Characterization of aroma-active volatile compounds in tom yum soup by gas chromatography-mass spectrometry combined with sensory evaluation techniques

Gas chromatography-mass spectrometry/olfactometry coupled with headspace-solid phase microextraction (HS-SPME-GC-O/MS) was applied for the characterization of volatile compounds in Tom Yum soup and its individual ingredients including lemongrass, kaffir lime leaf, chili, fish sauce and lime juice. Using HS-SPME with a 50/30 µm DVB/CAR/PDMS fiber and an extraction temperature of 40 °C for 50 min, along with an HP-5MS capillary column programmed from 50 to 200 °C at 3 °C/min and an MS electron impact ionization at -70 eV, 101 peaks in the HS-SPME-GC-MS chromatogram of Tom Yum soup were detected, and 96 volatile compounds were identified including alcohols, aldehydes, esters, ethers, and terpenes. These findings are based on the comparison of MS spectra with the NIST library as well as experimental and literature retention index data. In comparison with the volatile compound profiles of each individual ingredient of Tom Yum soup (both before and after cooking), five extra volatile compounds including p-mentha-3,8-diene, α-cyclocitral, iso-isopulegol, p-mentha-1,5-dien-8-ol and decyl acetate were found after the cooking process, possibly due to chemical reactions (such as cyclization) among the compounds in the mixed ingredients. Furthermore, eighteen aroma compounds that contribute to the impressive aroma of Tom Yum soup were detected and described. Especially, the seven dominant aroma compounds found were β-citral, geranial, β-linalool, geraniol, nerol, 3-(methylthio)propanal and 2-isobutyl-3-methoxypyrazine. The first four aromas detected in Tom Yum soup are from the lemongrass, kaffir lime leaf, and lime juice; the last three aroma compounds are from lemongrass, fish sauce and chili, respectively.