FOOD AND INDUSTRIAL MICROBIOLOGY

Module 7. Bioreactor configuration

Lesson 25
SOLID FERMENTOR SYSTEM AND THEIR TYPES

25.1 Introduction

The main difference between submerged and solid-state fermentations is the amount of free liquid in the substrate. Solid-state fermentations (SSF) exhibit a poor conductive gas phase between the particles as compared to submerged fermentation. The presence of a wide variety of SSF matrices in terms of composition, size of solid substrate, mechanical resistance to air flow, porosity, and water holding capacity renders bioreactor design and control more difficult for the regulation of two important parameters, namely temperature and water content of the solid medium. Other factors that influence the bioreactor design are fungal morphological characteristics, resistance to mechanical agitation, and degree of asepsis required for the fermentation process.

25.2 Categories of Bioreactor

Two categories of bioreactor exist for the SSF processes:

(i) At laboratory-scale, using quantities of dry solid medium from a few grams up to few kilograms, (ii) at pilot and industrial-scale, where several kilograms up to several tons are used. The first category comprises many designs, more or less sophisticated, while the second category, which is used mainly at industrial level, is markedly less varied .

However, based on similarities in design and operation, SSF bioreactors can be divided into groups on the basis of how they are mixed and aerated

Group I

Bioreactors in which the bed is static, or mixed only very infrequently (i.e., once or twice per day) and air is circulated around the bed, but not blown forcefully through it. These are often referred to as “tray bioreactors”.

Group II

Bioreactors in which the bed is static or mixed only very infrequently (i.e., once per day) and air is blown forcefully though the bed. These are typically referred to as “packed-bed bioreactors”.

Group III

Bioreactors in which the bed is continuously mixed or mixed intermittently with a frequency of minutes to hours, and air is circulated around the bed, but not blown forcefully through it. Two bioreactors that have this mode of operation, using different mechanisms to achieve the agitation, are “stirred drum bioreactors” and “rotating drum bioreactors”.

Group IV

Bioreactors in which the bed is agitated and air is blown forcefully through the bed. This type of bioreactor can typically be operated in either of two modes, so it is useful to identify two subgroups.

Group IV

A bioreactors are mixed continuously while Group IVb bioreactors are mixed intermittently with intervals of minutes to hours between mixing events. Various designs fulfill these criteria, such as “gas-solid fluidized beds”, the “rocking drum”, and various “stirred-aerated bioreactors”.

25.3 Laboratory Scale SSF Bioreactor

Small scale SSF equipment can be classified as those without forced aeration and agitation to include Petri dishes, jars, widemouth Erlenmeyer flasks, Roux bottles and roller bottles, and those incorporating continuous agitation of the solid medium such as a rotating drum bioreactor, a perforated drum bioreactor and a horizontal paddle mixer. The former are easy to operate in large numbers and commonly used for the screening of substrates or microorganisms for research purposes, while the latter offer the advantage of temperature control due to continuous agitation.

Fig. 25.1 Small scale SSF equipment (Okafor, 2007)

25.4 Industrial Scale SSF Bioreactor

Industrial scale SSF bioreactors can be built with or without aeration. Those without forced aeration can exhibit limitation of heat and mass transfer as the fermentation progresses, changing the properties of the microorganism involved, particularly in light of associated complexities like heat build up and inadequate oxygen transfer. However, with aeration strategies like circulation of air around the substrate layer or passing air through the substrate layer, these limitations are reduced to a certain extent.

25.4.1 SSF bioreactor without forced aeration

On an industrial scale, this bioreactor is generally a tray fermentor. The trays containing the solid medium are stacked in tiers and placed in humidity and temperature controlled chamber. This technology has the limitations of not conforming to asepsis conditions, and of high labour requirements. However, it is easily scaled up by the incorporation of additional trays.

Fig. 25.2 Tray type bioreactors for making Koji (Okafor, 2007)

1. Koji room, 2. water valve, 3. UV tube, (4, 8, 13) air blowers,

(5, 11) air filters, 6. air outlet, 7. humidifier, 9. heater,

10. air recirculetion, 12. air-inlet, 14. trays, 15. tray holders

24.4.2 SSF bioreactor with forced aeration and no mixing

In this type of bioreactor, no mechanical agitation is provided, but the medium can be manually agitated in situ or it can be transferred into a kneading machine and reloaded into the basket. However, this type of device without agitation is limited by the metabolic heat produced. Considerable temperature gradients can exist within the substrate bed. As the majority of the heat is removed and water is evaporated by forced aeration, the bed dries out, reducing fermentation efficiency. Periodic water addition in the form of spray is required to maintain the moisture content at desired levels.

25.4.3 SSF bioreactor with continuous mixing and forced aeration

A rotating drum bioreactor with continuous mixing maximizes the exposure of each substrate particle to a thermostatic air circulating unit in the headspace. A large reactor, capable of handling 10 kg of steamed wheat bran as substrate, has been reported. Large scale use of unagitated SSF is limited by the difficulty in maintaining temperature during the fermentation. However, in a rotating drum bioreactor, efficient heat transfer is possible by convective and evaporative cooling. As the scale of fermentation increases, evaporative cooling becomes significant, because the ratio of the heat produced to the surface area available for convection decreases. The inherent difficulties encountered in the operation of solid-state fermentation systems on a large scale has led to new developments aimed at improving the efficiency of the fermentation process.