Off-flavours are a major problem, especially for small and medium-sized businesses. If such raw cocoa batches contaminated with off-flavours get into production, they can cause immense economic damage. It is therefore essential to identify and sort out off-flavours immediately during the incoming goods inspection.

By Dr. Jörg Häseler

Until now, it was only possible to reliably detect off-flavours at incoming goods with the help of sensory-trained personnel. A way to objectively evaluate batches on the basis of measurable concentrations of the responsible odourants has been lacking until now.

A team of scientists from the Leibniz Institute for Food Systems Biology at the Technical University of Munich/Germany has identified the odorous substances responsible for such off-flavours as part of a research project. Companies can now use the research results to objectively assess the sensory quality of raw cocoa based on odourant concentrations.

The team led by food chemist Dr Martin Steinhaus has now made a significant contribution to solving the problem as part of the project. With the help of a molecular sensory concept established at the Leibniz Institute, the research group identified the key odorous substances that contribute significantly to typical off-flavour notes in raw cocoa.

In cocoa samples with a ham-like, smoky off-odour, the team identified six odour substances that are decisive for the off-flavour. The substances smelled smoky, ham-like, phenolic or horse stable-like. In nine other samples with a mouldy, musty smell, the researchers found four other off-flavour substances. These smelled mouldy, musty, like beetroot, faecal or like mothballs.

In addition, the research team determined the odour threshold values of the identified substances in order to be able to derive limit values from them. For example, the team recommends setting the maximum tolerable concentration for the off-flavour substance geosmin at 1.6 µg/kg raw cocoa. This substance is presumably of bacterial origin and even in very low concentrations causes a musty, earthy odour reminiscent of beetroot. For faecal, mothball-smelling 3-methyl-1H-indole, the researchers recommend not to exceed a limit of 1.1 µg/kg raw cocoa.

As the findings of the research team also show, about 40 to 65 % of the total geosmin is found in the seed coat of raw cocoa beans. In contrast, the off-flavour substance 3-methyl- 1H-indole was predominantly found inside the beans. “The unequal distribution of the two odorous substances on the seed coat and on the embryo is another important aspect that companies should consider when checking raw cocoa,” Martin Steinhaus notes. “At the moment, we cannot yet say how much geosmin is transferred from the coats to the seeds during the roasting process. However, it might make sense to remove the seed coats already before roasting.”