92 Introduction to Renewable Biomaterials
glucose by enzymes called amylases. Lignocellulose is the most challenging source
of carbohydrates. Combination of physical pre-treatment with extensive enzymatic
treatment is required to release fermentable sugars from lignocellulose. The process
of industrial ethanol fermentation is usually carried out in large fermenters at 30∘C
at slightly acidic condition of pH 4.8–5.0 to limit the growth of undesirable bacteria
that could compromise the efficiency of the fermentation process [34]. Acclimatised
yeast inoculums are added to carbohydrate solution, and fermentation is initiated.
Initially, rapid growth of yeast is observed until the concentration of 10^8 cells ml−^1 is
achieved. Since this stage, fermentation process takes off and ethanol concentration
increases in the fermentation tank, which is coupled with the generation of heat and
vigorous release of CO 2 , both side products of this metabolic activity. The process of
active fermentation takes about 12 h until majority of carbohydrates is utilised and is
followed by a slower phase that takes 40–48 h until carbohydrates are depleted and
concentration of ethanol reached 10–15% (w/w). After fermentation is completed, the
broth is distilled to recover ethanol. Fractional distillation is performed to separate
the ethanol at the concentration of 90–94% (azeotropic mixture with water). Further
purification step is required to obtain anhydrous ethanol of fuel grade (>99%). In the
industrial practice, dehydration is performed with zeolites – molecules that can entrap
water molecules within their pores decreasing the content of water to almost zero.
Ethanol fermentation yields co-products that are dependent on the type of feedstock.
Ethanol production from sugarcane is usually combined with production of table sugar
(sucrose). Corn ethanol production yields other products including DDGS (distiller’s
dried grains and solubles) – dry milling process, corn oil and corn gluten – wet milling
process. All ethanol fermentation processes generate high-purity CO 2 thatcanbecol-
lected and used in food, drink or chemical industries. Applications of ethanol extend
beyond traditional uses in food and fuel industries and may become valuable feedstock
for chemical industry in the synthesis of hydrocarbons, higher alcohols, acetaldehyde,
acetone, diethyl ether, and ethyl acetate [35].
3.4.9.2 ABE Fermentation
Production of butanol by solventogenic bacteria,Clostridium acetobutylicum,and
related is an example of strictly anaerobic metabolism. ABE (acetone–butanol–ethanol)
fermentation is an example of mixed fermentation where numerous fermentation prod-
ucts are formed. In ABE fermentation, there are two stages of the process: acidogenesis
and solvengenesis. In acidogenesis stage, two organic acids, acetate and butyrate, as
well as two gaseous products CO 2 and H 2 are produced. Acetogenesis produces an
ATP molecule in addition to those from glycolysis reaction, thanks to either acetate or
butyrate kinases [36]. Production of these organic acids results in an increase of growth
medium acidity that negatively impacts the producer strain, and in the second stage of
the fermentation these products are reassimilated and reenter metabolic pathway. The
reaction is catalysed by CoA transferase that does not require ATP utilisation; instead
the reaction is driven by the formation of acetoacetate and finally acetone [36]. Once
theseacidsarereassimilatedrespectivelyasacetylandbutyryl-CoA,thesynthesisof
ethanol and butanol can commence in solventogenic stage of the process.
Formation of acetyl-CoA occurs via decarboxyltion of pyruvate resulting from glycol-
ysis and assimilation of previously secreted acetate.
C 3 H 4 O 3 +CoA-SH↔CoA-S-C 2 H 3 O+CO 2 +H 2
