Beyond ethanol production, the modern mainstream fermentation industry manufac­turing primary metabolites including vitamins and amino acids, secondary metab­olites (including antibiotics), and recombinant proteins almost invariably opts for fed-batch fermentation technologies and has invested much time and expertise in devising feeding strategies for carbon sources (including sugars, oligosaccharides, and amylase-digested starches) to operate at a minimum concentration of free sug­ars and avoid carbon catabolite repressions. Some large-scale processes are run to vanishingly small concentrations of free glucose, and the feed rate is regulated not by direct measurement but indirectly by effects of transient glucose accumulation on physical parameters such as pH (responding to acid accumulation during glucose overfeeding) or the trends in dissolved O2 concentration.164

Yeast (S. cerevisiae in its “baker’s yeast” guise) cells are one of the three principal production platforms for recombinant proteins for the biopharmaceuticals market, the others being E. coli and mammalian cell cultures: of the 10 biopharmaceutical protein products achieving regulatory approval in the United States or the European Union during 2004, seven were produced in mammalian cell lines, two in E. coli, and the tenth in S. cerevisiae.165 In such cell systems, ethanol formation is either to be avoided (as a waste of glucose) or carefully regulated, possibly as a means of feedback control to a complex and variable sugar feed to high cell densities where O2 supply is critical to maintain anabolic, biosynthetic reactions rather than simple

fermentation.166,167

For fuel ethanol, in contrast, very high rates of sugar consumption and ethanol production are mandatory for competitive, commercial production processes. Tem­perature is, as always, an important parameter: under European conditions, 30°C is optimal for growth and 33°C for ethanol production.168 Rather than aiming at micro­aerobic conditions, a high-aeration strategy is beneficial for stabilizing a highly viable cell mass capable of high ethanol productivity.168 Glycerol accumulates as a major coproduct, but this waste of sugar metabolism can be minimized by several options for fermentation management: [38]

accumulation may (in addition to a role in osmotolerance) offer some degree of temperature protection to ethanol-forming yeast cells168

• High-aeration regimes greatly reduce glycerol accumulation169

• Maintaining a high respiratory quotient (the ratio of CO2 produced to O2 consumed) results in a high ethanol-to-glycerol discrimination ratio when the online data are used to feedback control the inlet sugar feed rate170

In addition to cane sugar, fed-batch technology has been used to produce ethanol from sugarcane molasses with an exponentially decreasing feed rate.171 The detailed mathematical model advanced by the Brazilian authors, incorporating the feed rate profile and two further process variables, is probably too complex for small-scale fermentation sites but well within the capabilities of facilities operating (and manag­ing) large, modern fermentors.

The bacterial ethanologen Z. mobilis is most productive both for biomass forma­tion and ethanol production from glucose when feeding avoids the accumulation of high concentrations of glucose; an important finding from this work was that attempts to regulate a constant glucose concentration do not optimize the process because of the complex relationship between specific growth rate and glucose supply.172 In gen­eral, experience in the fermentation routes to producing fine chemicals highlights the importance of accurately monitoring analyte levels inside fermentors to avoid exces­sive accumulations (or depletions) of key substrates and nutrients and triggering repres­sion mechanism and the appearance of metabolic imbalances, all of which negatively impact on process economics.173 In early 2007, a major collaboration was announced to provide automated, near real-time online monitoring of commercial fuel ethanol fermentations using proprietary methods to sample high-solids and highly viscous fer­mentation broths.174 A spectrum of measurements was included in the design remit, including methodologies for ethanol, sugars, and organic acids. Because fed-batch processes are widely considered to be the favored route to the contemporary yeast cell limit of 23% v/v ethanol production, bioprocess management using more sophisticated tools to ensure a steady and slow release of glucose and other monomeric sugars are a major future milestone for industrial-scale bioethanol production.121