FORMATION AND DISRUPTION OF ANIMAL-CELL AGGREGATES IN STIRRED VESSELS- MECHANISMS AND KINETIC-STUDIES

The kinetics and mechanisms of aggregate formation and disruption have been investigated for BHK natural aggregates grown in 250 cm(3) Comin g spinner flasks. The cells were cultivated as continuous cultures wit h a dilution rate of 0.005 day(-1). Hydrodynamics were manipulated var ying the agitation rate in a single step, and aggregate characteristic s were evaluated until a new stationary state was achieved. Steps incr easing agitation rate from 30 to 90 and 100 rpm (for disruption studie s) and decreasing from 100 to 30 45 and 60 rpm (for formation studies) were performed. The initial and final size distributions are essentia lly monomodal, with bimodal size distributions being observed during t he transition phase. Comparing the 30 and 100 and 100 to 30 rpm tests, almost complete reversibility in aggregate size was observed. Aggrega tes are hydrodynamically controlled, with a power dependence of aggreg ate size on the energy dissipated per unit of mass close to 0.25. Havi ng dimensions smaller than the Kolmogoroffs eddy size, aggregates are controlled by the action of viscous stresses in the viscous dissipatio n subrange. The variation in aggregate diameter and concentration with time has been mathematically described, based on kinetic models (firs t-order for diameter, first- and second-order for concentration), deri ved from several theories and correlations previously applied to the f ormation and disruption in different aggregate systems. Aggregate form ation is a faster process than disruption; disruption is occurring wit h two main mechanisms: breakage and cell shedding from the large aggre gates, leading to smaller aggregates and an increase in the fraction o f single cells. The breakage of large aggregates into smaller ones can occur due to the collisions between aggregates or due to the stress c aused by fluid motion. The increase on the agitation step leads to an increase on hydrodynamic stress, and thus the collisional fragmentatio n mechanism becomes less relevant. Formation is occurring by aggregate -aggregate collisions and, with a smaller relevance, single cell addit ion into the aggregates. Nevertheless reduced in number, more efficien t collisions are occurring between aggregates, leading to an increase in the rate of aggregate formation.