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Fermenter_Design.pdf

Aishwarya
April 07, 2013

 Fermenter_Design.pdf

Aishwarya

April 07, 2013
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  1. 1 Food Fermenter: Design and Process Control 1. Fermentation Fermentation

    technology is the oldest of all biotechnological processes. The term is derived from the Latin verb fevere, to boil—the appearance of fruit extracts, or malted grain acted upon by yeast, during the production of alcohol. Fermentation is a process of chemical change caused by organisms or their products, usually producing effervescence and heat. Microbiologists consider fermentation as ‘any process for the production of a product by means of a mass culture of micro organisms’. The process improves the digestibility, quality, safety and physico-chemical properties of the raw material and may be aimed at producing probiotics and functional foods or food ingredients. A fermented food can be considered as functional if it could be satisfactorily demonstrated to affect beneficially one or more target functions in the body, beyond adequate nutritional effects, in a way which is relevant to either the state of well-being and health or the reduction of the risk of a disease. Functional food science will serve to establish claims based either on enhanced function or disease risk reduction. Fermentation can also be applied to designing and manufacturing of functional foods. Some of the major categories of functional foods, such as the probiotic foods, contain live microbes, and fermentation may produce or release potential health- promoting compounds in the substrate medium. Several health-related effects associated with the intake of probiotics, including alleviation of lactose intolerance and immune enhancement, have been reported in human studies. Some evidence suggests a role for probiotics in reducing the risk of rotavirus-induced diarrhea and colon cancer. 2. Requirements of Microbial System Fermentation is the culmination of more than 8000 years of human experience using living organisms and the process of fermentation has been used to make products such as bread, cheese, beer and wine. When a fermentation process which is a biotechnological process, is implemented on a commercial scale there is every reason to believe that it will be in some bioreactor or fermenter. The entire process can be divided in three stages.  Stage I : Upstream processing which involves preparation of liquid medium, separation of particulate and inhibitory chemicals from the medium, sterilization, air purification etc.,  Stage II: Fermentation which involves the conversion of substrates to desired product with the help of biological agents such as microorganisms; and
  2. 2  Stage III: Downstream processing which involves separation of

    cells from the fermentation broth, purification and concentration of desired product and waste disposal or recycle. A fermentation process requires a fermenter for successful production because it provides the facilities for the process such as contamination free environment, specific temperature maintenance, maintenance of agitation and aeration, pH control, monitoring, dissolved oxygen (DO), ports for nutrient and reagent feeding, ports for inoculation and sampling, fittings and geometry for scale up, minimize liquid loss and growth facility for wide range of organisms. Depending on the type of product, the concentration levels it is produced and the purity desired, the fermentation stage might constitute anywhere between 5-50% of the total fixed and operating costs of the process. Therefore, optimal design and operation of bioreactor frequently dominates the overall technological and economic performance of the process. In any biological process, the following are unique features.  The concentrations of starting materials (substrates) and products in the reaction mixture are frequently low; both the substrates and the products may inhibit the process. Cell growth, the structure of intracellular enzymes, and product formation depend on the nutritional needs of the cell (salts, oxygen) and on the maintenance of optimum biological conditions (temperature, concentration of reactants, and pH) within narrow limits.  Certain substances e.g. inhibitors, precursors, metabolic products influence the rate and the mechanism of the reactions and intracellular regulation.  Microorganisms can metabolize unconventional or even contaminated raw materials (cellulose, molasses, mineral oil, starch, ores, wastewater, exhaust air, biogenic waste), a process which is frequently carried out in highly viscous, non-newtonian media.  In contrast to isolated enzymes or chemical catalysts, microorganisms adapt the structure and activity of their enzymes to the process conditions, whereby selectivity and productivity can change. Mutations of the microorganisms can occur under sub optimal biological conditions.  Microorganisms are frequently sensitive to strong shear stress and to thermal and chemical influences.  Reactions generally occur in gas-liquid -solid systems, the liquid phase usually being aqueous.  The microbial mass can increase as biochemical conversion progresses. Effects such as growth on the walls, flocculation, or autolysis of microorganisms can occur during the reaction. Accordingly, the general requirements of a bioreactor/fermenter can be narrated as follows:
  3. 3  The design and construction of biochemical reactors must

    preclude foreign contamination (sterility). Furthermore, monoseptic conditions should be maintained during the fermentation and ensure containment.  Optimal mixing with low, uniform shear achieved by proper designing of agitator and aerator.  Adequate mass transfer (oxygen) achieved by monitoring the speed of agitator and gentle heat transfer  Clearly defined flow conditions that can be maintained by proper opening valves and monitoring devices.  Feeding of substrate with prevention of under or overdosing by proper feed ports and monitoring suspension of solids.  Compliance with design requirements such as: ability to be sterilized; simple construction; simple measuring, control, regulating techniques; scale up; flexibility; long term stability; compatibility with up- downstream processes; antifoaming measures. 3. Fermenter Design 3.1 Components of a fermenter: Basic component includes drive motor, heaters, pump, etc.; vessels and accessories, peripheral equipment (reagent bottles), instrumentation and sensors. Fig. 1 shows the components of a fermenter. Functions of different components / parts of the fermenter are as follows: Component/ Parts Function Top plate Cover (made of steel) Clamp top plate compressed onto vessel using clamp Seal Separates top plate from vessel (glass) to prevent air leakage Vessel Glass, jacketed, steel with ports for various outputs, inputs, probes, etc. Drive motor Used to drive mixing shaft Drive shaft Mixes the medium evenly with its impeller Marine impeller For plant tissue culture Baffles Prevent sedimentation on sides and proper mixing Sparger Air supplier / after filtration via membranes – ensures efficient dispersal – by attached to impeller Exit gas cooler Like condenser remove as much moisture as possible from exhaust Inoculation needle Port to add inoculums
  4. 4 Feed pumps Regulates the flow rates of additives (medium,

    nutrients) variable speed Peristaltic pumps Fixed speed pumps – used for continuous sampling Syringe pump Using a syringe – mostly used in batch Exit gas analysis CO2 analyzer, O2 analyzer, mass spectrometer Sample pipe Through which samples are drawn 3 way inlet To insert different probes Fig 1. Ideal fermenter 3.2 Body construction:  Construction materials differ with small scale, pilot and large scale. In small scale for vessel construction glass or stainless steel may be used. For pilot and large scale process, stainless steel (>4% chromium), mild steel (coated with glass or epoxy material), wood, plastic or concrete may be used as vessel construction material. Any vessel used should
  5. 5 not have any corners and smooth surface is essential.

    The construction material must be non toxic and corrosion proof.  Glass vessel (borosilicate glass) Type I – glass vessel round or flat bottom with top plate. It can be sterilized by autoclaving and the largest diameter is 60cm. Type II – glass vessel flat bottom with top and bottom stainless steel plate. This type is used in in situ sterilization process and the largest diameter 30 cm.  Stainless steel Stainless steel is used as vessel construction material with the modifications. For example, >4% chromium (at least 10-13%) may be added; film of thin hydrous oxide - non-porous, continuous, self healing, corrosion resistance inclusion of nickel - improves engineering presence of molybdenum - resistance to halogen salts, brine, sea water, tungsten, silicone - improve resistance. 3.3 Sealing: Sealing between top plate and vessel is an important criteria to maintain airtight condition, aseptic and containment. There are three types of sealing. They are gasket, lip seal and ‘O’ ring. This sealing ensures tight joint in spite of expansion of vessel material during fermentation. The materials used for sealing may be fabric-nitryl or butyl rubbers. The seals should be changed after finite time. 3.3 Baffles: Baffles are metal strips that prevent vortex formation around the walls of the vessel. These metal strips attached radially to the wall for every 1/10th of vessel diameter. Usually 4 baffles are present but when the vessel diameter is over 3dm3 around 6-8 baffles are used. There should be enough gap between wall and baffle so that scouring action around vessel is facilitated. This movement minimizes microbial growth on baffles and fermentation walls. If needed cooling coils may be attached to baffles. 3.4 Aeration system (sparger): Sparger is a device for introducing air into fermenter. Aeration provides sufficient oxygen for organism in the fermenter. Fine bubble aerators must be used. Large bubbles will have less surface area than smaller bubbles which will facilitate oxygen transfer to a greater extent. Combined sparger-agitator is used to air supply via hollow agitator shaft. The air is emitted through holes in the disc or blades of agitator. There are three types of sparger viz. porous sparger, orifice sparger and nozzle sparger.  Porous sparger: made of sintered glass, ceramics or metal. It is used only in lab scale- non agitated vessel. The size of the bubble formed is 10-100 times larger than pore size. There is a pressure drop across the sparger and the holes tend to be blocked by growth which is the limitation of porous sparger.
  6. 6  Orifice sparger: used in small stirred fermenter. It

    is a perforated pipe kept below the impeller in the form of crosses or rings. The size should be ~ ¾ of impeller diameter. Air holes drilled on the under surfaces of the tubes and the holes should be atleast 6mm diameter. This type of sparger is used mostly with agitation. It is also used without agitation in some cases like yeast manufacture, effluent treatment and production of SCP.  Nozzle sparger: Mostly used in large scale. It is single open/partially closed pipe positioned centrally below the impeller. When air is passed through this pipe there is lower pressure loss and does not get blocked. 3.5 Exit gas cooler: Similar to liebig condenser, condenses the moisture from the exhaust gas in the fermenter. This removes as much moisture as possible, from the gas leaving the fermenter and prevents excess fluid loss. 3.6 Agitation: Agitation provides uniform suspension of cells in homogenous nutrient medium. This agitation provides bulk fluid and gas phase mixing, air dispersion, facilitates oxygen transfer and heat transfer and uniform environment throughout the vessel. 3.7 Stirrer glands and bearings: The entry point of stirrer into fermenter may be from top to bottom or sides. Mostly used from bottom so that it leaves more space for entry ports on top. There are four types of stirrer glands and bearings.  Stuffing box: Sealed by several layers of packing rings of asbestos or cotton yarn- pressed against the shaft by a gland follower; at high speeds- packing wears – pressure should be applied to ensure tightness, difficult to sterilize- satisfactory heat penetration, sufficient for containment.  Mechanical seal: Two parts, one is stationary in the bearing housing, and the other rotates on the shaft; two parts are pressed together by springs or expanding bellows, steam condensate use to lubricate and cool seals, safe for containment, double mechanical seal for level 2; at level 2 and 3 the condensate is piped to a kill tank, disinfectants flushed through the seal, steam condensate outlet monitoring indicates any seal failure.  Magnetic drives (some animal cell cultures): Shaft does not pierce the vessel, two magnets- one driving, held in bearing in housing on outside of head plate and one driven, placed on one end of impeller shaft held in bearing in suitable housing; ceramic magnets –magnetic power cross 16mm gap, 300 – 2000 rpm rotation possible.
  7. 7  Simple bush seals: Disadvantage of double seals are

    more difficult to assemble, difficult to detect failure of seal from normal and dead spaces and seals leading to contamination. Hence simple bush seal is preferred in some cases. 3.8 Valves and steam traps 3.8.1 Pressure control valves These types of valves are used for two purposes.  Pressure reduction  Pressure retaining 3.8.2 Safety valve There are types of safety valve by which the increase in pressure is released. They are,  A spindle lifted from its seating against the pressure – releases pressure  Bursting / rupturing of discs to release pressure In case of releasing the gas, the escaping gas must be treated before release. 3.8.3 Steam traps This steam trap is important to remove any steam condensate. There are two components viz. valve and seat assembly and open / close device. The operation of the component is based on,  Density of fluid: A float (ball / bucket) float in water, sink in steam. When it floats it closes and when it sinks it opens the valve.  Temperature of fluid: It has water / alcohol mixture which senses the change in temperature. This mixture expands in hot steam and closes the valve. When it contracts in cool water opens the valve.  Kinetic effect of fluid in motion: if a low density steam is flowing it will be high velocity. Likewise high density will flow with low velocity. The conversion of pressure energy into kinetic energy control the opening and closing 4. Chemical Engineering Aspects of Fermenter Design The objective of fermenter design and operation is to ensure that the desired activity of the microorganisms concerned shall not be restricted by the characteristics of the equipment. It is, therefore, useful to consider the problem in terms of the physical processes which might limit microbial activity. 4.1 Mass transfer: The transfer of mass within the system is fundamental to the whole operation. There are two aspects to this process -
  8. 8  the more-or-less uniform distribution of substrate and product

    molecules in the bulk of the fluid, and  Transfer between the bulk of the fluid and the microbial cells. For materials whose solubility is relatively close to the limiting concentration uniformity of distribution is more important. This could apply, for instance, to mineral salts of low solubility and, perhaps more importantly, to substrates such as steroids and hydrocarbons. Most importantly it applies to oxygen. When supplied in a stream of air, as is usually the case, the equilibrium concentration in fermentation media is only of the order of five to ten times the limiting concentration, even in the absence of microbial activity. In an actively respiring culture this margin is further reduced. The diffusional mass transfer of a component between two phases may be expressed as the product (mass transfer coefficient x interfacial area x concentration driving force). In dispersed systems it is frequently difficult to determine separately the interfacial area and the mass-transfer coefficient related to unit area, and these are combined to give a modified expression (volumetric mass-transfer coefficient x concentration driving force). When a component is added continuously during the course of the process, its availability to the organism may be determined either by the rate at which it is supplied to the nutrient fluid or by the rate at which it is transferred from the nutrient fluid to the cells. Clearly, if the overall rate of supply is below the demand of the organism microbial activity will decline. On the other hand, simply adding a component at an overall rate sufficient to match microbial demand will not be adequate unless the rate of transfer to and through the bulk fluid is sufficiently high. This is clearly seen for aeration, since, in many cases, high microbial demand rates cannot be met by sparging unassisted by mechanical agitation. A similar situation applies to the use of liquid or gaseous hydrocarbons as substrates. Although it has been suggested that hydrocarbons may be taken up directly by microorganisms, recent evidence strongly suggests that for these substrates, like others, the bulk of the transfer is through the aqueous phase. In an actively metabolizing culture, the concentration in the bulk fluid of any substrate present as suspended particles, bubbles or drops will be below the saturation or equilibrium concentration in the sterile medium under similar conditions. A steady-state or pseudo-steady-state concentration will prevail and will be determined by the balance between the rates of supply and demand. Thus we may write for the rate of supply dC = ksas Cs (1) dt s where dC = rate at which the substrate is being supplied to unit volume of culture
  9. 9 dt s ks = supply-side mass transfer coefficient Cs

    = concentration driving force as = interfacial area per unit volume For the demand, we may write dC = Uo Co dt d where dC = rate of consumption of substrate per unit volume dt d Uo = specific uptake rate, i.e. rate of consumption of substrate per unit mass of organisms Co = concentration of organisms Now Cs = Cb *- Cb Where Cb *= concentration of substrate at equilibrium in the absence of demand Cb = concentration at any time t Thus dC = dC - dC = ksas(Cb *- Cb) - Uo Co dt nett dt s dt d At steady state, dC = 0 dt nett i.e. ksas(Cb *- Cb) = Uo Co Cb = Cb *- Uo Co ksas It will be seen that the steady-state concentration falls below the equilibrium concentration in the absence of demand, by an amount determined by the ratio of demand to the volumetric supply mass-transfer coefficient. This will result in limitation of biological activity if the bulk concentration falls below the value at which activity becomes concentration-dependent. Equation 4 also shows that maintenance of a relatively high bulk concentration is assisted by producing a high interfacial area between the substrate and bulk liquid.
  10. 10 4.2 Heat transfer Heat is generated in submerged microbial

    systems, partly by the metabolic activity of the organisms and partly as a result of the mechanical work performed by the agitator and by the gas-bubbles as they expand in passing from the sparger to the head-space above the culture. In mechanically agitated systems the work performed by gas expansion is usually a small proportion of the total. Some heat is lost as a result of increased humidification of the gas stream in its passage through the culture and some by radiation from the outer surfaces of the fermenter. Frequently, the relative volumetric gas-flow rate will be reduced, thereby reducing relative loss of heat by evaporation. More importantly, for geometrically similar vessels the volume (and, therefore, the total heat generation) increases as the third power of the linear dimensions, whereas the surface area increases only as the second power. As a result, a scale is reached at which the loss of heat by evaporation and radiation is insufficient to prevent the temperature rising above the desired level. It is then necessary to provide additional cooling, either by passing a coolant over or around the wall of the vessel or by fitting cooling coils inside the vessel. This then serves to remove heat by conduction. The transfer of heat between the microorganisms and the surface of the vessel is somewhat analogous to that of mass transfer, involving interchange between the cells and bulk fluid and between the bulk fluid and the cooling surfaces. The direct determination of cell-to-fluid heat- transfer coefficients is much more difficult than the determination of mass-transfer coefficients, but it can be anticipated that the accumulation of heat within cell aggregates at a given level of metabolic activity will increase as the size of the cell aggregates increases. Thus, for a given temperature in the bulk fluid the average temperature of a cell will be expected to increase with increasing aggregation. Another effect of biological activity is the fouling of heat-transfer surfaces, either by attack by materials in solution or by deposition of cells. Such fouling reduces the overall heat-transfer coefficient. Apart from the choice of constructional materials, only the size and positioning of baffles will substantially affect the situation at the vessel walls. The design, location and supporting of coils, however, should be such as to minimize the risk of deposition by ensuring free flow of the suspension over the surface of the coils. 4.3 Mixing effects Mixing serves to minimize local variations in concentration and temperature. It arises from the random redistribution of elements of the culture suspension in the impeller zone, and from the random interaction, with the bulk of the suspension, of streams leaving the impeller. In a given system the rate of mixing is a direct, though not necessarily linear, function of the mass flow-rate
  11. 11 from the impeller. This may be expressed as a

    mean circulation rate, i.e. the number of times that a volume of suspension equal to the total volume passes through the impeller zone in unit time. For batch systems in which all particles are freely suspended, the distribution of circulation times is of little importance, since there will be adequate mass transfer throughout the suspension. The situation may be quite different, however, if the extent and nature of microbial growth is such as to modify the rheological properties substantially. For instance, physical interference among cell aggregates frequently results in pseudoplastic behaviour. In the impeller region the individual aggregates are freely suspended in the nutrient medium, whilst retaining their integrity. As they pass into the bulk of the culture, they assemble into clusters as a result of experiencing less intense hydrodynamic forces. Mass transfer between these clusters and the bulk of the fluid may be much less than that characteristic of the individual aggregates. Consequently, nutrients available in limited amounts may become exhausted, or there may be excessive local accumulation of metabolic products, if such an assembly experiences a longer-than-average circulation time. This may result in reversible or irreversible loss of biological activity. Such effects may be particularly important in the case of oxygen supply, since subjection of actively respiring organisms to conditions of oxygen deficiency may result in reduction of respiratory activity, or even a transition to anaerobic metabolism for facultative organisms. If growth of the organism results in a marked increase in viscosity and, more particularly, if the system becomes pseudoplastic, these effects are accentuated by two factors— the establishment close to the impeller of a zone of low effective viscosity, through which much of the air will be channelled, and the establishment near the walls of the vessel of stagnant zones. Such zones may become permanently depleted of oxygen, resulting, at best, in reduced productivity, or more seriously, in lysis, anaerobiosis or other undesirable metabolic activities. Channelling of air around the impeller, and around the impeller shaft in multi-impeller or top- entry configurations, may cause flooding of the impeller and marked reductions in mass transfer and mixing. Each impeller produces two mixing zones, one above and one below the impeller level. Each of these zones can be considered to be well-mixed, but the extent to which the system as a whole can be considered to be well-mixed depends on the rate of interchange of material between the two zones5. This effect and the formation of stagnant zones, mentioned earlier, have important implications for the location of sensors and sampling points, and arrangements for the addition and withdrawal of material during the course of fermentation. These considerations are of particular importance in relation to continuous culture, especially in systems operating as chemostats, in which deviations from perfect mixing may lead to unpredictable performance and difficulty of control.
  12. 12 5. Fermenter/Bioreactor Configurations The fermenter is the heart of

    any biochemical process in which microbial, plant cell systems are employed for the economic production of fermentation products. The main function of a fermenter is to provide a controlled environment for the growth of microorganisms to obtain a desired product. Fermenter can be classified into three groups based on the type of biochemical process employed (Verschoor, 1985).  Bioreactor with no agitation and aeration (anaerobic processes, e.g., production of wine and beer).  Bioreactor with agitation and aeration (aerobic submerged fermentation processes, e.g., production of citric acid and penicillin).  Bioreactor with aeration, but no agitation (aerobic solid state fermentation processes, e.g., production of food enzymes). However, in industrial practice, bioreactors are distinguished by their configuration and design. The common modes of bioreactor configurations are discussed below. 5.1 Submerged Fermenter Systems Submerged fermentation is the most popularly used technique for the production of a large number of products using a wide range of microorganisms. The medium used for submerged fermentation contains relatively highly processed ingredients. The water activity of the medium is high, making it prone to contamination if asepsis is not maintained. Rheological problems can be encountered at high substrate concentrations. Mass transfer from gas to liquid phase is usually a limiting factor, but due to better mixing, diffusional limitation of nutrients is not encountered in submerged fermentation. Better bioprocess control of fermentation process is possible with the help of online sensors. 5.1.1 Stirred Tank Fermenters These are the most commonly used fermenters. They are cylindrical vessels with a motor driven agitator to stir the contents in the tank. The Top-entry stirrer (agitator) model is most commonly used because it has many advantages like ease of operation, reliability, and robustness. The Bottom-entry stirrer (agitator) model is rarely used. Figure 5.1 a & b show different types of stirred tank fermenters. The Stirred tank reactor’s offer excellent mixing and reasonably good mass transfer rates. The cost of operation is lower and the reactors can be used with a variety of microbial species.
  13. 13 (a) (b) 5.1 .1 Stirred tank fermenters 5.1.2 Air-lift

    Fermenters Airlift fermenter (ALF) is generally classified as pneumatic reactors without any mechanical stirring arrangements for mixing. The turbulence caused by the fluid flow ensures adequate mixing of the liquid. The draft tube is provided in the central section of the reactor. The introduction of the fluid (air/liquid) causes upward motion and results in circulatory flow in the entire reactor. The air/liquid velocities will be low and hence the energy consumption is also low. ALFs can be used for both free and immobilized cells. The advantages of Airlift reactors are the elimination of attrition effects generally encountered in mechanical agitated reactors. It is ideally suited for aerobic cultures since oxygen mass transfer coefficient are quite high in comparison to stirred tank reactors. This is ideal for SCP production from methanol as carbon substrate. This is used mainly to avoid excess heat produced during mechanical agitation. Draft tubes are used in some cases to provide better mixing, mass transfer, and to reduce bubble coalescence by inducing circulatory motion. Fig 5.2 shows outline of ALFs. 5.1.3 Fluidised Bed fermenters/bioreactors Fluidized bed bioreactors (FBB) have received increased attention in the recent years due to their advantages over other types of reactors. The FBBs are generally operated in co-current upflow with liquid as continuous phase and other more unusual configurations like the inverse three phase fluidized bed or gas solid fluidized bed are not of much importance. Usually fluidization is obtained either by external liquid re-circulation or by gas fed to the reactor. In the case of immobilized enzymes the usual situation is of two-phase systems involving solid and liquid but the use of aerobic biocatalyst necessitate introduction of gas (air) as the third phase. A differentiation between the three phase fluidized bed and the airlift bioreactor would be made on
  14. 14 the basis that the latter have a physical internal

    arrangement (draft tube), which provides aerating and non-aerating zones. The circulatory motion of the liquid is induced due to the draft tube. Basically the particles used in FBBs can be of three different types: (i) inert core on which the biomass is created by cell attachment. (ii) Porous particles in which the biocatalyst is entrapped.(iii) Cell aggregates/ flocs (self-immobilization). In comparison to conventional mechanically stirred reactors, FBB (Fig.5.3) provide a much lower attrition of solid particles. The biocatalyst concentration can significantly be higher and washout limitations of free cell systems can be overcome. The smaller particle size facilitates higher mass transfer rates and better mixing. The volumetric productivity attained in FBBs is usually higher than in stirred tank and packed bed bioreactors. 5.1.2 Air-lift fermenter 5.1.3 Fluidised bed fermenter 5.1.4 Tower Fermenters Tower fermentors (Fig. 5.4) are simple in design and easy to construct. They consist of a long cylindrical vessel with an inlet at the bottom, an exhaust at the top, and a jacket to control temperature. They do not require agitation hence there are no shafts, impellers or blades. Tower fermentors are used for continuous fermentation of beer, yeast and SCP.
  15. 15 Fig 5.1.4 Tower fermenter 5.2 Solid-State Fermenter Systems Solid-state

    fermentation (SSF) is used for the production of bioproducts from microorganisms under conditions of low moisture content for growth. The medium used for SSF is usually a solid substrate (e.g., rice bran, wheat bran, or grain), which requires no processing. In order to optimize water activity requirements, which are of major importance for growth, it is necessary to take into account the water sorption properties of the solid substrate during the fermentation (Gervais & Molin, 2003.). In view of the low water content, fewer problems due to contamination are observed. The power requirements are lower than submerged fermentation. Inadequate mixing, limitations of nutrient diffusion, metabolic heat accumulation, and ineffective process control renders SSF generally applicable for low value products with less monitoring and control. There exists a potential for conducting SSF on inert substrate supports impregnated with defined media for the production of high value products (Ooijkaas et al., 2000). 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 (Pandey, 2003; Raghavarao et al., 2003). 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 (Durand, 2003.). Other factors that influence the bioreactor design are fungal morphological characteristics, resistance to mechanical agitation, and degree of asepsis required for the fermentation process. Fig .5
  16. 16 Fig 5.2 Solid-state fermentation apparatus 1 Steel cover, 2

    steel drum, 3 Iinlet sensors, 4 supports, 5 baffles, 6 sampling ports. The drum cross-sectional diameter (c) to drum length (a) ratio is 1/2. Other dimensions are as follows: sensor placement, b=c/3, Baffle height, d=c/12. Also indicated in the schematic are the placements of the outlet gas analyzer (G.A.), moisture controller (H) and water activity sensor (Aw) 6. Operational Process Control & Maintenance Measurement and control of operating variables is crucial for better performance of the reactors. Moisture, temperature, oxygen and gaseous products like CO2 concentrations are to be accurately measured on-line and controlled for good fermentation and product yield at optimum levels (Fernandez et al., 1997). The operating variables which can be manipulated to achieve optimum conditions depend on the type of reactors and the operating variables like flow rates, humidity of inlet air, frequency and intensity of agitation. Thermocouples can be used for online measurement of temperatures. Sensors will be effective to minimize the error in measurements so that control becomes easy and accurate. On-line sensors to measure relative humidity, pH, and pressure and concentration gradients across the reactors are the best bet for the accuracy and consistency (Durand, 2003). Smoothing algorithms may be used to account for noise in the measurement of reactor variables. Biomass estimation can be carried out based on oxygen consumption or CO2 evolution during the fermentation process. Applications of off-line measurement techniques for water activity, pH and biomass provide a check on online measurements as they suffer from less noise (Nagel et al., 2000; 2001).
  17. 17 7. Conclusion Biological activity depends basically on the microenvironment

    surrounding each cell. In practice, control of the microenvironment must be exercised rather indirectly through control of the overall conditions in the apparatus in which the process is conducted. It is clearly necessary to ensure that the overall supply of nutrients is adequate and that there should be provision for the removal of excess heat and of volatile products of metabolism, but this is insufficient to ensure optimal activity unless local variations in conditions can be kept within acceptable limits. Mechanical agitation is frequently employed to meet this latter need, but its effects are imperfectly understood and inadequately characterized. More needs to be known for example about the effects of mechanical forces on microorganisms, the extent to which aggregation is determined by biological and mechanical factors and about the distribution of circulation times on various scales of operation, especially in non-Newtonian systems. At the same time, more information is required about the response of biological systems to transient variations in conditions which may be experienced while a particular element of culture suspension is circulated around the vessel under the action of the impeller. It may then be possible to assess quantitatively the extent, if any, to which desired biological activity is limited by variations in the physical conditions in a particular situation, and the benefits which might result from modification of those conditions. This may be regarded as a distant and ideal objective, but benefits may accrue along the way. Fuller knowledge of physical conditions in microbial cultures and of their effects on biological activity will help to resolve these issues and to improve both equipment and process design. There seems no reason to believe that the sparged, agitated, baffled vessel will be displaced from its position of pre- eminence, but better characterization may lead to better exploitation, and to the identification on rational grounds of situations in which radically different types of contacting equipment may be justified. References Barnum, S. R. (1998). Biotechnology: An introduction. Belmont, CA: Wadsworth Pub. Co. Bu’Lock, J. and Kristiansen, B. (1987). Basic biotechnology. London; Orlando: Academic Press. Calderbank P. H. and Jones S. J. R. (1961). Trans. Inst. Chem. Engrs., 39, 363. Doran Pauline M. (2007). Bioprocess engineering principles. Academic press, California, Pp. 7. Durand, A. (2003). Bioreactor design for solid-state fermentation. Biochemical Engineering Journal l. 13: 113-125
  18. 18 Fernandez, M., Anania, J., Solar, I., Perez, R., Chiang

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