Unusual Profile of Thiol Precursors in Special Malts: First Evidence of Chemical Glutathione-/γGluCys- and CysGly-/Cys- Conversions

Evidence of Enzymatic and Chemical Interconversions of Barley Malt 3-Sulfanylhexanol Conjugates during Mashing

“…In a recent investigation of the 3SHol S -conjugate profile of 30 pale malts, the level of the CysGly-form was shown to be proportional to that of G-3SHol. This might be the first evidence of in situ γGT acting during the malting process . The high CysGly-3SHol content observed in wort by Molitor et al (109 μg/L in a 12°P wort) further suggests that γGT is still active during the mashing step …”

“…The analytical system consisted of a 1290 Infinity II UHPLC instrument (Agilent Technologies, Santa Clara, CA, USA) attached to a 6470B Agilent Triple Quadrupole mass spectrometer. Source parameters were as follows: gas temperature, 230 °C; gas flow, 4 L/min; nebulizer at 3.79 bar; sheath gas temperature, 400 °C; sheath gas flow, 11 L/min; capillary voltage, 3000 V in positive mode; nozzle voltage, 0 V; positive Delta EMV set at 400 V. Ionization was carried out by positive electrospray (ESI+), and detection was done by multiple reaction monitoring (MRM) as described by Chenot et al…”

“…Over the past 10 years, a dozen glutathionyl and cysteinyl S -conjugates of 5–7 C polyfunctional thiols (PFTs) have been evidenced in various hop varieties − and some malts. , These nonvolatile compounds have real aroma potential, as the PFTs that can be released from them are often very potent (sensory thresholds at ng/L level) and pleasant (descriptors such as exotic fruit, grapefruit, ...). ,− Their fates during malting, hop processing, wort boiling, and fermentation ,,− have been the focus of much recent research.…”

“…Chemical degradation of S -conjugates after heating or a pH increase has also been mentioned. More recently, chemical breaking of the Cys–Gly bond at high temperature was evidenced in roasted malts, leading to the occurrence of the γGluCys dipeptide (absent from pale malts) and to an unexpected correlation between the CysGly- and Cys- S -conjugates …”

“…In order to evidence potential actors capable of converting the different bound forms of PFTs to better yeast nutrients, the aim here was to identify all enzymatic and chemical interconversions of malt 3-sulfanylhexanol S -conjugates during mashing (these compounds having been identified as the most abundant and ubiquitous thiol precursors in all the studied malts) . In lab-scale mashing trials, the influences of the malt/water ratio, the temperature applied, the sample color (as a result of the kilning process), and acidity were studied.…”