Journal of Food Engineering 76 (2006) 291 302
www.elsevier.com/locate/jfoodeng
Application of solid-state fermentation to food industry A review
a,b a,*
Susana Rodríguez Couto , Ma Ángeles Sanromán
a
Department of Chemical Engineering, Isaac Newton Building, University of Vigo, Lagoas Marcosende, 36310 Vigo, Spain
b
Department of Chemical Engineering, Chemical Engineering School, Rovira i Virgili University, 43007 Tarragona, Spain
Received 9 November 2004; accepted 19 May 2005
Available online 20 July 2005
Abstract
Solid state fermentation (SSF) has become a very attractive alternative to submerged fermentation (SmF) for specific applications
due to the recent improvements in reactor designs. This paper reviews the application of SSF to the production of several metabo-
lites relevant for the food processing industry, centred on flavours, enzymes (a-amylase, fructosyl transferase, lipase, pectinase),
organic acids (lactic acid, citric acid) and xanthan gum. In addition, different types of biorreactor for SSF processes have been
described.
Ó 2005 Elsevier Ltd. All rights reserved.
Keywords: Bioreactors; Enzyme production; Food processing industry; Solid-state fermentation
1. Introduction liquid (Pandey, 1992). The low moisture content means
that fermentation can only be carried out by a limited
Microorganisms have long played a major role in the number of microorganisms, mainly yeasts and fungi,
production of food (dairy, fish and meat products) and although some bacteria have also been used (Pandey,
alcoholic beverages. In addition, several products of Soccol, & Mitchell, 2000a). Some examples of SSF pro-
microbial fermentation are also incorporated into food cesses for each category of microorganisms are reported
as additives and supplements (antioxidants, flavours, in Table 1.
colourants, preservatives, sweeteners,. . .). There is great SSF offers numerous advantages for the production
interest in the development and use of natural food and of bulk chemicals and enzymes (Hesseltine, 1977;
additives derived from microorganisms, since they are Pandey, Selvakumar, Soccol, & Nigam, 1999a; Soccol,
more desirable than the synthetic ones produced by Iloki, Marin, & Raimbault, 1994). This process is known
chemical processes. from ancient times and different fungi have been culti-
Solid-state fermentation (SSF) reproduces the natural vated in SSF for the production of food. Typical exam-
microbiological processes like composting and ensiling. ples of it are the fermentation of rice by Aspergillus
In industrial applications this natural process can be uti- oryzae to initiate the koji process and Penicillium roque-
lised in a controlled way to produce a desired product. fortii for cheese production. Also, in China, SSF has
SSF is defined as any fermentation process performed been used extensively to produce brewed foods (such as
on a non-soluble material that acts both as physical sup- Chinese wine, soy sauce and vinegar) since ancient time
port and source of nutrients in absence of free flowing (Chen, 1992). Also, in Japan SSF is used commercially
to produce industrial enzymes (Suryanarayan, 2003).
Since 1986 in Brazil a series of research projects for the
*
value-addition of tropical agricultural products and
Corresponding author. Tel.: [phone]; fax: [phone].
E-mail address: [e-mail] (Ma. Á. Sanromán). sub-products by SSF has been developed due to the high
0260-8774/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jfoodeng.2005.05.022
292 S.R. Couto, Ma. A. Sanromán / Journal of Food Engineering 76 (2006) 291 302
Table 1
& Nigam, 2003). Castilho, Alves, and Medronho (2000),
Main groups of microorganisms involved in SSF processes (extracted
have performed a detail economic analysis of the pro-
from Raimbault, 1998)
duction of Penicillium restrictum lipase in both SmF
Microflora SSF process
and SSF. They found that for a production scale of
Bacteria
100 m3 lipase concentrate per year, total capital invest-
Bacillus sp. Composting, natto, amylase
ment needed for SmF was 78% higher than that needed
Pseudomonas sp. Composting
for SSF. Also, SSF unitary product cost was 47% lower
Serratia sp. Composting
than the selling price. These studies pointed out that the
Streptococcus sp. Composting
great advantage of SSF processes is the extremely cheap
Lactobacillus sp. Ensiling, food
Clostridium sp. Ensiling, food
raw material used as main substrate. Therefore, SSF is
certainly a good way of utilising nutrient rich solid
Yeast
Endomicopsis burtonii Tape cassava, rice wastes as a substrate. Both food and agricultural wastes
Saccharomyces cerevisiae Food, ethanol
are produced in huge amounts and since they are rich in
Schwanniomyces castelli Ethanol, amylase
carbohydrates and other nutrients, they can serve as a
Fungi
substrate for the production of bulk chemicals and en-
Altemaria sp. Composting
zymes using SSF technique.
Aspergillus sp. Composting, industrial, food
The nature of the solid substrate employed is the
Fusarium sp. Composting, gibberellins
most important factor affecting SSF processes and its
Monilia sp. Composting
selection depends upon several factors mainly related
Mucor sp. Composting, food, enzyme
Rhizopus sp. Composting, food, enzymes,
with cost and availability and, thus, may involve the
organic acids
screening of several agro-industrial residues. In SSF pro-
Phanerochaete chrysosporium Composting, lignin degradation
cess the solid substrate not only supplies the nutrients to
Trichoderma sp. Composting, biological control,
the culture but also serves as an anchorage for the
bioinsecticide
microbial cells. Among the several factors, which are
Beauveria sp., Metharizium sp. Biological control, bioinsecticide
Amylomyces rouxii Tape cassava, rice
important for microbial growth and activity in a partic-
Aspergillus oryzae Koji, food, citric acid
ular substrate, particle size and moisture level/water
Rhizopus oligosporus Tempeh, soybean, amylase, lipase
activity are the most critical (Auria, Palacios, & Revah,
Aspergillus niger Feed, proteins, amylase,
1992; Barrios-Gonzalez, Gonzalez, & Mejia, 1993;
citric acid
Echevarria, Leon, Espinosa, & Delgado, 1991; Liu
Pleurotus oestreatus, sajor-caju Mushroom
Lentinus edodes Shii-take mushroom
& Tzeng, 1999; Pandey, Ashakumary, Selvakumar, &
Penicilium notatum, roquefortii Penicillin, cheese
Vijayalakshmi, 1994; Pastrana, Gonzalez, Pintado, &
Murado, 1995; Roussos, Raimbault, Prebois, & Lon-
sane, 1993; Sarrette, Nout, Gervais, & Rombouts, 1992;
amounts of agricultural residues generated by this coun- Smail, Salhi, & Knapp, 1995; Zadrazil & Punia, 1995).
try (Soccol & Vandenberghe, 2003). Thus, the produc- Generally, smaller substrate particles provide a larger
tion of bulk chemicals and value-added fine products surface area for microbial attack but if they are too
such as ethanol, single-cell protein (SPC), mushrooms, small may result in substrate agglomeration as well as
enzymes, organic acids, amino acids, biologically active poor growth. In contrast, larger particles provide better
secondary metabolites, etc. (Hölker, Höfer, & Lenz, aeration but a limited surface for microbial attack.
2004; Pandey, 1992; Pandey, 1994; Pandey, Azmi, Therefore, a compromised particle size must be selected
Singh, & Banerjee, 1999b; Pandey et al., 1999c; Pandey, for each particular process (Pandey et al., 1999a).
Nigam, & Vogel, 1988; Pandey et al., 2000b; Pandey Research on the selection of suitable substrates for
et al., 1999d; Vandenberghe, Soccol, Pandey, & Lebea- SSF has mainly been centred around agro-industrial resi-
ult, 2000) has been produced from these raw materials dues due to their potential advantages for filamentous
by means of SSF technique. fungi, which are capable of penetrating into the hardest
In recent years, SSF has received more and more of these solid substrates, aided by the presence of turgor
interest from researchers, since several studies for en- pressure at the tip of the mycelium (Ramachandran
zymes (Pandey et al., 1999a), flavours (Ferron, Bonna- et al., 2004). In addition, the utilisation of these agro-
rame, & Durand, 1996), colourants (Johns & Stuart, industrial wastes, on the one hand, provides alternative
1991) and other substances of interest to the food indus- substrates and, on the other, helps in solving pollution
try have shown that SSF can give higher yields problems, which otherwise may cause their disposal
(Tsuchiya et al., 1994) or better product characteristics (Pandey et al., 1999a).
(AcuÅ„a-Arguelles, Gutierrez-Rojas, Viniegra-González, SSF offers numerous advantages over SmF such as
& Favela-Torres, 1995) than submerged fermentation simpler technique and lower cost (Table 2). However,
(SmF). In addition, costs are much lower due to the effi- there are few designs available in the literature for biore-
cient utilisation and value-addition of wastes (Robinson actors operating in solid-state conditions. This is princi-
S.R. Couto, Ma. A. Sanromán / Journal of Food Engineering 76 (2006) 291 302 293
Table 2
Several researchers have studied the production of ar-
Advantages and disadvantages of SSF over SmF
oma compounds by SSF from several microorganisms
Advantages Disadvantages
such as Neurospora sp. (Pastore, Park, & Min, 1994),
Zygosaccharomyces rouxii (Sugawara, Hashimoto,
Higher productivity Difficulties on scale-up
Better oxygen circulation Low mix effectively
Sakurai, & Kobayashi, 1994), Aspergillus sp. (Ito,
Low-cost media Difficult control of process
Yoshida, Ishikawa, & Kobayashi, 1990), using pre-
parameters (pH, heat, moisture,
gelatinised rice, miso and cellulose fibres, respectively.
nutrient conditions,. . .)
Bramorski, Soccol, Christen, and Revah (1998) com-
Less effort in downstream Problems with heat build-up
pared fruity aroma production by Ceratocystis fimbriata
processing
Reduced energy and cost Higher impurity product, increasing
in solid-state cultures using several agro-industrial
requirements recovery product costs
wastes (cassava bagasse, apple pomace, amaranth and
Simple technology
soybean), determining that the media with cassava ba-
Scarce operational problems
gasse, apple pomace or soybean produced a strong fru-
It resembles the natural
ity aroma. Soares, Christen, Pandey, and Soccol (2000)
habitat for several
microrganisms
also reported the production of strong pineapple aroma
when SSF was carried out using coffee husk as a sub-
strate by this strain. Bramorski, Christen, Ramirez, Soc-
pally due to several problems encountered in the control col, and Revah (1998) and Christen, Bramorski, Revah,
of different parameters such as pH, temperature, aera- and Soccol (2000) described the production of volatile
tion and oxygen transfer and moisture. SSF lacks the compounds such as acetaldehyde and 3-methylbutanol
sophisticated control mechanisms that are usually asso- by the edible fungus Rhizopus oryzae during SSF on
ciated with SmF. Control of the environment within the tropical agro-industrial substrates.
bioreactors is also difficult to achieve, particularly tem- Kluyveromyces marxianus produced aroma com-
perature and moisture. pounds, such as monoterpene alcohols and isoamyl ace-
The aim of this paper is to review the potential appli- tate (responsible for fruity aromas), in SSF using
cation of SSF for the production of several metabolites cassava bagasse or giant palm bran as a substrate
of great interest to the food industry. In addition, differ- (Medeiros et al., 2001).
ent types of biorreactor for SSF processes are described. Esters are the source of the aromas and among them
pyrazines, which possess a nutty and roasty flavour, are
used as a food additive for flavouring (Seitz, 1994). Bes-
2. Some examples of applications of SSF to food industry son, Creuly, Gros, and Larroche (1997) and Larroche,
Besson, and Gros (1999) studied the production of
2.1. Flavours 2,5-dimethylpyrazine (2,5-DMP) and tetramethyl-
prazine (TTMP) using B. natto and B. subtilis, respec-
Flavours comprise over a quarter of the world mar- tively, on soybeans in SSF. They found that SSF
ket for food additives. Most of the flavouring com- was very suitable for the production of these
pounds are produced via chemical synthesis or by compounds.
extraction from natural materials. However, recent mar-
ket surveys have shown that consumers prefer foodstuff
2.2. Enzyme production
that can be labelled as natural. Plants have been major
sources of essential oils and flavours but their use de-
Recently, dos Santos, Souza da Rosa, DalÕBoit,
pends on natural factors difficult to control such as
Mitchell, and Krieger (2004) evaluated whether SSF is
weather conditions and plant diseases. An alternative
the best system for producing enzymes. They found that
route for flavour synthesis is based on microbial biosyn-
SSF is appropriate for the production of enzymes and
thesis or bioconversion (Janssens, de Pooter, Van-
other thermolabile products, especially when higher
damme, & Schamp, 1992). Several microorganisms,
yields can be obtained than in SmF.
including bacteria and fungi, are currently known for
their ability to synthesise different aroma compounds.
Attempts to use these microorganisms in SmF resulted 2.2.1. a-Amylase
in low productivity of aroma compounds (Yamguchi a-Amylases (endo-1,4-a-D-glucan glucanohydrolase
et al., 1993), which hampered their industrial applica- EC 3.2.1.1) are extra-cellular endo enzymes that ran-
tion. SSF could be of high potential for this purpose domly cleave the 1,4-a linkages between adjacent glu-
(Berger, 1995). Thus, Ferron et al. (1996) reviewed the cose units in the linear amylose chain and ultimately
prospects of microbial production of food flavours generates glucose, maltose and maltotriose units. Since
and the recommended SSF processes for their the 1950s, fungal amylases have been used to manufac-
production. ture sugar syrups containing specific mixtures of sugars
294 S.R. Couto, Ma. A. Sanromán / Journal of Food Engineering 76 (2006) 291 302
that could not be produced by conventional acid hydro- 2.2.2. Fructosyl transferase
lysis of starch. Amylases are extensively employed in Fructosyl transferase (EC 2.4.1.10) catalyses the
processed-food industry such as baking, brewing, prepa- formation of fructo-oligosaccharides from sucrose.
ration of digestive aids, production of cakes, fruit juices, Fructo-oligosaccharides are present in various com-
starch syrups, etc. monly consumed foods like fruits, vegetables, cereals
The production of a-amylases has generally been car- and honey in trace amounts. The production of fructosyl
ried out using SmF; however, SSF systems appear as a transferase derived from microorganisms has attracted
promising technology. Recently, Francis et al. (2003) attention in recent years by SmF using Aspergillus spp,
used spent brewing grains in SSF for the production Penicillium spp and Aureobasidium spp (Prapulla, Sub-
of a-amylase and determined that the supplement of fer- haprada, & Karanth, 2000) and SSF using both Aspergil-
mentation media with Tween-80 or calcium ions en- lus foetidus and A. oryzae (Hang, Woodams, & Jang,
hanced a-amylase activity. 1995; Sangeetha, Ramesh, & Prapulla, 2004).
Krishna and Chandrasekaran (1996) used banana Recently, Sangeetha et al. (2004) have studied the
fruit stalk as a substrate in SSF with Bacillus subtilis. production of fructosyl transferase by A. oryzae employ-
Different factors such as initial moisture content, parti- ing a great variety of agricultural by-products as sub-
cle size, thermal treatment time and temperature, pH, strates: Cereal brans (wheat bran, rice bran and oat
incubation temperature, additional nutrients, inoculum bran), corn products (corn cob, corn bran, corn germ,
size and incubation period on the production of a-amy- corn meal, corn grits and whole corn powder), coffee-
lase were characterised. Results obtained for the optimi- and tea-processing by-products (coffee husk, coffee pulp,
sation of process parameters clearly shown their impact spent coffee and spent tea), sugarcane bagasse and cas-
on the gross yield of enzymes as well as their indepen- sava bagasse. They found that, among them, the best re-
dent nature in influencing the organismÕs ability to syn- sults were obtained when rice bran, wheat bran, corn
thesise the enzyme. It is known that particle size (specific germ, spent coffee and tea were used supplemented with
surface area) is a critical factor in SSF. Banana fruit yeast extract and complete synthetic media.
stalk particles of 400 lm favoured maximal a-amylase
production compared to larger particles. A similar 2.2.3. Lipase
trend was reported for the production of glucoamylases Lipases (triacylglycerol acylhydrolases, EC 3.1.1.3)
with wheat bran (Pandey, 1991) and cellulases with coir are well known as efficient biocatalysts for the hydroly-
pith of small particle size (Muniswaran & Charyulu, sis of water-insoluble fatty-acid esters, being triacylgly-
1994). cerols of long chain fatty acids their natural substrates.
Nowadays, gelatinisation is coupled with liquefac- Lipases are nowadays widely used at industrial scale
tion, which is possible by the action of thermostable with applications in food, detergent, cosmetic and phar-
amylases, which have been reported in both SmF (Stam- maceutical industries (Jaeger & Reetz, 1998).
ford, Stamford, Coelho, & Araujo, 2001) and SSF Most studies on lipolytic enzymes production by bac-
(Babu & Satyanarayana, 1995). Sodhi, Sharma, Gupta, teria, fungi and yeasts have been performed in sub-
and Soni (2005) determined that the productivity of merged cultures; however, there are few reports on
thermostable amylases from Bacillus sp. was affected lipase synthesis in solid state cultures. In recent years,
by the nature of the solid substrate (wheat bran, rice increasing attention has been paid to the conversion of
bran, corn bran and combination of two brans), nature processing industry wastes in lipase by solid state
of the moistening agent, level of moisture content, cultures.
incubation temperature, presence or absence of sur- There are several reports dealing with extracellular li-
factant, carbon, nitrogen, mineral, amino acid and pase production by fungus such as Rizhopus sp., Asper-
vitamin supplements. Maximum enzyme production gillus sp., Penicillium sp. on different solid substrates
was obtained on wheat bran supplemented with glycerol (Christen, Angeles, Corzo, Farres, & Revah, 1995; Cor-
(1.0%, w/w), soyabean meal (1.0%, w/w), L-proline dova et al., 1998; Gombert, Pinto, Castilho, & Freire,
(0.1%, w/w), vitamin B-complex (0.01%) and moistened 1999; Kamini, Mala, & Puvanakrishnan, 1998; Miranda
with tap water containing 1% Tween-40. et al., 1999) under submerged conditions. However, few
Recently, Ramachandran et al. (2004) reported the researchers have investigated the synthesis of lipase by
use of coconut oil cake (COC) as a substrate for the pro- yeasts using SSF technique. Among them, Rao,
duction of a-amylase by A. oryzae under SSF condi- Jayaraman, and Lakshmanan (1993) determined that
tions. Raw COC supported the growth of the culture, the C/N ratio of the medium is an important parameter
resulting in the production of 1372 U/gds a-amylase in for lipase production by the yeast Candida rugosa.
24 h. Supplementation with 0.5% starch and 1% peptone Rivera-Muńoz, Tinoco-Valencia, Sanchez, and
to the substrate positively enhanced the enzyme synthe- Farres (1991), Ohnishi, Yoshida, and Sekiguchi (1994),
sis producing 3388 U/gds, proving COC a promising Christen et al. (1995) and Benjamin and Pandey
substrate for a-amylase production. (1996a, 1996b, 1997a, 1997b), compared SmF and SSF
S.R. Couto, Ma. A. Sanromán / Journal of Food Engineering 76 (2006) 291 302 295
systems for lipase production. All they found that en- showed more stable properties: they had a higher
zyme yields were higher and stable in the latter. stability to pH and temperature and they were less
Several factors can affect extracellular lipase produc- affected by catabolic repression than pectinases pro-
tion such as pH, temperature, aeration and medium duced by SmF (Acuńa-Arguelles et al., 1995).
composition. Furthermore, the presence of triglycerides
or fatty acids has been reported to increase lipolytic en- 2.3. Organic acids
zyme secretion by a certain number of microorganisms
(Marek & Bednarski, 1996). Therefore, in SSF the type Organic acids have been utilised for long time by the
of substrate could be used to enhance the production of food industry as food additives and preservatives for
enzymes, as several food and agroindustrial wastes are prevent deterioration and extending the shelf life of
rich in fatty acids, triglycerides and/or sugars. perishable food ingredients. Here, two common organic
Dominguez, Costas, Longo, and Sanroman (2003) acids widely used on the food industry have been
have reported the great potential of food-agroindustrial considered.
wastes (ground nut and barley bran) as support-sub-
strates for lipase production in solid state cultures of
2.3.1. Lactic acid
the yeast Y. lipolytica, since they led to much higher
Lactic acid fermentation has received extensive atten-
activities than those found using an inert support.
tion since long time (Benninga, 1990; Vickroy, 1985). It
has wide applications in food, pharmaceutical, leather
2.2.4. Pectinases
and textile industries and as a chemical feed stock. It
They constitute a heterogeneous group of enzymes
has two enantiomers L(+) and D( ) of which L(+) is
that catalyse the degradation of pectins, which are the
used by human metabolism due to the presence of L lac-
structural polysaccharides present in vegetable cells
tate dehydrogenase and is preferred for food. Nowa-
and are responsible for maintaining the plant tissues
days, lactic acid is in great demand due to its use as
integrity (Alkorta, Garbisu, Llama, & Serra, 1998). Pec-
starting material to produce biodegradable polymers
tinases are widely used in the food industry to clarify
used in medical, industrial and consumer products
fruit juices and wine, to improve oil extraction, to re-
(Bohlmann & Yoshida, 2000; Gross & Kalra, 2002;
move the peel from citrus fruit, to increase the firmness
Lichtfield, 1996; Malhotra, Raina, & Sanjay, 2000).
of several fruits and to degum fibres (Baker & Wicker,
Soccol, Marin, Rimbault, and Labeault (1994) stud-
1996; Chang, Siddiq, Sinha, & Cash, 1994).
ied the production of L(+)-lactic acid by Rhizopous ory-
Commercial pectinase preparations are produced
zae in solid-state conditions operating with sugarcane
from fungal microorganisms, mainly by Aspergillus
bagasse as a support. They obtained a slightly higher
niger strains. The use of SSF for pectinase production
productivity than in submerged cultivation. Also, Rich-
has been proposed using different solid agricultural
ter and Träger (1994) investigated the L(+)-lactic acid
and agro-industrial residues as substrates such as wheat
production by Lactobacillus paracasei in solid-state con-
bran (Castilho, Alves, & Medronho, 1999; Singh, Platt-
ditions using sweet sorghum as a support. More re-
ner, & Diekmann, 1999), soy bran (Castilho et al., 2000),
cently, Naveena, Altaf, Bhadrayya, Madhavendra, and
cranberry and strawberry pomace (Zheng & Shetty,
Reddy (2005) and Naveena, Altaf, Bhadriah, and Reddy
2000), coffee pulp and coffee husk (Antier, Minjares,
(2005) have reported the production of L(+) lactic acid
Roussos, & Viniegra-Gonzalez, 1993), husk (Antier,
by Lactobacillus amylophilus GV6 under SSF conditions
Minjares, Roussos, Raimbault, & Viniegra-González,
using wheat bran as both support and substrate.
1993; Boccas, Roussos, Gutierrez, Serrano, & Viniegra,
1994), cocoa (Schwan, Cooper, & Wheals, 1997), lemon
and orange peel (Garzón & Hours, 1991; Ismail, 1996; 2.4. Citric acid
Maldonado, Navarro, & Callieri, 1986), orange bagasse,
sugar cane bagasse and wheat bran (Martins, Silva, Da Citric acid is one of the most commonly used organic
Silva, & Gomes, 2002), sugar cane bagasse (Acuńa- acids in food and pharmaceutical industries. The food
Arguelles, Gutierrez-Rojas, Viniegra-González, & industry is the largest consumer of citric acid, using al-
Favela-Torres, 1994) and apple pomace (Hours, Voget, most 70% of the total production, followed by about
& Ertola, 1988a, 1988b). Also, Bai, Zhang, Qi, Peng, 12% for the pharmaceutical industry and 18% for other
and Li (2004) produced pectinase from A. niger by SSF applications (Shah, Chattoo, Baroda, & Patiala, 1993).
using sugar beet pulp as a carbon source and wastewater Its pleasant taste, high solubility and flavour-enhancing
from monosodium glutamate production as nitrogen properties have ensured its dominant position in the
and water source. This allowed not only reducing pro- market. Although citric acid can be obtained by chemi-
duction costs but also decreasing the pollution source. cal synthesis, the cost is much higher than using fermen-
It was found that SSF was more productive than tation. It is mainly produced by SmF, by the filamentous
SmF and, in addition, the pectinases produced by SSF fungus A. niger. Recently, in order to increase the
296 S.R. Couto, Ma. A. Sanromán / Journal of Food Engineering 76 (2006) 291 302
efficiency of citric acid production using A. niger, SSF the substrates, xanthan yields were comparable to those
has been studied as a potential alternative to SmF. obtained from conventional submerged cultivation. In
The production of citric acid depends strongly on an addition, the products were analysed by NMR spectros-
appropriate strain and on operational conditions. Oxy- copy, revealing a composition consistent with that of
gen level is an important parameter for citric acid fer- commercial xanthan.
mentation. Several researchers (Pintado, Lonsane,
Gaime-Perraud, & Roussos, 1998; Prado et al., 2004) 2.6. SSF bioreactors
have studied the influence of forced aeration on citric
acid production and the metabolic activity of A. niger The design of an efficient industrial-level reactor for
in SSF by respirometric analysis. They showed that SSF is of significance because SSF is more environmen-
citric acid production was favoured by a limited biomass tally friendly than SmF. However, it shows considerable
production, which occurred with low aeration rates. drawbacks such as transfer resistance, steep gaseous
Both works showed the feasibility of using the strain concentration and heat gradients that develop within
A. niger for citric acid production by SSF. the medium bed, which may adversely affect solid-state
Different agro-industrial residues such as apple fermentor performances (Ghildyal, Ramalaishna, Lon-
pomace, coffee husk, wheat straw, pineapple waste, sane, & Karantb, 1992; Lonsane, Saucedo-Castuneda,
mixed fruit, maosmi waste, cassava bagasse, banana, & Raimbault, 1992; Sargantanis, Karim, Murphy,
sugar beet cosset and kiwi fruit peel have been investi- Ryoo, & Tengerdy, 1992). Agitation and rotation in
gated for their potential to be used as substrates (Hang SSF were often carried out to improve mass and heat
& Woodams, 1985; Khare, Krishana, & Gandhi, 1995; transfers, but the shearing force caused by agitation
Kumar, Jain, Shanker, & Srivastava, 2003a, 2003b; and rotation has adverse effects on medium porosity
Shojaosadati & Babaripour, 2002). In addition, SSF and disrupts fungal mycelia.
gave high citric acid yield without inhibition related to There are four types of reactors to perform SSF pro-
presence of certain metal ions such as Fe2+, Mn2+, cesses and each in their own design tries to make condi-
Zn2+, etc. (Gutierrez-Rozas, Cordova, Auria, Revah, tions more favourable for fermentation under solid state
& Favela-Torres, 1995), although Shankaranand and conditions. The bioreactors commonly used, which can
Lonsane (1994) reported that addition of these minerals be distinguished by the type of aeration or the mixed
into the production media to a certain level enhanced system employed, include the following:
citric acid production by 1.4 1.9 fold with respect to Tray: It consists of flat trays. The substrate is spread
SmF. Therefore, SSF is a good way of using nutrient onto each tray forming a thin layer, only a few centime-
rich solid waste as a substrate. tres deep. The reactor is kept in a chamber at constant
temperature through which humidified air is circulated
2.5. Xanthan gum (Fig. 1). The main disadvantage of this configuration
is that numerous trays and large volume are required,
Xanthan gum is a hetero-polysaccharide produced making it an unattractive design for large-scale produc-
industrially by the bacterium Xanthomonas campestris, tion (Pandey, Soccol, Rodriguez-Leon, & Nigam, 2001).
fermenting commonly glucose or sucrose. It is the most Packed-bed: It is usually composed of a column of
important microbial polysaccharide from the commer- glass or plastic with the solid substrate retained on a per-
cial point of view, with a worldwide production of about forated base. Through the bed of substrate humidified
30,000 tons per year, corresponding to a market of $408 air is continuously forced (Durand et al., 1993; Raimba-
million (Demain, 2000; Sutherland, 1998). This water- ult, 1998; Rodríguez Couto, Rivela, MuÅ„oz, &
soluble microbial polysaccharide gives aqueous solu- Sanromán, 2000). It may be fitted with a jacket for
tions with several industrial applications in the food, circulation of water to control the temperature during
cosmetic, textile and pharmaceutical industries due to fermentation (Fig. 2). This is the configuration usually
their rheological properties. Because of these properties, employed in commercial koji production. The main
they have been used as emulsifiers, as stabilisers and as drawbacks associated with this configuration are: diffi-
texture enhancers in the food industry. culties in obtaining the product, non-uniform growth,
Recently, the feasibility of this exopolysaccharide poor heat removal and scale-up problems.
production using SSF has been reported by the group Horizontal drum: This design allows adequate aera-
of Stredansky and Conti (1999), Stredansky, Conti, tion and mixing of the substrate, whilst limiting the
Navarini, and Bertocchi (1999). X. campestris strains damage to the inoculum or product. Mixing is per-
were cultivated on a great variety of solid substrates or formed by rotating the entire vessel or by various agita-
by-products such as spent malt grains, apple pomace, tion devices such as paddles and baffles (Domínguez,
grape pomace and citrus peels, easily available and Rivela, Rodríguez Couto, & Sanromán, 2001; Nagel,
low cost substrates, in order to evaluate their ability to Tramper, Bakker, & Rinzema, 2001a, Nagel, Tramper,
produce the exopolysaccharide xanthan. With most of Bakker, & Rinzema, 2001b; Prado et al., 2004; Stuart,
S.R. Couto, Ma. A. Sanromán / Journal of Food Engineering 76 (2006) 291 302 297
Gas exit
Gas exit
3 rpm
Support susbstrate
Culture medium level
Air diffuser
Sampling port
Air inlet
Fig. 3. Scheme of a horizontal drum bioreactor (humidified air;
Trays
mechanical agitation).
Solid substrate
Air inlet
Fig. 1. Scheme of a tray bioreactor (passive aeration; static).
Mitchell, Johns, & Litster, 1999) (Fig. 3). Its main disad-
vantage is that the drum is filled to only 30% capacity,
otherwise mixing is inefficient.
Fluidised bed: In order to avoid the adhesion and
aggregation of substrate particles, this design supplies
a continue agitation with forced air (Fig. 4). Although
the mass heat transfer, aeration and mixing of the sub-
strate is increased, damage to inoculum and heat
build-up through sheer forces may affect the final
product yield.
The different disadvantages detected in the above-
mentioned bioreactor designs to perform SSF processes
have promoted the necessity of developing new bioreac-
tor configurations or modifying the already existing de-
signs. These bioreactor configurations should be able to
operate in continuous mode with high productivity for
prolonged periods of time without operational problems
as well as permit the scale-up of the process. Our re-
search group has been working in this field, resulting
Fig. 4. Scheme of a fluidised-bed bioreactor (humidified air; pneu-
matic agitation).
in the design of a new bioreactor, called immersion bio-
reactor. This bioreactor consists of a jacketed cylindrical
glass vessel with a round bottom, inside which several
wire mesh baskets filled with support colonised by the
fungus are placed. They moved upwards and down-
wards by means of a pneumatic system, remaining more
time outside than inside the medium (Rivela, Rodríguez
Couto, & Sanromán, 2000) (Fig. 5). It is noteworthy
that this bioreactor configuration was also able to run
in continuous mode without operational problems,
attaining high ligninolytic enzyme activities (Rodriguez
Couto, Barreiro, Rivela, Longo, & Sanroman, 2002).
Different studies were carried out for the production
of natural food and additives derived from micro-
Fig. 2. Scheme of a packed-bed bioreactor (humidified air; static). organisms in different bioreactor configurations. For
298 S.R. Couto, Ma. A. Sanromán / Journal of Food Engineering 76 (2006) 291 302
A packed-bed bioreactor with four stages was con-
structed and operated for microbial production of citric
Pneumatic system
Compressor
Compresso
r
acid by A. niger using apple pomace as a substrate.
Under the optimised conditions, 124 g citric acid was
produced from 1 kg dry apple pomace with yield of
Gas exit
80% based on total sugar (Shojaosadati & Babaripour,
2002).
Culture medium level
Water cooling
3. Conclusion
Mesh with substrate
Critical analysis of the literature shows that produc-
tion of relevant compounds for the food processing
Sampling port industry by SSF offers several advantages. It has been
Air inlet
well established that enzyme titres produced in SSF sys-
Fig. 5. Scheme of an immersion bioreactor (humidified air; mechanical tems are much higher than the achieved in SmF ones.
agitation).
Although the reasons for this are not clear, this fact is
kept in mind while developing novel bioreactors for
SSF processes.
example, the production of aroma compounds by K.
marxianus grown on cassava bagasse in solid state fer-
mentation using packed bed reactors, testing two differ- Acknowledgments
ent aeration rates was studied by Medeiros et al. (2001).
Headspace analysis of the culture by gas chromatogra- The authors are grateful to Xunta de Galicia (local
phy showed the production of 11 compounds. The pre- government of Spain) for the financial support of the re-
dominant compounds were ethyl acetate, ethanol and search post of Susana Rodríguez Couto under the pro-
acetaldehyde. The fruity aroma was attributed to the gramme Isidro Parga Pondal.
productions of esters.
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