Proteolytic enzymes find wide applications in food and other industries. They account for nearly 60% of the industrial market in the enzyme technology. Proteases are produced extracellularly by fungi and bacteria such as A. oryzae, A. flavus, R. oligosporus, Penicillium citrinum, P. chrysosporium, Bacillus subtilis, B. amyloliquefaciens, and Pseudomonas species (80,85,86). In recent years, different types of proteases such as acid, neutral, and alkaline proteases are being produced by SSF. Proteases production is generally inhibited by carbon sources, indicating the presence of catabolic repression of the biosynthesis. This fact makes it logical to use agro-industrial residues as substrates for proteases production; thus an SSF process becomes imperative. It is interesting to note that although a number of substrates such as wheat bran, rice bran, and oil cakes have been employed for cultivating different microorganisms, wheat bran has been the preferred choice (78-85).

Enzyme production in SSF is generally favoured under partial pressure of carbon dioxide. Studies on the effects of O2 and CO2 partial pressure on acid protease production by a strain of Aspergillus niger ANH-15 in SSF of wheat bran showed a direct relationship between pressure drop, production of CO2 and temperature increase (81). Acid protease production was increased when the gas had 4% CO2 (v/v) and it was directly related with the fungus metabolic activity as represented by the total CO2 evolved. Acid protease production on rice bran using a strain of R. oligosporus by making step changes in the gas environment and temperature during SSF to mimic those changes, which arose during SSF due to mass and heat transfer limitation, showed that a decrease of O2 concentration from 21 to 0.5% did not alter protease production (82,83).

Comparative study on protease production in SSF and SmF generally showed higher enzyme yields in SSF. For example, a study on acid protease production showed that total protease activity in SSF per gram of wheat bran was equivalent to 100 ml broth (79).

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