Solid state fermentation in a rotating drum bioreactor for the production of hydrolytic enzymes A.B. Díaz, I. de Ory, I. Caro and A. Blandino Dpt. of Chemical Engineering, Food Technology and Environmental Technologies Faculty of Sciences, Pol Rio S Pedro S/N. Puerto Real-Cádiz (Spain) Previous studies demonstrated that Aspergillus awamori produces pectinases, xylanases and cellulases by solid state fermentation. In order to evaluate the production of these enzymes in a laboratory-scale bioreactor, a rotating drum fermentor was used. Different air flow rates were tested demonstrating a positive effect of this parameter on enzyme activity. The production of the highest enzyme activity level was attained working in static or with an agitation of 1 min/day, using an aeration of 120 mL/min. 1. Introduction Solid state fermentation (SSF) involves the growth of microorganisms on moist solid substrates in the absence of visible water between the substrate particles (Pandey et al., 2001). Compared to submerged fermentation, the solid media used in SSF contain less water but an important gas phase exists between the particles (Durand, 2003). This feature is of great importance because of the poor thermal conductivity of the air compared to the water. As a result of the low Aw in SSF bioreactors, smaller fermenters are required and more concentrated products are produced, simultaneously reducing energy requirements for downstream processing (Robinson et al., 2001; Robinson and Nigam, 2003). However, this technique shows several disadvantages over submerged fermentation (SmF), which have discouraged its use for industrial production (Hölker et al., 2004). One of the major obstacles is the limited knowledge related to the design and operation of large-scale bioreactors (Ashley et al., 1999; Durand and Chereau, 1987). Difficulties in controlling important culture parameters, such as mass transfer and heat removal, have not been overcome completely (Fujian et al. 2002). The low moisture and poor thermal conductivity of the substrate make heat transfer and temperature control difficult in SSF (Bhargav et al. 2008). Many bioreactors have been traditionally used in SSF processes. These can be mainly classified in two groups: the ones which show an agitation system and the ones which work in static conditions. The first category comprises rotating drums, gas-solid fluidized beds, rocking drums, horizontal paddle mixer, etc, while the second one includes the packed-bed and the trays bioreactor. Static beds are required when the substrate bed must remain static throughout the growth phase or when the substrate particles have to be knitted together by the fungal mycelium, such as in the production of fermented foods like Tempe (Mitchell and von Meien, 2000). On the other hand, the use of mixed bioreactors improves the