Dhanus Micro Notes: Solid-substrate Fermentation or Solid State Fermentation

Solid-substrate Fermentation or Solid State Fermentation

Solid substrate fermentation or SSF involves the growth of microorganisms on solid organic materials in the absence or near absence of free water. It is the growth of microbes without free-flowing aqueous phase. The substrates used are cereal grains, bran, de-oiled oil seed cakes, legumes and lignocellulosic materials, such as straw, wood chippings, etc.

It has been used in traditional food fermentations and also in compost and silage making. Now, enzymes, organic acids and ethanol are produced by solid substrate fermentations.

SSF cannot be controlled by the sophisticated control mechanisms that are usually associated with submerged fermentations. Control of the environment, particularly the simultaneous maintenance of optimal temperature and moisture are difficult in SSF.

Solid-substrate fermentations involves the following steps

1. Pretreatment of a substrate by mechanical chemical or biological processing

2. Hydrolysis of polymeric substrates, e.g. polysaccharides and proteins

3. Utilization of hydrolysis products or fermentation

4. Separation and purification of end products

Microorganisms

The microorganisms associated with solid substrate fermentations are those that can tolerate relatively low water activity down to values around 0.7.

They may used as

1. monocultures, as in mushroom production, e.g. Agaricus bisporus

2. dual cultures, ‘e.g. straw bioconversion using Chaetomium cellulilyticum and Candida tropicalis

3. Mixed cultures, as used in composting and the preparation of silage, where the microorganisms may be indigenous or added mixed starter cultures (inoculants).

Physicochemical Parameters

Water - Water is lost during fermentation through evaporation and metabolic activity. This is normally replaced by humidification or periodic additions of water. If moisture levels are too low, the substrate will become less accessible and microbial growth will be reduced. If the moisture levels are too high, there will be reduction in the porosity of the substrate which lowers the oxygen diffusion rates and decrease gaseous exchange. This will decrease the rate of substrate degradation and increase the risk of microbial contamination.

Temperature: Heat generation has a major influence on relative humidity in SSF. The temperature is mainly controlled by aeration and/or agitation of the substrate.

Aeration: Most solid-substrate fermentations are aerobic, so aeration is necessary. The extent of aeration needed depends upon the microorganisms used, heat generation, generation of CO₂ and other volatile compounds, etc. The rate of oxygen transfer is influenced by the size of the substrate particles, the moisture level, etc.

Bioreactors Used for Solid-substrate Fermentations

Most solid-substrate fermentations do not require bioreactors, they simply involve spreading the substrate onto a suitable floor. A few anaerobic processes, such as silage production, require no agitation or aeration.

A few aerobic fermentations require aeration and agitation. Bioreactors commonly used for such SSF are the following

1. Rotating drum fermenters – This comprise a cylindrical vessel of around 100 L capacities mounted on its side onto rollers that both support and rotate the vessel. These are used in production of enzyme and microbial biomass. The main disadvantage is that the drum could be filled to only 30% capacity for efficient mixing.

2. Tray fermenters - These are used extensively for the production of fermented oriental foods and enzymes. Here substrates are spread onto each tray to a depth of few centimeters and then stacked in a chamber through which humidified air is circulated. These systems require numerous trays and large volume incubation chambers.

3. Bed systems – These are used in commercial koji production. This consist of a bed of substrate up to 1 m deep, through which humidified air is continuously forced from below.

4. Column bioreactors – This consist of a glass or plastic column, into which the solid substrate is loosely packed, surrounded by a jacket that provides temperature control. These vessels are used to produce organic acids, ethanol and biomass.

5. Fluidized bed reactors – These provide continuous agitation with forced air to prevent adhesion and aggregation of substrate particles. These are used for biomass production for animal feed.

Advantages and disadvantages of solid substrate Fermentation

Solid-substrate fermentations are the most suitable methods for the production of certain products. For example, most fungi do not form spores in submerged fermentations, but sporulation is done eaasily in solid substrate fermentations. So SSF is used for the production of Coniothyrium minitans spores for the biocontrol of the fungal plant pathogen ‘Sclerottnia sclerotiorum.

SSF is used for the production of microbial products such as feed, fuel, food, industrial chemicals, pharmaceutical products, several enzymes, organic acids, flavoring compounds etc. It have applications in bioprocesses such as bioleaching, bio-beneficiation, bioremediation, bio-pulping, etc.

Advantages	Disdvantage
Superior productivity	Slower microbial growth
Low cost in media preparation (Many domestic, industrial and agricultural wastes can be used), low energy requirements, Low capital cost	Problems of heat building up
Simple technology	Higher chances of bacterial contamination
Low waste water output	Difficult to scale up
No foaming issues	Difficult to control substrate moisture level