Onion Flavor Without Tears

Members of the allium family, which includes onions and garlic, are famous and infamous for the pungent, sometimes tear-inducing, sulfury volatile compounds they release when chopped or chewed. Another vegetable, the Chinese plant Toona sinensis, also has a sulfur-laden smell and taste, but it won’t make you shed a tear. Researchers led by Jana Pika at the flavor and fragrance company Firmenich Aromatics, in Shanghai, have delved into this intriguing but little-studied aspect of oniony flavor chemistry, identifying nonvolatile compounds in T. sinensis that are precursors to the volatiles that quickly form when the plant’s cells are disrupted (J. Agric. Food Chem. 2013, DOI: 10.1021/jf401946d). The team isolated several sulfur-containing compounds, including two isomers of (S,S)-γ-glutamyl-(S-1-propenyl)-thioglycine (shown), which the researchers say are the first norcysteine-containing plant metabolites discovered in nature. In the well-studied case of onions, the tear-generating volatile culprit propanethial S-oxide is produced when alliinase enzymes cleave the precursor alkyl-cysteine oxides. In T. sinensis, however, the major volatile compound is cis-1-propenethiol, which Pika and colleagues hypothesize is produced by the cleaving action of protease enzymes on the S-alkyl-norcysteine precursors.