November 2, 2021, by Lexi Earl

The problem of cadmium in cocoa beans

We are identifying ways to produce safer chocolate in order to help small-holder farmers, say David Salt, David Gopaulchan and Gabriel Castrillo

Cadmium is a potentially toxic heavy metal that can be found in various foods through bioaccumulation from the soil. It occurs naturally in soils (from volcanic activity, forest fires, and rock weathering), but can also be found because of agricultural and industrial soil contamination. When ingested in sufficient quantities, cadmium can contribute to the development of different diseases, including renal failure, cancers and bone demineralisation. As such, the European Union has restricted the amount of cadmium it allows in food, including in chocolate. Chocolate has recently been of particular focus, with three maximum levels established on the amount of cadmium allowed in the final chocolate products, which came into force in January 2019. The strictest maximum applies to chocolates targeted at children and young adults.

These new limits are concerning for cocoa growers as the EU is a major cocoa importer – consumers in the EU eat 50% of the world’s chocolate. While the limits apply to the finished chocolate product, rather than the beans, cocoa growers are more likely to encounter problems exporting their beans into the EU if they have elevated cadmium levels. This is of particular concern to cocoa growers in Latin America and the Caribbean, where cadmium accumulation in cocoa is a problem because of higher levels of cadmium in the soil and increased availability to the plant. Blending beans from different lots with lower levels of cadmium, can be used to reduce the overall cadmium level in the final chocolate product.  However, this is not a solution for producers growing “fine or flavour” cocoa for the bean-to-bar, artisan chocolate market where chocolates made from single origin beans are highly valued and lucrative.

Our researchers are working on ways to reduce the amount of cadmium in cocoa beans. Cocoa trees absorb cadmium from the soil and store it in their roots, leaves, and beans, but not all cocoa trees absorb cadmium at the same rate. Cocoa trees can store cadmium differently, with some storing more cadmium in the beans than others. The soils that cocoa plants grow in also vary in the level of acidity, with more acidic, lower pH soils giving rise to high levels of soluble cadmium that cocoa plants can absorb. Strategies such as adding lime to the soil to increase the pH can help to limit the accumulation of cadmium by the cocoa plants. Another strategy however, can be to use the variation in cadmium storage between the different types of cocoa plants to help reduce the amount of cadmium in the beans.

Our ongoing approach is to exploit genetic differences between cocoa varieties, focusing on the genetic differences that control the absorption and sequestration of cadmium into beans. The Cocoa Research Centre (CRC) at the University of the West Indies is the custodian of the International Cocoa Genebank (ICGT). This holds over 2400 cocoa tree varieties, which is regarded as the largest and most diverse cocoa collection in the public domain. Working in collaboration with the CRC, our researchers have sequenced the genomes of over 500 varieties of cocoa trees, and are examining these sequences to investigate genetic markers that may be linked to variation in cadmium accumulation in the bean. Once linked markers are identified, they can be used to find the genes responsible for controlling cadmium uptake into the beans, and to develop DNA tests to screen varieties to examine whether they have the markers for low or high cadmium accumulation. Once low cadmium cocoa tree varieties are identified they can be used in breeding programmes, for example by identifying low cadmium root stocks that can be used to graft cocoa trees that would then be low in cadmium. This would then help cocoa farmers in Latin America and the Caribbean produce beans with lower levels of cadmium, enabling them to sell their beans into the lucrative premium chocolate markets in the EU.

Further Reading:

(2020) Researching fermentation in cocoa. Future Food blog: https://blogs.nottingham.ac.uk/futurefood/2020/03/11/researching-fermentation-in-cocoa/

(2020) Doing science in the field: Stories from our Colombian cocoa project. Future Food blog: https://blogs.nottingham.ac.uk/futurefood/2020/01/16/doing-science-in-the-field-stories-from-our-colombian-cocoa-project/

(2019) Eating chocolate in the name of science. Future Food blog: https://blogs.nottingham.ac.uk/futurefood/2019/11/20/eating-chocolate-in-the-name-of-science/

(2019) Hot chocolate! Making artisan chocolate taste even better. University of Nottingham News: https://www.nottingham.ac.uk/news/making-artisan-chocolate-taste-even-better

(2019) Science adds flavour to unique chocolate made in Nottingham. University of Nottingham News: https://www.nottingham.ac.uk/news/science-adds-flavour-to-unique-chocolate-made-in-nottingham

David E Salt is the Director of the University of Nottingham’s multi-million pound Future Food Beacon of Excellence and a Professor of Genome-Enabled Biology. He spent more than twenty years as an academic in the US, before returning to the UK to establish and lead the Centre for Genome Enabled Biology at the University of Aberdeen (2011 – 2016). In 2016 he started at the University of Nottingham. He has long term research interests to understand the function of the genes and gene networks that regulate the plant ionome [defined as the elemental composition of an organism, tissue or cell], along with the evolutionary forces that shape this regulation. Over his career he has obtained 38 competitive research awards totalling £17.8 million (equivalent from US, UK and EU funding), and published 176 peer-reviewed papers in journals such as Nature, Science and PNAS that have been cited ~35,000 times (h-index = 88).

David Gopaulchan has a PhD in Biochemistry with expertise in genetics, genomics, molecular and cellular biology, and is a postdoctoral researcher with the Future Food Beacon. His research is focused on studying genetic diversity of cacao populations towards conservation, understanding sequestration of cadmium in cacao towards mitigation and controlling microbial communities involved in cocoa fermentation to improve quality and flavour. His interest also spans exploiting natural cacao variation to improve cacao yields, developing resilient varieties, and using more sustainable agriculture production systems.

Follow David Gopaulchan on Twitter: @DavidGopaulchan

Gabriel Castrillo is an Associate Professor in Plant Microbiome. Prior to this he was a Nottingham Research Fellow with the Future Food Beacon. He studied biochemistry (1995-2000) at Havana University, Cuba and completed his PhD (2004-2009) as a MAE-AECI fellow in the lab of Prof Javier Paz-Ares at the Spanish National Centre for Biotechnology. From 2009-2013, he was a Postdoctoral Fellow in the same institute, in the lab of Prof Antonio Leyva. He left Spain to undertake a second postdoc (2014-2018) at the University of North Carolina, Chapel Hill, USA, in the lab of Prof Jeff L. Dangl. He has published in Nature and Science.

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