Determination of Trimethylamine in Spinach, Cabbage, and Lettuce at Alkaline pH by Headspace Solid-Phase Microextraction



Abstract:  Trimethylamine (TMA) found in some leafy vegetables, such as spinach, cabbage, and lettuce, at alkaline pH was identified and quantified using headspace solid-phase microextraction and gas chromatography-mass spectrometry (HS-SPME and GC-MS). HS-SPME conditions were optimized at an adsorption temperature of 50 °C, equilibration time of 5 min, and adsorption time of 5 min with 65 μm of polydimethylsiloxane/divinylbenzene fiber. The TMA that was formed from spinach, cabbage, and lettuce was assayed at pH 7 to 11 for 0 to 4 h at 50 °C using HS-SPME. The results showed that the amount of TMA formed was dependent on pH. The amount of TMA formed increased dramatically at a pH greater than 9. TMA was not formed at a pH lower than 7. Spinach produced a higher amount of TMA than cabbage or lettuce. TMA was formed at alkaline pH from choline, betaine, and carnitine, which are TMA precursors. To confirm the SPME results, TMA was quantitated using the AOAC official method. Data obtained from chemical analysis were in good agreement with the SPME data. The formation mechanism of TMA is thought to be the Hofmann elimination reaction, which generates amine compounds at alkaline pH.

Practical Application:  Fishy off-flavor in foods is associated with trimethylamine (TMA), which is frequently found in fish and seafood. In this study, TMA was identified for the first time in some leafy vegetables, such as spinach, cabbage, and lettuce, at alkaline pH. The presence of TMA in leafy vegetables under certain circumstances such as high pH and temperature may affect the sensory properties of foods containing these vegetables.