ApplMicrobiol Biotechnol (1988) 29:536-543 Applied Microbiology Biotechnology © Springer-Verlag 1988 Kinetics of multiproduct acidogenic and solventogenic batch fermentations Mustafa Ozilgen Food EngineeringDepartment, Middle East TechnicalUniversity,06531 Ankara, Turkey Summary. Large amounts of data indicated that most of the metabolic processes of the acidogenic (acid producing) and the solventogenic (solvent producing) fermentations were regulated by prod- uct accumulation. A simple unstructured model simulated microbial growth, product formation and substrate utilization in six different fermenta- tions, where five different microorganisms pro- duced various combinations of ten different prod- ucts. Specific growth rates of these microorgan- isms decreased proportionally with overall prod- uct accumulation. The products were excreted in non-growth associated pattern. Excretion of some of these products were inhibited by the overall product accumulation similarly as the microbial growth. A substrate consumption model which considered the biomass and individually all the products as separate substrate sinks simulated the data satisfactorily. Introduction In a batch fermenter the specific growth rate of the microorganisms is generally defined as: 1 dX ~-X dt (1) where/.t is the specific growth rate, X is the micro- bial concentration and t is time. The parameter/.t is generally related to the substrate concentration with the well known Monod equation (Bailey and Ollis 1977; Wang et al. 1979): S 1-,£ = ~rnax Ks..]_S (2) where ~tma x is the maximum specific growth rate, Ks is a constant and S is the substrate concentra- tion. Another widely used expression for the pa- rameter # is the logistic equation (Weiss and Ollis 1980; Klimek and Ollis 1980; Lam and Ollis 1981; Ozilgen 1988): ~t=~s 1 Xmax (3) where #s and Xmax are the initial specific growth rate and the maximum attainable biomass con- centration, respectively. In Equation 3 the term (1--X/Xmax) represents slow-down of the growth with over-crowding of the medium. As the value of the parameter X approaches Xm~x the logistic equation predicts no growth (i. e. dX/dt = 0). Inhi- bitory product accumulation and competition for the limited nutrient are among the reasons of the slow-down of the growth in the over-crowded fer- menter. Acetate, butyrate, lactate and citric acid are among the most commonly studied acidic micro- bial products. Butanol, acetone, ethanol, sorbitol and butanediol are well known microbial sol- vents. Ethanol accumulation in the fermenters (and in the cells) was reported to inhibit various metabolic activities, such as specific growth rate, specific ethanol production rate, cell viability and sugar consumption (Novak et al. 1981 ; Ghose and Tyagi 1979; Lourerio and van Uden 1982; Leao and van Uden 1982). Ethanol accumulates in the cells as a consequence of production and kills them (Novak et al. 1981). Ethanol stress changes the lipid composition of the plasma membranes of the yeasts (Beaven et al. 1982; Thomas et al. 1978; Thomas and Rose 1979). It also inhibits maltose and glucose transport accross the plasma membrane (Thomas and Rose 1979).