Fermentation inhibitors

The fermentation inhibitors include furan aldehydes, aliphatic acids, and phenolic compounds. The furan aldehydes, furfural, and hydroxymethyl furfural (HMF), are formed from pentoses and hexoses, respectively (4,5). Several studies indicate that furfural inhibits Saccharomyces cerevisiae, at least when present in high concentrations (6-10). HMF has a similar effect (11,12). 

The breakdown of furan aldehydes leads to the formation of formic and levulinic acid. Moreover, acetic acid is formed during the degradation of hemicellulose. Partial breakdown of lignin can generate a variety of phenolic compounds (23), which also inhibit S. cerevisiae (14,15). In contrast to furan aldehydes and aliphatic acids, the toxic effect of specific phenolic compounds is highly variable (15). Different raw materials and different approaches to prepare lignocellulose hydrolysates will result in different concentrations of the fermentation inhibitors (16,17). 

Autohydrolysis enables the selective hydrolysis of hemicelluloses to a mixture mainly consisting of oligosaccharides and monosaccharides. The monosaccharide content can be increased under harsher reactor conditions, but then monosaccharides can undergo decomposition reactions, thereby increasing the content of potential fermentation inhibitors in hydrolysates. 

Diuretic drug preparations have promoted urine formation. They are derivatives of mercury propanol RCH2CH(OH)CH2HgX, where R is a polar hydrophilic group. The mercury diuretic preparations act as ferment inhibitors (latter containing-. They also inhibit adenosine triphosphate (ATP). These properties led to the use of mercury drug compounds in the treatment of bacterial infections. In these cases, they interacted with -SH groups of the bacteria proteins. 

Higher sensitivity to fermentation inhibitors than glucose-fermenting yeast strains (see Table 4). 

Benzoic acid, potassium salt EINECS 209-481-3 Potassium benzoate. Anti-corrosive, preservative, fermentation-inhibitor, antifungal agent for tobacco production, pyrotechnical additive. White solid soluble in H2O, EtOH. Am. Biorganics Lancaster Synthesis Co. Uallinckrodt Inc. Pentagon Chems. Lid VerdugtBV. 

Fig. 2. Fermentation inhibitors in lignocellulose hydrolysates. Some of the possible pathways by which the inhibitors are generated, either by acid hydrolysis or in the ethanolic fermentation, are indicated in the figure...

The Ce, Cg and Cio fatty acids are formed by yeast. As they are fermentation inhibitors at concentrations of only a few mg/1, they may be responsible for stuck fermentations (Volume 1, Section 3.6.2). 

Phenolic compounds from lignin degradation, furan derivatives (furfural and HMF) from sugar degradation and aliphalic acids (acetic acid, formic acid and levulinic acid) are considered to be fermentation inhibitors generated from pretreated lignocellulosic biomass (53). The formation of these inhibitors depends on the process conditions and the lignocellulosic feedstocks 54). Various methods for detoxification of the hydrolyzates have been developed (55). These include... 

Microorganisms metabolically engineered with improved inhibitor tolerance could reduce the need for detoxification process. Larsson et al. (766) developed a S. cerevisiae strain with enhanced resistance to phenolic fermentation inhibitors in lignocellulose hydrolyzates by heterologous expression of laccase. 

Uses Ferment inhibitor prohibited in beverages antimicrobial enzyme inhibitor ribonuclease inhibitor reagent for modification of nucleotides, histidine... 

Sodium bisulfite Sucrose fermentation aid, food Magnesium sulfate anhydrous Mineral oil Tristearin Urea fermentation aid, silage Formic acid Sodium formate fermentation control agent, beer/wine Potassium metabisulfite fermentation industry Abietic acid fermentation inhibitor Diethyl pyrocarbonate Potassium fluoride Propylene glycol fermentation media Agar... 

On the other hand, an acidic pretreatment will hydrolyze the hemiceUulose but the cellulose and lignin remains intact (21). An acidic pretreatment may also result in high concentrations of furfural compounds in the liquid phase (13). Such compmmds may act as fermentation inhibitors. 

It was established that a range of concentrations exists in which fermentation inhibitors derived from pretreatments of lignocellu-losic feedstocks are not affecting the yeast, but will inhibit further growth of lactic acid bacteria. By an optimization of the levels of fermentation inhibitors the fermentation of biomass can be conducted under non-sterile conditions. Here, the yield of ethanol is comparable to those achieved under sterile conditions (22). Optimized inhibitor levels can be achieved by controlling the ratio of water to biomass of a lignocellulosic biomass during and after the pretreatment. 

Even when these methods are successful, they are not ripe for commercial use. Moreover, in contrast to the clean sugar streams derived from starch and sucrose, hydrolysates derived from biomass tend to have fermentation inhibitors, such as acetic acid, or furfural. These compounds must be either removed when their concentrations are high. Or else, robust strains must be developed that are resistant to the above inhibitors. 

Macro-algal biomass types, which are containing significant quantities of carbohydrates, with low amounts of lignin and fermentation inhibitors are interesting alternative substrates for fermentation (43). 

Ethanol Production in the Presence of Fermentation Inhibitors Economic Outlook... 

Martin C, Marcet M, Ahnazin O, Jbnsson LJ. (2007). Adaptation of a recombinant xyloseutilizing Saccharomyces cerevisiae to a sugarcane bagasse hydrolysate with high content of fermentation inhibitors. Bioresour Technol, 98, 1767-1773. 

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