5. Conclusions

gets close to the maximum specific growth rate μmax, the productivity decreases, as

Comparison of suspension and fixed-bed cultures for Lactococcus lactis. Volume-specific lactate productivity q\*Lac versus dilution rate D. M17 medium (Difco) with 5 g L<sup>1</sup> lactose; carrier, 10 mL Multiferm; 100 mL, VitraPOR® 4 mm; 1 L, VitraPOR® 8 mm. The red arrow indicates the maximum specific growth rate of the

For the fixed-bed-cultures, the productivity increases further due to cell retention in the carriers. The highest value determined here is approx. 3–4 times higher than the maximum in chemostat cultivation. Obviously the maximum for fixed-bed

All fixed-bed systems used here can be described by the same spline. This is very important with respect to scale-up, as obviously data from small-scale systems can be used to predict the performance on a larger scale (for more details on scale-up,

For establishment of mathematical process model, biomass formation, lactose consumption, and lactate production during start-up of fixed-bed cultures with immobilized L. lactis were investigated experimentally and described by a reaction kinetic model [21]. Appropriate modeling and simulation of fixed-bed processes require biomass data. Therefore, a low-volume multiple fixed-bed reactor system (Multiferm) was used to investigate biomass formation of a L. lactis strain during the start-up phase of fixed-bed cultivation. The generation of data in parallel experiments was fast and easily compared to larger single reactor systems. Biomass data obtained from both fractions, retained and free suspended biomass, was used for modeling and simulation, together with data for lactose and lactate. The underlying Luedeking-Piret-like model structure was developed based on the results from suspension cultivations with the same strain. The fixed-bed system was described as perfusion culture with cell retention (Figure 7). For this, merely four additional parameters had to be defined to extend the suspension model to fixed-bed cultures. Experimental trends and steady states of both biomass fractions besides substrate and product could be described very well. Thus, this model could be used for

4.3 Reaction kinetic model for start-up of fixed-bed reactors

washout of cells occurs.

see [20]).

88

Figure 6.

strain.

cultures has not been reached so far.

Growing and Handling of Bacterial Cultures

process layout during process development.

The goal of the studies was to evaluate the performance of fixed-bed bioreactor systems on different scales compared to suspension culture. The suggested concept for development of fixed-bed processes could be confirmed. The multi-fixed-bed bioreactor Multiferm provides an ideal downscaled and economical system that can be used for basic studies with low requirements on medium and cells. Here, questions such as optimal carrier design, appropriate medium, and process parameters (e.g., technique for immobilization, initial cell density, flow rate, temperature, oxygen, pH) can be evaluated. Especially the start-up phase can be investigated. The next step, a 100 mL fixed-bed system, provides data on the performance and long-term stability of the culture. Problems that might not have been shown up in the Multiferm, e.g., insufficient long-term stability, can be detected here. The 1 L fixed-bed can be regarded as a pilot scale already because medium requirement was already at 27.6 L per day at the highest dilution rate. Additionally, the radial-flow geometry can be easily scaled up further.

As expected, fixed-bed bioreactors could be operated in a perfusion mode at a steady state with dilution rates higher than the maximum specific growth rate. By this, very high volume-specific productivity with respect to lactate can be reached and maintained for long periods of time. The fixed-bed processes with lactic acid bacteria on macroporous carriers could be transferred on a pilot scale without loss in productivity. Furthermore, the productivity could be described by a spline, indicating that the maximum growth rate was not reached in this study.

Therefore, a process development tool for fixed-bed processes is now at hand that will pave the way for an industrial application of this promising technology.

#### Abbreviations and symbols



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