After the preparatory stage, the glucose solution can be fermented in ethanol. The temperature of the glucose is lowered to around 35°C, and then the yeast (usually Saccharomyces cerevisiae) is added and the anaerobic fermentation process begins, which converts the glucose into ethanol and carbon dioxide.

Glucose S^cchfrrfycesce:7la > 2 EtOH + 2 CO, (8)

As a rule, the preferred method is to conduct the saccharification and fermentation steps during the same stage of the production process. Fermentation can be completed in two stages (Verma et al., 2000) using starch treated with a-amylase and glycoamylase. Fermentation may be continuous or discontinuous, it makes no difference. When the fermentation broth reaches an ethanol content of around 8-10% v/v (beyond which the yeast can no longer survive), the ternary mixture is distilled by adding benzene or cyclohexanone, or using molecular sieves. After distillation, the ethanol is 95% pure.

In 2006, a research group (Robertson et al., 2006) experimented with the so-called "cold hydrolysis" of starch, concluding that the potential use of this method relies on the discovery and characterization of more efficient enzymes and the development of processes with a high level of integration, such as simultaneous liquefaction, saccharification and fermentation, along with other factors. Figure 2 shows the flow chart for bioethanol production from materials containing starch.

3.3 Raw materials containing saccharose

For the purposes of bioethanol production, the most important raw materials containing saccharose are unquestionably sugar cane and sugar beet. Two thirds of the world’s sugar production derives from cane, the other third from beet.