2-[3-(2-Thioxopyrrolidin-3-ylidene)methyl]-tryptophan (TPMT) is a yellow pigment of salted radish roots (takuan-zuke) derived from 4-methylthio-3-butenyl isothiocyanate (MTBITC), the pungent component of radish roots. Here, we prepared salted radish and analyzed the behavior of the yellow pigment and related substances in the dehydration process and long-term salting process. All salted radish samples turned yellow, and their b^* values increased with time and temperature. The salted radish that was sun-dried and pickled at room temperature turned the brightest yellow, and the generation of TPMT was clearly confirmed. These results indicate that tissue shrinkage due to dehydration, salting temperature, and pH play important roles in the yellowing of takuan-zuke.

Stability of 4-methylthio-3-butenyl isothiocyanate (MTBI) was studied in an aqueous medium. Most of (E)-MTBI was degraded within 3h by shaking or sonicating and within 6h on an agar plate. (Z)MTBI was more stable. Antimicrobial activity of MTBI on 8 bacteria, 3 yeasts, and 5 fungi was estimated in a gaseous phase. The growth of fungi were strongly inhibited by MTBI in the dose range 2.5-7.5 μmol/ plate. The fact that the maximum amount of MTBI in head space was only 0.2% of the dose suggested that the antimicrobial activity of MTBI was very high.

The structure of the yellow pigment found in salted radish roots was studied. It was found that 1-(2-thioxopyrrolidin-3-yl)-1,2,3,4-tetrahydro-β-carboline-3- carboxylic acid (TPCC) was unstable under neutral pH, and was easily converted into the yellow pigment. The yellow pigment was isolated and identified as 2-[3-(2-thioxopyrrolidin-3-ylidene)methyl]-tryptophan (TPMT) by IR, MS, 1H-, and 13C-NMR spectroscopy. In addition, we proved that this compound was the main yellow pigment in salted radish roots. This compound induced no mutagenicity in Salmonella typhimurium TA98 and TA100, either with or without prior activation.