title: Synthesis and structure determination of covalent conjugates formed from the sulfury-roasty-smelling 2-furfurylthiol and di- or trihydroxybenzenes and their identification in coffee brew.
abstract: Recent investigations demonstrated that the reaction of odor-active thiols such as 2-furfurylthiol with thermally generated chlorogenic acid degradation products is responsible for the rapid aroma staling of coffee beverages. To get a clear understanding of the molecular mechanisms underlying this aroma staling, the existence of putative phenol/thiol conjugates needs to be verified in coffee. The aim of the present study was therefore to synthesize such conjugates for use as reference substances for LC-MS screening of coffee. To achieve this, catechol, 3-methyl-, 4-methyl-, and 4-ethylcatechol, pyrogallol, hydroxyhydroquinone, 5-O-caffeoylquinic acid, and caffeic acid, respectively, were reacted with 2-furfurylthiol in the presence of iron(III) chloride and air oxygen. After purification, the structures of 25 phenol/thiol conjugates were identified by means of LC-MS/MS and 1D/2D NMR experiments. Using these compounds as reference materials, four conjugates, namely, 3-((2-furylmethyl)sulfanyl)catechol, 3-((2-furylmethyl)sulfanyl)-5-ethylcatechol, 4-((2-furylmethyl)sulfanyl)hydroxyhydroquinone, and 3,4-bis((2-furylmethyl)sulfanyl) hydroxyhydroquinone, were identified for the first time in coffee brew by means of HPLC-MS/MS(MRM). These findings clearly demonstrate catechol, 4-ethylcatechol, and hydroxyhydroquinone as the primary thiol trapping agents involved in the aroma staling of coffee beverages.
authors.affiliation: Institut für Lebensmittelchemie, Universität Münster, Corrensstrasse 45, D-48149 Münster, Germany.
authors.forename: Christoph
authors.initials: C
authors.lastname: Müller
authors.forename: Sarah
authors.initials: S
authors.lastname: Hemmersbach
authors.forename: Gordon Van't
authors.initials: GV
authors.lastname: Slot
authors.forename: Thomas
authors.initials: T
authors.lastname: Hofmann
mesh_terms: D002851:Chromatography, High Pressure Liquid; D003069:Coffee / Q000737:chemistry*; D005663:Furans / Q000032:analysis / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D013058:Mass Spectrometry; D015394:Molecular Structure; D009812:Odorants / Q000032:analysis*; D019800:Phenol / Q000032:analysis / Q000737:chemistry*; D013438:Sulfhydryl Compounds / Q000032:analysis / Q000737:chemistry*
chemical_list: D003069:Coffee; D005663:Furans; D013438:Sulfhydryl Compounds; C455106:furfuryl mercaptan; D019800:Phenol