Thermophiles and fermentation technology

Thermophilic microorganisms have been of great scientific interest for several decades, principally in regard to their biotechnological potential and also of the thermostable enzymes they produce. Optimal cultivation techniques for these organisms are required, therefore, not only for basic study but also for evaluation of their thermostable microbial products. Operating a fermentor at elevated temperatures may be advantageous in terms of increased solubility of substrates, improved mass transfer due to decreased viscosity, and increased diffusion rates. However, the cultivation of thermophiles also has many associated problems. A high cultivation temperature can give unexpected problems affecting the choice of reactor design and construction materials, and with the heating and cooling of the fermentor. Other problems may be caused by the low solubility of gases and the instability of substrates and other reagents used. Furthermore, high productivity requires high cell densities to be achieved and in many cases thermophiles are characterised by low growth rates, low growth yields and susceptibility to substrate and product inhibition at low concentrations. Different ways to circumvent some of these problems, such as using gas-lift fermentors, dialysis fermentors or cultivation with cell recycling are discussed.