Enzymes are among the most important products obtained for human needs through microbial sources. A large number of industrial processes in the areas of industrial, environmental and food biotechnology utilize enzymes at some stage or the other. Current developments in biotechnology are yielding new applications for enzymes. Solid state fermentation (SSF) holds tremendous potential for the production of enzymes. It can be of special interest in those processes where the crude fermented products may be used directly as enzyme sources. This review focuses on the production of various industrial enzymes by SSF processes. Following a brief discussion of the micro-organisms and the substrates used in SSF systems, and aspects of the design of fermenter and the factors affecting production of enzymes, an illustrative survey is presented on various individual groups of enzymes such as cellulolytic, pectinolytic, ligninolytic, amylolytic and lipolytic enzymes, etc.
Solid state fermentation (SSF) holds tremendous potential for the production of enzymes. It can be of special interest in those processes where the crude fermented product may be used directly as the enzyme source1. In addition to the conventional applications in food and fermentation industries, microbial enzymes have attained significant role in biotransformations involving organic solvent media, mainly for bioactive compounds. This system offers numerous advantages over submerged fermentation (SmF) system, including high volumetric productivity, relatively
higher concentration of the products, less effluent generation, requirement for simple fermentation equipments, etc.
Microorganisms used for the production of enzymes in solid state fermentation systems
A large number of microorganisms, including bacteria, yeast and fungi produce different groups of enzymes. Selection of a particular strain, however, remains a tedious task, especially when commercially competent enzyme yields are to be achieved. For example, it has been reported that while a strain of Aspergillus niger produced 19 types of enzymes, a -amylase was being produced by as many as 28 microbial cultures3. Thus, the selection of a suitable strain for the required purpose depends upon a number of factors, in particular upon the nature of the substrate and environmental conditions. Generally, hydrolytic enzymes, e.g. cellulases, xylanases, pectinases, etc. are produced by fungal cultures, since such enzymes are used in nature by fungi for their growth. Trichoderma spp. and Aspergillus spp. have most widely been used for these enzymes. Amylolytic enzymes too are commonly produced by filamentous fungi and the preferred strains belong to the species of Aspergillus and Rhizopus. Although commercial production of amylases is carried out using both fungal and bacterial cultures, bacterial a -amylase is generally preferred for starch liquefaction due to its high temperature stability. In order to achieve high productivity with less production cost, apparently, genetically modified strains would hold the key to enzyme production.
Substrates used for the production of enzymes in SSF systems
Agro-industrial residues are generally considered the best substrates for the SSF processes, and use of SSF for the production of enzymes is no exception to that. A number of such substrates have been employed for the cultivation of microorganisms to produce host of enzymes . Some of the substrates that have been used included sugar cane bagasse, wheat bran, rice bran, maize bran, gram bran, wheat straw, rice straw, rice husk, soyhull, sago hampas, grapevine trimmings dust, saw dust, corncobs, coconut coir pith, banana waste, tea waste, cassava waste, palm oil mill waste, aspen pulp, sugar beet pulp, sweet sorghum pulp, apple pomace, peanut meal, rapeseed cake, coconut oil cake, mustard oil cake, cassava flour, wheat flour, corn flour, steamed rice, steam pre-treated willow, starch, etc.. Wheat bran however holds the key, and has most commonly been used, in various processes.
Aspects of design of fermenter for enzyme production in solid state fermentation systems
Over the years, different types of fermenters (bioreactors) have been employed for various purposes in SSF systems. Pandey8 reviewed the aspects of design of fermenter in SSF processes. Laboratory studies are generally carried out in Erlenmeyer flasks, beakers, petri dishes, roux bottles, jars and glass tubes (as column fermenter). Large-scale fermentation has been carried out in tray-, drum- or deep-trough type fermenters. The development of a simple and practical fermenter with automation, is yet to be achieved for the SSF processes.
Factors affecting enzyme production in solid state fermentation systems
The major factors that affect microbial synthesis of enzymes in a SSF system include: selection of a suitable substrate and microorganism; pre-treatment of the substrate; particle size (inter-particle space and surface area) of the substrate; water content and aw of the substrate; relative humidity; type and size of the inoculum; control of temperature of fermenting matter/removal of metabolic heat; period of cultivation; maintenance of uniformity in the environment of SSF system, and the gaseous atmos-phere, i.e. oxygen consumption rate and carbon dioxide evolution rate.
Enzymes produced by solid state fermentation processes
Ideally, almost all the known microbial enzymes can be produced under SSF systems. Literature survey reveals that much work has been carried out on the production of enzymes of industrial importance, like proteases, cellulases, ligninases, xylanases, pectinases, amylases, glucoamylases, etc.; and attempts are also being made to study SSF processes for the production of inulinases, phytases, tannases, phenolic acid esterases, microbial rennets, aryl-alcohol oxidases, oligosaccharide oxidases, tannin acyl hydrolase, a -L-arabinofuranosidase, etc. using SSF systems (cf. Table 2). In the following sections, a brief account of production on various enzymes in SSF systems is discussed.
L-glutaminase is considered a potent anti-leukamic drug and has found application as a flavour-enhancing agent in food industry. In a maiden report, Prabhu and Chandrasekaran reported L-glutaminase production by SSF using marine Vibrio costicola. Polystyrene was used as the inert substrate. They also evaluated several organic substrates for their ability to produce glutaminases by SSF using the same strain. Among the tested materials, wheat bran and rice bran were found superior in comparison to saw dust, coconut oil cake, and groundnut cake. However, use of polystyrene as the substrate offered several advantages over organic substrtes. For example, leachate from polystyrene-SSF system was not only less viscous but also showed high specific activity of the enzyme.