After acid hydrolysis (or some other pretreatment) and cellulase digestion, the product of the process is a mixed carbon source for fermentation by an ethanologenic microbe. Few details have been made public by Iogen about their development of nutritional balances, nitrogen sources, or media recipes for the production stage fer — mentation.1 This is not surprising because, for most industrial processes, medium optimization is a category of “trade secret,” unless patenting and publication priori­ties deem otherwise. Few industrial fermentations (for products such as enzymes, antibiotics, acids, and vitamins) rival the conversion efficiency obtained in ethanol production; one notable exception — about which a vast literature is available — is that of citric acid manufacture using yeasts and fungi.110 Many of the main features of citric acid fermentations have echoes in ethanol processes, in particular the use of suboptimal media for growth to generate “biological factories” of cell populations supplied with very high concentrations of glucose that cannot be used for further growth or the accumulation of complex products but can be readily fluxed to the simple intermediate of glucose catabolism — the biochemistry of Aspergillus niger strains used for the production of citric acid at concentrations higher than 100 g/l is as limited (from the viewpoints of biological ingenuity and bioenergetics) as ethanol production by an organism such as Z. mobilis (see figure 3.4).

Nevertheless, interest in media development for ethanol production has been intense for many years in the potable alcohol industry, and some innovations and developments in that industrial field have been successfully translated to that of fuel ethanol production.