INTRINSIC FERMENTATION KINETICS OF LACTOSE IN ACIDOGENIC BIOFILMS

The intrinsic fermentation kinetics of lactose in acidogenic biofilms were investigated in situ in a continuous flow fermentor at 35-degrees-C and pH 4.6. The external and internal mass transfer resistances to lactose molecules from bulk solution to inside the biofilms were experimentally minimized or eliminated in a thin biofilm and recycled medium. In a chemically defined culture medium, the immobilized acidogens converted lactose mainly to acetate and butyrate; the minor products included ethanol, propionate, lactate, and hydrogen. The utilization rate of lactose, as a function of lactose concentration in the fermentor, can be described by a Michaelis-Menten equation, as can the formation rates of acetate, butyrate, and ethanol. The production rates of propionate and lactate had a linear relationship with lactose concentration under the experimental conditions. The low pH (4.6) of culture medium could depress the formation of propionate, an intermediate which is most difficultly digested by acetogenic bacteria located in the second fermentor in a two-phase process. Production rate of acetate quickly reached a constant, and additional utilization of lactose produced more butyrate and other minor products.