Understanding the Onion Lachrymatory Factor: Structure, Formation, and Flavor Implications

This article presents a thorough examination of research on the onion lachrymatory factor (LF), covering aspects such as its structural determination, confirmation of stereochemistry, formation pathways, analytical methods, and its impact on onion flavor. Identified as the onion LF, propanethial S-oxide primarily adopts a (Z)-configuration and is generated through the enzymatic breakdown of its precursor, (+)-S-(E)-1-propenyl-L-cysteine S-oxide, with alliinase serving as the catalyst. Following this, an isomerization process takes place involving an intermediate compound, (E)-1-propenesulfenic acid, catalyzed by lachrymatory factor synthase. To address challenges in conventional gas chromatography (GC) analysis posed by high temperatures, alternative methods such as UV spectroscopy, chemical derivatization, GC with cold injection, high-performance liquid chromatography, direct analysis in real-time mass spectrometry, proton-transfer reaction-mass spectrometry, and secondary electrospray ionization high-resolution orbitrap mass spectrometry have been employed for qualitative and quantitative LF analysis. Beyond its tear-inducing properties, LF significantly contributes to the distinctive onion flavor. This review is crucial for advancing our understanding of the onion LF and sulfur-containing flavor components in the Allium vegetable family. It helps for improving the flavor of onions and refining various onion varieties.