Application of solid-state fermentation to food industry—A review
Introduction
Microorganisms have long played a major role in the production of food (dairy, fish and meat products) and alcoholic beverages. In addition, several products of microbial fermentation are also incorporated into food as additives and supplements (antioxidants, flavours, colourants, preservatives, sweeteners, …). There is great interest in the development and use of natural food and additives derived from microorganisms, since they are more desirable than the synthetic ones produced by chemical processes.
Solid-state fermentation (SSF) reproduces the natural microbiological processes like composting and ensiling. In industrial applications this natural process can be utilised in a controlled way to produce a desired product.
SSF is defined as any fermentation process performed on a non-soluble material that acts both as physical support and source of nutrients in absence of free flowing liquid (Pandey, 1992). The low moisture content means that fermentation can only be carried out by a limited number of microorganisms, mainly yeasts and fungi, although some bacteria have also been used (Pandey, Soccol, & Mitchell, 2000a). Some examples of SSF processes for each category of microorganisms are reported in Table 1.
SSF offers numerous advantages for the production of bulk chemicals and enzymes (Hesseltine, 1977, Pandey et al., 1999a, Soccol et al., 1994). This process is known from ancient times and different fungi have been cultivated in SSF for the production of food. Typical examples of it are the fermentation of rice by Aspergillus oryzae to initiate the koji process and Penicillium roquefortii for cheese production. Also, in China, SSF has been used extensively to produce brewed foods (such as Chinese wine, soy sauce and vinegar) since ancient time (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 sub-products by SSF has been developed due to the high amounts of agricultural residues generated by this country (Soccol & Vandenberghe, 2003). Thus, the production of bulk chemicals and value-added fine products such as ethanol, single-cell protein (SPC), mushrooms, enzymes, organic acids, amino acids, biologically active secondary metabolites, etc. (Hölker et al., 2004, Pandey, 1992, Pandey, 1994, Pandey et al., 1999b, Pandey et al., 1999c, Pandey et al., 1988, Pandey et al., 2000b, Pandey et al., 1999d, Vandenberghe et al., 2000) has been produced from these raw materials by means of SSF technique.
In recent years, SSF has received more and more interest from researchers, since several studies for enzymes (Pandey et al., 1999a), flavours (Ferron, Bonnarame, & Durand, 1996), colourants (Johns & Stuart, 1991) and other substances of interest to the food industry have shown that SSF can give higher yields (Tsuchiya et al., 1994) or better product characteristics (Acuña-Arguelles, Gutierrez-Rojas, Viniegra-González, & Favela-Torres, 1995) than submerged fermentation (SmF). In addition, costs are much lower due to the efficient utilisation and value-addition of wastes (Robinson & Nigam, 2003). Castilho, Alves, and Medronho (2000), have performed a detail economic analysis of the production of Penicillium restrictum lipase in both SmF and SSF. They found that for a production scale of 100 m3 lipase concentrate per year, total capital investment needed for SmF was 78% higher than that needed for SSF. Also, SSF unitary product cost was 47% lower than the selling price. These studies pointed out that the great advantage of SSF processes is the extremely cheap raw material used as main substrate. Therefore, SSF is certainly a good way of utilising nutrient rich solid wastes as a substrate. Both food and agricultural wastes are produced in huge amounts and since they are rich in carbohydrates and other nutrients, they can serve as a substrate for the production of bulk chemicals and enzymes using SSF technique.
The nature of the solid substrate employed is the most important factor affecting SSF processes and its selection depends upon several factors mainly related with cost and availability and, thus, may involve the screening of several agro-industrial residues. In SSF process the solid substrate not only supplies the nutrients to the culture but also serves as an anchorage for the microbial cells. Among the several factors, which are important for microbial growth and activity in a particular substrate, particle size and moisture level/water activity are the most critical (Auria et al., 1992, Barrios-Gonzalez et al., 1993, Echevarria et al., 1991, Liu and Tzeng, 1999, Pandey et al., 1994, Pastrana et al., 1995, Roussos et al., 1993, Sarrette et al., 1992, Smail et al., 1995, Zadrazil and Punia, 1995).
Generally, smaller substrate particles provide a larger surface area for microbial attack but if they are too small may result in substrate agglomeration as well as poor growth. In contrast, larger particles provide better aeration but a limited surface for microbial attack. Therefore, a compromised particle size must be selected for each particular process (Pandey et al., 1999a).
Research on the selection of suitable substrates for SSF has mainly been centred around agro-industrial residues due to their potential advantages for filamentous fungi, which are capable of penetrating into the hardest of these solid substrates, aided by the presence of turgor pressure at the tip of the mycelium (Ramachandran et al., 2004). In addition, the utilisation of these agro-industrial wastes, on the one hand, provides alternative substrates and, on the other, helps in solving pollution problems, which otherwise may cause their disposal (Pandey et al., 1999a).
SSF offers numerous advantages over SmF such as simpler technique and lower cost (Table 2). However, there are few designs available in the literature for bioreactors operating in solid-state conditions. This is principally due to several problems encountered in the control of different parameters such as pH, temperature, aeration and oxygen transfer and moisture. SSF lacks the sophisticated control mechanisms that are usually associated with SmF. Control of the environment within the bioreactors is also difficult to achieve, particularly temperature and moisture.
The aim of this paper is to review the potential application of SSF for the production of several metabolites of great interest to the food industry. In addition, different types of biorreactor for SSF processes are described.
Section snippets
Flavours
Flavours comprise over a quarter of the world market for food additives. Most of the flavouring compounds are produced via chemical synthesis or by extraction from natural materials. However, recent market surveys have shown that consumers prefer foodstuff that can be labelled as natural. Plants have been major sources of essential oils and flavours but their use depends on natural factors difficult to control such as weather conditions and plant diseases. An alternative route for flavour
Conclusion
Critical analysis of the literature shows that production of relevant compounds for the food processing industry by SSF offers several advantages. It has been well established that enzyme titres produced in SSF systems are much higher than the achieved in SmF ones. Although the reasons for this are not clear, this fact is kept in mind while developing novel bioreactors for SSF processes.
Acknowledgments
The authors are grateful to Xunta de Galicia (local government of Spain) for the financial support of the research post of Susana Rodríguez Couto under the programme Isidro Parga Pondal.
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