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Managing cadmium in cacao products from farm to fork

Book - Dissertation

A new EU regulation was enforced in 2019 which sets the maximum allowed cadmium (Cd) concentration in chocolates and cacao powders sold to the European consumers. The cacao market in Central and South America is affected by this regulation because Cd concentrations are markedly higher in cacao beans from that region compared to beans from Africa, the main global cacao producing region. Lowering the cacao nib Cd concentrations by a factor 1.3 or more could largely improve the sustainability of the cacao sector in Central and South America. Cadmium in cacao beans originates from the soil and several agronomic mitigation strategies are being explored to lower the soil-plant transfer of Cd. However, none of these have had large success to date. Mitigation strategies can also be implemented during the extensive postharvest process of cacao because the new EU Cd regulations only apply to the final product, but such strategies had not been explored thus far. The objective of this dissertation was to study the effect of conventional postharvest processing on the Cd concentrations in the different cacao bean tissues, with specific focus on fermentation, and to reveal potential postharvest mitigation strategies to lower the Cd concentration in the final product. As a first step, a study was set up to confirm the suspected effect of the cacao origin on the Cd content of chocolate products. The relation between the elemental composition of chocolates and the origin of the cacao used was studied in 139 single origin chocolates. The EU Cd regulations were exceeded in 10 % of the samples, which were all produced with cacao from Central or South America. Increasing cacao content of the product was associated with increased concentrations for most elements, indicating cacao as the main source of minerals and trace elements (incl. Cd) in chocolate. Classification and Regression Tree (CART) analysis resulted in a decision tree that could effectively classify chocolate samples by cacao origin based on the concentrations of five elements (Ba, Cd, Mo, Sr and Zn). Samples of South America were differentiated from the other samples based on their Cd concentration, indicating the geogenic origin of Cd. The second step was devoted to the Cd distribution in the cacao fruit and its changes during fermentation. Chocolate products are made of the cacao nibs, the central part of the fermented cacao bean. Cadmium concentrations in the different tissues of unfermented cacao fruits decrease in the order testa > nib ~ placenta ~ pod husk > mucilage. Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) was used to visualise the Cd distribution within cacao beans and confirmed higher Cd concentrations in the testa than in the nib. The impact of fermentation on the distribution of Cd in cacao was studied using a top-down approach, starting with a full scale fermentation setup, followed by lab scale and micro-fermentations, and finally controlled incubations to mimic the fermentation conditions. Fermentation in full scale commercial setups (> 200 kg) induced an outward Cd migration from the nib to the testa, i.e. from a lower Cd concentration tissue to a higher Cd concentration tissue, which decreased nib Cd concentrations by maximally a factor 1.3. This migration occurred only if fermentation was sufficiently extensive to acidify the nibs to pH < 5.0, which was the first indication that pH-driven nib Cd mobilisation occurs during fermentation. Next, a micro-fermentation experiment was performed to study the fermentation effect with higher precision than in the commercial setups. Single pod derived cacao beans in mesh bags were embedded in a full scale commercial fermentation box, and four days of fermentation reduced the nib Cd concentration by a factor 1.25. The mobility of Cd within the cacao beans was mapped with imprints of transversal cuts exposed to a metal binding gel, followed by LA-ICP-MS analysis. That showed that fermentation enhances the Cd mobility in cacao nibs and that it establishes a mobile concentration gradient from inside the nibs to the outer testae and mucilage. In subsequent lab scale fermentation experiments (5 kg), lactic and acetic acid were applied during or after fermentation in an attempt to enhance the nib Cd mobilisation. Total nib Cd concentrations did not decrease due to organic acid treatments, which was likely related to the lower temperatures reached in these small fermentation vessels in comparison to full scale commercial setups. It was hypothesised that loss of structural integrity in the cacao nibs due to fermentation heat, acetic acid and/or ethanol is required for acidification-driven nib Cd mobilisation. That hypothesis was tested in an incubation experiment using combinations of different incubation temperatures, and acetic acid and ethanol concentrations in the media. Mobilisation of Cd in the nibs was most pronounced when acetic acid addition was associated with higher temperatures, whereas ethanol did not have statistical effects. The incubation of cacao beans in typical fermentation conditions (45 °C and 20 g L-1 acetic acid) reduced the nib Cd concentration by a factor 1.3, and this reduction factor increased to 1.6 in more extreme conditions. Fine-tuning of fermentation parameters for optimal production of heat and acetic acid thus holds promise for lowering cacao nib Cd concentrations. In a final step, water and chelating agent extractions of cacao nibs were evaluated as mitigation measures. Water extractions of ground cacao nibs had initially been used in this work as a diagnostic technique to compare Cd mobility among samples. These data, however, indicated large water extractable Cd fractions in fermented cacao nibs, but not in unfermented nibs. Water and chelating agent (EDTA) extractions with short contact times (1 or 2 hours) and small liquid:solid ratios (1 - 6 mL g-1) were performed on broken cacao nibs derived from a commercial cacao fermentation. Incubation for 1 hour in 10 mM EDTA at liquid:solid ratios of 2 mL g-1 or higher removed 50 - 60 % of nib Cd. Water and/or chelating agent washing prior to nib roasting likely influences the flavour quality of the final product but implementation of such technique is not implausible because similar practices are already adopted in selected cacao processing technologies (i.e. alkalinisation or Dutch processing). In conclusion, this study revealed that cacao nib Cd concentrations can be decreased by fermentation due to pH-driven mobilisation and that this effect only occurs with adequate fermentation heat, likely because it requires loss of structural integrity in the nib tissue related to bean death, i.e. loss of germination potential due to heat and acetic acid. Conventional fermentation practices can reduce nib Cd concentrations by a factor 1.3, while fine-tuning of fermentation parameters can potentially increase this reduction factor to 1.6. Follow-up experiments are required to reveal if such fermentation conditions are practically feasible. It is also suggested to identify to what extent the breakdown of phytate during cacao fermentation is a driving factor for the enhanced Cd mobilisation. Washing of fermented cacao nib fragments with a solution containing low concentrations of a chelating agent, e.g. EDTA, can reduce nib Cd concentrations by a factor > 2. Both fine-tuning of fermentation parameters and nib washing can be readily and widely implemented and are, therefore, highly promising as mitigation strategies to address the Cd issue in Central and South America.
Publication year:2021
Accessibility:Open