A mathematical model was developed and tested to simulate the generation and transfer of heat in solid substrate fermentation (SSF). The experimental studies were realized in a 1-L static bioreactor packed with cassava wet meal and inoculated with Aspergillus niger. A simplified pseudohomogeneous monodimensional dynamic model was used for the energy balance. Kinetic equations taking into account biomass formation (logistic), sugar consumption (with maintenance), and carbon dioxide formation were used. Model verification was achieved by comparison of calculated and experimental temperatures. Heat transfer was evaluated by the estimation of Biot and Peclet heat dimensionless numbers 5-10 and 2550-2750, respectively. It was shown that conduction through the fermentation fixed bed was the main heat transfer resistance. This model intends to reach a better understanding of transport phenomena in SSF, a fact which could be used to evaluate various alternatives for temperature control of SSF, i.e., changing air flow rates and increasing water content. Dimensionless numbers could be used as scale-up criteria of large fermentors, since in those ratios are described the operating conditions, geometry, and size of the bioreactor. It could lead to improved solid reactor systems. The model can be used as a basis for automatic control of SSF for the production of valuable metabolites in static fermentors.

Differential scanning calorimetry (DSC), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) were used to determine morphological, structural and surface changes (biodegradation) on thermo-oxidized (80°C, 15 days) low-density polyethylene (TO-LDPE) incubated with Aspergillus niger and Penicillium pinophilum fungi, with and without ethanol as cosubstrate for 31 months. TO-LDPE mineralization by fungi was also evaluated. Significantly morphological and structural final changes on biologically treated TO-LDPE samples were observed. Decreases to three units on crystallinity and crystalline lamellar thickness (0.4 -1.8 Å), and increases in small-crystals content (up to 3.2%) and mean crystallite size (8.4 -14 Å) were registered. An oxidation decrease (almost twice) on samples without ethanol with respect to the control was observed, while in those with ethanol it was increased (up to 2.5 times). Double bond index increased more than twice from 21 to 31 months. The higher TO-LDPE changes and fungi-LDPE interaction was observed in samples with ethanol, suggesting that ethanol favors the TO-LDPE biodegradation, at least in case of P. pinophilum, probably by means of a cometabolic process. Mineralization of 0.50 % and 0.57 % for A. niger, and of 0.64 % and 0.37 % for P. pinophilum were obtained, for samples with and without ethanol, respectively. A model to explain morphological and structural changes on biologically treated TO-LDPE is also proposed.

A study was made to compare the production of pectinase by Aspergillus niger CH4 in solid-state (SSF) and submerged (SmF) fermentations. Production of endo-(endo-p) and exo-pectinase (exo-p) by SSF was not reduced when glucose, sucrose or galacturonic acid (up to 10%0) were added to a culture medium containing pectin. Moreover, both activities increased when concentrations of the carbon sources were also increased. In SmF, these activities were strongly decreased when glucose or sucrose (3%) was added to culture medium containing pectin. The addition of galacturonic acid affected endo-p activity production to a lesser extend than exo-p. Final endo-p and exo-p activities in SSF were three and 11 times higher, respectively, than those obtained in SmF. The overall productivities of SSF were 18.8 and 4.9 times higher for endo-p and exo-p, respectively, than those in SmF. These results indicate that regulatory phenomena, such as induction-repression or activation-inhibition, related to pectinase synthesis by A. niger CH4 are different in the two types of fermentation.

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BACKGROUND: Heat removal is one of the major constraints in large-scale solid-state fermentation (SSF) processes. The effect of internal air circulation by forced convection on heat and water transfer has not been studied in SSF tray bioreactors. Formulation of a mathematical model for SSF requires a good estimation of the mass and heat transfer coefficients.

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