as having the highest piney and green hop aromas, while Citra and Simcoe were characterized as being very fruity, citrusy, and tropical (especially Citra). The Hallertau Mittlefrueh (HHA) beers were similar to the East Kent Goldings, and these two were more floral and rose-like than the Saaz sample with more melon character than the American cultivars. Volatile analysis of the beer samples was performed using a stir-bar sorptive extraction (SBSE) of the beer samples followed by quantification by gas chromatography mass spectrometry (GC-MS). In general, the beers brewed with the American hop varieties were higher in aroma and in monoterpene alcohols.
In addition to hop oil-derived aroma, previous studies have demonstrated that non-volatile hop-derived precursors, specifically glycosides, survive the boil process and can be hydrolyzed to release volatile aglycones capable of contributing to aroma. To investigate this, twelve single hopped pilot scale beers were brewed using pellet, supercritical extract, and spent hop fractions of Citra, Simcoe, Centennial, or Cascade cultivars in order to investigate the contribution of these different hop fractions to the aroma of kettle hopped beers. The spent hop treatments produced beers that had noticeable, albeit low, hop aroma which suggest that the water-soluble components
left behind in the spent hops may contribute to hop aroma. The intensity and nature of
the hop aroma in the Spent treatments was hop variety. However, contributions of
water soluble components from spent hops to hop aroma in beer was very subtle,
especially compared to the pellet and extract treatments.
Aqueous extracts of the spent material from pilot scale supercritical CO₂ fluid
extraction (SFE) of hop pellets were treated to investigate the impact of different
hydrolysis treatments and on the aroma and volatile profile. Aroma profiles were
evaluated using descriptive analysis by a trained panel. Volatiles arising from
hydrolysis treatments of aqueous extracts of the spent materials were measured using
SBSE and GC-MS. The intensity and nature of the hop aroma was treatment specific.
Acidic hydrolysis of water soluble extracts produced the most intense Overall and
Pine aroma. Differences in the aroma intensities due to the hydrolysis from the
addition of different enzyme preparations were present but subtle. Aromas liberated by
ale yeast produced different profiles than the lager yeast. All treatments showed
increases in aglycone content and changes in aroma profile when treated with
hydrolytic enzymes preparations.
However, fundamental studies that examine the extraction of glycosides during
brewing and their subsequent hydrolysis by yeast have not been fully investigated.
Furthermore, extraction of other hop-derived compounds into beer show a strong
dependency on the hop cultivar being used and the point at which it is added.
Therefore, the extent of glycoside extraction due to hopping regime, cultivar, and their
hydrolysis due to yeast β-glucosidase activity was investigated. The glycoside
concentration of worts made with three different hopping regimes and three cultivars
was measured. Additionally, β-glucosidase activities for 80 different yeast strains and
their effect on aglycone concentration in wort was determined. Glycoside content was
measured by the difference in volatile aglycone concentrations between samples
treated with purified β-glucosidase and untreated samples. Aglycone concentration
was measured by SPME GC-MS. Results showed that yeast have a wide range of
abilities to hydrolyze glycosides with a maximum hydrolysis occurring after three
days of fermentation regardless of yeast activity. Although it was shown that yeast are
capable of glycoside hydrolysis, glycoside concentrations in wort are low and have
small contributions to hop aroma. These results help explain the extent to which
different brewing yeasts and hopping regimes contribute to hoppy beer aroma through
the hydrolysis of non-volatile hop-derived compounds.
Finally, in order to investigate the effect of hopping regime on the
monoterpene alcohol content and sensory attributes of beer, 6 single hop beers were
made using different hop additions and evaluated by sensory and instrumental
analysis. Beers were brewed while varying two factors: hop cultivar (Simcoe and
HHA) and timing of hop addition (60 min. boil, 25 min. whirlpool, or 48-hour dryhopping).
Additionally, the impact of yeast strain on treatment was investigated. Each
treatment was compared to an unhopped control using SBSE GC-MS and descriptive
sensory analysis. Multivariate statistical analysis were used to described the between
relationships between instrumental and sensory results. Whirlpool additions produced
beers with the highest concentrations of geraniol, linalool, and β-citronellol; beers
brewed with highly aromatic Simcoe hops produced more intense and individually
distinct aromas for each hopping regime compared to the HHA hopped beers.
Conversely, beers brewed with HHA hops showed less intense aromas with less
distinction between hopping regimes, except for the dry-hopped treatment, which was
characterized by a more floral type aroma than the other HHA. This research shows
that despite the popularity of dry-hopping as an aroma hopping method, whirlpool
additions can also produce intensely aromatic beers.