Examining How the Fermentation Medium Influences Thiol Expression and Its Perceived Aroma in Commercial Brewing Yeast Strains.

In Saccharomyces, the IRC7 gene encodes for a cysteine S-conjugate ß-lyase enzyme which can release polyfunctional thiols from their cysteinylated precursor forms, thereby promoting thiol aroma in beer. This study examined the thiol production of 10 commercial yeast strains in two different media, a hopped yeast extract-peptone-dextrose (YPD) medium and a 100% barley malt wort to explore how differences in yeast strain and medium conditions influence the release of polyfunctional thiols. 3-Sulfanylhexan-1-ol was most affected by medium conditions, and its concentrations were highest in wort fermentations. The higher nitrogen content and pH of the YPD medium relative to the wort fermentations were notable differences, and significant correlations between these variables and the extent of free thiol production were observed. A strong association existed between polyfunctional thiol concentrations and the fermentation-derived, malt, and hop-derived compounds 2-phenylethanol, ß-damascenone, and ß-ionone. The sensory impressions of thiol character in beer were influenced by the presence of other aromatic compounds such as esters and terpene alcohols, and aroma attributes such as "tropical" were not the most suitable for describing beers brewed with yeasts that fully express homozygous IRC7F. Sensory attributes "sweaty", "vegetal", and "overripe fruit" were more strongly associated with these strains.

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.

Impact of harvest maturity on the aroma characteristics and chemistry of Cascade hops used for dry-hopping.

The impact of ripening on the dry-hop aroma potential and chemical development of Cascade hops is not well understood. Therefore, 5-6 weekly hop samples were collected over the 2014, 2015 and 2016 harvests. Concentrations of humulones did not change as a function of harvest date, while total hop essential oil content displayed significant positive trends. Concentrations of thiol precursors decreased over harvest while concentrations of free thiols increased. These weekly samples were used to dry-hop an unhopped base beer. Overall hop aroma intensity and citrus quality attributed to beer during dry-hopping increased as a function of harvest date. These results suggest that for brewers to maximize the efficiency of hop usage, early harvested Cascades might be better for bittering, while, later harvested Cascades might be better for dry-hopping or aroma additions because they attributed more intense citrusy aromas to beer and had higher concentrations of free thiols and terpene alcohols.

Innovative analysis of 3-mercaptohexan-1-ol, 3-mercaptohexylacetate and their corresponding disulfides in wine by stable isotope dilution assay and nano-liquid chromatography tandem mass spectrometry.

Both 3-mercaptohexan-1-ol (3MH) and 3-mercaptohexyl acetate (3MHA) were largely studied for the last 20 years due to their pleasant olfactory notes conferred to wine. Until now, many analytical methods focused only on the free forms of both 3MH and 3MHA in wine that provided partial information in the wine aroma evolution. Our study proposes new analytical measurements which allow quantification of both free and disulfide forms of 3MH and 3MHA to better understand the redox phenomenon occurring in wines and further, to orientate wine aroma evolution. Free thiols were analyzed by an original method based on maleimide derivatization allowing in-situ disulfide reduction followed by SIDA-LC-MS/MS analyses exhibiting excellent performances. Indeed, the accuracy ranged from 95 to 110% in three different wine matrices and the repeatability and intermediate reproducibility were inferior to 15% (RSD measurements). Our method exhibited very low limits of detection, which are below to 0.5ng/L and inferior to the perception thresholds of both compounds. Then, this method was applied to three different wines exposed to several oxidative conditions. On the one hand, it was demonstrated that copper sulfate treatment firstly destroyed the total amount of free 3MH to the benefit of thioether and disulfides compounds, with proportions that could be slightly modified by glutathione addition. On the other hand, oxygenation of wines resulted in partial free 3MH destruction to the benefit of thioether compounds. We proposed for the first time an innovative analysis that gives a complete picture of wine aroma, which can be really useful to winemakers to manage wine aroma evolution and to take advantage of the disulfide reservoir.

Development of a routine analysis of 4-mercapto-4-methylpentan-2-one in wine by stable isotope dilution assay and mass tandem spectrometry.

The 4-mercapto-4-methylpentan-2-one (4MMP) is a key aroma compound in wines, especially in Sauvignon Blanc ones. Its accurate quantification is quite difficult due to its traces levels and its reactivity in wine conferred by the thiol function. In this paper, we proposed a new method for its quantification in wine without any sample preparation, based on automated derivatization procedure by methoximation and SIDA-SPME-GC-MS/MS analysis. The derivatization procedure was adapted from a previously published method in order to decrease the amount of reagents and the volume of wine (only 3mL are required). The use of SPME and the detection conditions have also been optimized to reach the best sensitivity as possible. The method was then validated according to the International Organization of Vine and Wine recommendations and exhibited excellent performances. Indeed, this method allowed us to quantify the 4MMP in wine at traces levels (LOD=0.19 ng L(-1)) with reproducible results (RSD<15%) and a very good accuracy (recovery=102%).