One of the reasons for substituting ethanol for petrol is that ethanol can be produced from biological material and hence is both renewable and sustainable, and reduces carbon emissions.

The ability of microorganisms to produce alcohol from sugars has been known since Egyptian times and could be regarded one of the first uses of biotechnology. The best-known and most widely used microorganism involved with the production of ethanol has been the yeast Saccharomyces cerevisiae, but it is not the only one. In the absence of oxygen, the yeast will switch its metabolism to fermentation producing ethanol and carbon dioxide. The lack of oxygen inhibits the citric acid cycle so pyru­vate would be expected to accumulate (Fig. 6.7). However, under these conditions

Fig. 6.6. Typical process for the production of ethanol from sugarcane in Brazil. (Modified from Poole and Towler, 1989.)

pyruvate is converted to acetaldehyde by the enzyme pyruvate decarboxylase with the release of carbon dioxide. Acetaldehyde is then converted to ethanol by the enzyme alcohol dehydrogenase. The overall equation is given below:

C6H12O6 = 2C2H5OH + 2CO2

theoretical yield of ethanol from this equation is 51% of the substrate added but some energy is required to maintain the cells so that the yield is about 95% of the theoretical yield with pure substrates. However, with industrial systems the best yields are around 91%. The concentration of ethanol obtained by fermentation is normally from 5 to 10% as ethanol begins to inhibit growth above 5%. Concentrations of 10% ethanol can be obtained with pure substrates. The reason for the loss of viability as the ethanol concentration increases is that ethanol is a solvent and dis­rupts the cells’ lipid-protein membrane making it increasingly leaky. Yeast strains with a higher tolerance to ethanol have membranes containing a higher proportion of longer-chain unsaturated fatty acids. Higher concentrations of ethanol can be obtained using high concentrations of substrates, and ethanol tolerant (10-18%) strains but the process is much slower. The limitation of S. cerevisiae is that it cannot utilize pentoses such as xylose and arabinose and more complex carbohydrates like starch and cellulose.