Ultra-high-performance liquid chromatography (UHPLC) accurate mass
tandem mass spectrometry is a powerful tool for identifying and profiling
plant metabolites. Here, we describe an approach to characterize glycosidically
bound precursors of monoterpenoids, norisoprenoids, volatile phenols,
aliphatic alcohols, and sesquiterpenoids in grapes. Chromatographic
separation of glycosylated compounds was evaluated using phenyl–hexyl
(reverse phase), glycan/hydrophilic interaction, and porous graphitic
carbon (PGC) stationary phases. PGC provided the best UHPLC separation
for 102 tentatively identified aroma precursors in Vitis vinifera L. cv. Riesling and Muscat of Alexandria
berries. Monoterpene-triol, monoterpene-tetraol, and sesquiterpenol
glycosides were tentatively identified for the first time in grapes,
and a C6-alcohol trisaccharide was tentatively identified for the
first time in any plant. Comparison of glycosylated aroma molecules
in Riesling and Muscat of Alexandria grapes showed that the two varieties
were distinguishable based on relative abundances of shared glycosides
and the presence of glycosides unique to a single variety.
Pentose-hexose monoterpene alcohol glycosides were isolated and semiquantitatively measured in dried Humulus lupulus cones using UHPLC-qTOF-MS/MS and HPLC fractionation followed by GC−MS. The samples evaluated included hop cones from five important dual-purpose cultivars (varieties) in the United States, from two locations (farms) per variety and from three distinct harvest time points (maturities) per location, as dictated by dry-matter (% w/w) at the time of harvest. Hop variety accounted for the biggest variation among the concentrations of pentose-hexose monoterpene alcohol glycosides as well as other volatile and nonvolatile chemical factors measured in the samples. This indicates that genetics plays a major role in hop flavor production. Interestingly, "maturity", or ripeness at the time of harvest, was the next most significant factor impacting the concentrations of pentose-hexose monoterpene alcohol glycosides along with most of the other volatile and nonvolatile factors (such as total oil concentration and composition). However, maturity notably had a bigger impact on some cultivars such as Sabro, Mosaic, Simcoe, and Citra. Surprisingly, farm (i.e., location, farming practices, etc.) accounted for the least amount of variation among the concentrations of the different analytical factors. These results highlight the importance of breeding/genetics as well as considering hop maturity/ripeness at the time of harvest on the production and subsequent development of analytical chemical factors associated with driving hoppy beer flavor. It is essential for future studies assessing the impact of different farming practices and locations (i.e., regionality, terroir, etc.) on the constituents in hops important for hoppy beer flavor to consider and account for the impact of hop maturity as well as genetics.
Volatile aroma compounds found in grapes and hops may be present as both free volatiles and bound glycosides. Glycosides found in the raw materials are transferred to their respective fermented beverages during production where the odorless compounds may act as a reservoir of free volatiles that may be perceived by the consumer if hydrolyzed. A review of the literature on grape and wine glycosides and the emerging literature for glycosides in hops is presented in order to demonstrate the depth of history in grape glycoside research and may help direct new research on hop glycosides. Focus is brought to the presence of glycosides in the raw materials, the effect that winemaking and brewing have on glycoside levels, and current methods for the analysis of glycosidically linked aroma compounds.
The
isomeric nature of monoterpenyl glycosides makes unambiguous
identification of intact glycosides difficult. As a result, it is
challenging to relate the changes in free monoterpenol concentrations
to the corresponding glycosides during wine fermentation and storage.
In this study, we isolated and identified linalool, nerol, and geraniol
monoterpenyl glycosides fromVitis viniferacv. Riesling grapes through fractionation followed by acid or enzyme
hydrolysis. Changes in the composition of identified monoterpenyl
glycosides and their respective free volatiles were then monitored
during alcoholic fermentations of Riesling juice with four different
yeast strains across two successive years. The relative concentrations
of the volatiles were monitored by solid-phase microextraction gas
chromatography mass spectrometry, while ultrahigh-performance liquid
chromatography quadrupole time-of-flight mass spectrometry was used
for intact glycosides. Glycoside hydrolysis during fermentation could
be related to relative concentrations of the corresponding free aglycones.
However, other sources of free monoterpenols were also observed. Differences
in glycoside hydrolysis among yeast strains and across years were
observed and may be related to grape maturity and/or nutrient levels.
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