Hydrodynamic mechanisms use liquid kinetic energy to mix the
fermenter contents, which is achieved by using an external liquid pump
for external circulation and reinjection, e.g. deep-jet fermenters.
Fermenter control and monitoring
 Fermentation systems must be efficiently controlled in
order to optimize productivity and product yield, and
ensure reproducibility.
 The key physical and chemical parameters involved largely
depend on the bioreactor, its mode of operation and the
microorganism being used.
 They are primarily aeration, mixing, temperature, pH and
foam control.
 Control and maintenance at optimum levels inside the
reactor is mediated by sensors (electrodes), along with
compatible control systems and data logging.
Operating modes
 Industrial fermentations are operated as batch, fedbatch or
continuous cultures.
 Most are batch processes, which are closed systems where
there are no additions following inoculation, apart from
acid or alkali for pH control and input of air for aerobic
fermentations.
 In batch fermentations there is a definite beginning and end
to the process.
 A fermenter is loaded, sterilized and inoculated, and the
organism is grown through a typical batch profile.
 Examples are antibiotics, beers, wine etc
 The advantages of batch systems are that initial capital
expenditure is lower and, if contamination occurs, it is
relatively simple to terminate and restart a new
fermentation cycle.
Air sterilization
 To prevent contamination of either the fermentation by airborne microorganisms or the environment by aerosols
generated within the fermenter,
 For the purpose, both air input and air exhaust ports have
air filters attached. These filters are designed to trap and
contain microorganisms.
 Filters are made of glass fibre, mineral fibres,
polytetrafluoroethylene (PTFE) or polyvinyl chloride
(PVC), and must be steam sterilizable and easily changed.
Media and vessel sterilization
 For pilot-scale and industrial aseptic fermentations the
fermenter can be sterilized empty.
 The vessel is then filled with sterile medium, prepared in a
batch or continuous medium ‘cooker’ that may supply
several fermentations.
 Alternatively, the fermenter is filled with formulated
medium and the two are sterilized together in one
operation.
 However, some industrial fermentations are not aseptic, but
microbial contamination is still maintained at a minimum
level by boiling or pasteurization of the media.
 Small laboratory-scale fermenters of 1–5 L capacity are
usually filled with medium and then sterilized in a steam
autoclave.
Solid-substrate fermentations
 Solid-substrate fermentations have been used for producing
various fermented foods in Asia for thousands of years, but
this method is rarely used in Europe and North America.
 It involves the growth of microorganisms on solid,
normally organic, materials in the absence or near absence
of free water.
 The substrates used are often cereal grains, bran, legumes
and lignocellulosic materials, such as straw, wood
chippings, etc.
 Traditional processes are largely food fermentations
producing oriental tempeh and sufu, cheeses and
mushrooms along with compost and silage making.
 In addition, enzymes, organic acids and ethanol are now
produced by solidsubstrate fermentations.
 Solid-substrate fermentations lack the sophisticated control
mechanisms that are usually associated with submerged
fermentations.
 Solid-substrate fermentations are normally multistep
processes, involving:
1. pretreatment of a substrate that often requires
mechanical, chemical or biological processing;
2. hydrolysis of primarily polymeric substrates, e.g.
polysaccharides and proteins;
3. utilization of hydrolysis products; and
4. separation and purification of end-products.
Rotating drum fermenters
- comprising a cylindrical vessel of around 100 L capacity
mounted on its side onto rollers that both support and rotate
the vessel.
- These fermenters are used in enzyme and microbial
biomass production.
- Their main disadvantage is that the drum is filled to only
30% capacity, otherwise mixing is inefficient.
Tray fermenters
- which are used extensively for the production of fermented
oriental foods and enzymes.
- Their substrates are spread onto each tray to a depth of only
a few centimetres and then stacked in a chamber through
which humidified air is circulated.
- These systems require numerous trays and large volume
incubation chambers of up to 150m3 capacity.
Bed systems
- consisting of a bed of substrate up to 1 m deep, through
which humidified air is continuously forced from below.
Column bioreactors
- consisting of a glass or plastic column, into which the solid
substrate is loosely packed, surrounded by a jacket that
provides a means of temperature control.
- These vessels are used to produce organic acids, ethanol
and biomass.
Fluidized bed reactors
- which provide continuous agitation with forced air to
prevent adhesion and aggregation of substrate particles.
- These systems have been particularly useful for biomass
production for animal feed.