Page 131 - Lignocellulosic Biomass to Liquid Biofuels

Bioconversion of lignocellulosic biomass to bioethanol and biobutanol 105

Phenolic compound commonly found in a typical spruce hydrolyzates
includes gallic, vanillic, syringic and ferulic acids, catechine, picein and
pungenin, taxifolin, and coniferyl aldehyde [255]. All these phenolics are
strong inhibitors to cellulolytic enzymes and to yeast cells, affecting nega-
tively the hydrolysis and fermentation processes [240].
The phenols are also indicated as more toxic compounds than furan
aldehydes and weak acids, even at lower concentrations. The high toxicity
of phenols is related to their low molecular weight (MW) that allows
them able to cross cell membranes and damage internal structures by cel-
lular morphological changes. Phenolics indeed can perturb the cell mem-
brane, increasing its fluidity and causing a significant drop of intracellular
potassium concentrations. They can also promote the DNA breakdown
and inhibition of RNA and protein synthesis [237].
The toxicity of phenolics is very variable, and it not necessarily
depends on the functional groups. Adeboye et al. [277] investigated the
severity of the inhibitory effects of 13 phenolic compounds (commonly
present in spruce hydrolyzates) on the S. cerevisiae growth, biomass, and
ethanol yield. They observed different toxicity limits even between phe-
nolic compounds that share the same functional groups. For instance, tox-
icity limits of ferulic and p-coumaric acids are 1.8 and 9.7 mM,
respectively. Moreover, the concentration and the nature of phenolic
compounds affect the fermentability of substrates rich in phenols. The
inhibition mechanisms among phenols are dissimilar and cannot be identi-
fied on the basis of main classes of phenolic compounds (aldehydes, acids,
alcohols, and ketones) [277]. Li et al. [280] investigated the different
inhibitor effects of three phenols, such as syringaldehyde, vanillin, and
phenol, on cell growth and fermentation of glucose and xylose. The van-
illin showed the strongest inhibitory effect on glucose fermentation, fol-
lowed by phenol and then syringaldehyde, while phenol had the strongest
inhibition on xylose fermentation among the three phenols considered.
Moreover, the results obtained from this research work indicated that low
MW phenols inhibit xylose fermentation more strongly than high MW
phenols [280].

3.5.4 Interaction effects
In according to work of Palmqvist et al. [250], in the presence of mixtures
of acetic acid and furfural in the fermentation medium, the specific
growth rate decreased more than the sum of the individual effect,