In HPLC analysis of foods to which sorbic acid is added as preservative, a peak of unknown substance is often observed before that of sorbic acid. Especially, the peak was observed very often in fish-paste products, Japanese pickles, foods boiled down in soy, pickles in vinegar and others. The rate of appearance exceeded 83% in the above foods. The structural analysis of this unknown substance was carried out with GC/MS, GC/FTIR, NMR. The mass spectrum of the unknown substance showed fragments ions at m/z 112 (M+), 97 (M+-Me), 67 (M+-CHO2), 45 (CHO2), 41 (CH3-CH=CH-), the fragmentation pattern being nearly identical with that of sorbic acid (M+, 112). The 13C-NMR of the unknown substance showed one methyl carbon at δ14.120 (q), four olefin carbons at δ120.22 (d), δ127.32 (d), δ137.07 (d), δ141.47 (d) and one carboxyl carbon at δ172.62 (s). The 1H-NMR showed one methyl proton at δ1.91, four olefin protons at δ5.87, δ6.00, δ6.19 and δ7.73. The following coupling constants due to olefilic protons were observed : J2, 3=15.23 Hz (trans bond), J3, 4=11.0 Hz and J4, 5=11.0 Hz (cis bond), and J5, 6=7.0 Hz and J4, 6=1.5 Hz. From the above spectral data, the structure of the unknown substance was identified to be trans-2, cis-4-hexadienoic acid, a geometrical isomer of sorbic acid.
