In spite of the considerable efforts devoted to the fermentative alcohols, indus­trial applications have been delayed because of the high cost of production, which depends primarily on the energy input to the purification of dilute end-products, the low productivities of cultures, and the high cost of enzyme production. These issues are directly linked to inhibition phenomena.

Along with the conventional unit operations, liquid-liquid extraction with biocompatible organic solvents, distillation under vacuum, and selec­tive adsorption on the solids have demonstrated the technical feasibility of the extractive fermentation concept. Lately, membrane separation pro­cesses that decrease biocompatibility constraints have been proposed. These include dialysis [85] and reverse osmosis [65]. More recently, the concept of supported liquid membranes has been reported. This method minimizes the amount of organic solvents involved and permits simultaneous realiza­tion of the extraction and recovery phases. Enhanced volumetric produc­tivity and high substrate conversion yields have been reported [86] via the use of a porous Teflon® sheet (soaked with isotridecanol) as support for the extraction of ethanol during semi-continuous fermentation of Saccharomyces bayanus. This selective process results in ethanol purification and combines three operations: fermentation, extraction, and re-extraction (stripping) as schematically represented in Figure 4.13. As shown and suggested, novel process ideas can further accomplish maximized alcohol production, energy savings, and reduced cost in production.