When you think about culturing bacteria or fungi in large quantities, you likely envision flasks shaking or maybe bioreactors filled to the brim with liquid media. But did you know that many bacteria and fungi can grow on solid carriers without being submerged in liquid? Enter solid state fermentation (SSF). In this article, I’ll introduce the concept of SSF, common real-world processes that use it, and some of the considerations that go into successfully growing microbes on solid substrates.
What is Solid State Fermentation?
SSF is defined as a fermentation process on a solid support or matrix where there is little free water. The ‘carrier’ can be several nutrient-rich things like rice, wheat bran, oats, or soil. The solid substrate is somewhat moist (moisture content ~15-25%) and nutrient-rich to support cellular growth. For many microbes, especially those that are isolated from soils and ‘dry’ surfaces, SSF better simulates a natural habitat than liquid cultures. Many bacteria, yeast, and fungi can all be grown using SSF for a variety of applications (more on this later!).
SSF can be compared to submerged fermentation, or the more traditional route of growing microbes in a liquid media. Of special interest to biological engineers working with microbes at a large scale, SSF processes take a great deal less energy input. In addition, yields of target products like secondary metabolites or enzymes are often higher in SSF compared to submerged fermentations.
The SSF Workflow and Important Considerations
As with liquid cultures, solid state fermentations require very careful sterile technique. For the inoculation of the solid carrier, a relatively small volume of liquid ‘starter’ culture is added to the solid material (or ‘carrier’) followed by extensive mixing. The carrier is then incubated in environmental conditions with controlled temperature and humidity until it is ready for harvesting!
Though this process sounds simple, there are actually a plethora of variables to think about when developing a successful SSF process. The rate of inoculation, moisture content of the carrier, growth conditions, and length of fermentation are all key parameters that influence final titer and product formation. The choice of solid substrate is also a huge factor that can make or break SSF. Not only is the nutrient profile of the carrier clearly important for growth, physical parameters like porosity and water absorptivity must be considered. This list of variables only scratches the surface of what goes into SSF and doesn’t even include factors that influence how easily cells can be harvested!
Because of the complexity involved in designing and optimizing SSF, it is not often used at the laboratory scale. However, it can be an efficient way of generating products at the industrial level!
How Can SSF be Used on a Large Scale?
Food: Likely the most ubiquitous SSF process you are familiar with is mushroom cultivation. Mushrooms are grown by mixing solid substrates like compost with fungal “spawn”, or mycelium, which grows and spreads throughout the medium before farmers induce fruit production by temperature and water shocking the mycelium. The result: delicious fungi you can get at your local grocery store! In addition, Aspergillus fungi are produced by SSF for various fermented foods and sake in Japan (the Koji process).
Antibiotics and antifungals: The literature abounds with examples of common secondary metabolites that can be produced using SSF technology. These include compounds like penicillin, various tetracyclines, and cyclosporins, to name just a few. While antibiotics are typically produced in liquid media on the industrial scale, we may see future antibiotics and natural products produced by SSF since product yields tend to be higher in SSF compared to liquid fermentations. However, a major challenge of producing bioactive compounds like antibiotics using SSF is that the resulting product must be extracted from solid media, which can also result in several impurities in the final extract.
Waste treatment: SSF of fungal species is commonly used to degrade biomass that accumulates in industries, including forestry, agriculture, and pulp and paper. Several species of fungi produce enzymes like amylases, cellulases, peroxidases, and more in response to being cultivated on waste products containing lignocelluloses. Without SSF, these industries would be much less sustainable.
Enzymes: Production of enzymes is at the heart of industrial biotechnology as demonstrated by gradual yet continual growth of the enzyme market since the 1970s. As with other bioactive molecules and natural products, SSF often requires less energy and results in higher yields than in submerged fermentations.
Why Use SSF?
Though not common in academic labs, solid state fermentation is a promising technology due to its cost effectiveness and high yield. Substrates for SSF are often abundant and widely available, and the reduction in water compared to submerged fermentations makes it an appealing choice for sustainable production of various products. Current limitations of SSF are largely based on the engineering required to optimize and scale up SSF processes, but SSF may become the preferred technique for many biotech industries in the future.
Have you ever used solid state fermentation? Tell us about it below!
- Pandey, Ashok & Selvakumar, P & Soccol, Carlos & Nigam, Poonam. Solid State Fermentation for the Production of Industrial Enzymes. Current Science. 1999; 7: 149-162.
- Chundakkadu Krishna. Solid-State Fermentation Systems—An Overview. Critical Reviews in Biotechnology, 2005, 25:1-2, 1-30.
- Yazid, Noraziah Abu et al. Solid-State Fermentation as a Novel Paradigm for Organic Waste Valorization: A Review. Sustainability 2017, 9(2), 224.
- Couto, Susana Rodriguez et al. Application of solid-state fermentation to the food industry: A review. Journal of Food Engineering. 2006, 76, 291-302.
- Soccol, Carlos et al. Recent developments and innovations in solid state fermentation. Biotechnology Research and Innovation 2017, 1(1): 52-71.