Potential degradation pathways of polyfunctional thiol precursors

Details

Supervisors

Collin, Sonia

Faculty

Faculté des bioingénieurs

Degree label

Master [120] : bioingénieur en chimie et bioindustries, à finalité spécialisée

Abstract

Following the identification of a significant aromatic pool represented by cysteinylated (Cys-) and glutathionylated (G-) precursors of polyfunctional thiols (PFTs) in hop, many studies have been carried out in order to develop methods to release volatile thiols. As the ability of chemical hydrolysis and yeasts to release free thiols have already been demonstrated on hop precursors in beer, the aim of the present work was to evaluate and optimize the release activity of commercially available exogenic enzymes, another option seen as promising. For this purpose, the enzymes of interest were first incubated in different media (model, beer and hop media) under different conditions (pH, temperature and time) in the presence of spiked synthetic Cys- or G- precursors. Free thiols were then extracted with bidistilled dichloromethane and quantitated by gas chromatography– pulsed flame photometry detector. From Cys-adducts, the release of thiols in the presence of exogenous enzymes (apotryptophanase or cystathionine) appeared at its best in a pH 7.7 medium, compared to beer and hop media. Yet, in this model medium (pH 7.7), a slight proportion of the release appeared to be resulting only from the chemical hydrolysis. In hop medium, the removal of polyphenols confirmed their inhibitory role on the release of free PFTs by both chemical hydrolysis (at pH 7.7) and enzymatic activity. From G-adducts, the combination of GGT and apotryptophanase enzymes in model media highlighted for the first time the ability of b-lyase activity to release free PFTs from CysGly-adducts. It also confirmed that the presence of an acceptor and a higher incubation temperature optimized the GGT activity, as well as the key-role of pH (7.7). In fermentation media, the combination of GGT enzyme and K-97 yeast appeared to be more efficient in releasing free thiols than K-97 yeast alone. In this way, fermentations of low-polyphenol wort in presence of GGT could constitute interesting new dry-hopping strategies for increasing the utilization of the huge amount of G-adducts in hop.