HPLC-PAD analysis of ethylenethiourea (ETU) and its destruction during retorting of canned beans and kale

The percentage recovery and precision for the analysis of ethylenethiourea (ETU) from beans in a tomato sauce and kale, spiked with 1 and 5 ppm ETU, were determined. The percentage destruction of ETU in the two vegetables at the two spiking levels caused by thermal processing (at 116.7°C for 95 min) of the canned products was analyzed and the effect of thermal processing on pH was measured.

ETU was extracted from the vegetables with a mixture of water (pH 11-12), NaCl, methanol and Celite during a 16 hr maceration before filtration. Part of the extract was mixed with Gas Chrom S and chloroform:methanol (96:4, V/V), and ETU was eluted from an alumina column with chloroform:methanol (96:4, V/V). The eluate was analyzed by HPLC using a pulsed amperometric detector (PAD), a Hypercarb Graphitized Carbon column and solvent of 0.022M H₃PO₄:acetonitrile (94:6, V/V) (1.0 mL/min). Averaged across vegetables and ETU spiking levels, ETU recovery was 101.0 ±6.1%(n=12).

ETU was not found in the beans, but was found in the kale (0.02 ppm). When pH values were 7.17 or higher, 44 to 48% of the ETU in the spiked samples was destroyed (pIn kale, with and without an ETU spike, thermal processing decreased the pH of the products by 0.79 to 1.03 units. Significant decrease in pH value during thermal processing, except for the pH decrease in unspiked kale, appeared to be associated with ETU destruction by thermal degradation. Thermal processing can decrease ETU residues in the food supply and that pH might play a more important role in thermal degradation of ETU than previously known.