Abstract: The kinetic parameters for thermal decomposition of alliin in aqueous solution were determined in order to explore its thermal stability. The activation energy and pre-exponential factor were separately determined as 80.5 kJ/mol and 1.27 × 107. By analysis of the thermal decomposition products using gas chromatography-mass spectrometry (GC-MS), it was found that as the temperature rose, the content of diallyl disulfide kept decreasing from 91.59% to 53.62%, the content of diallyl trisulfide firstly increased and then decreased and the content of diallyl thioether kept increasing, illustrating that high temperature could promote the fracture and rearrangement of sulfur and ether compounds. By mass spectrometry (GC) and the 2,4-dinitrophenylhydrazine method, the presence of S-allyl-L-cysteine sodium and pyruvic acid was detected in the liquid phase products. At the B3LYP/3-21+G(d,p) level, the structures of the reactants and products were optimized, and the transition state during alliin decomposition was calculated theoretically. By also investigating the decomposition kinetics and thermal decomposition products, it was inferred that alliin could form a five-membered ring intermediate during the decomposition process. Afterwards, hyposulfurous acid and pyruvic acid were generated via Cope elimination reaction.Diallyl disulfide and diallyl trisulfide were formed from hyposulfurous acid through further reaction.

Key words: alliin, kinetics, garlic, activation energy