Cadmium migration from nib to testa during cacao fermentation is driven by nib acidification

Abstract

Previous work has shown that cacao nib cadmium (Cd) concentrations decrease during fermentation, but only when reaching sufficiently low nib pH. In this work, lab-scale experiments (5 kg units) with lactic and acetic acid amendments were ineffective at reducing the total nib Cd concentration. In contrast, the water-extractable fraction of the nib Cd concentration clearly increased when the pH was decreased. When single pod derived beans were embedded inside a full-scale fermentation box to monitor the effect of the fermentation effect with high precision, nib Cd concentrations decreased by a factor 1.25 (P-value <0.05) after four days of fermentation. Visualisation of the mobile Cd gradient within beans with LA-ICP-MS (using imprints of transversal cuts exposed to a metal binding gel) revealed that fermentation enhances the Cd mobility in the nibs.