Open and continuous fermentation: products, conditions and bioprocess economy.

Microbial fermentation is the key to industrial biotechnology. Most fermentation processes are sensitive to microbial contamination and require an energy intensive sterilization process. The majority of microbial fermentations can only be conducted over a short period of time in a batch or fed-batch culture, further increasing energy consumption and process complexity, and these factors contribute to the high costs of bio-products. In an effort to make bio-products more economically competitive, increased attention has been paid to developing open (unsterile) and continuous processes. If well conducted, continuous fermentation processes will lead to the reduced cost of industrial bio-products. To achieve cost-efficient open and continuous fermentations, the feeding of raw materials and the removal of products must be conducted in a continuous manner without the risk of contamination, even under 'open' conditions. Factors such as the stability of the biological system as a whole during long cultivations, as well as the yield and productivity of the process, are also important. Microorganisms that grow under extreme conditions such as high or low pH, high osmotic pressure, and high or low temperature, as well as under conditions of mixed culturing, cell immobilization, and solid state cultivation, are of interest for developing open and continuous fermentation processes.