Fluid shear effects on suspension cultures of Morinda citrifolia

The shear susceptibility of cell suspension cultures of the plant cell Morinda citrifolia was investigated by subjecting the cells to the well-defined shear field generated in turbulent flow through a capillary. Suspensions were circulated using a peristaltic pump and average shear stresses between 25 and 350 N m(-2) were generated in the capillary test section. Control experiments were performed to assess the possible contribution of the peristaltic pump to the observed cell damage. There was clear evidence of pump-induced damage at the more severe test conditions and all viability measurements were corrected accordingly. Both shake flask suspension cultures (aged between 9 and 15 days) and repeated batch fermentation cultures, grown in a stirred tank reactor (STR) under a variety of controlled agitation conditions, were tested in the capillary shear loop. The cell damage incurred was evaluated in terms of suspension viability, as determined by a dye exclusion technique. Viability loss was found to conform closely to a first-order model in which the rate constant was observed to increase with the imposed shear stress. Furthermore, a linear relationship was identified between the specific death constant and the cumulative energy dissipated. Post-shear morphological measurements showed that the chain length distribution is shifted toward markedly lower values. In comparison with shake flask cultures, repeated batch fermentation cultures exhibited a marked increase in sensitivity to capillary shear. Based upon the determined morphological characteristics, this result is primarily attributable to the increased chain lengths characteristic of the repeated batch cultures.