F uel ethanol is currently made by large-scale yeast fermentation of sugars that are extracted or prepared from crops followed by separation of the ethanol by distillation 1,2 . The first major fuel-ethanol programme (ProAlcool) started in Brazil in 1975 3 , followed by pro- grammes in the USA in 1978 and, more recently, in Canada (Table 1). Although many other countries pro- duce ethanol for fuel and other purposes (http://www. distill.com/berg/), major production has only occurred in those countries with especially favourable agricul- tural and economic conditions. This article will examine current production, put the work into the context of changing environmental and economic realities and describe the latest technological developments. Feedstocks Sugar extraction from sugar cane Sugar cane (Saccharum officinarum) is a drought-tolerant, tropical and subtropical crop that contains 12–17% total sugars on a wet-weight basis with 68–72% moisture (90% sucrose and 10% glucose or fructose). The aver- age extraction efficiency to produce cane juice by crushing is approximately 95% and the remaining solid residue is cane fibre (bagasse). In factories that only produce ethanol, the cane juice is heated up to 110°C to reduce microbial contami- nation, decanted, sometimes concentrated by evaporation and then fermented. In combined sugar–ethanol plants (annexed distilleries), sucrose crystals that are formed after cane-juice concentration are removed by centrifu- gation, leaving a syrup (molasses) that contains up to 65% w/w sugars. Both sugar-cane juice and molasses (after adjusting the sugar concentration) normally contain sufficient minerals and organic nutrients to be immediately suitable for ethanol production by fermentation with Saccharomyces cerevisiae 3 . Starch extraction and saccharification from grains Maize (Zea mays) and other less commonly used grains can be processed by wet or dry milling. Wet milling was originally devised for the starch industry and adapted for fuel-ethanol production. The grain is soaked (steeped) in water with sulphur dioxide for up to 40 h, followed by grinding and separation of starch and co-products (Table 2). When wheat is used, the valuable bran and germ are usually removed first by dry processing in a flour mill before steeping in water. The starch fraction is gelatinized by cooking at low and high temperatures, followed by the addition of -amylase, which yields dextrin oligosaccharides. In the final sac- charification process, glucoamylase converts the starch to glucose, which can eventually be fermented to ethanol. The main difference with dry milling is that the entire grain is milled to a median diameter of approxi- mately 1 mm and the different components of the cereal grain are not fractionated before the water and enzyme are added; the slurry is processed as in wet milling. In both systems, the sugar-containing juice that leaves the processor (mash or wort) is essentially sterile, and this is a crucial point in the subsequent successful downstream processing. Fermentation and distillation In Brazil, 70% of the sugar-cane distilleries use a batch process with a fermentation capacity of up to 1.5 million (l ethanol) d -1 ; a ‘continuous’ version of the same process is also being used in approximately 350 distilleries, the latest of which was built in 1995. In both processes, yeasts are separated from the fer- mented medium by centrifugation and reused in sub- sequent fermentations after dilute-sulphuric-acid washing to reduce bacterial contamination. Very high cell densities (8–17% v/v) and temperatures of 33–35°C 482 0167-7799/99/$ – see front matter © 1999 Elsevier Science Ltd. All rights reserved. PII: S0167-7799(99)01384-0 TIBTECH DECEMBER 1999 (VOL 17) FOCUS Fuel ethanol after 25 years Alan E. Wheals, Luiz C. Basso, Denise M. G. Alves and Henrique V. Amorim After 25 years, Brazil and North America are still the only two regions that produce large quantities of fuel ethanol, from sugar cane and maize, respectively. The efficiency of ethanol production has steadily increased and valuable co-products are produced, but only tax credits make fuel ethanol commercially viable because oil prices are at an all-time low. The original motivation for fuel-ethanol production was to become more independent of oil imports; now, the emphasis is on its use as an oxygenated gasoline additive. There will only be sufficient, low-cost ethanol if lignocellulose feedstock is also used. A. E. Wheals (bssaew@bath.ac.uk) is at the Department of Biology and Biochemistry, University of Bath, Bath, UK BA2 7AY and also at the Department of Biology, Federal University of Lavras, 37200-000, Lavras, MG, Brazil. L. C. Basso, D. M. G. Alves and H. V. Amorim are at the Department of Biological Sciences, ESALQ/University of São Paulo, 13418-900 Piracicaba, SP, Brazil. D. M. G. Alves and H.V. Amorim are also at Fermentec, Rua Treze de Maio, 768, 13400-900 Piracicaba, SP, Brazil. Table 1. Major fuel-ethanol producers Country Crop Fuel-ethanol production (10 9 litres) Brazil Sugar-cane (juice and 10.5 hydrous molasses) 6.5 anhydrous (in 1999) USA Maize (95%) plus 5.3 anhydrous some wheat and (in 1998) barley Canada Maize plus 15% 0.24 anhydrous wheat (in 1998)