Formic acid fermentation inhibitor

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... 

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). 

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. 

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. 

Formalin, a 40 per cent solution of formaldehyde in water, has been used as a fermentation inhibitor. It is applied either on its own or more effectively with an acid such as sulphuric or formic acid. Typical results of a sulphuric acid/formalin mixture applied to ryegrass at the rate of 4.6 1/tonne are shown in Table 19.7 the formaldehyde combines with the protein, protecting it from hydrolysis by plant enzymes and microorganisms in the silo. The acid in the mixture acts as a fermentation inhibitor, preventing, in particular, the development of undesirable bacteria in the silage. Formalin is now used only in conjunction with formic acid, which has proved to be more effective than sulphuric acid. In Furope, the use of formaldehyde as an additive has been banned owing to concern about its carcinogenic properties. 

Cantarella et al. (2004) determined the effects of several ehemicals in the three main classes of toxins mentioned above, on subsequent enzymatic hydrolysis and simultaneous saccharifieation and fermentation (SSF) for the production of ethanol by Saccharomyces cerevisiae. Vanillin (0.5 g/1) was found to be the most potent inhibitor in SSF when compared to similar concentrations of 5-HMF and acetic acid. Longer lag phases and the most pronounced reduction in fermentation productivity were found when higher concentrations of acetic acid (i.e. 2 g/1) were used, while levulinic and formic acids at 1 g/1 were found to reduce ethanol production by 38% and 48%, respectively. Efficient and selective removal or substantial dilution of vanillin and the organic acid inhibitors found in steam-exploded hydrolysates were concluded to be the most important pretreatment considerations for improving process productivity. 

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