Significance of bed porosity, bran and specific surface area in solid-state cultivation of Aspergillus oryzae Yovita S.P. Rahardjo a,b, * , Fenna Jolink b , Sebastiaan Haemers b , Johannes Tramper b , Arjen Rinzema a,b a Wageningen Centre for Food Sciences, P.O. Box 557, 6700 AN Wageningen, The Netherlands b Wageningen University, Agrotechnology and Food Sciences Department Food and Bioprocess Engineering Group, P.O. Box 8129, 6700 EV, Wageningen, The Netherlands Received 29 September 2004; received in revised form 25 May 2005; accepted 25 May 2005 Abstract In this paper, the effects of bed porosity, bran and specific surface area on the oxygen uptake rate and a-amylase production during growth of Aspergillus oryzae on wheat grain and wheat–flour substrate are reported. The high oxygen uptake rate found during cultivation of A. oryzae on wheat–flour substrate was not reached on wheat grain. This is mainly due to the bran of the wheat grain. Using wheat–flour substrates, it was shown that extra bed porosity increased the a-amylase production and oxygen uptake rates. Furthermore, the peak oxygen uptake rate decreased with increasing surface area–volume ratio of the substrate particles, while the a-amylase production and the cumulative oxygen uptake per gram of initial substrate dry matter increased. The present work does not support a direct correlation between aerial mycelia and enzyme production. There is, however, a correlation between the a-amylase yield and the cumulative oxygen uptake (not the uptake rate). This implies that aerial mycelia could accelerate a-amylase production even if they do not increase the yield. # 2005 Elsevier B.V. All rights reserved. Keywords: Aspergillus oryzae; a-Amylase; Solid-state fermentation; Particle size; Bran; Substrate surface area; Porosity 1. Introduction Oxygen supply into the layer of microbial cells covering the substrate particles is a major bottleneck in solid-state fermentation (SSF) [1,2]. In previous work, it has been shown that Aspergillus oryzae can apparently overcome this limitation by forming abundant aerial mycelia. During cultivation on a model substrate (wheat– flour disks), formation of aerial mycelia increased the oxygen uptake rate by a factor 7 [3] and the a-amylase production by a factor 2–3 [4]. However, it appears that A. oryzae does not form abundant aerial mycelia during cultivation on wheat grain, as oxygen uptake rates in continuously mixed [5] and unmixed packed-bed fermenters (M.J. Hoogschagen, personal commununication) were comparable to those in wheat– flour disk cultures in which aerial mycelia formation was suppressed [3]. In continuously mixed fermenters no aerial mycelia were observed at all [5], probably due to the shear. In packed-bed fermenters there is no shear, but nevertheless oxygen uptake rates as well as the appearance of the fermented mass indicated that aerial mycelia were by far not as abundant as observed on wheat–flour disks. This study aims at elucidating the reasons for the limited aerial mycelia formation in grain beds, and at finding ways to promote their formation and thereby perhaps speed up the fermentation process and increase enzyme yields. There are three significant differences between wheat– flour disks and wheat grain: (1) There is an abundant amount of open space available for the formation of aerial mycelia above the wheat–flour www.elsevier.com/locate/geneanabioeng Biomolecular Engineering 22 (2005) 133–139 * Corresponding author. Tel.: +31 317 483770; fax: +31 317 482237. E-mail address: Yovita.Rahardjo@wur.nl (Yovita S.P. Rahardjo). 1389-0344/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.bioeng.2005.05.002