# Lignocellulosic Biomass as a Substrate for Oleaginous Microorganisms: A Review
## 5 Pretreatment By-Products Affecting Enzymatic Hydrolysis and Fermentation
\begin{table}
\begin{tabular}{l c c c c c c c c c c c c c c c c c c c c c c c c} \hline \hline  & \multicolumn{4}{c}{**Furan Compounds**} & \multicolumn{4}{c}{**Phenolic Compounds**} & \multicolumn{4}{c}{**Weak Acids**} & \\ \cline{2-13} \cline{13-13}  & \multicolumn{4}{c}{**Furfural**} & \multicolumn{4}{c}{**5-HMF**} & \multicolumn{4}{c}{**Vanillin**} & \multicolumn{4}{c}{**Syngingdehyde**} & \multicolumn{4}{c}{**4-HB**} & \multicolumn{4}{c}{**Formic**} & \multicolumn{4}{c}{**Acetic**} & \multicolumn{4}{c}{**Levillancic**} \\ \cline{3-13}  & \multicolumn{4}{c}{**\%**} & \multicolumn{4}{c}{**\%**} & \multicolumn{4}{c}{**\%**} & \multicolumn{4}{c}{**\%**} & \multicolumn{4}{c}{**\%**} & \multicolumn{4}{c}{**\%**} & \multicolumn{4}{c}{**\%**} & \multicolumn{4}{c}{**\%**} \\ \cline{3-13}  & \multicolumn{4}{c}{**g/l**} & \multicolumn{4}{c}{**Inhibition**} & \multicolumn{4}{c}{**g/l**} & \multicolumn{4}{c}{**Inhibition**} & \multicolumn{4}{c}{**g/l**} & \multicolumn{4}{c}{**Inhibition**} & \multicolumn{4}{c}{**g/l**} & \multicolumn{4}{c}{**Inhibition**} & \multicolumn{4}{c}{**g/l**} & \multicolumn{4}{c}{**Inhibition**} & \multicolumn{4}{c}{**g/l**} & \multicolumn{4}{c}{**Inhibition**} & \multicolumn{4}{c}{**g/l**} & \multicolumn{4}{c}{**Inhibition**} \\  
Other sugar derivatives formed during acid pretreatment include polyols generated during the hydrogenation of cellulose and hemicellulose sugars, such as sorbitol, mannitol, xylitol, galacitol, arabitol, ribitol (adonitol) and erythnitol [238; 239]. However, these compounds usually have no negative effect. On the contrary, it has been suggested that polyols may mimic water and maintain an artificial sphere of hydration around macromolecules and also act as scavengers of reactive oxygen species, thereby preventing lipid peroxidation [238]. Aromatic carboxylic acids derived from lignin are also generated during pretreatment, but in lower amounts. These compounds, represented by cinnamic and benzoic acid derivatives, are commonly produced from herbaceous lignin rather than hardwood and softwood lignin. The common cinnamic acid derivatives include p-coumaric, ferulic and sinapic acids. Benzoic acid derivatives include 4-hydroxybenzoic acid. Additionally, by-products from lignin are three typical aliphatic phenolic aldehydes, namely vanillin, syringaldehyde and 4-hydroxybenzaldehyde, which can also be considered as lignin derivatives of the guaiaacyl group (G), syringyl group (S) and hydroxyphenyl group (H), respectively [15; 73; 109]. Aromatic carboxylic compounds derived from lignin are present in hydrolyzed lignocellulosic biomass in relatively low concentrations compared to carbohydrate degradation products, such as aliphatic carboxylic acids and furan aldehydes. However, their inhibitory effect is usually stronger than aliphatic carboxylic acids, causing a loss of integrity of biological membranes, thus affecting their ability to serve as selective barriers and enzyme matrices [232]. Among phenolic compounds, vanillin is the most toxic inhibitor for microorganisms [15; 235; 235]. Phenolic aromatic compounds can also affect enzymatic hydrolysis. Although the treatment of lignocellulosic hydrolysate with reducing agents, such as bisulfite and dithionite, decreases inhibitory effects due to sulfonation of phenolics, aromatic compounds remain a problem for cellulolytic enzyme activity. It has also been shown that the inhibitory effect of phenolic compounds increases with increasing hydrophobicity [239]. The effect of the pretreatment by-products on microorganisms has been widely studied in the case of ethanol-producing microorganisms. However, since different yeasts have different responses to inhibitors, the effect of the various inhibitory compounds on microbial metabolism merits further investigation in oleaginous species.