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Nestlé-backed study unlocks cocoa fermentation acetate mystery

By Nicola Cottam , 13-Jun-2014

Nestlé studies explores best starter cultures for flavor during cocoa fermentation. Photo credit: Sustainable Cocoa Initiative
Nestlé studies explores best starter cultures for flavor during cocoa fermentation. Photo credit: Sustainable Cocoa Initiative

Exploring the mechanisms controlling acetate formation during cocoa fermentation may help identify the best starter cultures to optimize flavor attributes, according to a team of scientists supported by Nestlé.

"Our studies have unravelled the metabolism of the rather unexplored acetic acid bacteria in the complex fermentation environment," said one of the researchers Christoph Wittmann, of Saarland University, Saarbruecken, Germany.

“This discovery reveals a fine-tuned collaboration of a multi-species consortium during cocoa fermentation - and that may help improve selection of natural strains for better-balanced starter culture.”

Acetic acid bacteria and flavor

Acetic acid bacteria (AAB) play a key role in cocoa fermentation and the development of desired flavors. During fermentation a combination of yeast and lactic acid bacteria (LAB) turn cocoa pulp sugars into ethanol and this in turn is converted into lactate and acetic acid.

The acid penetrates the cocoa husk and sets off a chain of biochemical reactions, which is the precursor to flavor development.

Previous studies have analysed AAB’s contribution to cocoa fermentation but there is scant knowledge concerning its metabolic pathway affecting acetate production, the authors explained.

Balance of micro-organisims

Nestlé researchers in Switzerland investigated the metabolism of AAB under simulated conditions, using two AAB strains - acetobacter pasteurianus and a. ghanensis - selected as representative strains of two major AAB species naturally occurring in cocoa pulp.

They observed that acetate was almost exclusively produced by ethanol (96-98%), while lactate manufactured acetoin (carbon molecule for storing energy) and biomass building blocks. However in order to maintain optimal AAB performance there needs to be a balanced ratio of lactate and ethanol, they noted.

“This indicated a surprising separation of metabolism. Both strains exhibited two functional pathway modules, related to the utilization of lactate and ethanol with only weak exchange fluxes.”

Results also revealed that the efficiency of AAB was highly dependent on the compositional traits of LAB and yeast at the initial fermentation stage.

“As lactate and ethanol are individually supplied by lactic acid bacteria and yeasts, during the initial phase of cocoa fermentation, respectively, this underlines the importance of a well-balanced microbial consortium for a successful fermentation process.”

Source: Applied and Environmental Microbiology
Doi: 10.1128/AEM.01048-14
‘The key to acetate: Metabolic fluxes of acetic acid bacteria under cocoa pulp fermentation simulating conditions’
Authors: Philipp Adler, Lasse Jannis Frey, Antje Berger, Christoph Josef Bolten, Carl Erik Hansen, Christoph Wittmann

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