Modeling lactic acid fermentation of gowé, a sorghum-based fermented beverage

The Gowe is a Beninese traditional cooked paste pre pared from sorghum. It is sold as is and then diluted and consumed in as a sweet, slightly acidic drink. The traditional manufacture of Gowe includes the unit operations of malting, fermentation and cooking. A Gowé of high quality relies on a sufficient acidification during fermentation and th e hydrolysis of starch by the malt amylases which produces fermentable substrates and imparts a sweet taste. The kinetic of lactic fermentation thus depends on the malt amylasic activities that are in return inhibited by the acidification due to the production of lactic acid. In addition, the Gowe sh all present a good sanitary quality that will be promoted by a fast and sufficient acidification. Th e objective of this study is to propose a global model combining predictive microbiology and predictive biochemistry, for predicting and optimizing the preparation of a Gowe with high organoleptic an d sanitary qualities. Two strains of Lactobacilli ( Lb casei and Lb brevis ), known to have different potentialities for acidification were grown on liquid medium MRS. A pathogenic strain of Bacillus cereus was grown on nutrient broth to test the resistance to contamination of the gowe and thus ensure the sanitary quality. We modelled the growth of the strains at constant pH according to a logistic model that determines the growth rate and the duration of the lag phase. Secondly, a cardinal model was used to model the effect of temperature and pH on the gr owth. The kinetics of mortality of B. cereus with pH has also been modelled using reaction of order 1 . We then determined the speed of production of lactic acid, which increases linearly with the r ate of growth of lactic acid bacteria, and modeled the effect of lactic acid on the pH of the medium. The variation of α-amylase activity with temperature and pH was also modeled with empirical laws and the rate of the hydrolysis of starch and of the production of free sugars (mainly glucose and maltose) was modelled by enzymic kinetic with competitive inhibition by the products of reactions (sugar free). A global fermentation simulator was then built. The comparison of measured and predicted values showed that the model gave a fairly good prediction of the microbial populations (Lactobacilli and B. cereus ), but also the acidity and the rate of free sugars . This tool thus allowed proposing scenarios of fermentation to optimize both the organoleptic and sanitary qualities of gowe. Several optimized scenarios could be offered to economic operators, depending on their technological level. (Résumé d'auteur