kernel, cassava, water chestnut, wheat, and wheat bran. Arum and wheat bran gave the highest yields (51).

Although the sources of a-amylases are fairly extensive, the principal commercial preparations are derived from some bacterial and fungal species. These include Bacillus sp., Bacillus amyloliquefaciens, B. coagulans, B. licheniformis, B. megatarium, Aspergillus sp., A. niger, A. oryzae, A. kawachii, Aeromonas caviae, Pycnoporus sanguineus, and Saccharomycopsis capsularia (28). The Bacillus species is considered the most prolific producer of a-amylase by SSF. B. amyloliquefaciens and B. licheniformis are considered potent species for thermophilic a-amylase (24,28,45,52,53). Among filamentous fungi, Thermomyces lanuginosa was reported to be an excellent producer of a-amylase (54). A strain of Aspergillus oryzae was used for a-amylase production using spent brewing grain in SSF and the process was considered economically promising (52). Irrespective of enzyme properties (such as temperature, pH optima, and range), SSFs are typically performed at mesophilic temperatures such as 30°C using agro-industrial residues for 24-96 h.

GA production in SSF was first demonstrated in 1914. The production of GA was carried out in shallow trays with Aspergillus sp., A. awamori, A. niger, A. oryzae, and Rhizopus sp., R. oligosporus, under SSF. An extensive study was carried out on the production of GA in solid cultures by a strain of A. niger (7,12,13,38-44,55-58). The study included screening of various agro-industrial residues, including wheat bran, rice bran, rice husk, gram flour, wheat flour, corn flour, tea waste, and copra waste, individually and in various combinations. Apart from the substrate particle size, which showed profound impact on fungal growth and activity, substrate moisture and water activity also significantly influenced the enzyme yield. Different types of bioreactors, including flasks, trays, rotary reactors, and columns (vertical and horizontal), were used to evaluate their performance. Enzyme production in trays occurred in optimum quantities in 36 h in comparison to the 96 h typically required in flasks. Supplementation of wheat bran medium with yeast extract increased glucoamylase synthesis by the fungal culture.

Several attempts have been made to compare the GA production in SSF and SmF (59-62). Generally SSF yielded higher enzyme titres. However, contrary to the general findings, Rhizopus A-11 showed a 4.6-fold lower GA yield from a conventional SSF on wheat bran medium than the yield in SmF, which used metal ion supplemented medium (60). A similar trend was found with a fungal strain of A. niger, which produced higher GA titres in SmF (102 U/ml) than in SSF (66 U/ml) in a shorter period (66 h in comparison to 96 h). In an aqueous biphasic system comprising polystrene glycol (PEG 6000) and potassium phosphate, however, both the GA recovery and yields were twice as high as the control system comprising an aqueous phase only (59).

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