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 ...
The effect of organic solvents on the equilibrium position of lipase-catalyzed esterification of glycerol and decanoic acid has been investigated. The reaction is carried out in an aqueous-organic two-phase system. In polar solvents, high mole fractions of monoacylglycerol and low mole fractions of triacylglycerol and measured, while in nonpolar solvents, the measured differences in the mole fractions of monodi-, and triacylglycerols are less. There is a good correlation between the ester mole fractions at equilibrium and the log P of the solvent (partition coefficient in n-octanolwater), however, only if the group of tertiary alcohols is excluded. In the plot of the easter mole fractions as a function of the logarithm of hte solubility of water in the organic solvent, the tertiary alcohols can be included; however, in this case other deviations appear.For the prediction of the effect of organic solvents on the ester mole fractions at reaction equilibrium in nondilute reaction systems with a water activity below 1, the program TREP (Two-phase Reaction Equilibrium Prediction) is developed, which is based on the UNIFAC group contribution method. With this model the equilibrium data are essentially predicted from basic thermodynamic data. The required equilibrium constants are estimated from experiments without an organic solvent in the reaction medium. The mole fractions calculated by TREP show the same trends as the experimentally measured mole fractions; however, some variation is observed in the absolute values. These deviations may be due to inaccuracies in the UNIFAC group contribution method. TREP is found to be a correct method to predict within some limits the ester mole fractions at equilibrium for all mixtures of solvents, substrates, and products. The production of monoester can be enhanced in reaction system with a sufficient high concentration of a polar solvent. In experiments with a triglymeto-decanoic acid ratio of 5, almost no di-and triesters can be detected at equilibrium.
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