The formation of 2-alkanones by free and microencapsulated P. roquefortii spores in an aqueous-organic two-phase system was investigated by using substrates supplied as a solution in decane. It was shown that the spores remained catalytically active after entrapment within permeable polyamide microcapsules and readily catalyzed the formation of 2-pentanone, 2-heptanone, and 2-undecanone from short-chain alkyl esters of hexanoic, octanoic, and lauric acid, respectively, with the rate of reaction being markedly dependent on the type and concentration of the ester substrate used. in general, the optimal concentration of the esters in decane was found to be much higher than that of the respective fatty acid substrates and, in the case of alkyl dodecanoates, the biotransformation could be carried out efficiently even in the absence of added solvent Further analysis revealed a significant difference in the reaction rates observed with free and microencapsulated spores at 0.5 but not at 3.0 M methyl dodecanoate, suggesting that at high substrate concentrations the biotransformation was no longer limited by mass transfer. (C) 2000 Elsevier Science Inc. All rights reserved.