The following is a summary of the stated research mentioned above. The content summarized here, including the figures and tables, all belong to the researchers (unless otherwise indicated). The summary attempts to stay as close to the original paper as much as possible with some adjustments in regards to jargon, length, or to focus on bean to bar aspects.

This paper was published in 2021.

1. Introduction

Cadmium (Cd) accumulates in the human body primarily through food in non-smokers. Cadmium is a trace metal that is not known to benefit humans biologically, and in high amounts over long periods of time cadmium ingestion can contribute to adverse health effects such as renal tubular dysfunction (kidney damage) and osteomalacia (bone softening) (WHO, 2010). Trace metals are naturally occurring in the environment, and are found in an array of foods. Staple foods such as rice, wheat, and potatoes are known to contain very low levels of cadmium yet still contribute to dietary cadmium due to being a highly consumed staple food. Chocolate is also another food which contain traces of cadmium.

In the 2010’s many countries including EU, New Zealand, Australia, Russia, and others began to set new regulations for cadmium concentrations in chocolate and cacao powders. The EU in 2014 set it to 0.10 to 0.80 mg of Cd per kg (depending on the product and cocoa solid content). There are repots of products available on the market which exceed these limits (Abt et al., 2018; Vanderschueren et al., 2019). Due to these new regulations and findings, research on Cadmium in cacao has greatly risen, from 29 papers in 2014 up to over 100 papers by 2020.

Where does Cadmium in cacao originate?

It originates from the bean itself, not from contamination during processing. There are strong correlations between cadmium concentrations in chocolate vs cacao content of the chocolate, as well as chocolate vs the origin of the cacao (Villa et al., 2014; Yanus et al., 2014; Abt et al., 2018; Lo Dico et al., 2018; Vanderschueren et al., 2019).

All plants can uptake cadmium, but cacao trees are effective at this, and so can uptake higher amounts than other plants we consume from. The earliest study on cadmium concentrations in cacao beans dates back to 1979. Cacao high in cadmium are often found in Latin American origins, as well as in specialty origin chocolates (Knezevic, 1979).

Cadmium mitigation has been studied a great deal for other food crops (durum wheat, spinach, potatoes, and rice) more so than for cacao. This review summarizes existing knowledge on cadmium concentrations in cacao-derived products.

2. Cadmium in soils and its relation to cacao plants

2.1 Origin of cadmium in cacao beans: soil, air, fertilizer, and irrigation water

Most cadmium in plant tissue in general originations from the soil, and is taken up by the root system. Only a very small amount is taken up through the air via foliar (leaf) uptake. This is likely true from cacao as well due to the strong correlations between soil and cocoa bean cadmium concentrations (well above what is expected from foliar uptake).

Cadmium occurs naturally in the environment, with soil concentrations ranging from 0.1 to 1.0 mg Cd per kg, with a global mean of 0.1 to 0.3 mg Cd per kg (Smolders and Mertens, 2013). However, polluted soils can contain much higher levels cadmium. The cadmium concentrations in soils differ geographically, with high amounts in geographically young soils of Latin America (between 0.22 and 10.8 mg kg-1). There is no reliable information to date about cadmium concentrations in soils of cacao growing regions in Africa, even though most cacao is produced in Western Africa. However, the soils there are old and weathered, and so expected to have lower cadmium concentrations. Keep in mind that although cadmium concentrations are higher in Latin American soils, the soil in cocoa producing areas is mostly under 1 mg kg-1, and not categorized as contaminated soils. That said, many researchers have found that cacao from Latin America origin was generally higher than other origins (such as Africa). See Table 1. The strong correlation to geographical origin does not appear to be from anthropogenic activity (human activities, fertilizing…), as these activities occur worldwide. A study by Gramlich et al. (2018), Argüello et al. (2019) and Scaccabarozzi et al. (2020) found that high cadmium concentrations in cacao farms were associated with alluvial soils (soil deposited from surface water) from sedimentary materials. Sedimentary rocks generally contain higher cadmium concentrations than is found in igneous rocks (Thornton, 1981; Birke et al., 2017). It’s also important to note that alluvial soils in Africa do not exhibit higher cadmium concentrations as Latin American counterparts. Rodriguez Albarrcin et al. (2019) suggests this may be due to mining activities in Latin America.

Mineral P-Fertilizer has been suggested to contribute to high cadmium levels, but a long-term study by Gramlich et al. (2017) suggests otherwise. As well, most cacao farmers do not use P-fertilizer due to cost, and also not used on such a large scale enough to make an impact on raising soil cadmium concentrations in cacao growing regions.

Irrigation water containing cadmium may be a source of cadmium in soil and plants grown in these soils. For instance, the Puyango-Tumbes river basin between Ecuador and Peru, with dissolved Cd concentrations from 1-10 ug L-1 due to upstream gold mining. These situations have been mentioned as potential source of cadmium in cacao fields (Chavez et al., 2015).

Profile studies reported that soil cadmium concentrations decline with soil depth in cacao plantations. The cadmium in 0-15 cm of soil is a factor 1.5 larger than cadmium levels at a depth of 15-60 cm. This is likely due to cycling effects of decomposing biomatter, and not likely anthropogenic. The average cacao leaf cadmium concentration is 2.6 mg kg-1 dry weight, with a biomass production of 3.9 Mg ha-1 y-1, therefore the cadmium returning to the soil via leaf litter decomposition is about 10g Cd ha-1 y-1. This is much greater than the worst-case inputs from contaminated P fertilizers or irrigation water.

Considering the above information, increased cadmium in cacao soils generally appears to be mostly geogenic (from the soil) as opposed to anthropogenic (from human activities), although mining activities, sedimentary materials, contaminated irrigation water may increase local Cd concentrations. As well, the annual cycling of Cd from leaf litter to soil explains the greater Cd concentrations in surface soil.

2.2 Bean cadmium concentrations and their relationship with soil properties: a meta-analysis

Table 1 shows some general trends of cadmium in cocoa bean in various regions. For instance, cacao found in the Andean countries in South America (so not Brazil) had higher levels compared to other regions in Asia and Africa. The way the cacao is processed also has an impact on Cd concentrations (fermentation, peeling, and roasting - see section 4.3). There is much variation in bean Cd concentrations geographically. Within the fruits of the same tree, there is no data to suggest much variability. However, there is some variability among trees from a single field (Arguello et al., 2019).

The Cadmium concentrations of crops in general depend on soil cadmium, as well as other factors such as increasing soil acidity, decreasing organic matter in soil, and sometimes zinc deficiency and chloride salinity (Smolders and Mertens, 2013). This also applies to cacao. For this study here, the researchers analyzed the raw data of 7 out of 400 studies (based on criteria used to select for the studies) on soil-plant Cd relationships. A total of 785 paired soil-plant data points were gathered from surveys in Latin America. The 7 selected studies reported total soil Cd, soil pH, soil organic carbon (SOC) and bean Cd, and other variables. Keep in mind that in studies I and VII bean Cd referred to peeled nibs (no testa), whereas studies II, III, IV, V, and VI were not peeled and so bean Cd included the nib and testa (see Table 2). The results here emphasize how great an impact the EU Cd regulations will have on cacao producers.

In each study, the Cd concentrations in topsoil were within the range of non-polluted soils (0.43 +/- 0.41 mg Cd/kg, n=776), but 50 soil samples across all the studies exceeded the 1.0 mg Cd/kg and represent only 6% of the data. The trends observed cocoa bean Cd concentration increases with increasing soil Cd, with decreasing pH (except in study II), and with decreasing soil organic carbon (except study VI). Soil with high organic carbon allows for more sorption sites for the cadmium and less likely to be taken up by the tree. In regards to pH, it affects the Cd2+ ability to bind to oxygen in the carboxylic or phenolic groups contained in the organic matter (Smolders and Mertens, 2013).

Cadmium in cacao is an interaction of several factors which control availability of Cd to the plant. Predicting cadmium concentrations in plant matter is not best predicted by total soil Cd alone, but by available soil Cd. This is because there are various particles in the soil which absorb/adsorb cadmium, making this Cd unavailable for uptake by the plant. That said, Soil pH is not a significant variable when looking at available soil Cd, but is used when referring to total soil cadmium.

A multivariate regression model was applied to analyze the data. A regression model is a mathematical equation which helps identify which variables have an impact. The equation used was:

log10 [Bean Cd] = 1.34 + 0.86* log10 [Total Soil Cd] - 0.18*pH-0.25*log10[SOC]

The multivariate model suggests:

Bean Cd increases nearly proportionally to total soil Cd
Bean Cd increases by a factor of 1.5 per unit decrease in soil pH (Fig. 2)
Doubling SOC (soil organic carbon) reduces bean Cd by a factor of 1.8

Other factors such as zinc and liming the soil also impact Cd concentrations. It’s suggested that Zinc triggers plant Cd uptake (Oliver et al., 1994; Chaney et al., 2006; Chaney, 2010). In a study by Souza dos Santos et al. (2020) they reported an increase in soil zinc inhibited uptake of cadmium by CCN-51, but was only found in soils spiked to cadmium concentrations above environmentally relevant soil Cd concentrations. Cadmium uptake in cacao was enhanced by surface liming of the soil, which was suggested to be a response of the roots to cope with micronutrient deficiency in the topsoil (i.e. Zn) caused by the liming (Arguello et al., 2020). Studies II and VII report a statistically significant impact of soil Zn on bean Cd (Table 2).

Transfer factors (TF)

Transfer factors indicate to what extent a crop accumulates cadmium in hits edible parts. It is calculated as a ration of the Cd concentration in plant leaf/edible portion over the total Cd concentration of the soil. The TF for cacao in the various studies ranged from 0.26 to 19, and are affected by soil pH (Fig. 3), where TF in acidic soils are higher, which confirms the importance of soil pH to available cadmium for uptake. TF are also influenced by plant genetics. Generally, the average bean Cd will remain below 0.60 mg Cd/kg if soil Cd is below 0.32 mg/kg (pH of 7-8), below 0.29 mg/kg (pH6-7), below 0.19 mg/kg (pH 5-6) or below 0.10 mg/kg for acidic soils with a pH <5.0.

2.3 Agronomic factors affecting Cd uptake in cacao

Although some have argued that Cd uptake is impacted by cacao management, such as monoculture vs agroforestry, there is not enough information to properly assess this, and not enough standardization among the studies. Arguello et al. (2019) found no significant effect on cropping systems to bean Cd concentration from a study done in Ecuador. Gramlich et al. (2017) concluded something similar, but did find Cd concentrations in cacao leaves higher when grown as monoculture vs agroforestry. His suggestion as to why is based on the idea that in agroforestry there is a higher density of plants, and so more competition for nutrients from the soil and a lower growth rate for cacao. He proposes that plants with lower growth rates take up less nutrients overall, and Cd is taken up as a “hitchhiker” element along with other essential nutrients.

The impact of fertilizers on Cd uptake has not been widely studied. Gramlich et al. (2017) found no significance in soil and bean Cd concentrations between soils treated with organic or conventional fertilizers. However, Arguello et al. (2019) reported higher Cd concentrations in cacao beans from trees that received organic fertilizers versus those that received conventional fertilizers. This may be due to the fact that organic fertilizers is mostly composed of compost of local vegetation which may contain various amounts of trace metals. This would impact the trees depending on the the rate of application of this compost and it’s quality, which may result in higher Cd concentrations. Zug et al. (2019) found higher Cd concentrations in cacao beans from trees that received conventional nitrogen fertilizers, but no effect from phosphorus fertilizers. An increase in Nitrogen has been linked to Cd uptake in plants by Yang et al., 2020, suggesting since nitrogen increases above ground vegetative growth, it results in more nutrient uptake and therefore more hitchhiker elements such as Cd to be taken up by the plant. As well, increased N may lead to a decrease in soil pH, causing mobilization of soil cadmium.

3. Uptake, translocation and partitioning of cadmium within the cacao tree

3.1 Soil-plant transfer of Cd in cacao compared to other plant species

Theobroma cacao L. is a perennial tree, and displays cauliflorous flowering where the flowers/pods develop on the drink and thicker branches (Toxopeus, 1985). The pod is made up of a woody husk, containing 20-50 seeds all surrounded by a whitish sweet mucilaginous pulp (Fig. 4). The beans are connected through a tissue called the placenta. Each cocoa bean is comprised of two cotyledons (which we refer to as the nib - the part used to make chocolate), a germ or embryo (the radicle), and an outer shell (the testa) (Wood, 1980). The nib is the part that is retained for chocolate making.

Soil-plant TFs (transfer factors) reported in cacao are higher compared to TFs for most other crops (Table 3), but similar to those reported for sunflower kernels. Therefore, the accumulation potential of cacao is high, but not unusual. Because of this, cacao has been described as a Cd accumulator, although this term lacks a clear definition. Some criteria to indicate whether a plant is a Cd accumulator are:

In natural conditions, the plant should accumulate at least 100 mg Cd/kg (Cacao doesn’t meet this criteria)
Both the TF and internal translocation factor should be larger than 1 (TF greater than 1 have been reported for cacao)
Extreme metal tolerance due to efficient biochemical detoxification (van der Ent et al., 2013) (additional research required)

For these reasons, the authors here suggest cacao be termed a moderate Cd accumulator. Table 4 shows that the mean Cd concentration in the cacao plant has a relatively homogeneous distribution between roots, scions, and leaves. Only 3% of the data from Kramer (2010) and Kabata-Pendias (2010) report leaf Cd concentrations in cacao exceeding the critical level of 10 mg Cd/kg, and suggests Cd toxicity to cacao plants is unlikely to occur at a large scale. Only one study discussed cadmium toxicity in cacao seedings, but the soils were spiked to high Cd concentrations which were not in the range of leaf Cd concentrations found in most fields.

3.2 Uptake and translocation of Cd within the cacao plant: trends mechanisms

Cadmium is a hitchhiker element, using transporters at all steps of the nutrient pathways from root uptake to seed loading (Clemens and Ma 2016). Metal ion transporter protein gene families linked to Cd uptake in plants include: Zinc-Iron Permease (ZIP), Natural Resistance Associated Macrophage Proteins (NRAMPs) and heavy metal transporting ATPases (HMAs). The identification of these genes can be used to develop mitigation strategies such as genetic engineering or through marker-assisted breeding programs.

NRAMP5 has been reported to play a role in the entry of Cd by the root cells for rice, barley, Polish wheat, and tobacco. Ullah et al. (2018) cloned 5 TcNRAMP genes and expressed them in yeast strains. The yeast cells expressing TcNRAMP5 accumulated up to 3 times more cadmium compared to empty vector control cells. Therefore, TcNRAMP likely plays a role in the regulation of Cd uptake in cacao plants, but these findings relevant to cacao soil-plant systems is limited. Also keep in mind that experiments using hydroponic environment can boost activities 100 times higher than natural Cd conditions, so what is observed in a lab may be quite different in the field in natural conditions. Also, uptake in yeast may not be able to predict uptake in cacao.

uptake of cd in the cacao root system

From the root, Cd is transported radially across the root cells and loaded into the xylem, long-distance transport via xylem and phloem, and then transferred to plant organs (Clemens and Ma, 2016). The rate of the translocation of Cd from the root to above ground tissue depends on vacuolar sequestration (“hiding” Cd away in vacuoles as a protective measure to reduce mobility of Cd to other tissues), xylem loading, and intervascular and xylem-to-phloem transfer (Fig. 4). HMA3 (Heavy Metal transporting ATPase 3) is a key transporter of Cd into the vacuoles and observed in rice (Wang et al., 2019), soybean (Wang et al., 2012), and Chinese cabbage and pak choi (L. Zhang et al., 2019). However, the role of HMA3 in Cd sequestration in cacao has not been determined.

Translocation of cd inside the cacao plant

These observations highlight the need for more studies to focus on understanding the mechanism for Cd translocation from roots to above tissues. Since cacao is a cauliflorous tree, mechanisms for Cd loading into seeds may be different from other plants (rice, wheat, etc.). Most studies report leaf Cd concentrations to be higher than the corresponding seed Cd concentrations, with a clear correlation between the two suggesting the leaf Cd concentrations can be used as a proxy for bean Cd. However, the use of leaf Cd as a proxy can only be justified when analyzing a genetically homogenous sample set. Engbersen et al. (2019) and Lewis et al. (2018) reported only moderate correlations.

Studying the translocation of Cd within plants can also be observed using isotope discrimination between different types of tissues (Table 5). For instance, Moor et al. (2020) enriched leaves in heavy Cd isotopes compared to the roots. A greater fraction between roots and above ground tissue suggest that perhaps the additional Cd retention mechanisms are invoked to inhibit the translocation of Cd to plant parts above ground. In most plants, Cd becomes enriched in heavy isotopes in the order of roots<stem<leaf<seed (Table 5). The only study on Cd isotope discrimination suggests the transfer of Cd in cacao is different compared to other plant species in the studies currently available (Barraza et al., 2019). This same study reported lighter isotopes in the beans compared to the leaves, suggesting a hypothesis that the cauliflorous character of cacao leads to Cd being transferred from the xylem to phloem in developing cocoa beans without first passing through the leaves.

distribution of cadmium within the cacao fruit

Table 4 summarizes the Cd concentrations for different cacao fruit tissues. Vanderschueren et al. (2020) reported Cd concentrations decreased from testa>nib~placenta~pod husk>mucilage in cacao fruits and reported that testa Cd was 1.5-18. times larger than nib cadmium. Similar findings from Lewis et al. (2018), but the authors reported the ratio between test and nib concentrations depending on the cacao genotype. Although the testa contains on average higher levels of Cd, it only makes up a small fraction of the total bean weight.

Little is known about the role of the outer pod husk in Cd transport to cacao beans. Several surveys have compared the two and found similar Cd concentrations in both tissues. Engbersen et al. (2019) reported Cd concentrations within the cacao fruit may change during maturation of the fruit. Their data indicated as pods matured there was a decrease in Cd concentrations in the pod husk and increase in the unpeeled cocoa bean Cd concentrations. However, additional research is required as the observed difference may be related to the variation in Cd concentration among the fruits of one tree, rather than impacted by maturation.

Cultivar-related differences in Cd uptake and partitioning

The cultivar effect was first reported by Lewis et al. in 2018, suggesting there are cultivar-specific differences in xylem-to-phloem transfer of Cd to the various tissues. Engbersen et al. (2019) suggested the loading of Cd into the beans as being influenced by the cultivar of the cacao for similar reasons. However, the trees in Engbersen’s study were grafted onto rootstocks of unknown genetic identity, which may have affected the uptake of the metals. Further research is required in this area, but the observations so far suggest that an isotope approach pay provide more insight.

4. Mitigation strategies to lower the cadmium concentration in the final product

The cadmium regulations for cacao products apply to the final food product, not the cacao beans. Because of this, mitigation strategies can be applied to various stages of chocolate making starting from growing the tree to the final bar. Most likely, multiple strategies would apply to multiple points along the process in order to have a product which complies to current Cd regulation standards.

Mitigating can be based on:

soil amendments to reduce Cd uptake (liming for example)
selecting specific cultivars which reduce Cd translocation to the seeds
fermentation techniques
winnowing

4.1 Mitigating Cd uptake using soil amendments

Lime

Applying liming materials [CaCO3, CaO, Ca(OH)2, CaMg(Co3)2] has been used and recommended as a way to reduce Cd uptake and accumulation in various crops. Liming increases soil pH and increases competition between Ca2+ and Cd2+ at the root, Thereby decreasing the solubility of cadmium and makes it less available for uptake by the plant. One problem with liming is that by increasing the concentration of Ca2+ can result in a release of surface bound cadmium, which is likely why liming does not consistently work in reducing cadmium uptake. This application has worked for rice crops in China, but it is unknown if it will have the same impact on cacao.

In a study by Ramtahal et al. (2019) they found that liming young potted plants (6 month old cuttings) had an impact on lowering Cd, but in the field with 30 year old trees the result was negligible, and Cd concentrations measured in the leaf even rose in both control and lime treated trees over time resulting in a reduction by a factor of only 1.1. One explanation for it’s lack of effect in the field is that it’s difficult to incorporate lime in deeper soils, and that the cadmium was taken up by roots reaching deeper soils, and it would be difficult to reach this level without damaging the root system of the trees. Contrastingly, Arguello et al. (2020) found that liming cacao seedlings in pots (lime on the top compartment only) enhanced cadmium uptake from the non-limed bottom compartment.

Gypsum

Gympsum is capable of reaching deeper soil layers and overcome the limitations of liming. Gypsum has been documented to reduce Cd uptake, but the mechanism behind why is unknown. It has yet to be applied to cacao, but could be a possible contender.

Biochar

Biochar has been effective at immobilizing metals such as cadmium, but due to it’s complex composition it’s metal sorption capability is not quite understood. However, through experiments it seems that biochar is able to increase soil pH (Rees et al., 2014). It’s effectiveness also depends on the soil properties (such as acidic pH, coarse texture, and a good amount of organic carbon content) as well as the type of crop.

An experiment by Ramtahal et al. (2019) reported a reduction of cadmium concentration in the leaves of cacao seedlings, and was dose dependant on the amount of biochar used, but in the field with 30 year-old trees, the result was not significant. However, in studies on rice, the lasting effect was not very long, and so repeated application would be necessary which would likely not be cost effective for cacao farming.

Fertilizer Management & Zinc supplementation

There have been no studies with cacao no zinc supplementation, but has been successful with other crops. The effect of zinc fertilization on crop cadmium depends on the status of zinc already present in the soil. The greatest impact has been soils which are already deficient in zinc. In studies where soil zinc levels were not deficient, the zinc fertilization did not have a significant impact on reducing cadmium uptake by the plants (in this case wheat). However, a contrasting study by McLaughlin et al. (1995) on potatoes showed that high levels of zinc to non zinc-deficient soils reduced cadmium uptake by the tuber. It’s suggested that the reason zinc is effective is that it competes with cadmium in regards to uptake by the roots.

4.2 Genetics as a potential mitigation strategy

The research regarding using genetics of trees and specific cultivars to mitigate Cd uptake is still in its early phase. However, there are some indications this could be an effective strategy. For instance, Ullah et al. (2018) and Moore et al. (2020) identified TcNRAMP5 as a potentially important gene for Cd uptake in cacao, and possibly a candidate for genetic selection. A greenhouse experiment with 53 wild and domesticated cacao genotypes suggested 11 cacao clones as “low Cd accumulators" (Arevalo-Hernandez et al., 2020). However it’s important to note that in this experiment, the soil was spiked with unnaturally high cadmium levels, so experiments with more natural levels found in field would be required before making conclusions.

Lewis et al. (2018) found significant differences in bean cadmium concentrations among various cultivars as well. However, the differences might be related to other factors such as soil variability at the plantation, or differences in leaf flushing and pod development cycles, rather than a difference in cadmium translocation based on cultivar.

The concept of using low-Cd accumulating rootstocks for grating may be a promising strategy. Grafting is already used a great deal in cacao cultivation. The rootstock effects on above-ground cadmium concentrations seem to be species dependent. Arao et al. (2008) grafted eggplants onto the rootstock of turkey berry (Solanum torvum) and reduced cadmium in the eggplant fruit by 63-74%. As well, cadmium concentration in the xylem sap of the turkey berry was significantly lower than in the eggplant xylem sap, which suggests S. torvum can limit translocation of cadmium from the root to the shoot. However, a study by Mengist et al. (2018) with potato cultivars demonstrated that although rootstock was an important factor, it was the scion which regulates the distribution of cadmium to other parts of the plant.

The use of cacao rootstocks with low cadmium uptake potential is promising. In a way, it can “renovate” old cultivars to reduce Cd concentrations in the beans while manintaining the flavour specific cacao cultivars (which is likely under control of the scion).

4.3 postharvest processing

There are four main approaches (Fig. 6) to reduce cadmium in postharvest processing:

Fermentation
Testa Removal
Recipe/Product
Cacao Bean Mixing

As we know, acidity can impact the mobility of cadmium, and so during fermentation where acidity increases (pH decreases) cadmium is mobilized in the seed. The potential for using fermentation as a strategy has been pointed out by Meter et al. (2019) but not much research on this to date. There are some studies where cadmium concentrations were reported on unfermented beans versus products made from these beans. Barraza et al. (2017) observed that unpeeled unfermented cocoa beans contained roughly 1.02-1.37 mg/kg of cadmium versus in cacao liquor (ground up peeled often roasted nibs) to be around 1.47-3.88 mg/kg of cadmium. Yanus et al. (2024) reported Cd concentrations in unfermented nibs (~0.072 mg/kg), the testa (~0.085 mg/kg), and cocoa powder (~0.125 mg/kg) all of which suggest some sort of cadmium enrichment through processing. However, it’s not clear if these trends recorded were related to the variability of samples or indeed from processing. A study by Vanderschueren et al. (2020) on the impact of fermentation on cadmium indicated a migration of cadmium from the nib to the testa during fermentation, lowering the concentration of cadmium in the nib. It’s important to point out this decrease in nib concentration during fermentation only occurred when pH at the end of fermentation was less than 5.

Removing of the testa can also reduce final cd concentrations in the final product by ~ a factor of 2. The testa is removed during breaking and winnowing the beans, after which the nibs are ground into a cacao liquor. Keep in mind that cacao liquor is allowed to contain up to 5% residual testa and/or radicle on a fat-free dry weight bases, or about 2.5% of the total dry weight. Since testa makes up about 10% of the dry weight, removing the testa can reduce the cadmium concentration by a factor of 1.16. Also keep in mind that the new EU limits may unnecessarily reject cacao due to a decision base don total bean cadmium (including testa) instead of only the nib cadmium level.

Cadmium is also contained in the non-fat cacao solids, and therefore this needs to be considered as well in regards to cadmium limits. Final consumer products made with the same cocoa beans will have very different cadmium concentrations depending on their intended product. For instance, cadmium concentrations in white chocolate are negligible, while at the other end is cocoa powder (which has much of the fat removed) can contain very high levels. Therefore, cadmium concentrations are often reported to be higher in cocoa powders than in cacao liquor/cocoa beans. So it is important for regulators and manufacturers to take into consideration the final product. The new EU limits on cadmium depends on the final product. A milk chocolate with 35% cacao solids (EU limit of 0.30 mg/kg), cacao beans can be used with a nib cadmium concentration up to 0.86 mg/kg. For a dark 70% chocolate (EU limit 0.80 mg/kg), cacao beans with nib cadmium concentrations up to 1.14 mg can be used. A webtool was recently created at https://platform.climaloca.org/chocosafe that allows farmers to calculate acceptable cacao nib Cd concentrations based on the cacao solids content of the product.

It is common to mix cacao from different geographical sources during the production process. Therefore, since there are differences in cadmium concentration depending on geographical origin, cacao from different origins can be mixed to allow for acceptable cadmium concentrations in the final product. For example, mixing cacao from West Africa which is reported to contain low Cd concentrations with cacao from South America where it may be higher. This strategy may also work on a national scale, where cacao from various parts of the same country can be blended before export to ensure acceptable overall cadmium concentrations. For example, in a report by the Ecuadorian government between 2014-2019, the average bean cadmium concentration exceeds the recommended limits in 11 out of 23 provinces which equates to 15% of the countries production. Blending this at the national scale would result in the average Cd concentration in the beans to comply with Cd requirements. Although this may not always be a feasible strategy, there is potential such as where larger co-ops purchase cacao from small scale farmers and then sell the mixed product to international customers.

5. From chocolate bar to body burdens

Different regulatory bodies are implementing their own limitations in regards to the maximum allowed cadmium concentration in chocolate and cocoa products. However, there is controversy regarding the tolerable intake levels, and the relationship between dietary intake and body burden (the amount of said chemical in the body at any given time). JECFA set a provisional monthly tolerable intake of 25 ug Cd kg-1 body weight. This corresponds to a tolerable daily intake of 0.83 ug Cd kg-1 body weight (FAO/WHO, 2010).

Children and infants have been identified as a high-risk group, however, adverse effects of Cd intake are due to accumulation in the human body over a lifetime of exposure, so focusing on these young age groups may not be relevant. Research has indicated that only a small part of dietary cadmium is likely to be absorbed by the human body (European Chemicals Bureau, 2007), but regulations and health-based guidelines are based on dietary Cd intake rather than resulting Cd body burden. Most dietary Cd studies have shown that Cd body burden increases less proportional with increasing dietary Cd intake. This contrasts the assumption in the risk assessment models. This is likely related to lower cadmium bioavailability or changes in micronutrient status with diet when relying on high Cd diets. There is very little information on the gastro-intestinal absorption rate of cadmium from cacao products. Studies so far focused on in vitro experiments (Mounicou et al., 2002 and 2003). Considering the strong influence of dietary status (such as dietary zinc, iron, and calcium) on the gastro-intestinal absorption of cadmium, it remains to be demonstrated if long term high chocolate consumption actually increases body burden cadmium to levels of concern.

Mitigating the level of cadmium in cacao products: Reviewing the transfer of cadmium from soil to chocolate bar