Heat transfer in packed-beds of agricultural waste with low rates of air flow applicable to solid-state fermentation

A Critical Evaluation of Recent Studies on Packed-Bed Bioreactors for Solid-State Fermentation

Packed-bed bioreactors are often used for aerobic solid-state fermentation, since the forced aeration supplies O2 and removes metabolic heat from the bed. Motivated by the potential for applications in biorefineries, we review studies conducted on packed-bed bioreactors over the last decade, evaluating the insights these studies provide into how large-scale packed beds should be designed and operated. Many studies have used low superficial air velocities and suffer from preferential airflow, such that parts of the bed are not properly aerated. Moreover, some studies have proposed ineffective strategies, such as reversing the direction of the airflow or introducing air through perforated pipes within the bed. Additionally, many studies have used narrow water-jacketed packed-bed bioreactors, but these bioreactors do not reflect heat removal in wide large-scale packed beds, in which heat removal through the side walls makes a minor contribution. Finally, we conclude that, although some attention has been given to characterizing the porosities, water sorption isotherms and volumetric heat and mass transfer coefficients of substrate beds, this work needs to be extended to cover a wider range of solid substrates, and work needs to be done to characterize how these bed properties change due to microbial growth.

A novel method to assess heat transfer and impact of relevant physicochemical parameters for the scaling up of solid state fermentation systems

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New method to study heat transfer in SSF under forced aeration.

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Granulometry and Carr index have the greatest impact on heat transfer.

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Substrates with low CI and high GR facilitate the scaling up of a SSF process.

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Bulk density has poor effectiveness in heat removal.

Heat production during fermentation is undesirable. It can affect the growth of biomass, sporulation, products formation and the scaling-up. Physico-chemical properties of substrates influence heat and mass transfer in Solid State Fermentation. Heat is chemically produced into substrates without micro-organism to allow better reproducibility. A 2^4–1 fractional factorial design is chosen to study the influence of four physicochemical parameters on heat transfer: Granulometry, Bulk Density, Carr Index (compressibility index) and Water Absorption Capacity. Results show that the two main physicochemical parameters which influence heat transfer are Granulometry and Carr Index. High Granulometry and low Carr Index have influence on maximum temperature reached during the test, warm-up speed and cooling speed. These two parameters allow efficient air flow through the substrate bed with large interparticle spaces enhancing exchange surface between air and particles. A substrate with these characteristics facilitates heat transfers in forced-aerated reactors.

Current developments in the resistance, quality, and production of entomopathogenic fungi

There is a worldwide concern to achieve food security with a sustainable approach, including the generation and implementation of techniques for the production of high-quality chemical-free crops. This food revolution has promoted the development and consolidation of programmes for integrated pest management. Some of those programmes include the use of diverse organisms (biological control agents) to suppress populations of pests potentially harmful to the crops. Among these biological control agents are entomopathogenic fungi that are highly effective in suppressing a diversity of insects and have, therefore, been produced and marketed throughout the world. However, the bottleneck for applying entomopathogenic fungi is the production of propagules (blastospores and conidia) with resistance to environment conditions and abiotic factors, maintaining high quality in terms of virulence. Therefore, this manuscript presents recent studies related to increasing resistance and quality using different bioreactors to produce conidia. The above presents a global panorama related to current developments that contribute to improving the resistance, quality, and production of entomopathogenic fungal propagules.

Bioconversion of agricultural waste into poly-γ-glutamic acid in solid-state bioreactors at different scales

With the purpose of developing a novel approach of agricultural waste treatment and overcoming bottlenecks for upscaling solid-state fermentation processes, the type of aerated, continuously stirred solid-state bioreactors were used for the production of γ-PGA by Bacillus amyloliquefaciens JX-6. Using corn stalk and soybean meal, the most common agricultural waste in China, as solid substrates, the maximum production of γ-PGA was 116.88 ± 5.05 g/kg and 102.48 ± 3.30 g/kg in 50 L and 150 L bioreactors, respectively. Production of γ-PGA in 50 L bioreactor was higher than in 150 L bioreactor, demonstrating that a reduction in γ-PGA production occurred as the fermentation system enlarged. An analysis of the interactions among fermentation parameters (temperature, moisture, and pH), γ-PGA production, solid substrates and bacterial communities indicated that different bioreactor capacities caused changes in fermentation parameters and bacterial communities, which in turn affected substrate utilization and γ-PGA production. Overall, obtaining considerable amounts of γ-PGA under non-sterilized fermentation expressed that JX-6 has excellent abilities to adapt to these substrates and conditions. Bioconversion of agricultural waste into γ-PGA in scale-up fermentation was successfully conducted by creating a more stable and suitable fermentation environment in bioreactors.