beta-Fructofuranosidases are powerful tools in industrial biotechnology. We have characterized an extracellular beta-fructofuranosidase from the yeast Schwanniomyces occidentalis. The enzyme shows broad substrate specificity, hydrolyzing sucrose, 1-kestose, nystose and raffinose, with different catalytic efficiencies (k(cat)/K(m)). Although the main reaction catalysed by this enzyme is sucrose hydrolysis, it also produces two fructooligosaccharides (FOS) by transfructosylation. A combination of (1)H, (13)C and 2D-NMR techniques shows that the major product is the prebiotic trisaccharide 6-kestose. The 6-kestose yield obtained with this beta-fructofuranosidase is, to our concern, higher than those reported with other 6-kestose-producing enzymes, both at the kinetic maximum (76gl(-1)) and at reaction equilibrium (44gl(-1)). The total FOS production in the kinetic maximum was 101gl(-1), which corresponded to 16.4% (w/w) referred to the total carbohydrates in the reaction mixture.