Bioreactor designs for solid state fermentation

Biochemical Engineering Journal13(2003)113–125Bioreactor designs for solid state fermentationA.Durand∗Platform for Development in Biotechnology,UMR-INRA1082(IBQ),17Rue Sully,Dijon21065,FranceReceived17December2001;accepted after revision24July2002AbstractSolid state fermentation has gained renewed attention not only from researchers but also from industry.This technique has become a more and more attractive alternative to submerged fermentation for specific applications due to the recent improvements,especially in the design.This paper reviews the various reactor designs and focuses on the differences between lab-scale and industrial-scale designs.It highlights the main designs that have emerged over the last10years and the potential for scaling-up for each category of reactor.©2002Elsevier Science B.V.All rights reserved.Keywords:Solid state fermentation;Bioreactor design;Engineering;Scale-up strategies1.IntroductionIs solid state fermentation(SSF)a new challenge for a very ancient technology?The question is worth asking be-cause this“ancient art”is in the process of becoming a mod-ern technology.During the last10years,many articles have been pub-lished,several books have been edited showing a spurt of SSF processes even in western countries.The fact that this process is particularly well adapted to the metabolism of fungi,the micro-organisms most commonly in SSF pro-cesses,is an important feature because of the characteristics of these micro-organisms(apical growth,enzymatic ac-tivities).Moreover,in western countries,recent important problems has emerged such as:pollution of soils and the potential use of bioremediation,BSE epidemic and the ne-cessity tofind alternative for animal feeding,to cite only two examples.Thus,SSF has gained a new interest from researchers and manufacturers over the past10years. Many papers have appeared on the use of solid state fer-mentation,with studies on the effects of different factors on fungus metabolism,and the potential for producing differ-ent metabolites[1–3].The great majority of these papers were SSF processes at laboratory-scale.Conversely,very few works have been carried out on the engineering aspects and problems of scale-up.Compared to submerged fermentation,the solid media used in SSF contain less water but an important gas phase ex-ist between the particles.This feature is of great importance ∗Tel.:+33-3-8069-3061;fax:+33-3-8069-3229.E-mail address:durand@dijon.inra.fr(A.Durand).because of the poor thermal conductivity of the air compared to the water.Another point is the wide variety of matrices used in SSF which vary in terms of composition,size,me-chanical resistance,porosity and water holding capacity.All these factors can affect the reactor design and the control strategy for the parameters.Indeed in submerged fermenta-tion,we can consider roughly that all the media are made up essentially of water.In this environment,the temperature and pH regulations are trivial and pose no problem during the scaling-up of a process.In submerged fermentation,only one major difficulty is encountered:the transfer of oxygen to micro-organisms which depends upon the shape,the size of the reactor and the agitation/aeration system used.To characterise this transfer,a parameter,K L a(oxygen trans-fer coefficient),has been defined.It can be considered as a“similarity invariant”,i.e.its value expresses the capac-ity of the equipment to transfer oxygen independently of the volume of the reactor and so,constitutes an important parameter used for the scale-up studies in submerged fer-mentation.In SSF,besides the oxygen transfer which can be a limiting factor for some designs,the problems are more complex and affect the control of two important param-eters:the temperature and the water content of the solid medium.Other factors also affect the bioreactor design:(i)the mor-phology of the fungus(presence or not of septum in the hyphae)and,related to this,its resistance to mechanical ag-itation,(ii)the necessity or not to have a sterile process. Before analysing the various types of bioreactors,their advantages and drawbacks,it is important to specify that in a general way,many types of reactors are able to run at laboratory-scale with small quantities of medium.But,the1369-703X/02/$–see front matter©2002Elsevier Science B.V.All rights reserved. PII:S1369-703X(02)00124-9114 A.Durand /Biochemical Engineering Journal 13(2003)113–125scale-up is complicated mainly by intense heat generation and heterogeneity in the system [4].In this paper,emphasis will be put on the differences between bench-scale bioreactors and pilot or industrial units and also between non-sterile and sterile process.2.Bioreactor classificationTwo 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.Within each category,some of the bioreactors can operate in aseptic conditions.boratory-scale bioreactorsSeveral types of equipment are used for SSF.Petri dishes,jars,wide mouth Erlenmeyer flasks,Roux bottles and roller bottles offer the advantage of simplicity [5,6].Without forced aeration and agitation,only the temperature of the room,where they are incubated,is regulated.Easy to use in large numbers,they are particularly well adapted for the screening of substrates or micro-organisms in the first steps of a research and development program.One of the interesting lab-scale units is the equipment de-veloped and patented by an ORSTOM team between 1975and 1980[7].It is composed of small columns (Ø4cm,length 20cm)filled with a medium previously inoculated and placed in a thermoregulated water-bath (Fig.1).Water saturated air passes through each column.This eqiupment is widely used by many researchers and offers the possibilityto Fig.1.Typical lab-scale column reactor.Several columns detailed on the right part of the figure are located in a water-bath for temperature control.aerate the culture and also analyse the micro-organism res-piration by connecting the columns to a gas chromatograph with an automated sampler that routinely samples each col-umn.This equipment is convenient for screening studies,op-timisation of the medium composition and measurement of CO 2produced.The small quantity of medium (few grams)used and the geometry of the glass column is suitable for maintaining the temperature in the reactors (the heat removal through the wall seems to be sufficient).The design of this reactor,however,does not permit sampling during fermenta-tion and so it is necessary to sacrifice one entire column for each analysis during the process.This equipment,with its advantages (forced aeration,cheap,relatively easy to use),can constitute a first step in the research.A new generation of small reactors was developed by an INRA-team in France a few years later.The first model de-veloped [8]addressed problems concerning the regulation of the water content of the medium.A second model built during 2000has been tested but has not been reported in the literature.As shown in the photograph (Fig.2),this re-actor has a working volume of about pared to the first model,the principal changes were the introduction of a relative humidity probe,a cooling coil on the air circuit and a heating cover for the vessel.These changes improved the regulation of the water content during the process.As for the ORSTOM columns,the mini-reactors are filled with a medium previously inoculated in a sterile hood.Each reactor is automatically controlled by a computer.Moreover,sam-ples can be taken by opening the cover in the presence of a flame without problem of contamination.In this type of re-actor,the temperature and the water amount of the medium can be monitored by means of the regulation of the tempera-ture,relative humidity and flow rate of the air going through the substrate layer.Different profiles for the air-inlet tem-perature and flow rate can be elaborated and generate useful information for the scaling-up studies.A.Durand/Biochemical Engineering Journal13(2003)113–125115Fig.2.Photography and schematic of a lab-scale sterile reactor.(1)Heating cover,(2)medium temperature probe,(3)stainless steel sieve,(4)air-inlet temperature probe,(5)relative humidity probe,(6)resistive heater,(7)water temperature probe,(8)massicflow meter,(9)level probe,(10)insulating jacket.Fig.3.Rotating drum bioreactor.(1)Air-inlet,(2)rotating joint,(3)coupling,(4)air nozzles,(5)air line,(6)rollers,(7)rotating drum,(8)solid medium, (9)rim.116 A.Durand /Biochemical Engineering Journal 13(2003)113–125Fig.4.Perforated drum bioreactor.Another concept,based on continuous agitation of the solid medium,was developed by several teams mentioned below.The bioreactors can be a rotating drum (Fig.3),a per-forated drum (Fig.4)or an horizontal paddle mixer (Fig.5).With or without a water-jacket,this type of reactor is re-quired to be continuously mixed to increase the contact be-tween the reactor wall and the solid medium and also to provide oxygen to the micro-organism.For rotating drum bioreactors,as an horizontal cylinder,the mixing is pro-vided by the tumbling motion of the solid medium which may be aided by baffles on the inner wall of the rotating drum (perforated or not).However,in all these reactors,the mixing is less efficient than with a paddle mixer [9].In-deed,agglomeration of substrate particles during the growth of the mycelium can occur which increases the difficultyofFig.5.Photography of an horizontal paddle mixer used in the Wageningen University of Agriculture.Schematic of a stirred horizontal bioreactor.(1)Air-inlet,(2)temperature probes,(3)water-jacket,(4)paddles,(5)air outlet,(6)agitation motor,(7)reactor,(8)solid medium,(9)agitation shaft.regulating the temperature of the solid medium.Moreover,the oxygen transfer inside these balls of medium,agglomer-ated by the fungal hyphae and also very often by the stick-iness of the substrate used,may be very low or nil.In ad-dition,from an engineering point of view,a water-jacket on a moving body of a reactor causes problems that increase with scale [10].A continuous mixing horizontal paddle mixer (Fig.5)was developed by a Dutch team at Wageningen University.This aseptic fermenter was used for different purposes and to improve simultaneous control of temperature and moisture content.Although heat transport to the bioreactor wall was improved,this device becomes inefficient for larger volume [11]because heat removal only through the wall becomes increasingly inefficient as the volume increases.Generally,a continuous agitation,even if it is gentle,can modify the structure of the solid medium to a pasty texture.Depending upon the nature of the particles (clay granules as support for example),this agitation can also be abrasive and so be harmful for the mycelium especially if the hyphae have no septa.For processes in which the substrate bed must remain static,a reactor designed by ORSTOM team in France and named Zymotis is an interesting equipment [12,13].It con-sists of vertical internal heat transfer plates in which cold water circulates (Fig.6).Between each plate the previously inoculated solid medium is loaded.Thermostated air is in-troduced through the bottom of each partition.This reactor,which looks like a tray reactor where the layers of substrate would be set vertically,appears difficult to work in aseptic conditions.Very often in SSF a shrinkage of the volume of medium occurs during the mycelium growth.With this type of device,A.Durand /Biochemical Engineering Journal 13(2003)113–125117Fig.6.Photography of the Zymotis showing heat exchanger plates for the thermostated water circulation (at left)and during a culture (at right).the risk is that the contacts with the vertical plates will de-crease as the fermentation progresses,which would lead to poor heat transfer and air channelling.Finally,the scale-up of such a design appears very difficult.2.2.Pilot and industrial-scale bioreactorsAs mentioned before,the number of reactor types used at pilot scale and in industry is less wide due,at once to some important reasons and necessities which arethat:Fig.7.Koji-type reactor:(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 recirculation,(12)air-inlet,(14)trays,(15)tray holders.•above some critical quantity of substrate,the heat removal becomes difficult to solve and restricts the design strate-gies available.The solid medium becomes compacted or creates air channelling,shrinkage,etc.All these factors affect heat and mass transfer,•the properties of the micro-organism with respect to its resistance to mechanical stirring,its oxygen requirement and temperature range.When the mycelium hyphae do not have septa,they can be destroyed by a mechanical stirring.So,the culture layer will be thin to allow heat118 A.Durand/Biochemical Engineering Journal13(2003)113–125removal which automatically orientates to a category of reactor,•the nature of the substrate and the need to pretreat or not it,appropriate procedures for the inoculation,the sterility or the level of contamination acceptable for the process and the application,•the economy of the country where the process is devel-oped especially with respects to the labour cost.Indeed some technologies need more manpower than others,•handling poses different problems such as the ease of filling,emptying and cleaning the reactor.The heat and mass transfer problems identified above can be attributed to poor aeration.This problem can be addressed using the following strategies:(i)the air circulates around the substrate layer or(ii)it goes through it.Within the sec-ond strategy,three possibilities are available:unmixed,in-termittently or continuously mixed beds.2.2.1.SSF bioreactors without forced aerationThis category is ancient and the simplest.Probably differ-ent ancient civilisations have used this technology domesti-cally for fermenting miscellaneous raw agricultural products in baskets.The microbial starter culture might be transferred in the form of a“mouldy medium”.Although this technology has advanced,it is still based on the same principle.Applied on commercial scale,it corresponds to the tray fermenters (Fig.7)as typified by the famous Koji process[14–18].Made of wood,metal or plastic,perforated or not,these trays,con-taining the solid medium at a maximum depth of15cm,are placed in thermostated rooms.The trays are stacked in tiers, one above the other with a gap of a few centimetre.This technology can be scaled-up easily because only the num-ber of trays is increased.Although it has been extensively used in industry(mainly in Asian countries),this technol-ogy requires large areas(incubation rooms)and is labour intensive.It is difficult to apply this technology to sterile processes except if sterile rooms are built and if procedures and equipment for the employees are provided,which will be prohibitive.An alternative could be to use polypropylene semi permeable sterilizable bags to maintain sterility.More-over,some bags have a microporous zone which allows a passive airflow rate from20to2000cm3/(cm2/min).2.2.2.Unmixed SSF bioreactors with forced aerationThe basic design feature of packed-bed bioreactors is the introduction of air through a sieve which supports the sub-strate.In this way,a bioreactor was developed at pre-pilot scale(Fig.8)for defining the control strategy and optimis-ing the air-inlet temperature,the airflow rate,the addition of water and agitation during a SSF process[8].Located in a clean room,the reactor can be pasteurised in situ by steam generated by the water-bath used for the air humidi-fication.This reactor is very simple and can process a few kilograms of dry solid medium.These reactors constitute an interesting tool that can be used in two ways:(i)toanalyse Fig.8.General view and schematic of the unmixed bioreactors with forced aeration.(1)Basket conateining the solid medium,(2)valves for airflow adjustment,(3)air temperature probe,(4)relative humidity probe, (5)draincocks,(6)heating box,(7)humidifier,(8)coil for circulation of cold water,(9)resistive heater.empirically the global evolution of a process and determine the environmental parameters for regulating the temperature and the moisture of the solid medium,(ii)to study mass and heat transfer phenomena and oxygen diffusion[19].Both the reactor diameter and the height of the substrate layer are around40cm,so the quantity of solid medium is suffi-cient to predict what can happen in a larger volume[20].In the absence of mathematical models for the scale-up,these reactors are very useful.No mechanical agitation exists in-side these reactors,but the medium can be manually agi-tated 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 produc-tion.Temperature gradients from the bottom to the top ofA.Durand /Biochemical Engineering Journal 13(2003)113–125119the layer appear unavoidable.As the majority of the heat is eliminated by convection and water evaporation [21],the bed dries out and the water additions needed have to be cal-culated and agitation is required in order to distribute the added water uniformly.To reduce the need for a strong aeration,another concept was recently developed.It consists of the introduction of heat exchangers directly beneath the perforated plates which support the substrate and/or inside the substrate layers.A similar strategy on a lab-scale but with vertical heat exchang-ers has been demonstrated for the Zymotis bioreactor [12].Based on this principle,a first bioreactor was patented [22]and used by a German company (Prophyta)for producing biopesticides in sterile conditions.The fermenter is a tower with perforated plates on which the solid medium is located.Sterile air can go through each plate.Beneath each plate,heat exchangers are located to remove heat during the cultivation (Fig.9).A similar bioreactor,Plafractor TM ,was patented by an Indian company,Biocon,[23].Metabolic heat,as in the previous reactor,is mainly removed by conduction.This reactor was constructed by stacking and interconnecting in-dividual modules (Fig.10).Non-communicatingchannelsFig.9.Schematic of the patented industrial bioreactor showing the exchanger plates under each tray [22].deliver cooling and heating fluids sandwiched between two municating channels can deliver fluids for ster-ilising (with steam or ethylene oxide for example),for ad-justing the moisture and oxygen content and for extracting the compound of interest after the cultivation.Moreover,the interior of each module has a mixing arm that revolves about the central axis of the module while rotating.This reactor has been used mostly for metabolite production.In the patent,a maximum quantity of 20kg of wheat bran was mentioned with approximately 22,600cm 2plate area,but the number of modules was not mentioned.Finally,no de-tails were given about the disposal of the waste product,the cleaning procedure which seems relatively complicated and generally the possibility to scale-up this device.2.2.3.Continuously mixed SSF bioreactors with air circulationThis category is essentially a rotating drum because con-tinuous mixing is necessary to maximize the exposure of each substrate particle to the thermostated air circulating in the headspace.Different teams have worked on this design and it is mostly used at lab and pre-pilot scale.Although120 A.Durand /Biochemical Engineering Journal 13(2003)113–125Fig.10.Scheamtic of the Plafractor TM reactor [23].rotating drums have been described in the past,the largest reactor recently cited in the literature was a 200l stainless steel rotating drum (Ø56cm and 90cm long)which used 10kg of steamed wheat bran as substrate [24]for kinetic studies of Rhizopus .Researches were carried out at lab-scale to study the efficiency of this design,the role of the baffles and the influence of the filling (amount of substrate per unit volume)on the mass transfer by using tracer or image analysis [25–27].These works introduced the rational design and scale-up of this type of reactor.In several cases,the mycelium and the substrate particles,particularly starchy and sticky materials,agglomerate.Under these conditions,even with baffles inside the drum,it was very difficult to separate these aggregates,consequently,the heat,mass and oxygen transfers were greatly reduced.When the rotation rate of the drum is increased,it can affect the mycelium growth presumably because of shear effects [25].For a discontinuously rotating drum,the design is iden-tical to the reactor described above but between two agi-tations,it operates like a tray reactor.So,it is absolutely necessary to limit the height of the substrate layer,other-wise it will be necessary to continuously agitate due to the heat accumulation and,taking into account the poor ther-mal conductivity of the air,the medium temperature will in-evitably increase.Very few studies have been published on this ing this rotating drum,a strategy for regulating the medium temperature was described in a thesis [28]and in a publication [29].It consists of activating the rotation of the drum in response to the temperature measured by a thermocouple in the medium.Efficient for a 4.7l workingA.Durand /Biochemical Engineering Journal 13(2003)113–125121Fig.11.Discontinuously rotating drum [28].volume (Fig.11),on soy beans with Rhizopus ,for tempe production,no scale-up studies have been attempted.2.2.4.Intermittently mixed bed bioreactors with forced aerationIn general,these bioreactors can be described as packed beds in which conditioned air passes through the bed.An agitation device is periodically used to mix the bed and at the same time,water is sprayed if necessary.The design of these reactors,the capacity of which varies from afew Fig.12.Photography and schematic of the Koji making equipment:(1)Koji room,(2)rotating perforated table,(3)turning machine,(4,11)screw and machine for unloading,(5)air conditioner,(6)fan,(7)air outlet,(S)dampers (9)air filter,(10)machine for filling,(12)control board.kilograms to several tons,is influenced the necessity or not to operate in sterile conditions.For non-sterile processes ,a number of advances have been done in the design and application of such bioreactors.One design is represented by the rotary type automatic Koji making equipment marketed by Fujiwara in Japan (Fig.12).The treated substrate is heaped up on a rotary disc.De-pending on the diameter of this disc,different working volumes are available but always with a layer of maximum thickness 50cm.This non-sterile reactor operates with a122 A.Durand /Biochemical Engineering Journal 13(2003)113–125microcomputer which controls all the parameters (tempera-ture of the air-inlet,air flow rate and agitation periods).The main drawback of this equipment is the need to prepare and inoculate the substrate in other equipment before filling the reactor.Nevertheless this type of design is widely used in Asian countries.Similar to the reactors used in the barley malting process,huge equipment has been built for the first step of the process for making soy sauce.A specific building contains the solid state reactor which is generally rectangular with a length of several meters.Several tons of pretreated and inoculated substrate are put on a wire mesh and conditioned air is forced through the layer.An agitator trolley periodically mixes the solid medium.Although this kind of reactor is very simple and basic,it is widely used in many Asian manufacturers of soy sauces.An INRA team in Dijon (France)has developed a non-sterile process strategy based on the following principle (Fig.13).The temperature (T m )and the moisture (W A m )of medium are maintained by a regulation of the temperature,relative humidity and flow rate of the air input.It is also nec-essary to spray water (E)and agitate (A)periodically.TheFig.14.Pilot plant reactor [30]:Photography showing a general view of the reactor (left),a detail of the swelling joints.A schematic diagram of this pilot plant.(1)Carriage motor,(2)screw motor,(3)valves for inoculum and water spraying,(4)temperature probes,(5)weight gauges,(6)relative humidity probe,(7)cooler,(8)humidifier by steam injection,(9)airflow meter,(10)fan,(11)heater,(12)air filter,(13)cooler.Fig.13.General schematic of the intermittently packed bed reactor with forced aeration.(T in ,HR in and D in )respectively the temperature,relative humidity and flow rate of the air-inlet,(T out ,HR out and D out ),respectively,the temperature,relative humidity and flow rate of the air outlet,(T m )temperature of the solid medium,(WA m )water amount of the solid medium,(M g )total mass of the solid medium,(A)agitation,(E)water spray.A.Durand /Biochemical Engineering Journal 13(2003)113–125123volume of water sprayed is calculated from on line measure-ments of the total mass of the medium (M g )and by estimat-ing the mass losses due to respiration (CO 2).On this basis,a reactor of 1.6m 3capacity using 1t of sugar beet pulp (with 25%of dry matter)per batch was reported [20].This reactor has been successfully used for different applications includ-ing protein enrichment of agro-industrial by-products,pro-duction of enzymes or biopesticides [30].The system has been since continuously pared to the first re-actor,the main changes (Fig.14)were in the equipment it-self with some accessories (pump for inoculation and water spraying),the air conditioning and the software for the con-trol and the regulation of the parameters during the process.Particularly,a mathematical model has been established for maintaining the moisture of the medium during the process by taking into account the dry matter losses and the evapora-tion due to the aeration.Moreover,this reactor is closed by a cover in three parts allowing the passage of the screw axis during the mixing.By this way,it is possible to pasteurise the reactor and/or to treat the substrate in situ,near 80◦ing the same design,two larger reactors (each with overall dimensions of 17.6m ×3.6m ×2.0m)and a maxi-mum working capacity of 50t (20%dry matter)were con-structed [31]to demonstrate the potential industrial-scale-up of this fermenter.For sterile processes ,the reactors based on this same prin-ciple are smaller and,currently,no industrial application has been published in the literature.Such sterile processes are necessarybecause:Fig.16.Photography and schematic diagram of the sterile reactor developed by the National Institute of Agronomic Research in Dijon [30].(F)Air filter,(HC)humidification chamber,(HB)heating battery,(BP)by-pass,(CB)cooling battery,(HM)probe for air relative humidity measurement,(TP)probe for medium temperature measurement,(WG)weight gauges,(SH)sterile sample handling,(JR)water temperature regulation in the double jacket,(AD)planetary agitation device,(M)motor for agitation,(IS)sterile system for adding inoculum and solutions,(CO)water aircondenser.Fig.15.Photography of the ribbon mixer developed by Wageningen University of Agriculture [32].124 A.Durand/Biochemical Engineering Journal13(2003)113–125•the product obtained must be sterile for application and legislative reasons(food and pharmaceutical industries for example),•the micro-organism used has a very slow growth rate and so must be cultivated in clean conditions.Indeed,usually, it is considered that beyond3days cultivation,it is very difficult to work in non-sterile conditions.An original design has been developed by the Wa-geningen University in Holland and used at pilot scale for studying production of a biopesticide[32].This reactor with a total capacity of50l can hold up to20kg of wet cereal grains substrate.It consists of a conical vessel mixed by a ribbon at the wall(Fig.15).This reactor,developed in co-operation with Hosokawa Micron,is sterilisable in situ by steam.Sensors register the temperature at several heights in the bed,also during mixing.Another bioreactor with a50l bed was patented by Du-rand et al.[33].This reactor,which has a planetary mixing device(Fig.16)is entirely piloted by a micro-computer dur-ing the different process steps:sterilisation of the bioreac-tor while empty,sterilisation of the medium,process control during the fermentation and data acquisition.Among oth-ers,this reactor was used for the production by fed-batch SSF of gibberellic acid(Bandelier et al.,1997)and for the production(after15days cultivation)of conidia for biolog-ical control application[34].Up to date,no scale-up of this device was carried out in the industry.3.Conclusions and perspectivesOver the last10years,significant progress has been made in the design of solid state reactors.More and more,studies appear in the literature with promising perspectives.Some designs,such as the rocking drum bioreactor,appear more difficult to scale-up than others[13].When we analyse the evolution of the solid state processes in the world,we ob-serve that it is not a matter of chance that the research and the industrial applications are most important in the Asian countries.An ancient and important tradition,mainly for the food industries,promoted this evolution.But,despite this success,little improvements were carried out from the tradi-tional Koji equipment and,in most cases,the scaling-up has only consisted in mechanising the tray fermenter.It can also be observed that if a pilot reactor exists,empirical studies can be conducted for effective process control design and successful industrial-scale-up[2].Up to now,the majority of the scale-up was done using the rules of thumb,but taking into account the great advan-tages of the SSF process,several research teams are work-ing around the world and significant advances have been made in the development of quantitative approaches for heat and mass transfer([35,36];Ashley et al.,1999),mathe-matical modelling[13],measurement of some process vari-ables[37,38]and for the development of growth models[6].Interesting approaches have been used by several workers not only to characterise the oxygen transfer and the effects of air pressure oscillation amplitude[39],but also to attempt to better understand the behaviour of the micro-organism in such solid state systems[40].However,some aspects of SSF processes still need re-search:development of probes mainly for moisture measure-ment,control and regulation of process variables,analytical procedures well adapted to these specific media,measure-ment of the gaseous environment.Finally,and especially if the aim of the studies is not only academic but for industrial applications,it is important to keep in mind that some phe-nomenon studied at laboratory-scale are not representative of large-scale because the scaling-up is impossible.Very of-ten,the problems linked to the increase in the volume(mass compaction,shrinkage,diminution of the heat transfer,etc.) do not appear at the bench scale and serious errors can be made in the choice of bioreactor design or process control strategy.If an industrial submerged fermentation process is run-ning,it would be unrealistic to imagine its replacement by a SSF process taking into account the level of investment already done by the industry[1].Despite a volumetric pro-ductivity,very often greatly higher obtained in SSF(around 10times more for enzyme production for example),it is difficult on an economical point of view to change the pro-duction tool.But by improving the reactor and the process control;the industry could consider the solid state fermen-tation as a very attractive alternative when new investments are decided or when new processes are developed.Finally, for some industrial applications(production of spores for cheese making for example),SSF constitutes the sole so-lution for a production process and now,more viable and optimised tool can be proposed to the industry. 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