BACKGROUNDMagnetic nanomaterial has been considered a promising carrier for cell immobilization, because of its low mass transfer resistance, high load rate, high specific surface and facility for reusability. In this study, carboxyl functioned magnetic nanoparticles (CMNPs) were prepared by chemical co-precipitation and oleate modification methods, and then immobilized with Clostridium tyrobutyricum for the production of butyric acid. RESULTSThe immobilization conditions were systematically optimized to achieve maximum efficiency and the highest cell relative activity, in which the mass ratio, immobilization temperature and time were determined as 0.72 g g(-1), 35 degrees C and 30 min, respectively. The endurance of CMNPs-cells was also evaluated in a repeated-batch mode of eight continuous batches. Stable and reliable production of butyric acid was obtained in each batch with an average butyrate titer and yield of approximate to 11.02 g L-1 and approximate to 0.43 g g(-1) utilized sugar. Compared with the free-cell fermentation, higher butyrate yield and concentration were obtained with immobilized cells of C. tyrobutyricum in repeated-batch fermentation, suggesting that the coated cells did not experience a mass transfer problem. CONCLUSIONConsidering the convenient separation of CMNPs-cells from fermentation broth via an external magnetic force, as well as its excellent long-term performance overcoming the possible accumulation of dead cells and culture degeneration, the developed usage of CMNPs-cells could be very benign for industrial application. (c) 2018 Society of Chemical Industry