Does your cocoa smell like horse dung or mothballs?

Researchers have identified the odourants responsible for off-flavour cocoa, which make it smell like horse dung or mothballs.

Unsurprisingly, musty, mouldy, smoky or horse dung-like smelling cocoa is not suitable for chocolate production.

Off-flavours are a significant problem, especially for small and medium-sized enterprises. However, also for larger companies, off-flavour-tainted cocoa batches can cause economic damage, posing the need for their reliable identification during the incoming goods inspection.

Until now, this has only been possible with some limitations by using a trained sensory panel, since no approach to objectively evaluate off-flavours in cocoa based on the concentrations of the causative odourants was available.

As part of a larger research project, a team of scientists led by Martin Steinhaus from the Technical University of Munich found the key odourants that chiefly contribute to typical off-flavours in fermented cocoa.

In cocoa samples with hammy-smoky off-flavours, the team identified six crucial odourants. The substances smelled smoky, hammy, phenolic or horse stable-like. In samples with a mouldy-musty odour, the researchers found an additional four off-flavour compounds. These smelled mouldy, musty, beetroot-like, faecal or mothball-like. The research team determined the odour threshold values of the identified substances to derive maximum concentration limits.

For example, in their latest publication, the team recommends a maximum tolerable concentration for off-flavour substance geosmin of 1.6 micrograms per kilogram of fermented cocoa. Even in the lowest concentrations, this odourant has a musty, earthy smell reminiscent of beetroot. Geosmin is presumably of bacterial origin. For faecal, mothball-like smelling 3-methyl-1H-indole, the researchers recommend a limit of 1.1 micrograms per kilogram of fermented cocoa.

Odourant distribution is also important

The research team’s findings also show that about 40–65% of total geosmin is found in the seed coat (testa) of fermented cocoa beans. In contrast, the off-flavour substance 3-methyl-1H-indole was predominantly found inside the beans.

“The uneven distribution of the two odourants between the seed coat and the embryo is another important aspect that chocolate manufacturers should consider when testing fermented cocoa,” said Steinhaus.

“At the moment, we do not know how much geosmin is transferred from the shells into the embryo during the roasting process. Therefore, it might be reasonable to remove the seed coats before roasting rather than after roasting.”

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