BACKGROUND: Fructooligosaccharides are important sweeteners produced by sucrose biotransformation. Although fructooligosccharides production has been reported widely, most studies have been carried out at laboratory level. This study evaluates semibatch and continuous fructooligosaccharides production by Aspergillus sp. N74 at bench scale in a mechanically agitated airlift. RESULTS: Sucrose biotransformation to fructooligosaccharides was carried out with biomass harvested after 24 or 48 h of culture. For 6.21 +/- 0.33 or 9.66 +/- 0.62 g biomass dry weight L-1, the highest FOS yields were obtained at batch operating 62.1 and 66.4% after 26 or 6 h of reaction, respectively. Reduction in fructooligosaccharides yield was observed for both biomass concentrations at semibatch operating, while a comparable yield was obtained during continuous operating (62.1% for 6.21 +/- 0.33 g L-1 and a dilution rate 0.016 s(-1), and 62.8% for 9.66 +/- 0.62 g L-1 and a dilution rate 0.032 s(-1)). Nevertheless, 1-kestose formation was favored with biomass harvested after 24 h under any operating mode. CONCLUSION: Biomass concentration, reaction time and operating mode have a notable effect on fructooligosaccharides yield and composition. 1-kestose, the most valuable fructooligosaccharide, was obtained in greatest proportion at a biomass concentration 6.21 +/- 0.33 g L-1. Under the different operating modes, Aspergillus sp. N74 mycelia and the reactor described are presented as a feasible alternative for scaling up fructooligosaccharides production. (C) 2008 Society of Chemical Industry