From Cellular Function to Industrial Products
41
and 3) there may be compounds present that are undesirable. With a requirement for increased
documentation and reproducibility in the fermentation industry there is trend towards application
of more defined media, and often minimal media with a carbon and energy source (glucose,
sucrose or starch), an inorganic nitrogen source (ammonia or urea), a mixture of minerals and a
perhaps a few vitamins may replace complex media to the benefit to the producer. In this process
it is important to consider also the benefits of using a defined medium in the subsequent
downstream processing. Also in the pharmaceutical sector there is a desire to use defined media,
and particularly the use of serum free medium is today considered a standard requirement in
connection with production of heterologous proteins using mammalian cell cultures. In the future
it is expected that lignocellulose containing material may serve as a cheap and efficient carbon
and energy source. Lignocellulose basically consists of three components: lignin, cellulose, and
hemicellulose (a complex pentose rich polymer), and it is present in many different plant fibers
like com cob, bagasse, straw and wood. The exploitation of lignocellulose as raw material may
allow many commodity products like fuels, polymers, and chemicals to be produced through
fermentation processes in a cost efficient manner.
2.2.3 Criteria for Design and Optimization
The criteria used for design and optimization of a fermentation processes depends on the product.
Thus, the criteria used for a high volume/low value added product are normally completely
different than the criteria used for a low volume/high value added product. For products
belonging to the first category (which includes most whole cell products, most primary
metabolites, many secondary metabolites, most industrial enzymes, and most polysaccharides)
the three most important design parameters are:
Yield of product on the substrate (typical unit: g product per g substrate)
Productivity (typical unit: g product per L reactor volume per hour)
Final titer (typical unit: g product per L reactor volume)
Yield of product on the substrate is here very important since the raw materials often account for
a significant part of the total costs. Thus in penicillin production the costs of glucose alone may
account for up to 15% of the total production costs. Productivity is important since this ensures
an efficient utilization of the production capacity,
i.e.
the bioreactors. Especially in an increasing
market it is important to increase the productivity since this may prevent new capital
investments. Final titer is of importance for the further treatment of the fermentation medium,
e.g.
purification of the product. Thus if the product is present in a very low concentration at the
end of the fermentation it may be very expensive even to extract it from the medium with a
satisfactory yield.
In Chapter 3 we are going to introduce reaction rates and yield coefficients that enables a
quantification of productivity and yield, but the concept can be illustrated if we return to Fig. 2.1.
which is a representation of the overall conversion of substrates into metabolic products and
biomass components (or total biomass). As we will discuss in Section 3.1 the rates of substrate
consumption can be determined during a fermentation process by measuring the concentration of
previous page 64 Bioreaction Engineering Principles, Second Edition  read online next page 66 Bioreaction Engineering Principles, Second Edition  read online Home Toggle text on/off