Controlling off-flavours in cocoa processing

Fermented cocoa occasionally exhibits off-flavours that can compromise the quality of chocolate and other cocoa products. But what causes these undesirable flavours? Researchers at the Leibniz Institute for Food Systems Biology at the Technical University of Munich have now shown that overfermentation of cocoa beans can cause smoky, leathery and cheesy off-flavours. Smoky flavours, therefore, are not necessarily attributable to smoke contamination.

It has only been in the last few years that the odourants responsible for many cocoa off-flavours have been identified. However, how and under what conditions they arise is not yet fully understood. To get to the bottom of this, Franziska Krause and principal investigator Martin Steinhaus examined cocoa samples from various stages of fermentation and growing regions. Part of the work took place under real-world conditions on cocoa plantations in Nicaragua and Costa Rica, where Krause monitored the fermentation process onsite.

Using modern odourant analysis techniques in the laboratory, the researchers compared samples of optimally fermented cocoa beans with samples of intentionally overfermented beans. In doing so, they detected elevated concentrations of short-chain carboxylic acids in the overfermented samples. These compounds can cause off-flavours that smell like cheese, sweat or even vomit.

The overfermented samples also contained significantly higher concentrations of the compounds 2-methoxyphenol and 4-ethylphenol. Both substances have a smoky odour. While 2-methoxyphenol has a rather sweet, smoky smell, 4-ethylphenol also exhibits leathery, phenolic and horse stable-like notes.

In an earlier study, the research team had already identified and quantified off-flavour compounds typical of wood smoke contamination in cocoa beans. The new results now show that overfermented cocoa beans contain key smoke-like-smelling phenols in concentrations similar to those found in wood smoke-contaminated cocoa. “Our results thus demonstrate for the first time, objectively and at the molecular level, that smoky off-flavours can arise not only from smoke contamination but also from overfermentation,” said food chemist Martin Steinhaus.

Krause said: “Another important finding of our study is that the quantitative spectra of the odour-active phenols differ between overfermented and wood-smoke-contaminated cocoa. These differences could help to identify the causes of smoky off-flavours more reliably in the future.”

The study not only provides new insights into the causes of off-flavours but also offers a practical approach to process control. At both study sites, the pH of the cocoa bean pulp in the fermentation mass increased significantly once the optimal fermentation duration was reached. A simple measurement using test strips could therefore complement the standard temperature, colour and odour checks, helping to detect the onset of overfermentation early on and prevent the development of off-flavours.

The food industry can now use these results of the study, which has been published in the Journal of Agricultural and Food Chemistry, to objectively assess the sensory quality of fermented cocoa based on odourant concentrations.

Future research will aim to clarify which microorganisms are involved in the formation of the identified compounds and at what concentrations these are actually perceived as off-flavours by consumers.

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