A multiproduct kinetic model for non-growing Clostridium thermocellum JW20 in constant fed-batch culture

摘要

No kinetic models exist to quantitatively describe the complex fermentation activities of non-growing C. thermocellum cells, even though it has been shown that they differ in metabolism (i.e. product shifts, yield, and selective cellular inactivation) from growing Clostridium thermocellum cells. The main objective of this study was to develop a comprehensive macrokinetic model that mathematically describes the formation kinetics of ethanol, acetate, lactate, glucose, and the consumption of cellobiose in constant fed-batch fermentation. Experimental data were collected from ceils harvested late in the exponential growth period. These cells were maintained in batch culture without yeast extract to prevent further growth. An intensive input variablecalled the specific cellobiose feeding rate (q_f, mmole cellobiosecentre dot g dry cells~(-1) centre dot h~(-1)) was used to quantify input process variables. The intracellular metabolic state of the cells was quantified through an intensive pseudo-variable that served to estimate the specific lumped intracellular materials (SLIM, mmole cellobiose equivalencecentre dot g dry cells~(-1)). This pseudo-variable represented the specific amount of cellobiose taken in by the cells but not converted into the measured end-products. These two intensive variables simultaneously controlled the specific production rates of ethanol, acetate, and lactate and the cellular uptake rate of cellobiose. On the other hand, the extracellular production of glucose was indirectly affected by them. The production of ethanol was favored over the other end-products when the values of q_ f was equal to or less than 0.39 mmole cellobiosecentre dot g dry cells~(-1) centre dot h~(-1). The production of lactate drastically decreasedwhen SLIM was lower than approx 3.12 mmole cellobiose equivaience ceutre dot g dry cells~(-1). The cells rapidly lost their ability to form end-products when the values of SLIM was equal or greater than approx 10 mm mole-g dry cells~(-1). Several constant fed-batch fermentations were simulated by the model. The overall model predictions were satisfactory. There were, however, some deviations between the model predictions and the experimental data. These deviations could be caused by small deviations inthe process variables and conditions. The main objective of this study was to develop a comprehensive macrokinetic fermentation model for the constant fed-batch cultivation of C. thermocellum on cellobiose. The formation of the major products ethanol, acetate, lactate, and glucose were described over the course of the fermentation. The consumption of ceilobiose was also predicted. The model performance was evaluated against both the calibration data set and an independent data set. Improvements to themodel and the effective use of non-growing C. thermocellum cells were addressed.