3. Kinetic model The specific growth rate (μ) can be calculated for continuous fermentation with cell recycle as (Brandberg et al., 2007): X dX dt D X X 1 . 1 o μ = + ( ) where, X and Xo are the cell mass concentrations in the bioreactor and outlet broth, respectively; μ is the specific growth rate (h1 ); and D is the dilution rate (h1 ). At steady state conditions (dX/ dt¼0), μ can be represented as: D X X . 2 o μ = ( ) where D F V 3 i = ( ) where V is the broth volume in the bioreactor; Fi is the flow rate at the inlet of bioreactor (l h1 ). The volumetric ethanol productivity, qp (g l1 h1 ); specific ethanol productivity, qsp (g g1 h1 ) and ethanol yield, YP/S (g g1 ) can be calculated from Fig. 1B as: q FP FP V 4 p ( ) o c o c = + ( ) or, q FP FP XV 5 sp ( ) o c o c = + ( ) Y FP FP FS F S 6 P S o c o c i o i o / ( ) ( ) = + − ( ) where Po and Pc are the product concentrations in the outlet flow and condensate, respectively (g l1 ) and Fi and Fc are the product flow rates at the inlet and condensate, respectively (l h1 ); Si and So are the substrate concentrations in the bioreactor inlet and outlet, respectively (g l1 ). F