PROBLEMS ASSOCIATED WITH FERMENTATION AND RECOVERY
The microbial fermentation process has become an integral part of the recent
advances in genetic engineering and biotechnology. Most of the fermentation processes
are complex processes that involve multiple substrate, gas-liquid mass transfer, and
sometimes nowNewtonian broth rheology. The final desired product concentration in
the aqueous medium can vary over a wide range. Some of the organic acid fermentations
such as citric acid and glutamic acid, a concentration of 100 grams per liter or
above can be achieved. On the other hand, some more complex macromolecular
product fermentations, such as vitamin B,, and human insulin, a concentration of 1 gram per liter or less is satisfactory because these products command premium prices.
As shown in FIGURE 1,the fermentation product cost is more or less dictated by the
final product concentration in the fermentation broth. It is always desirable to increase
the final product concentration in the fermentation broth. The primary causes of the
dilute nature of the fermentation end product are that the product itself can stop its
own synthesis (product regulation) and product degradation in the fermenting broth.
Even though these effects that occur at high product concentration are known,
approaches for major improvement have been primarily limited to genetic manipulation.
By means of random mutation and selection, strains tolerant to a high concentration
of product may be selected to avoid such problems.
Isolation of microbial products produced by fermentation is a specialized field
because of the unique character of the fermentation broth from which the products are
synthesized. The complexity of the heterogenous mixture of soluble and insoluble
materials is required to maintain or improve the fermentation, but it also causes
problems for the separation scientists and engineers. Although not always, the product
usually is present in a relatively low concentration compared to the concentration of
other materials present. The desired product is also prone to degradation by extremes
of temperature and pH. Most important of all, the product must be protected from
both enzymatic/microbial and chemical degradations. Depending on whether the
fermentation product is intracellular or extracellular, the sequence of recovery and
purification steps is shown in FIGURE 2. The extraction efficiency of a product recovery
and purification scheme is dictated not only by the final fermentation product
concentration but also by the number of steps required to achieve the specified product
purity that, in turn, is dictated by the market demand. Increasing the number of steps
in the isolation of microbial product from the fermentation broth will increase the product purity, but it also decreases the overall extraction efficiency (FIG. 3). The
initial isolation steps such as cell removal and volume reduction generally cause a more
drastic drop in the overall extraction efficiency as depicted in FIGURE