Kinetics of Gibberella fujikuroi Growth and Gibberellic Acid Production by Solid-State Fermentation in a Packed-Bed Column Bioreactor Cristina M. M. Machado,* ,† Bruno O. Oishi, ‡ Ashok Pandey, § and Carlos R. Soccol ‡ Laborato ´rio de Processos Biotecnolo ´gicos, Departamento de Engenharia Quı ´mica, Universidade Federal do Parana ´ (UFPR), 81531-970 Curitiba, PR, Brazil, Embrapa Hortalic ¸ as, BR 060 km 09, 70359-970 Brası ´lia, DF, Brazil, and Biotechnology Division, Regional Research Laboratory, CSIR, Trivandrum-695 019, India In this work the growth of Gibberella fujikuroi and gibberellic acid (GA 3 ) production were studied using coffee husk and cassava bagasse as substrates in a packed-bed column bioreactor connected to a gas chromatograph for exit gas analysis. With the respirometric data, a logarithmic correlation between accumulated CO 2 and biomass production was determined, and the kinetics of the fungal growth was compared for estimated and experimental data. The solid medium consisted of coffee husk (pre- treated with alkali solution), mixed with cassava bagasse (7:3 dry weight basis), with a substrate initial pH of 5.2 and moisture of 77%. Cultivation was carried out in glass columns, which were packed with preinoculated substrate and with forced aeration of 0.24 L of air/[h (g of substrate)] for the first 3 days, and 0.72 L of air/[h (g of substrate)] for the remaining period. The maximum specific growth rate (µ m ) obtained was 0.052 h -1 (between 24 and 48 h of fermentation). A production of 0.925 g of GA 3 / kg of substrate was achieved after 6 days of fermentation. Introduction Gibberellic acid (GA 3 ) is the most important gibberel- lin, a class of diterpenoid acids that function as plant growth regulators. It affects stem elongation, germina- tion, elimination of dormancy, flowering, sex expression, enzyme induction, and leaf and fruit senescence and is a high-valued plant growth regulator with various applica- tions in agriculture. Its high price, however, has limited its use for high-premium crops (1). The industrial process currently used for the produc- tion of GA 3 is based on submerged fermentation (SmF) techniques. Despite the use of the best process technol- ogy, the yield of GA 3 is low. The presence of product in dilute form in SmF was recognized as a major obstacle in economic manufacture of the product, mainly due to the consequent higher costs of downstream processing and disposal of wastewater (2, 3). Recently, different studies have been carried out to decrease the production costs, using several approaches such as screening of the fungi, optimization of the nutrients and culture condi- tions, and development and utilization of alternative substrates (4-10). Brazil has one of the most important agricultural- based economies in the world, producing coffee, sugar- cane, soybeans, cassava, fruits, etc. Almost every product is exported, which is definitely an excellent contribution to its economical development. However, this great production is responsible for the generation of very high amounts of residues that cause serious environmental problems (11-14). Solid-state fermentation (SSF) systems permit the growth of microorganisms on solid material in the ab- sence or near absence of free water. Part of the water is absorbed within the solid matrix (15). General and micro- biological aspects of SSF, operation system conditions, and scaling-up strategies have been reviewed in pertinent literature (15-17). One important step in the develop- ment of an SSF process is the proper description of bio- logical activity. Results of these descriptions can be used for modeling, optimization, and scale-up (18). The impossi- bility of separation of biomass from the substrate and the heterogeneous characteristics of SSF processes are the principal difficulties found when kinetics accomplish- ment is attained. These facts impede the acquirement of representative samples and are particularly acute in the case of fungal growth and mycelia production. Even with the difficulties that are encountered in the SSF process, the kinetic procedure cannot be substituted by good will, subjectivity, or even the simple and overall process description (15). Automatic on-line analysis of CO 2 and O 2 in the exit gases from SSF reactors allows real time information on the physiological state of the cultures to be obtained and correlation with other factors, such as biomass, to be es- tablished. It is also useful for the monitoring of diverse biotransformations and has possible applications to the scale-up of processes. Furthermore, gas measurements in aerobic cultures allow for the calculation of the respiratory activity rate from the natural logarithm of the total production of CO 2 . Since this parameter is obtained from a larger number of data points, a more accurate value of respiratory activity rate than that * To whom correspondence should be addressed. Phone: +55-61- 3859081. Fax: +55-61-5565744. E-mail:cristina@cnph.embrapa.br. † Universidade Federal do Parana ´ (UFPR). ‡ Embrapa Hortalic ¸ as. § Regional Research Laboratory, CSIR. 1449 Biotechnol. Prog. 2004, 20, 1449-1453 10.1021/bp049819x CCC: $27.50 © 2004 American Chemical Society and American Institute of Chemical Engineers Published on Web 08/18/2004