Formic acid, fermentation inhibition

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. 

Normally, the strains of P. pentosaceum, P. freudenreichii and P. shermanii studied produced 10-11 mmoles of propionic acid per 100 ml, although the vitamin Bn levels in these strains varied greatly, being 56, 1650 and 1070 pg per g biomass, respectively. Inhibition of vitamin Bn biosynthesis resulted in a reduced propionic acid production, with acetic, or acetic and formic acids, becoming the main end products of fermentation. As a result, the ratio of propionic/acetic acid in the cells that produced insignificant quantities of vitamin Bn (growing in cobalt-free medium with methionine) was low, 1 4, as compared with the normal ratio of 2 1 (Vorobjeva, 1976). 

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). 

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