First Principles of Pretreatment and Cracking Biomass to Fundamental Building Blocks 197
Hydroxy methyl furfural is a toxic compound originating from hexose degradation.
Hydroxy methyl furfural is considered less toxic than furfural, and its concentration
in hemicellulose hydrolyzates is normally lower than furfural. The decomposition of
hemicellulose is responsible for the formation of inhibitors. Kinetic studies have shown
that the production of furfural increases after pretreatment for longer time and at
highertemperature. The temperature higher than 160∘C and residence time of acid
pretreatment longer than 4 h have been reported to be adequate to produce furfural
or hydroxymethyl furfural, and their formation is significant at higher temperature or
longer residence terms (McKillip and Collin, 2002).
6.4.7 Lignin Degradation Products
A large variety of compounds (aromatic, polyaromatic, phenolic, and aldehydic) are
released from lignin during hydrolysis of lignocellulosic biomass. Phenolic compounds
have a considerable inhibitory effect on the fermentation of lignocellulosic hydrolyzates
and their toxicity (even at low concentrations) is higher than furfural and HMF, espe-
cially those with low molecular weight are the most toxic ones. Phenolic compounds
cause a partition and loss of integrity of biological membranes of the fermenting
organisms, reducing cell growth and sugar assimilation (Mussatto and Roberto, 2004).
The main influencing factors for phenolic compounds formation are process time and
temperature. Lignin degradation is negligible at temperatures lower than 180∘Cinthe
absence of strong acid or alkaline conditions.
6.4.8 Acetic Acid
Acetic acid is derived from acetyl groups present in the hemicellulose structure. When
thepHofthemediumislow,aceticacid(pka=4.7) appears in the undissociated form
that is liposoluble and diffuses across the plasma membrane. The toxicity of acetic acid
varies according to the fermentation conditions. Since the formation of acetic acid
begins with hemicellulose hydrolysis, its formation cannot be prevented. However, a
higher fermentation pH can decrease this effect or the acid can be neutralized before
the fermentation process.
6.4.9 Inhibitory Extractives
Extractive compounds derived from lignocellulosic materials are acidic resins, taninic
acids, and terpene acids. The toxicity of these materials is lower than lignin degradation
products and acetic acid.
6.4.10 Heavy Metal Ions
Heavy metal ions such as iron, chromium, nickel, and copper can originate from
corrosion of hydrolysis equipment during pretreatment process, and their toxicity
inhibits the enzymes in the fermenting microorganism’s metabolic pathways.
6.4.11 Separation
The separation phase is done after the fermentation stage and produced ethanol is
extracted from the fermentation medium by different methods.
