Stuart et al. (1999) undertook studies in a bioreactor of 85 cm length and 19 cm internal diameter, giving a volume of approximately 20 L. The bioreactor was operated with various substrate loadings and rotational speeds. Aspergillus oryzae was used, in some cases grown on wheat bran, in others on an artificial gel substrate. The temperature in the chamber was maintained at 30-32°C and air was supplied at this temperature. Relevant observations were:
• Regarding the effect of rotational speed on growth on the gel substrate. Between 0 and 10 rpm, there was a beneficial effect of rotation, with an increase in the maximum specific growth rate, while the amount of protein enrichment that occurred remained constant (Fig. 8.5). As the rotational speed used in the fermentation increased from 10 to 50 rpm, there were decreases in both the amount of protein enrichment obtained during the fermentation and the maximum specific growth rate observed. • Regarding the effect of the substrate used on the temperatures reached within the bed. During fermentations with the gel substrate, the bed temperature did not exceed 35°C, while during fermentations with the wheat bran substrate the temperature peaked at values between 45 and 50°C. This effect is probably related to the availability of the carbon sources in the two substrates. The gel substrate contained slightly less than 5% starch by weight (on a wet basis) while the wheat bran substrate contained 15% starch by weight (on a wet basis) and in addition had protein and fat available. Also note that the majority of wheat bran particles were flat and smaller than 4 mm diameter while the gel substrate consisted of 6 mm cubes, such that the wheat bran substrate had a much larger surface area to volume ratio.
De Reu et al. (1993) built a rotating-drum bioreactor in which one end was fixed, in order to allow the insertion of sensors into the bed (Fig. 8.6(a)). This complicates the design, as it is necessary to have a seal between the rotating body and the fixed end-plate. The bioreactor had an inner diameter of 20 cm and a length of 15 cm, giving a total volume of 4.7 L. Although the bed volume was not specifically mentioned, fermentations were undertaken with 1 kg of cooked soybeans (inoculated with Rhizopus oligosporus), meaning that the bed volume was probably between 2 and 2.5 L. Air was introduced into the headspace through the central axis. The bioreactor was placed in a room with an air temperature of 30°C.
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