Acetic acid fermentation inhibitor

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). 

Economically feasible processes for biomass conversion to ethanol requires the fermentation of the sugars generated in the pretreatment and hydrolysis steps. In agricultural residues and hardwoods, xylose constitutes about 45% of the total sugars, and therefore xylose conversion to ethanol is important for high yields. Dilute acid hydrolysis of cellulosic biomass generates inhibitory compounds such as furfural, hydroxymethyl furfural, and acetic acid [1], These inhibitors affect the ability of yeasts to ferment the hydrolyzates, and therefore a detoxification step is usually included in fermenting acid... 

Product Challenged Growth Studies. To study the inhibitory factors of the acetone-butanol fermentation, the growth rates of Cl. acetobutylicum in the presence of each fermentation product were determined. The end products used in this study included ethanol, butanol, acetone, acetic acid, and butyric acid. From the slopes of the least squares regression lines of optical density vs. time data, the maximum specific growth rates in the presence of varying concentrations of each inhibitor ()j ) were determined. The results for each fermentation product are shown in Figures 1 - 3. There appears to be a threshold concentra-... 

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... 

The main drawback of the acid hydrolysis processes is the formation of undesirable by-products. This not only lowers the yield of sugars, but several of the by-products severely inhibit the formation of ethanol in the fermentation process. Potential inhibitors are furfural, 5-hydro ethylfiirfural (HMF), levulinic acid, acetic acid, formic acid, uronic acid, 4-hydroxybenzoic acid, vanillic acid, vanillin, phenol, cinnamaldehyde, formaldehyde, etc. (I, 36). Some inhibitors, such as terpene compounds, are initially present in the wood, but apparently most of the inhibitors are formed in the hydrolysis process. 

Acetic acid is sometimes mentioned as an important inhibitor (47, 48). Since acetic acid has a dissociation constant of 4.75 in water, it will be partly dissociated at a pH of 5-5.5 (typical values for the fermentation). It is generally accepted that the effect of the undissociated part of the acid is larger than the effect of the dissociated part (49). The undissociated carboxylic acids can diffuse through the cell membrane (50, 51). Since the intracellular pH is higher than diat of the extracellular medium (52), the undissociated acid which has diffused into die cell is partly dissociated into acetate and hydrogen ions, thereby potentially lowering the intracellular pH (53, 54). The optimum extracellular pH for growth... 

Formic and levulinic acids are other weak ctuboxylic acids (dissociation constants 3.75 and 4.4 respectively), which are found in hydrolyzates. Formic acid is most likely formed from degradation of HMF (56, 57), although other parallel formation routes are possible. Formic acid is a stronger inhibitor than acetic acid (57), and acts inhibitory to the fermentation process above a concentration of about 1 g/1 (58). Levulinic acid is a degradation product of HMF (56), and was shown to have a negative effect on fermentability of the hydrolyzates (57). However, due to the low concentration of formic and levulinic acid normally found in hydrolyzates, they are probably of secondary importance with respect to inhibitory effects. 

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. 

Although acetic acid is known to be inhibitory to some strains of fermentatively growing Saccharomyces (Rasmussen et al., 1995 Fugelsang et al., 1993), it is generally believed that other inhibitory compounds are produced as well, and the final stuck fermentation represents the contribution of these multiple insults (Edwards, 1996). However, the nature of these additional inhibitors has yet to be determined. 

Aside from potential sensory implications, acetic acid and associated products of LAB metabolism represent potent inhibitors to fermentatively growing Saccharomyces, delaying the onset of fermentation and potentially causing fermentation to stick (see previous discussion of Microbial Antagonism). At pH >3.5, bacterial carbohydrate metabolism (Peynaud and Domercq, 1968) or MLF (Giannakopoulis et al., 1984 Zeeman et al., 1982) yielded higher concentrations of acetic acid than parallel lots at a lower pH. 

The latter approach to remove fermentation inhibitors in lignocellulosic hydrolysate using whole cells was also the focus of study of Wierckx et al. (2010). In this study, the bacterium Cupriavidus basilensis HMF14 was identified as a promising detoxification bacterium, as it consumes HMF, furfural, acetic acid, and a wide... 

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