Biotechnological Applications of Hemicellulosic Derived Sugars: State-of-the-Art - Sustainable Biotechnology: Sources of Renewable Energy - page 71

Compared to other pretreatment methods, it is especially useful for the conversion
of hemicellulose into xylose, which can be fermented into ethanol by specialized
microorganisms [3, 4]. Most dilute acid processes are limited to a sugar recovery
efficiency of around 50%. It has been reported that the cell wall structure and com-
ponents may be significantly different in different plants, which may influence the
digestibility of the biomass [23]. A broad dilute acidic hydrolysis on a variety of
lignocellulosic materials with respective ethanol production has been reviewed by
Chandel et al. [3].
Formation of Inhibitors During Acid Hydrolysis
During acid hydrolysis of lignocellulosics, aliphatic acids (acetic, formic, and lev-
ulinic acid), furan derivatives, and phenolic compounds are formed in addition to
the sugars. Furfural and 5-hydroxymethyl furfural (HMF) are the most important
furans, formed by decomposition of pentoses and hexoses respectively [24]. Acetic
acid has been reported in the hydrolysis of the acetyl groups into hemicellulose as a
consequence of deacetylation of acetylated pentosan [25]. Multiple phenolic com-
pounds are derived from lignin, including vanillin, vanillic acid, vanillyl alcohol,
4-hydroxybenzoic acid, 4-hydroxybenzaldehyde, coumaric acid, syringaldehyde,
syringic acid, cinnamaldehyde, dihydroconiferyl alcohol, hydroquinone, catechol,
veratrole, acetoguaiacetone, homovanillic acid, and Hibbert's ketones [25]. HMF
is converted at a lower rate than furfural, which may be due to lower membrane
permeability and cause a longer lag-phase in the growth of microorganisms [26].
The phenolic compounds penetrate biological membranes and cause them to lose
integrity, thereby affecting the membranes' ability to serve as selective barriers. The
microbial growth was found to be inhibited in the presence of acetic acid (>3.5 g/l)
in hemicellulosic hydrolysates, this phenomenon may occur due to the inflow of
undissociated acid into cytosol [26].
Removal of Fermentation Inhibitors from the Hemicellulosic Hydrolysates
In order to enhance the efficiency of hydrolysate fermentation, several detoxification
methods have been employed, including chemical, physical, and biological methods
[25]. These methods include neutralization, overliming, use of ion exchange resins,
adsorption onto activated charcoal or tin oxides, and treatments with enzymes such
as peroxidase and laccase [3, 25]. Since detoxification increases the cost of the pro-
cess, it is important to either overcome the need for detoxification steps or develop
cheap and efficient detoxification methods. Overliming with CaO or Ca(OH) 2 is
a classical chemical detoxification method. It efficiently removes furans and phe-
nolics with marginal loss of sugars [24]. Organic solvents such as ether or ethyl
acetate have also been applied to extract most of the inhibitors, such as phenolics,
weak acids, and furans [25].
Activated charcoal treatment is an efficient and economical method of remov-
ing phenolic compounds, acetic acid, aromatic compounds, furfural, and HMF
by adsorption [25]. Biological detoxification is another method that enhances the