In the case of growth of yeast cells nutrient rich medium can be fully utilized to give cell mass if dissolved oxygen concentration can be maintained at a desired level in the direct vicinity of the cells. During biotransformation of benzaldehyde to l-phenyl acetyl carbinol (l-PAC) by yeast cells the level of oxygen should be such that it should maintain organisms in the active form to maximize the production of l-PAC and reduce the production of other by-products. Continuous stirred tank reactor (CSTR), is commonly used in many of the bioprocess as it allows efficient contacting of three phases, i.e. gas, liquid medium and solid cells. In the case of CSTR, the aspect ratio is maintained more than 1, to ensure high residence time of gas phase, increase the transfer efficiency and to ensure less power input on introduction of gas, uniform power dissipation. The shear can be reduced by operating the impellers at lower speed. For the same volume of liquid, multiple impellers on a single shaft with appropriate combination and spacing are being suggested as optimum [2]. In such cases mixing and mass transfer are dependent on the flow rate of gas, type of agitator and its speed and properties of liquids. Power consumption per impeller decreases with an increase in the number of impellers [2] and this increases the uniformity of energy dissipation. A combination of DT–PTD has been reported to give highest values of fractional gas hold-up and of gas–liquid mass transfer coefficient with least power input for air–water system by Arjunwadkar et al. [3].