Khan, Saad Ullah; (1997) The effects of fermentation scale on the cell qualities of Saccharomyces cerevisiae and Saccharopolyspora erythraea. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This report investigates the effect of scale-up on the cell quality of Saccharomyces cerevisiae and Saccharopolyspora erythraea. Cell quality was defined as the chemical and physical composition of a cell at a particular point during fermentation. Chemically this includes the concentration of the cells internal and externally secreted molecules, whereas physically this includes the nature of the cells structure, in terms of morphology and wall strength, and its influence on the broth rheology. Knowledge of the cell quality is useful because it can influence the operation of the fermenter and the subsequent downstream processing operations; cell quality profiles can be used to determine the best harvesting point of the fermentation, to obtain the optimum mix of cell qualities. For instance mass, heat and momentum transfer are influenced by the broth rheology which in turn may depend on the cell morphology; whereas the cells' product location and concentration determines the required scale of the fermenter and specifies and sizes the downstream processing chain. Batch fermentations of the two organisms were carried out in defined media, using fermenters between 7 and 1500 litres scale. The fermentations were sampled regularly to produce cell quality profiles for the duration of the fermentations. The cell qualities monitored for S. cerevisiae were the dry cell weight, alcohol dehydrogenase and glucose-6-phosphate dehydrogenase activities, the cell soluble protein content and the cell wall strength (indirectly determined by the homogenisation of the cells). For S. erythraea the cell qualities monitored were the dry cell weight, the erythromycin production, the broth rheology, the cell morphology, the cell soluble protein content and the cell wall strength. The cell quality profiles of S. cerevisiae were found to be scale independent over the range studied. For S. erythraea the cell quality profiles were dependent on the operating conditions and the scale of fermentation; particularly, cell fragmentation was noted to increase with stirrer speed and fermentation scale. The method of scale-down of fermentation appears to be suitable for reproducing the conditions and behaviour of cells from production scale fermenters in the laboratory; these models reproduce the most important mechanisms operating at the large scale. Scale-down models based on the recirculation of broth through a high shear zone, are suggested as a means of reproducing the cell quality profiles obtained in the large scale fermenters used in this study; especially to investigate the effects of shear on cell qualities such as cell wall strength and morphology.

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