Abstract
Carbon nanotubes are increasingly finding application in a wide range of industries. The focus of this study was to investigate the immobilisation of yeast cells onto carbon nanotubes, using a flocculation method, for possible use in fermentation processes. Carbon nanotubes, which are long thin cylinders of carbon, were used as artificial agents to induce flocculation of yeast cells. The immobilisation experiments on carbon nanotubes were conducted under different process conditions and compared with control experiments done on free cells. The resultant immobilised cells or flocs were recovered and freeze dried before analysis was performed. The flocculated cells were characterised by scanning electron microscopy to confirm that flocculation had occurred. Conditions that gave optimum flocculation on carbon nanotubes were found to be: a pH between 5.0 and 5.8, a temperature between 25 °C and 30 °C, an agitation speed of about 110 rpm, and a concentration of carbon nanotubes (in powder form) of between 44 mg/mL and 54 mg/mL. The addition of calcium ions and glucose decreased the rate of flocculation and delayed the onset of flocculation. Our study has demonstrated that carbon nanotubes have great potential to improve the flocculation capacity of brewer’s yeast.
Highlights
There are numerous biotechnological processes that make use of immobilisation techniques to manipulate cells
A typical example is the yeast cells flocculation that occurs at the end of a fermentation process in the stationary phase, where the flocculated cells either sink to the bottom of the fermenter or rise to the surface attached to carbon dioxide bubbles
Growth curves were investigated for stationary phase and showed that the yeast cell concentrations were on average 65.75 x 106 colony forming units (CFU)/mL after growth and 62.63 x 106 CFU/mL on average before flocculation
Summary
There are numerous biotechnological processes that make use of immobilisation techniques to manipulate cells. These techniques can be divided into four major categories based on the physical mechanism at play in bringing about the immobilisation of the cells. These categories are (1) attachment or adsorption on solid carrier surfaces, (2) entrapment within a porous matrix, (3) natural aggregation by flocculation, and (4) artificially induced cross-linking by agents and/ or cell containment behind barriers.[1] Amongst the available yeast cell immobilisation techniques, the flocculation of microorganisms is very attractive, because of its simplicity and low cost. Flocculation can be induced by an artificial agent to increase the efficiency of the process and/or lower the overall process cost
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