Modelling fungal solid-state fermentation: the role of inactivation kinetics

Abstract

The theoretical mathematical models described in this paper are used to evaluate the effects of fungal biomass inactivation kinetics on a non-isothermal tray solid-state fermentation (SSF). The inactivation kinetics, derived from previously reported experiments done under isothermal conditions and using glucosamine content to represent the amount of biomass, are described in different ways leading to four models. The model predictions show only signi®cant effects of inactivation kinetics on temperature and biomass patterns in the tray SSF after long fermentation periods.The models in which inactivation is triggered by low speci®c growth rates can predict restricted biomass evolution in combination with a fast temperature increase followed by a slower temperature decrease. Such inactivation might occur when substrate is limiting or products are formed in toxic concentrations.Temperature is predicted to be the key parameter. Oxygen concentration is predicted to become limiting only at high heat conduction and low oxygen diffusion rates. Desiccation of the substrate is predicted not to occur. IntroductionSolid-state fermentations (SSF) are applied for the production of, for example, enzymes, biopesticides, food, feed and ®ne chemicals. In SSF, microorganisms grow on a moist solid substrate without free-¯owing water. The chemical reactions necessary for growth are exothermic and thus temperature control is necessary. It depends strongly on the type of fermenter system used (tray, packed bed, rotating drum) what the effect of heat production is on temperature in the fermenter bed and thus on the results of the fermentation. A typical temperature course during fermentation in a tray system shows a steep increase to 40±45 C after a short lag phase, followed by a decline at a rate of 0.1 [5]±0.5 C [6] per hour, depending on the microorganism and type and size of system used.Simulation models can provide insight into the complex interaction between microbial growth, oxygen consumption and concomitant heat production, and oxygen and heat transport. Ideally, these models should take into account the effects of growth, maintenance and decay on respiration activity, each as functions of, for instance, time, temperature, nutrient and water availability.Several examples of models for fungal SSF processes can be found in the literature, describing different fermentation systems [7±12]. These reports focus on the biomass production phase of the fermentation and do not properly consider the period after growth has virtually stopped. Especially for secondary metabolite production, the latter period may be very important. Besides, in most SSF systems fungal biomass is present growing at different rates, depending on combined effects of several parameters, such as temperature, water activity, and oxygen and carbon dioxide concentration. Thus, all biomass is not necessarily in the same growth phase. This emphasizes the need for considering description of the post-growth stage in modelling SSF.In our models we describe the respiration ...

Recent developments in modeling of solid-state fermentation: heat and mass transfer in bioreactors

“…The later model of Rajagopalan and Modak [6] explicitly describes the various steps in O 2 transfer to and within a biofilm growing at the particle surface and therefore is more mechanistic, providing the basis for an investigation into whether diffusion of O 2 within the pores, transfer from the pores to the biofilm or diffusion within the biofilm is the limiting step in O 2 transfer. However, if there is good empirical data on which to express O 2 consumption rates, then the approach of Smits et al [7] might be preferable due to its simplicity.…”

“…The more recent models of Rajagopalan and Modak [5,6] and Smits et al [7] consider both mass and energy balances and do not make the pseudo-steady-state assumption. In these two models, the systems are visualized slightly differently (Fig.…”

“…2. Comparison of the manners in which the tray models of (A) Rajagopalan and Modak [5,6] and (B) Smits et al [7] treat the tray system.…”

“…To date only the model of Smits et al [7] describes the water balance in the substrate bed within the tray. It is described by the following equation:…”

“…However, due to the different ways in which the system was visualized, the equations appear slightly differently. The equation used by Smits et al [7] to describe the O 2 balance within the bed is relatively simple: being an effective diffusivity that accounts for the fact that the pores represent only part of the overall volume of the bed and provide a tortuous path for diffusion.…”