Acceptability and economic considerations

With the very strong incentive to produce microbial products as cheaply as possible, considerable efforts have gone into identifying cheap feedstocks and into optimizing fermentation processes. From a consideration of the yields of microbial oils (section 9.2), it can be appreciated that a minimum of 5 tonnes of substrate (or feedstock) are needed to produce 1 tonne of oil under optimal fermentation conditions. Very often the lipid yield will be less than this 20% thus necessitating an even higher outlay for the purchase of the carbon source. 

Whatever substrate is used for a fermentation process, some simple caveats must be remembered. For example, the substrate must be available for a minimum of 10 months each year. Many suggested possible waste materials suitable for fermentation are seasonal and available for a relatively short time. The feedstock should also be storable and not be subject to rapid degradation by indigenous microorganisms. It should also be remembered that wastes are only zero cost when they are not wanted. Any would-be user of a waste may find that the price for the material is increased once it is learnt that it is of value. Thus, the provider of the waste should at least, in theory, be a partner to any planned fermentation scenario. 

The possible design of a SCO-producing process has been considered in some detail by Davies (1988,1992a,b) and Davies and Holdsworth (1992) for the development of a cocoa butter equivalent (CBE) process using Candida curvata. Davies (1992a) has identified three key areas that require successful development before large-scale production can be achieved. These are  

optimization of the fermenter operation to maximize oil yield (g oil per 100g substrate used)  

extraction of the oil from the yeast concentrate while still in the wet state. 

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