| dc.description.abstract | Solid-state fermentation (SSF) processes have long been employed in the production and preservation of a variety of foods, and in the production of enzymes, organic acids, antibiotics and other microbial products. They are fermentation processes that take place due to the action of microorganisms on solid or semisolid substrates. The substrate is a nutritionally inert solid support, whose presence is advantageous to the microorganism with respect to access to nutrients.
Certain SSF products are used as foods, and the texture developed during fermentation is an important attribute of the product. A static tray fermenter is one of the types of fermenters used in producing food products of desired quality. Major problems in SSF fermenters in general, and static bed fermenters in particular, are heat build-up and oxygen diffusional limitations. A detailed knowledge of the interrelationship of heat and mass transfer is essential for the rational design and control of SSF processes. In the present investigation, experiments have been conducted to understand fungal growth on solid substrates, and four models have been proposed to describe heat and mass transport and cell growth.
Experiments were conducted in a Perspex tray fermenter to understand fungal growth on wheat bran as the solid substrate. The measured temperature profiles have been compared qualitatively with model predictions.
The pseudo-homogeneous model considers a stationary biofilm on individual substrate particles. A pseudo?steady state is assumed between oxygen transfer onto the film surface and subsequent consumption by cells. The resulting partial differential equations were solved using orthogonal collocation. Model predictions have been compared with available literature data. The effect of varying several operational parameters on fermenter performance has been studied.
The heterogeneous model considers an expanding biofilm on each substrate particle, which reduces porosity and increases oxygen diffusional limitation. The resulting moving boundary was immobilised using a suitable transformation, and the pertinent differential equations were solved. The theoretical prediction on the maximum possible utilisation of the available space for cell growth has been incorporated, and the effect of variation of intrinsic parameters has been studied.
Finally, glucose limitation on cell growth has been investigated by proposing a model for a single-particle system, following earlier work in the literature. The conclusion drawn from the single-particle model is that glucose acts as a growth-limiting substrate. To check the validity of this conclusion in the actual fermenter, a unified model has been proposed, combining the characteristic features of the heterogeneous model and the single-particle substrate model. It has been found from this model that oxygen is more severely growth-limiting than glucose, and in the presence of oxygen limitation, glucose does not limit the cell growth process.
It is hoped that the mathematical models developed will help in a better understanding of the transport processes involved, and thus improve the design of the fermenter and the control of the fermentation process. | |
