Solid State Fermentation – Part 1: Processes and Application

Abstract

Solid State Fermentation (SSF) has emerged as a prominent method for producing value-added products such as enzymes, biofuels, organic acids, and more. Utilising agro-industrial wastes as substrates, SSF offers several environmental and economic benefits.

This review provides an in-depth analysis of SSF, highlighting its advantages over Submerged Liquid Fermentation (SLF), the types of substrates used, the key microorganisms involved, and the applications of SSF in various sectors. Additionally, the expertise of NIRAS in designing process equipment and optimising fermentation processes is emphasised.

Introduction

Solid State Fermentation (SSF) is a process involving the cultivation of microorganisms on solid substrates without free-flowing water. Historically, SSF has been employed for traditional fermentation processes such as koji and tempeh production. However, modern SSF has expanded its applications, significantly impacting industrial biotechnology, environmental management, and agro-industrial waste utilisation.

NIRAS, with its extensive experience in fermentation technology, has proven to be an invaluable partner in optimising SSF processes and designing state-of-the-art equipment.

Process descriptions

Process steps in SSF:

1. Substrate preparation:
• Agro-industrial residues such as wheat bran, rice husks, and fruit peels are pre-treated to enhance their accessibility for microbial degradation. This may involve grinding, steaming, or chemical treatments to break down complex structures and increase surface area.
2. Inoculation:
• Selected microbial strains, including fungi, bacteria, or yeasts, are inoculated into the prepared substrate. The choice of microorganism depends on the desired product and the nature of the substrate.
3. Incubation:
• The inoculated substrate is incubated under controlled environmental conditions, including temperature, humidity, and aeration, to promote microbial growth and product formation. This step is critical for achieving high yields and quality.
4. Harvesting:
• After the fermentation period, the fermented product is harvested. This may involve drying, milling, or solvent extraction, depending on the nature of the product.
5. Product recovery:
• The final product is purified and concentrated through various downstream processes, including filtration, centrifugation, and chromatography. The recovery process aims to maximise yield and ensure product purity. 

Case studies:

• Enzyme production:
• Amylase production: Wheat bran is used as a substrate for the production of amylase by Aspergillus niger. The process involves substrate preparation, inoculation with fungal spores, and incubation at 30°C with controlled humidity. The harvested product is purified to obtain high-quality amylase.
• Biofuel production:
• Bioethanol from sugarcane bagasse: Sugarcane bagasse undergoes pre-treatment with dilute acid to hydrolyse lignocellulosic components. The pre-treated substrate is then inoculated with Saccharomyces cerevisiae and incubated under anaerobic conditions to produce bioethanol. The ethanol is distilled and purified for use as a renewable fuel.

Advantages of SSF

• Environmental and economic benefits:

• Lower water activity in SSF minimises contamination risks, thus reducing the need for stringent aseptic conditions.
• SSF processes generate less wastewater compared to SLF, making them more environmentally friendly.
• Utilising agro-industrial wastes as substrates helps in waste management and adds value to otherwise low-value by-products​​​​.

• High product yields:

• SSF often results in higher yields of desired products due to the closer simulation of natural habitats for many microorganisms, particularly fungi.
• The concentrated nature of substrates in SSF simplifies downstream processing and product extraction​​​​.

• Lower energy requirements:

• SSF typically requires less energy for agitation and aeration, leading to reduced operational costs.
• The solid nature of the substrates in SSF eliminates the need for extensive mechanical agitation and sterilisation​​​​.

References

1. Mienda, B. S., Ahmad, I., & Umar, A. (2011). Microbiological Features of Solid State Fermentation and its Applications - An overview. Research in Biotechnology, 2(6), 21-26.
2. Pandey, A. (2003). Solid-state fermentation. Biochemical Engineering Journal, 13(1), 81-84.
3. Sadh, P. K., Duhan, S., & Duhan, J. S. (2018). Agro-industrial wastes and their utilization using solid state fermentation: a review. Bioresour. Bioprocess., 5(1).
4. Hongzhang, C. (2013). Modern Solid State Fermentation - Theory and Practice. Springer.
5. Recent developments and innovations in solid state fermentation. (2017).
6. Mattedi, A., Sabbi, E., Farda, B., Djebaili, R., Mitra, D., Ercole, C., Cacchio, P., Del Gallo, M., & Pellegrini, M. (2023). Solid-State Fermentation: Applications and Future Perspectives for Biostimulant and Biopesticides Production. Microorganisms, 11, 1408.
7. Solid-State Fermentation as a novel paradigm for organic waste valorization: A review. (2021).
8. Newly designed multi-stacked circular tray solid-state bioreactor analysis of a distributed parameter gas balance during solid-state fermentation with influence of variable initial moisture. (2021).

NIRAS, with its extensive experience in SSF, is a leading collaboration partner in designing process equipment and optimising fermentation processes, ensuring high efficiency and sustainability in industrial applications.