2.1. Batch and semicontinuous processes
The Melle-Boinot process is the typical process for fuel ethanol production by batch fermentation. This process comprises the weight and sterilization of feedstock, followed by the adjustment of pH with H2SO4and of the degrees Brix to values of 14–22. Obtained wort is fermented by yeasts. The produced wine is decanted, centrifuged and sent to ethanol separation stage, whereas the yeasts are recycled to the fermentation in order to reach high cell concentration during cultivation (Kosaric and Velikonja, 1995).
Fed-batch culture implies low levels of substrate concentration during the course of fermentation, while ethanol is accumulating in the medium. This type of cultivation regime, along with the cell recycling, is the most employed technology in Brazil for bioethanol production due to the possibility of achieving higher volumetric productivities. Control of the flow rate of medium feed is quite advantageous because the inhibitory effect caused by high substrate or product concentrations in the fermentation broth can be neutralized. It was observed that the addition of sucrose in linear or exponentially decreasing way leads to 10–14% increase in ethanol productivity (Echegaray et al., 2000). The optimization of feeding policy plays a crucial role for increasing both productivity and ethanol yield of fed-batch fermentations. This issue was analyzed in a previous review (Cardona and Sánchez, 2007). For fed-batch cultures, Alfenore et al. (2004)showed that higher ethanol concentrations (147 g/L) can be obtained in cultures without oxygen limitation (0.2 vvm) during only 45 h in comparison to microaerobic conditions.