Anaerobic fermentations--some new possibilities.

Anaerobic fermentations start with the major advantage that, unlike aerobic processes, they do not incur the cost penalties associated with the need to sustain high rates of culture aeration, agitation and cooling. Even so, many of the more traditional fermentations that yield high volume, low value products (such as alcohols or fatty acids) are currently only economically viable if they utilize biological wastes and agricultural surpluses as substrates. To achieve maximum conversion efficiency it is necessary to select the appropriate species/strain of micro-organism which will perform best under the intended conditions of operation (which could include elevated temperatures and extremes of pH). Knowledge of the mechanisms whereby the fermentation is physiologically controlled can then suggest means, including genetically engineered strain improvements and/or the establishment of stable co-cultures with other microbes, whereby the yield of a desired product can be optimized. The biology of a fermentation process can thus be rendered sufficiently consistent and predictable for the biochemical engineer to be able to achieve optimal fermenter productivity. Anaerobic processes are particularly well suited to a continuous flow mode of operation (with cell retention) and new techniques for selective product removal and concentration can reduce the energy costs associated with 'downstream processing'. Hitherto, most interest has been centred on those anaerobic fermentations of renewable biomass which yield immediately useful products (fuel alcohols, methane). However, by combining biological and chemical procedures the profitability of even some superficially unpromising fermentations could be substantially enhanced. Collaborative research by microbial physiologists, geneticists and biochemical engineers is a prerequisite for marketable success in new applications of fermentative anaerobes and their enzymes.