Identification of cysteinylated aroma precursors of certain volatile thiols in passion fruit juice

Together with 3-mercaptohexan-1-ol and S-mercaptohexyl acetate, already known to contribute to the aroma of passion fruit (Passiflora edulis), 3-mercapto-3-methylbutan-1-ol and 3-mercapto-3methylbutyl acetate have been identified for the first time in this fruit. 3-Mercaptohexan-1-ol and 3-mercapto-3-methylbutan-1-ol may be produced in vitro from nonvolatile extracts of this fruit by the enzymatic action of a cell-free extract of Eubacterium limosum, which has a beta-lyase activity on S-cysteine conjugates (EC 4.4.1.13). This release strongly suggests that these volatile thiols are present in combined form, as S-cysteine conjugates. It was possible to identify the precursor of 3-mercaptohexan-1-ol as S-(3-hexan-1-ol)-L-cysteine, in the form of trimethylsilylated derivatives from the juice of this fruit, using GC/MS analysis. The presence-of free and combined forms of these volatile thiols in this fruit has now been demonstrated.