Crude castor oil hydrolysates mainly consisting of ricinoleic acid can be biologically converted to 11-hexyloxy-9-undecenoic acid (C-18) with an unsaturated bond and an ester bond which is utilized as a fertile chemical platform. The platform chemical of 11-hexyloxy-9-undecenoic acid can be converted to omega-hydroxyundec-9-enoic acid or dicarboxylic acid via simple chemical or biological conversion. In this study, the biotransformation was directly performed from crude castor oil hydrolysates containing 82% ricinoleic acid using the recombinant Escherichia coli BL21 with two plasmids containing Micro coccus luteus alcohol dehydrogenase and Pseudomonas putida KT2440 Baeyer-Villiger monooxygenase, respectively. The recombinant cells were cultured in Riesenberg medium at pH 6.9 and 20 degrees C for 28.5 h. For biotransformation at 35 degrees C for 2 h, 2.54 g/L 11-hexyloxy-9-undecenoic acid was obtained from crude castor oil hydrolysates with the final concentration of 2.98 g/L ricinoleic acid. The yield and volumetric productivity were 81.34% and 1.22 g/L/h, respectively. Herein, direct conversion of crude castor oil hydrolysates to value-added 11-hexyloxy-9-undecenoic acid was successfully demonstrated. (C) 2015 Elsevier Ltd. All rights reserved.