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Millions of cocoa pods are harvested annually to satisfy huge demand for chocolate, said Nicholas Westwood and colleagues reporting in the American Chemical Society’s ACS Sustainable Chemistry & Engineering journal. While the beans and pulp are used, more than 21m tonnes of leftover cocoa pod husks are thrown away.
Waste husks have been explored as a source of carbohydrates and sugars, but they also contain lignin, a tough lipid polymer found in many woody plants. Lignin could be a renewable replacement for some substances typically derived from petroleum, such as flame retardants.
While most methods to produce lignin have focused on hardwood trees, some scientists have processed other plant materials that would otherwise go to waste, such as rice husks and pomegranate peels. Westwood and his team wanted to see if high-quality lignin could be extracted from cocoa pod husks, and to determine whether it could make valuable, practical materials.
The researchers obtained cocoa husks and milled them into a powder. After rinsing to remove fatty residues, they boiled the powdered husks in a mixture of butanol and acid, a standard lignin extraction method. They next confirmed the isolated lignin’s quality and high purity, finding no evidence of carbohydrates or other contaminants.
They then modified the pure lignin biopolymer to have flame-retardant properties by attaching a fire suppressant molecule called DOPO. In experiments when the modified lignin was heated, it charred but did not burn – a sign that it could act as a flame retardant.
The team plans to conduct human safety tests after the next phase of testing. They also aim to optimise the properties of the flame retardant materials.
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