The primary source of cadmium in cocoa beans has been linked to its direct uptake by the cacao plant from cadmium contaminated soils. This research was conducted to evaluate and interpret significant relationships between cadmium levels in tissues of the cacao plant and soils from cocoa-growing areas in Trinidad and Tobago. Total (HNO 3-extractable) concentrations of cadmium in both tissues and soils were determined. The levels of cadmium measured varied in the order: leaves > pods > shells > nibs > soil. Cadmium levels in all the cacao tissues analyzed were significantly (p < 0.05), positively and strongly correlated with each other. Additionally, significant (p < 0.05) positive relationships were also identified between Cd in cacao tissues and corresponding total HNO 3-extractable Cd levels in soils. These findings suggest that they can possibly be used as predictive tools for assessing Cd levels in cacao.
Contamination of soils with heavy metals is an increasing problem globally and a threat to environmental biota, as these metals accumulate in soils and plants [
Generally, the intake of Cd in very small amounts by humans is unavoidable, with agricultural produce being a major source [
Over the years, foods such as vegetables [
It is claimed that Cd levels in cocoa beans are due to their uptake by cacao plants from soils [
In this study, Cd levels among cacao leaves, pods, bean nibs, testae/shells and soils, on which cacao trees were grown, were determined and compared, to identify whether it was possible to relate soil Cd with cacao tissues and especially cacao beans (nibs) from which edible products were derived. Such correlations can be used to assess the effectiveness of soil treatments to reduce Cd uptake and redistribution in cacao tissues, especially the nibs. The potential to estimate cadmium levels in cacao beans via correlations with other cacao tissues was also investigated, to provide a basis for subsequent studies in Cd absorption and distribution in cacao trees. A recent study of cadmium in cacao tissues and soils in Ecuador [
All water used for sample preparation and cleaning of glassware in this study was glass-distilled and deionized. To avoid trace metal contamination, all laboratory glassware and utensils used were washed with a commercial detergent and tap water, immersed in a 2 M nitric acid HNO3 (J.T. Baker, USA) bath for at least 24 h, then rinsed with distilled deionized water before drying in an oven at 50˚C. All reagents used for sample preparation and analyses in this study were of analytical grade and were tested in blank analyses, to correct for Cd background levels.
Cacao samples were collected from 45 plantations throughout different cacao-growing locations on all major soil types (I-VII) in Trinidad and Tobago (
Characteristic | Soil type | ||||||
---|---|---|---|---|---|---|---|
I | II | III | IV | V | VI | VII | |
Sand (%) | 31 | 64 | 47 | 35 | 33 | 25 | 19 |
Silt (%) | 18 | 7 | 21 | 20 | 17 | 14 | 10 |
Clay (%) | 42 | 25 | 29 | 49 | 32 | 59 | 33 |
pH | 4.9 | 4.5 | 4.9 | 4.9 | 5.3 | 7.6 | 6.2 |
EC (mhos × 106) | 37 | 43 | 138 | 16 | 41 | 99 | 55 |
CEC (meq/100g) | 11.5 | 4.7 | 8.2 | 8.5 | 4.4 | 6.3 | 34.2 |
OM (%) | 1.55 | 1.03 | 2.41 | 0.86 | 1.89 | 3.96 | 0.52 |
Base saturation (%) | 18 | 13 | 49 | 28 | 32 | 100 | 100 |
Ca (meq/100g) | 1.3 | 0.5 | 3.5 | 1.8 | 0.8 | 8.0 | 15.6 |
Mg (meq/100g) | 7 | 0.2 | 0.7 | 0.9 | 0.7 | 0.4 | 31.1 |
From each cacao pod, the beans were removed and rinsed in deionized water to remove excess mucilage. The beans were then oven-dried on glass plates at 105˚C for 4 - 5 h, separated into nibs and shells, and each ground separately in ceramic mortars to <1 mm diameter particle size. Pod husks were rinsed with deionized water, grated finely (<1 mm) on a plastic grater and dried on glass plates at 105˚C for 3 h. Leaves were washed with deionized water, chopped to <1 mm in a food processor, then oven-dried on aluminum foil at 105˚C for about 24 h. Soil samples were air-dried for 48 h, ground and sieved through a 2 mm sieve, as recommended for soil analyses [
The method of determination for Cd in cacao tissues was optimized for highest recovered levels, using local samples of cacao leaves, pods, nibs and shells or testae. Sample mass: acid volume, digestion conditions and extraction times were systematically varied, to determine the highest levels of Cd recovered from each sample type. For these analyses, optimal recoveries were obtained for 0.5 g aliquots of nib, shells, pod and leaf samples, to each of which 10 mL concentrated HNO3 (J.T. Baker, USA) were added and samples allowed to pre-digest at room temperature for 12 h, followed by complete digestion on a heating block at 130˚C for 6 h. Digested extracts were cooled, diluted with 5mL distilled water, filtered through Whatman No. 542 filters and made up to 25 mL, for Cd determination by flame atomic absorption spectrometry (FAAS).
Reference Cocoa Materials certified for Cd were unfortunately unavailable to our laboratory at the time of the study, necessitating the use of NIST SRM 1570a Spinach Leaves, (National Institutes of Standard and Technology, Maryland, USA), to validate the test method. Replicate analyses (n = 10) of this SRM were done. Once validated, this method was used to analyze all cacao tissues samples, with appropriate instrumental and sample quality control measures taken, to maintain the accuracy of analyses. Quality control results shown in
As for cacao tissues, the method of determination of total Cd in soil was similarly optimized, using a soil sample from a local cocoa farm. For optimal extractions, to triplicate 0.5 g aliquots of soil samples, 10 mL concentrated HNO3 was added to each and samples pre-digested at room temperature for 12 h, followed by exhaustive digestion on a heating block at 130˚C for 8 h. Digested extracts were cooled, diluted with 5 mL distilled water, filtered through Whatman No. 542 filters and made up to 25 mL, for Cd determinations by FAAS.
A Certified Reference Soil, namely NIST SRM 2710 Montana Soil was analyzed in replicate (n = 10) with this method and % recoveries for Cd determined. Appropriate quality control measures, as done with cacao
Certified value (μg/g) | Mean, n = 23 triplicates (μg/g) | % Recovery | CV | |
---|---|---|---|---|
SRM 1570a (Spinach leaves) | 2.89 ± 0.07 | 2.69 ± 0.05 | 95.5 | 4.71 |
SRM 2710 (Montana soil) | 21.80 ± 0.2 | 21.58 ± 0.31 | 99.00 | 1.43 |
tissues were also taken to maintain accuracy of analyses. Results of quality control for soil samples are shown in
Correlation analyses used to evaluate and interpret significant relationships between the selected heavy metal concentrations in various cacao sample types were done with Minitab 16 software.
Data generated from 23 sets of replicate analyses of leaf and soil CRM over the 18-month experimental period, including 10 replicates for method validation, are shown in
More than 50% of the locations sampled contained detectable (>0.3 µg/g) Cd in the cacao tissues sampled. Generally, Cd levels measured in the tissues of the cacao plants varied in the order: leaves (0.54 - 5.21 µg/g) > pods (0.53 - 4.49 µg/g) > shells (0.44 - 4.41 µg/g) > nibs (0.35 - 3.82 µg/g). This trend is consistent with some previous reports of differing Cd levels among organs or tissues in the same plant [
Conversely, the total HNO3-extractable concentrations of Cd in soils sampled across cacao plantations ranged from 0.3 - 1.7 µg/g. These levels were generally lower than those of cacao tissues of trees grown on the soils, consistent with previous reports [
Correlation analyses were used to evaluate and interpret significant relationships for Cd concentrations in the various cacao tissues (
Cadmium levels in all the cacao tissues analyzed were significantly (p < 0.05), positively and strongly correlated with each other: (Nib/Shell) r = 0.86; (Nib/Leaf) r = 0.79; (Nib/Pod) r = 0.76; (Shell/Leaf) r = 0.83; (Shell/Pod) r = 0.68); and (Leaf/Pod) r = 0.80. This indicates that the concentrations of Cd concentrations in cacao nibs, shells, leaves and pods vary proportionately with each other. It also suggests that the Cd distribution and concentration ratios within tissues are related. Such strong correlations suggest that cadmium levels in cacao beans can be estimated through its other tissues, when pods are not available. This may offer a means of early screening of cacao cultivars for cadmium accumulation, for selecting low-cadmium varieties, to allow cacao growers to meet the proposed European Union standard [
Cd (Nib) | Cd (Shell) | Cd (Leaf) | Cd (Pod) | |
---|---|---|---|---|
Cd (Shell) | 0.86 | |||
Cd (Leaf) | 0.79 | 0.83 | ||
Cd (Pod) | 0.76 | 0.68 | 0.80 | |
Cd (Soil) | 0.35 | 0.43 | 0.34 | 0.50 |
Bold values are significant at p < 0.05.
Significant (p < 0.05) relationships were also identified between Cd in cacao tissues and corresponding total HNO3-extractable Cd levels in soils: (Nib/Soil) r = 0.35; (Shell/Soil) r = 0.43; (Leaf/Soil) r = 0.34; and (Pod/Soil) r = 0.50. It is notable that such correlations were obtained for a wide variety of cacao cultivars and soil types and should be applicable to other cacao-growing countries.
It is well known that most plants which accumulate heavy metals in their aerial tissues primarily reflect the concentrations of their respective metal in the soil [
This study demonstrated that strong and significant correlations exist between Cd levels in cacao tissues over a wide range of cacao cultivars. These findings suggest that it may be possible to predict Cd levels in cocoa beans from those in mature leaves and pods. This can be especially useful in assessing cacao trees, growing areas and soil types for possible elevated Cd levels in beans, even when pods are unavailable. Such an approach can be used by cacao farmers and purchasing agents of cocoa beans to estimate whether beans from any farm or area are likely to meet specific Food Safety Standards for Cd. Similarly, the effectiveness of soil treatments to minimize Cd uptake from soils by cacao plants can be monitored through tissue Cd levels before and following remedial soil treatments.
The authors are grateful for the generous financial and logistical support provided by the Government of Trinidad and Tobago, through the Ministry of Food Production and staff members Davinan Ramnath and Ian Rampersad. The assistance of the Cocoa and Coffee Industry Board of T & T, the Cocoa and Coffee Marketing Co-Operative Society Ltd. and local cocoa farmers in the provision of samples and the administrative support of the University of the West Indies, St. Augustine are also highly appreciated.
GideonRamtahal,Ivan ChangYen,IsaacBekele,FrancesBekele,LawrenceWilson,KamaldeoMaharaj,LisaHarrynanan, (2016) Relationships between Cadmium in Tissues of Cacao Trees and Soils in Plantations of Trinidad and Tobago. Food and Nutrition Sciences,07,37-43. doi: 10.4236/fns.2016.71005