Microbial conversion of ethanol

Utilization of a Computer in On-Line Control and Optimization of a Batch Process for Microbial Conversion of Ethanol to Protein... 

In 1789 the chemist Lavoisier discovered the microbial conversion of sugar into ethanol and only successively was this ability attributed to yeasts, as explained by Pasteur [15]. It is not surprising that, since then, yeasts have been extensively studied and their capacity to convert sugars into ethanol and carbon dioxide exploited to produce beer, wine, and bread. 

Ethanol is the key reactant in Eq. (1), and also in Eq. (2) because it is readily converted to acetaldehyde. The process based on Eq. 1 was developed in Russia and the process based on Eq. 2 was developed in the United States. The yield of butadiene for the Russian process is about 30-35%. It is about 70% if mixtures of ethanol and acetaldehyde are employed as in the U.S. process. Equation (3) represents a process that involves 2,3-butylene glycol, a product from the microbial conversion of biomass. The process is carried out in two sequential steps via the glycol diacetate in overall yields to butadiene of about 80%. The process of Eq. (4) starts with a biomass derivative, the cyclic ether tetrahydrofuran, and can be carried out at high yields. When this process was first operated on a large scale in Germany, acetylene and formaldehyde were the raw materials for the synthesis of intermediate tetrahydrofuran. It is manufactured today from biomass feedstocks by thermochemical conversion, as will be discussed later. 

In direct microbial conversion of lignocellulosic biomass into ethanol that could simplify the ethanol production process from these materials and reduce ethanol production costs, Clostridium thermocellum, a thermoanaerobe was used for enzyme production, hydrolysis and glucose fermentation (755). Cofermentation with C thermosaccharolyticum simultaneously converted the hemicellulosic sugars to ethanol. However, the formations of by-products such as acetic acid and low ethanol tolerance are some drawbacks of the process. Neurospora crassa produces extracellular cellulase and xylanase and has the ability to ferment cellulose to ethanol 139). 

Malolactic fermentation (MLF) in wine is by definition the enzymatic conversion of L-malic acid to L-lactic acid, a secondary process which usually follows primary (alcoholic) fermentation of wine but may also occur concurrently. This reduction of malic acid to lactic acid is not a true fermentation, but rather an enzymatic reaction performed by lactic acid bacteria (LAB) after their exponential growth phase. MLF is mainly performed by Oenococcus oeni, a species that can withstand the low pFi (<3.5), high ethanol (>10 vol.%) and high SO2 levels (50 mg/L) found in wine. More resistant strains of Lactobacillus, Leuconostoc and Pediococcus can also grow in wine and contribute to MLF especially if the wine pH exceeds 3.5 (Davis et al. 1986 Wibowo et al. 1985). The most important benefits of MLF are the deacidification of high acid wines mainly produced in cool climates, LAB contribute to wine flavour and aroma complexify and improve microbial sfabilify (Lonvaud-Funel 1999 Moreno-Arribas and Polo 2005). 

Vast amounts of renewable biomass are available for conversion to liquid fuel, ethanol. In order to convert biomass to ethanol, the efficient utilization of both cellulose-derived and hemicellulose-derived carbohydrates is essential. Six-carbon sugars are readily utilized for this purpose. Pentoses, on the other hand, are more difficult to convert. Several metabolic factors limit the efficient utilization of pentoses (xylose and arabinose). Recent developments in the improvement of microbial cultures provide the versatility of conversion of both hexoses and pentoses to ethanol more efficiently. In addition, novel bioprocess technologies offer a promising prospective for the efficient conversion of biomass and recovery of ethanol. 

A total of almost 250 ISPR projects in microbial whole cell biotechnology are listed in Table 2. Over one third of these projects have dealt with the production of organic solvents such as ethanol, butanol, acetone or propanol (90 projects). Ethanol (70% of all the solvents) has been by far the most important microbial product for which different ISPR techniques have been applied. The second most important class of products involved in ISPR projects have been organic acids such as lactic, acetic, butyric, or propionic acid (54 projects). Most of effort in this product class has focused on lactic acid (55% of all organic acids). Important ISPR activities have also been reported for the microbial production of various aromas and fine chemicals (30 projects in each product category). A considerable amount of ISPR approaches have been shown in steroid conversions (17 projects) and the production of secondary metaboHtes and various enzymes (13 projects in each product category). 

---