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Content archived on 2024-05-29

Development of mixed starter cultures for the production of high quality cocoa in West Africa

Final Activity Report Summary - COCOSTARTER (Development of mixed starter cultures for the production of high quality cocoa in West Africa)

The main purpose of the COCOSTARTER project was to improve the quality of cocoa through the development of starter cultures to control fermentation.

Currently, cocoa beans in the cocoa producing countries in West Africa ferment spontaneously by the actions of contaminating microorganisms, without any apparent control of the types of the involved microflora. This has been observed to consequently result in variations in the quality of dried cocoa beans, which negatively influences the price and thus the economy of the cocoa producing countries.

During the incoming phase of the project a number of previously isolated yeasts, acetic acid bacteria and lactic acid bacteria were selected to formulate starter cultures. The purpose of the return phase was to test the developed starter cultures in the field. The viability of the cultures, which were preserved by freeze drying for over 13 months, was tested through culturing in selective media to observe the growth of the respective microbial types for yeasts, acetic acid bacteria and lactic acid bacteria. The culture was then used to develop an inoculum, using a sterilised diluted cocoa pulp as the growth medium. The inoculum was subsequently applied in cocoa fermentation trials at the Cocoa Research Institute in traditional cocoa fermentation protocols.

In a typical fermentation trial, cocoa beans were extracted from fresh pods obtained from farms of the Cocoa Research Institute or nearby individual farmers and sampled into two comparable sets of similar weights. One set was inoculated with the inoculum and the other with similar aliquots of the sterilised cocoa pulp medium without the starter culture, as a control treatment. In one of the setups, the amount of inoculum used was varied to investigate the effect of the inoculation rate on the process. Inoculation was performed using a Swiss mess knapsack sprayer and the control samples were always inoculated before the test samples. Typically, the inoculum was measured into the spray tank and sprayed uniformly on layers of the cocoa beans. The beans were then mixed by hand and covered as typically done in the traditional processes. Samples were taken during the course of fermentation to determine some fermentation characteristics. Samples were also processed into chocolate at the Cocoa Processing Company Ltd. in Tema, Ghana, for sensory evaluation.

The starter cultures were very viable after 13 months of storage. The viability had only fallen from about 1013 to about 1010 per gram for yeast and from 1010 to 107 for acetic acid bacteria. The obtained data showed that the addition of the inoculum with the starter culture significantly increased the yeast, acetic acid bacteria and lactic acid bacteria counts of the cocoa. There was a rapid increase in the counts of these species to maximum counts within 48 hours in the heap with the starter culture, as compared to the controls in which maximum counts occurred at 96 hours. The changes in pH of cocoa samples during the fermentations in both cocoa pulp and nibs were generally similar to reported patterns. However, the drop in nibs' pH was sharper and faster in the heaps with the starter culture, suggesting an enhanced effect due to the increased microbial content. Cut test results suggested that the starter culture significantly affected the progression of the fermentation. The amount of inoculum that was used also seemed to affect the fermentation. There were in general significantly fewer slaty beans after 48 hours of fermentation in the heaps with the starter-culture. The pattern of the cut-test results suggested that, with the starter-culture, more than 50 % of the beans became fermented by the 48th hour of fermentation. The liquor of cocoa samples fermented with the starter culture yielded significantly more cocoa fat, which had lower free fatty acid content compared to the controls. Sensory evaluation of the produced chocolate samples also suggested significant changes in the cocoa samples due to the starter culture, resulting in differences in the sensory attributes. The use of the starter-culture significantly suppressed off flavour development and generally improved the chocolate attributes.

In general, there seemed to be much correlation among the various observations, namely the observed physical appearance of the fermenting beans, the pattern of pH changes, the physical quality of the dried cocoa beans, the cut test results, the free fatty acid content of the liquor and the sensory attributes of the chocolate, all suggesting a positive enhancement of the fermentation by the starter culture. The observations could be linked to the rapid establishment of the technologically desirable yeast, acetic acid bacteria and lactic acid bacteria, which could prevent the rapid proliferation of moulds that would be responsible for the deep coloration of the beans during the fermentation and lipase production to cause fat degradation with the consequent higher free fatty acid content and the subsequent intense off flavour of the fermented beans.

In conclusion, it could be said that the application of the starter culture had the potential of accelerating the fermentation process and improving the flavour characteristics of chocolate.