Continuous Cultivation - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 648

12.5. SOLID SUBSTRATE FERMENTATIONS
Our focus so far has been on the liquid media fermentations with very little discussion on
solid substrates. Solid substrate fermentations imply a general method of fermentations in
which moisture content may or may not need to be low, but the substrate is in the form of
solid particles, where the media can be in liquid phase. Bacterial ore leaching (i.e. growth
and microbial oxidation on surfaces of mineral sulfide particles) or fermentation of rice in
a packed column with circulating liquid media is an example of solid substrate fermenta-
tions. When the solid substrate is submerged in the liquid medium, the fermentation is
usually referred to as submerged fermentation (SmF). However, when no free liquid presents
in the system, the fermentation is usually referred to as solid-state fermentation (SSF).
SSFs are a special form of solid substrate fermentations for which the substrate is solid and
the moisture level is low, which usually refers to as no free flowing water in the medium. The
water content of a typical SmF is more than 90%. The water content of a solid mash in SSF
often varies between 40% and 80%. SSFs are usually employed for the fermentation of agri-
cultural products or foods, such as rice, wheat, barley, durum, oat, peas, millet, corn, broad
beans, and soybeans. The unique characteristic of SSFs is operation at very low moisture
levels, which provides a selective environment for the growth of mycelial organisms, such
as molds. In fact, most SSFs are mold fermentations producing extracellular enzymes on
moist agricultural substrates. Since bacteria and yeasts cannot tolerate low moisture levels
(water activities), the chances of contamination of fermentation media by bacteria or yeast
are greatly reduced in SSF. Although most SSFs are mold fermentations, SSFs based on
bacteria and yeast operating at relatively high moisture levels (75 e 90%) are also used.
SSFs are used widely in Asia for food products, such as tempeh, miso, or soy sauce fermen-
tations, and also for enzyme production.
The choice of using SSF was quite intuitive at the dawn of food processing. Tofu and grains
go “bad” under moisture and our ancestors discovered that they could be turned to tasty
food. The major advantages of SSF over SmF systems are 1) small volume of fermentation
mash or reactor volume to deal with for the same amount of final product, leading to low
capital and operating costs, 2) a lower chance of contamination by unwanted microorganisms
due to low moisture levels, 3) easy product recovery, 4) energy efficiency, 5) the allowing of
the development of fully differentiated structures, which is critical in some cases to product
formation, and 6) easy to implement with simple capital structure and minimum operation
requirements. The major disadvantage of SSFs is the heterogeneous nature of the media due
to poor mixing characteristics, which result in poor ability to control the conditions (pH, DO,
and temperature) within the fermentation mash. To eliminate these control problems,
fermentation media are usually mixed either continuously or intermittently. For large
fermentation mash volumes, the concentration gradients may not be eliminated at low agita-
tion speeds, and mycelial cells may be damaged at high agitation speeds. Usually, a rotating
drum fermentor is used for SSF systems, and the rotational speed needs to be optimized for
the best performance.
Table 12.5 lists some of the traditional food products produced via aerobic SSF. The koji
process is an SSF system that employs molds ( Aspergillus , Rhizopus ) growing on grains or
foods (wheat, rice, and soybean). A typical SSF process involves two stages. The first and