Thermally-induced formation of taste-active 2,5-diketopiperazines from short-chain peptide precursors in cocoa

2,5-diketopiperazines (DKPs) are cyclic dipeptides responsible for the specific bitter taste of cocoa formed during roasting. The 2,5-diketopiperazine and peptide composition of four different cocoa bean samples from different origins were studied using LC-MS techniques. 34 diketopiperazines were identified, of which 10 are newly reported in cocoa. Their formation was followed during two different roasting time-series using a zero-order and an alternative Prout-Tompkins solid-state kinetic models. The activation energies of diketopiperazine formation showed a distribution close to normal with individual values depending on the nature of the substituents. The relative concentrations of the DKPs were correlated with their putative peptide precursors in unroasted cocoa bean samples. The results showed a significant positive correlation, indicating that oligopeptides formed in cocoa bean fermentation are taste-precursors for bitter tasting diketopiperazines. Unexpectedly, for most diketopiperazines, a single major peptide precursor could be suggested.