Cacao is the second most important crop with significant economic and social relevance in Colombia. National production generates 60000 tons on average, and is cultivated over 176050 hectares 1. While cultivated in several regions, the district of Santander is subject to special attention since this district represents 26 % of the national production. However, one of the biggest challenges in exporting cacao is the presence of cadmium (Cd) in cacao beans at specific spots in farms located in Santander.
As described in the Codex Alimentarius 2,3 the levels set by the EU regulation are similar to those being proposed for inclusion in Codex of 0.8 mg/kg for chocolate with ≥ 50 % to ≤ 70 % cocoa solids, and 0.9 mg/kg for chocolate with > 70 % cocoa solids. The categories and limits for products with < 50 % total cocoa solids and for cocoa powder (100 % total cocoa solids) have yet to be defined.
The European Union has been a pioneer in legislating Cd content in foods because several of its member countries have reported an annual per capita consumption of chocolate greater than 5.5 kg 4. Therefore, it is necessary to reduce the risks of food safety that can be incurred due to the presence of heavy metals in the product. Meanwhile, in Colombia, there is still no clear national legislative framework regarding the maximum permissible levels of Cd in either soils or chocolate and cocoa derivatives. To help understand the issue, it is important to monitor and regulate the metal content in the entire cacao system to ensure products remain below established thresholds, and thus improve the competitiveness of the Colombian cacao for export, and also protecting public health and establishing a national quality control for trade based on cocoa safety. To understand Cd fluxes, it is useful to analyse the behaviour of metal migration from the soil to chocolate. The following sections are a journey from soil to chocolate production within a single farm.
Subsoil and topsoil Cd distribution
Very few studies have focused on assessing both the soil agronomic and post-harvest factors affecting Cd content in cacao in terms looking at the entire process in a single farm. Relevant factors include i. Cd subsoil distribution, ii. soil pH, soil organic matter (SOM), phosphorus content, iii. the application of chemical P-based fertilisers and iv. post-harvest treatments 5–7.
All of these factors might have an effect on final Cd content in chocolate. Due to the fact that the distribution of Cd in soils tends to be highly heterogeneous, assessing the distribution of both soil and post-harvest Cd contents requires the use of multi-method approaches, involving an assessment of the physical, chemical and microbiological properties of the cocoa crop system. To assess heavy metal soil distribution, the two-dimensional electrical resistivity tomography technique (2D–ERT) is an accurate tool for use in analysing crop systems based on previous reports 8, and it recently used for assessing Cd content in cacao soils in Colombia 9. This technique has been used in the assessment of soils where clay minerals and the weathering of granite rock were noticed on the ground surface 10. This approach can help in identifying areas potentially high in Cd spots to aid in soil sampling to quantify Cd in the soil.
pH, Soil Organic Matter (SOM) and phosphorus content in soil
The variability of Cd concentrations in cocoa beans from different sites has been attributed to the ‘total’ soil Cd content and critical soil factors influencing Cd phytoavailability, such as pH, texture and SOM 11. Despite the importance of identifying the factors that govern Cd accumulation in cocoa beans and the need to find options to reduce its concentration, only a few studies have investigated the effects of soil and other environmental factors on heavy metal uptake by cacao plants, especially under field conditions 12,13. Several studies have determined that, among the diverse soil parameters, pH is the most relevant for controlling plant-available Cd 14. However, other critical soil parameters, such as the presence of clay, organic matter content, texture, and iron or aluminium oxide levels have also been indicated as useful properties that can help to predict Cd uptake by plants 15.
Soil pH has also been associated with Cd availability in soil 14, and this is particularly evident in the case of cacao crops 5,16,17. There is evidence that acidic soils are associated with the presence of Cd and lead in plants 16,18. Interestingly, acidic soils from the Santander district of Colombia have also exhibited SOM contents close to 85 % 5. Furthermore, it has been observed that when zinc contents were high (average values of 11.6 ± 0.2 mg/kg) in farms located in Santander, there was an increase of available Cd 5. Therefore, these physical parameters should be considered when studying Cd in soils and its potential accumulation in cocoa beans.
The soil parameters above represent a picture of the complexity of soil management in the case of cacao crops. Nevertheless, in addition to these factors, fertiliser application can influence Cd phytoavailability when contaminated with this heavy metal. Regular fertiliser application could also be a key factor in bioavailability, acting as a long-term anthropogenic source of contamination and affecting both soil pH and ionic ligand interactions 14 with the biota in the soil solution. Hence, fertilisation, mainly in P-like and NPK-like forms, might influence Cd speciation and complexation, thus increasing the mobility of the available Cd from the soils into the cacao roots.
Cocoa in the post-harvest phase
The type of post-harvest transformation of cocoa from seeds (fresh material removed from the pods) to beans (fermented and dried cocoa) along with the transformation of beans into nibs significantly influences the quality of the final product such as chocolate. This is due to the biochemical reactions involved due to the mass and heat transfer phenomena occurring inside the seeds during fermentation as well as the effects of drying and roasting 19. It is possible to describe the seeds’ transformation into chocolate in terms of two main steps:
From seeds to beans: During seed fermentation, the temperature rises from 28 to 50°C, and the pH drops from 6.5 to 4.5 units. Interestingly, this is due to the microbial succession of yeasts and bacteria during the fermentation of the cocoa beans 20, where yeasts, lactic acid and acetic acid bacteria are the main populations interacting in diauxic metabolic ratios. During this process, 40 % of seed weight is lost to evaporation due to the liquefaction of the seed mucilage. Another 30 % of weight is lost through drying 21.
From beans to chocolate: During roasting, the beans are processed at temperatures ranging between 110 to 160°C and a time duration between 5 to 120 minutes 22.
Despite the importance of the post-harvest and processing stages with respect to cocoa quality, there has been little research 23 assessing how artisanal, non-technified post-harvest operations, might reduce the bean’ Cd content. Some studies have noted Cd decreases comparing fresh whole seeds and fermented dried testa 21, as well as derived products such as cocoa powder and chocolate; in such cases, a relationship has been established between Cd content, single-origin cacao and genotypes 7,21,24−26. The findings from a previous study 21 indicate that there is a higher Cd content in the testa (1.83 mg/kg on average) than in the cotyledon (0.88 mg/kg on average), with considerable variation in the genetic material 21. This final aspect in the post-harvest operation influences the Cd values of chocolate within the range of 0.004–3.15 mg/kg 24.
Therefore, the aim of this study was to analyse the Cd flux using cacao from a single farm by considering the influence of i. soil parameters, ii. fertiliser amendments and iii. post-harvest operating processes. The study describes the movement of Cd from soil to beans to fermented beans to chocolate, in a single process.