Formation and stability of isothiocyanate protein conjugates at different pH values and bread types enriched with nasturtium (Tropaeolum majus L.)

Abstract

Brassicaceae vegetables are rich in glucosinolates (GLS), which degrade into various breakdown products, including isothiocyanates (ITC), during food processing. ITC are associated with health-promoting properties; therefore, producing food products enriched with a high content of these compounds is of interest for improving and maintaining human health. The present study aimed at evaluating a potential increase in ITC formation in Brassicaceae-enriched bread while minimizing ITC-protein conjugates. The influence of pH on GLS degradation was evaluated in different bread types and pH-adjusted model breads. In all samples, ITC, nitriles, and ITC-amino acid conjugates were analyzed by GC-MS and LC-ESI-MS/MS, respectively. The highest ITC-Lys levels with 33.9 µmol BITC-Lys/g bread could be found in a (more alkaline soda) bread with a pH of 7.2, while "free" BITC content was the lowest. However, this finding could not be directly correlated to pH value, because in model breads no significant relation between the pH and the BITC content could be identified. Especially the baking process impacted the ITC content as it was reduced from dough to the bread by 98%. Therefore, a heated food product is not suitable for an enrichment with GLS-rich vegetables to achieve a high content on BITC. Nevertheless, in the bread matrix itself the degradation products and BITC-Lys conjugates were stable during storage.

Keywords: Benzyl isothiocyanate; Glucosinolates; Nasturtium; Thiourea; pH stability.