Furfuryl alcohol confirmed as key intermediate for furfurylthiol in coffee

Furfurylthiol (FFT) is a key odorant and largely determines the aroma of fresh roasted coffee. Earlier studies showed formation of FFT from furfuryl alcohol (FFA) and glutathione in aqueous model reactions. It was suggested that FFA and coffee proteins are precursors for sulphur odorants in coffee. The present study investigated the role of FFA in the formation of FFT during coffee roasting by “in-bean” experiments. Green coffee beans were soaked with FFA and freeze-dried. After roasting FFT was quantified by stable isotope dilution analysis. Spiking with FFA increased the FFT level in roasted coffee. In another experiment beans were spiked with deuterium labelled d2-FFA. After roasting the isotopologue distribution of FFT was determined. Beans spiked with labelled d2-FFA generated labelled d2-FFT and unlabelled furfural. These results unambiguously confirmed FFA as key intermediate for FFT in coffee. On the other hand, furfural, which does not originate from FFA, seems not related to FFT formation. The suggested formation pathway comprises formation of furfuryl cation from FFA, which reacts with protein-bound cysteine, and subsequent elimination of FFT from the protein-bound S-furfurylcysteine.