Operational parameters for packed beds in solid-state cultivation

The influence of process parameters in production of lipopeptide iturin A using aerated packed bed bioreactors in solid-state fermentation

The strain Bacillus iso 1 co-produces the lipopeptide iturin A and biopolymer poly-γ-glutamic acid (γ-PGA) in solid-state fermentation of substrate consisting of soybean meal, wheat bran with rice husks as an inert support. The effects of pressure drop, oxygen consumption, medium permeability and temperature profile were studied in an aerated packed bed bioreactor to produce iturin A, diameter of which was 50 mm and bed height 300 mm. The highest concentrations of iturin A and γ-PGA were 5.58 and 3.58 g/kg-dry substrate, respectively, at 0.4 L/min after 96 h of fermentation. The low oxygen uptake rates, being 23.34 and 22.56 mg O2/kg-dry solid substrate for each air flow rate tested generated 5.75 W/kg-dry substrate that increased the fermentation temperature at 3.7 °C. The highest pressure drop was 561 Pa/m at 0.8 L/min in 24 h. This is the highest concentration of iturin A produced to date in an aerated packed bed bioreactor in solid-state fermentation. The results can be useful to design strategies to scale-up process of iturin A in aerated packed bed bioreactors. Low concentration of γ-PGA affected seriously pressure drop, decreasing the viability of the process due to generation of huge pressure gradients with volumetric air flow rates. Also, the low oxygenation favored the iturin A production due to the reduction of free void by γ-PGA production, and finally, the low oxygen consumption generated low metabolic heat. The results show that it must control the pressure gradients to scale-up the process of iturin A production.

Overview of solid state bioprocessing

Solid-state fermentation has centuries of history, but it is only in the last two decades that there has been a concerted effort to understand the bioprocessing issues involved and to apply them to a wide range of new products. This article provides an overview of the knowledge of solid-state bioprocessing that has been gained over this time. It shows that, although significant advances have been achieved in understanding of what controls process performance, much research is still required.

A two-phase model for water and heat transfer within an intermittently-mixed solid-state fermentation bioreactor with forced aeration

A two-phase dynamic model is developed that describes heat and mass transfer in intermittently-mixed solid-state fermentation bioreactors. The model predicts that in the regions of the bed near the air inlet there can be significant differences in the air and solid temperatures, while in the remainder of the bed the gas and solid phases are much closer to equilibrium, although there can be differences in water activity of around 0.05. The increase in the temperature of the gas as it flows through the bed means that it is impossible to prevent the bed from drying out, even if saturated air is used at the air inlet. The substrate can dry to water activities that severely limit growth, unless the bed is intermittently mixed, with the addition of water to bring the water activity back to the desired value. Under the conditions assumed for the simulation, which was designed to mimic the growth of Aspergillus niger on corn, two mixing events were necessary, one at 17.4 and the other at 27.9 h. Even though such a strategy can minimize the restriction of growth by water-limitation, temperature-limitation remains a problem due to the rapid heating dynamics. The model is obviously a useful tool that can be used to guide scale-up and to test control strategies. Such a model, describing the non-equilibrium situation between the gas and solid phases, has not previously been proposed for solid-state fermentation bioreactors. Models in the literature that assume gas-solid temperature and moisture equilibrium cannot describe the large temperature differences between the gas and solid phase which occur within the bed near the air inlet.

Biochemical engineering aspects of solid state bioprocessing

Despite centuries of use and renewed interest over the last 20 years in solid-state fermentation (SSF) technology, and despite its good potential for a range of products, there are currently relatively few large-scale commercial applications. This situation can be attributed to the complexity of the system: Macroscale and microscale heat and mass transfer limitations are intrinsic to the system, and it is only over the last decade or so that we have begun to understand them. This review presents the current state of understanding of biochemical engineering aspects of SSF processing, including not only the fermentation itself, but also the auxiliary steps of substrate and inoculum preparation and downstream processing and waste disposal. The fermentation step has received most research attention. Significant advances have been made over the last decade in understanding how the performance of SSF bioreactors can be controlled either by the intraparticle processes of enzyme and oxygen diffusion or by the macroscale heat transfer processes of conduction, convection, and evaporation. Mathematical modeling has played an important role in suggesting how SSF bioreactors should be designed and operated. However, these models have been developed on the basis of laboratory-scale data and there is an urgent need to test these models with data obtained in large-scale bioreactors.

Special transformation processes using fungal spores and immobilized cells

Although many microbial processes have been described which are able to produce interesting aroma compounds, the number of industrial applications are limited. Reasons for this are in most cases low final product yield, low biotransformation rates, substrates and/or end-products inhibition, toxicity towards the microorganisms themselves and difficulties of recovery from the bioreaction mixture. This means that the development of specific catalysts and processes is an important challenge for researchers in this field. This review presents two special kinds of catalysts, fungal spores and immobilized cells, with emphasis on their production and on their use in the production of aroma compounds. The production of fungal spores by solid state fermentation is described in greater detail. In the second part, this review also offers examples of development of three production processes, the production of methyl ketones of spores of Penicillium roquefortii, the hydroxylation of beta-ionone by immobilized Aspergillus niger cells, and the production of alkyl pyrazines by bacteria in liquid and solid media. For each of these processes, the analysis of limiting steps-biological and/or physico-chemical-is presented and the significant role of process conditions to increase aroma yield is discussed.