Figure 1. (a) Concentration of substrate [S] and product [P] as a function of time during a discontinuous batch process. Also shown is the enzyme activity [E] as a function of time due to inactivation. (b) Enzyme activity as a function of temperature with the different effects of increasing reaction rate (curve 1) and increasing inactivation rate (curve 2).

Figure 2. (a) Inactivation of a protease (Alcalase) that is stable under alkaline conditions at different pH and temperatures (activity given in logarithmic scale as a function of time in min) (Adler-Nissen, 1986). (b) Correlation according to Arrhenius for t _D (t _D in min corresponds to the time during which the activity decreases by a factor of 10, thus the practical maximal time of operation) (Adler-Nissen, 1986).

Figure 3. Structure of starch molecules: (a) amylose; (b) amylopectin.

Figure 4. Activity (%) of an alkaline protease as a function of pH (substrate: casein, reaction at 40 °C, solid line; hemoglobin (denatured), reaction at 25 °C, broken line).(after International Bio-Synthetics, 1988).

Figure 5. Principles of membrane systems. (a) Membrane reactor with integrated semipermeable membrane; (b) continuous stirred tank reactor; (c) cascade; (d) tubular reactor, each system with membrane module/unit and recycling of enzyme.

Figure 6. (a) Scheme of production system with membrane unit, with mixing and storage tanks (1), pumps (2), filters (3), heat exchangers (4), enzyme tank (5), ultrafiltration module (6), analytical monitors (7), product vessel (8), and control units for pH, temperature (TI), pressure (PIC), and volumetric flow (FIC) (regulated control: dashed and solid lines). (b) Unit for amino acid production.(reproduced with kind permission of Degussa, Hanau, Germany).

Figure 7. Examples of different membrane configurations (Tutunjian, 1985).

Figure 8. Reaction scheme for continuous production of l-leucine from a keto acid (α-ketoisocaproate) with leucine DH (B), formate DH (A), and polymer-bound cofactor.

Figure 9. Continuous production of l-leucine in a single enzyme membrane reactor; conditions (feed concentrations): 100 mM α-ketoisocaproate, 400 mM ammonia, and 400 mM formic acid; LeuDH 2.5 U ml ^–1, FDH 2.5 U ml ^–1; 0.2 mM PEG-NAD ⁺; residence time 1.25 h. The arrows indicate supplementation of PEG-NAD ⁺ when the cofactor concentration decreased to 0.1 mM (Kragl et al., 1996).

Figure 10. Total amounts of enzyme consumed and product obtained as a function of process time; example used for calculation: substrate 500 mM, enzyme deactivation 3% per day, molecular weight of product 100 g mol ^–1. Residence time 1 h (Kragl et al., 1996).