Characterization of Humulus lupulus glycosides with porous graphitic carbon and sequential high performance liquid chromatography quadrupole time-of-flight mass spectrometry and high performance liquid chromatography fractionation.

Monoterpenes contribute to the characteristic aroma of several hop varieties and may occur as nonvolatile glycosides. Upon hydrolysis, the volatile terpenes are released from the glycoside precursors. Little is known, however, about the glycoside composition of hops. Seven pentose-hexose monoterpene alcohol glycosides from dried Humulus lupulus L. cv. Citra cones were isolated using high performance liquid chromatography separation and fractionation on a reverse phase phenyl-hexyl column. Further evaluation of each isolated fraction through HPLC qTOF MS with porous graphitic carbon (PGC) showed that the seven isolated monoterpenyl glycoside fractions could be further resolved into 20 isomers. Isolation on phenyl-hexyl followed by separation on PGC was needed to distinguish each isomer present. Additionally, the hop cones were screened for potential aroma glycosides. Using the PGC column combined with a database of over 900 potential glycosides, the identification of 21 additional monoterpene-polyol, norisoprenoid, volatile phenol, and aliphatic alcohol glycosides is reported.

Characterization of Free and Bound Monoterpene Alcohols during Riesling Fermentation.

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.

The Occurrence of Glycosylated Aroma Precursors in Vitis vinifera Fruit and Humulus lupulus Hop Cones and Their Roles in Wine and Beer Volatile Aroma Production.

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.

Evaluation of Variety, Maturity, and Farm on the Concentrations of Monoterpene Diglycosides and Hop Volatile/Nonvolatile Composition in Five Humulus lupulus Cultivars.

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.

Direct Analysis of Glycosidic Aroma Precursors Containing Multiple Aglycone Classes in Vitis vinifera Berries.

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.