Immobilization of Bacillus firmus strain 37 in inorganic matrix for cyclodextrin production

Cyclodextrins (CDs) are produced industrially from starch, using bacterial cyclodextrin glucanotransferases (CGTase). Cells of Bacillus firmus strain 37, immobilized by adsorption in different inorganic matrix, were used for β-CD production. The matrices were prepared by the sol–gel method and scanning electron microscopy observations showed that SiO2/TiO2 and SiO2/MnO2 were the most suitable matrices for effective immobilization. The operational stability of the immobilized cells was studied by repeated batch cultivation in four cycles of 144 h each. At the end of the second cycle, the cells immobilized in both matrices had maintained 60% of the β-CD production from the initial cycle, and a gradual fall in β-CD production occurred up to the fourth cycle, due to cell desorption from the matrices. Storage stability was studied for 7 months and the maximum β-CD production by immobilized cells in SiO2/MnO2 and SiO2/TiO2 occurred within 90 days of storage at 4 °C (4.4 ± 0.4 and 6.0 ± 0.3 mM β-CD, respectively). After 150 days of storage, they presented the same β-CD production as freshly prepared biocatalysts. When control cycles using free cells were carried out, they produced on average 3.0 ± 0.1 mM β-CD; hence, the immobilized cells showed a one-and-a-half to twofold increase in β-CD production compared to the free cells.