Fungal solid state fermentation on agro-industrial wastes for industrially relevant enzymes production

Large amounts of enzymes are needed to make their application feasible at industrial scale, thus considerably increasing costs of industrial processes. Reducing enzymes costs is a key research priority to make industrial bioprocesses competitive in comparison to the conventional ones. From this point of view, solid state fermentation (SSF) holds tremendous potential for the production of industrial enzymes, because of its economical advantages. SSF is defined as a fermentation process taking place on moist solid supports, in absence or near absence of free water (Couto Rodriguez et al, 2005). The aim of the SSF is to bring the cultivated microorganisms into tight contact with the substrate and thus achieve the highest substrates concentration for the fermentation (Hölker et al, 2004). The materials used in the SSF can be inert supports, having only attachment function and requiring nutrient supplementation, or non-inert supports, named also support-substrates, that provide both a source of nutrients and attachment (Couto Rodriguez, 2008). For development of any SSF process there are some important aspects that must be considered, such as the choice of microorganisms and substrates.
Bacteria, fungi and yeasts can be used in SSF, even if the former must be previously manipulated because of their high water activity requirement (Pandey, 2003). Filamentous fungi are the most important group of microorganisms used in SSF process. The hyphal mode of fungal growth and their good tolerance to low water activity and high osmotic pressure conditions make fungi efficient and competitive in natural microflora for bioconversion of solid substrates. As a matter of fact, the hyphal mode of growth gives a major advantage to filamentous fungi over unicellular microorganisms in colonization of solid substrates and utilization of available nutrients. The basic mode of fungal growth is a combination of apical extension of hyphal tips and the generation of new hyphal tips through branching. An important feature is that, although extension occurs only at the tip at a linear and constant rate, the frequency of branching makes the biomass growth kinetic exponential, mainly in the first steps of the vegetative stage. The hyphal mode of growth gives the filamentous fungi the power to penetrate into the solid substrates. Fungi cannot transport macromolecular substrates but the hydrolytic enzymes for their utilization are excreted at the hyphal tip, without large dilution like in the case of Submerged Fermentation (SmF), what makes the action of hydrolytic enzymes very efficient and allows penetration into most solid substrates increasing the accessibility of all available nutrients (Raimbault, 1998).
The selection of the substrate in a SSF process depends upon several factors, mainly related with cost and availability of the substrate. Lignocellulosic wastes (mainly agricultural, food farming residues and, paper, organic and green fractions of urban wastes) are produced in high amounts whose disposal is a major problem for their producers and public authorities. These materials are usually treated as wastes and disposed of as such, either by burning, landfilling or by using them as feed for animals. These management practices cause environmental and economical problems due to the low calorific power and high water content of the wastes, the high costs for their transport, and the evidence that not every material is appropriate for feeding every animal. Besides, due to legislation and environmental reasons, the industry is increasingly being forced to find an alternative use for its residual matter, thus triggering the reutilization of biological wastes in economically and ecologically sustainable ways.