| Extensive empirical research on the fermentation of vegetables has been published, but with little concern for predictive modeling. The main objectives of this work were to develop a mathematical model of cucumber fermentation using a mixed culture of lactic acid bacteria and yeast, and to apply the model for optimization and control of the process. To pursue this goal, effects of key variables involved in cucumber fermentation were defined and models for predicting growth, substrate depletion and product formation by Lactobacillus plantarum and Saccharomyces rosei were developed in liquid medium. Variables studied included lactic acid, acetic acid, sodium chloride, malic acid, hexose, and hydrogen ion concentrations.; Media used included one containing yeast extract, trypticase, ammonium sulfate, and minerals (YTA); 50% cucumber juice (CJ); and brained, blanched whole cucumbers. Growth and fermentation studies were conducted at 30{dollar}spcirc{dollar}C in batch and continuous culture under a nitrogen atmosphere. Growth was determined spectrophotometrically, by dry weight or by plate count enumeration in MRS agar. Substrate and product concentrations were determined by HPLC. The dynamic model system was solved using the Fehlberg Fourth-fifth order Runge-Kutta method.; Liquid phase studies indicated that limiting conditions for growth of L. plantarum in CJ were pH 3.37 (lower limit), 69 mM undissociated lactic acid, 150 mM undissociated acetic acid, or 11.8% NaCl. Growth was stimulated by addition of acetic acid up to 40 mM, but was inhibited thereafter. Growth-associated and non-growth-associated coefficients for lactic acid production during CJ fermentation were 51.9 mmoles/g cells and 7.2 mmols/g cells.hr, respectively. Models for growth, substrate utilization, and acid production in batch culture in CJ were tested and shown to reasonably predict the observed data. The influence of malic acid (normally present in cucumbers) on fermentation kinetics was studied in YTA, with or without added malic acid. Malic acid reduced the maintenance coefficient and increased the biomass yield in continuous culture, based on calculations from the Ludeking and Peret model. The biomass yield coefficient was 27.4 and 38.5 mg cells/mmoles hexose in the absence and the presence of 12 mM malic acid, respectively.; Also in liquid phase studies S. rosei was shown to be more acid resistant than was L. plantarum. Extrapolation of the model suggested that limiting conditions for growth of S. rosei in CJ were pH 2.6 (lower limit), 172 mM undissociated lactic acid, 219 mM undissociated acetic acid, or 16.9% NaCl.; To use such a model in whole cucumber fermentation, a dynamic model to predict mass transfer in/out of the cucumber was constructed. |
