The next step is represented by distillation and rectification. An azeotropic
solution of 95% (v/v) ethanol is obtained. Further concentration to absolute ethanol
is finally achieved by molecular sieves or azeotropic distillation (using benzene or
cyclohexane) (Chiaramonti 2007).
1.4.2 Bioethanol Production from Starch
As reported in Table 1.3, grains (corn, wheat, or barley) mainly provide starch. This
is made up of long chains of glucose units. The amylose structure contains 1000
monomeric units, while the amylopectin structure contains 1000–6000 units. Starch
is the most utilized for ethanol production in North America and Europe. To produce
ethanol it is necessary to hydrolyze the starch into monomers. The hydrolytic
reaction is catalyzed by glucoamylase enzyme. D-glucose, which is an isomer of
glucose, is obtained as final hydrolysis product. Enzymatic hydrolysis is then
followed by fermentation, distillation, and dehydration to yield anhydrous ethanol
(Kumar et al. 2010).
There are two distinct methods for processing corn: wet milling and dry milling.
Dry mills are usually smaller in size and are built primarily to produce only ethanol.
Wet mill facilities also produce a list of high-valued co-products such as highfructose corn syrup, corn oil, and corn gluten.
Corn dry-milling process is carried out in five steps (Vohra et al. 2014):
(i) Biomass handling (milling)
(ii) Liquefaction
(iii) Hydrolysis (saccharification)
(iv) Fermentation
(v) Distillation and recovery
In dry-grind process, the corns are milled to a powder and heated with water at
85
C (Kojima and Johnson 2005). Then hot water and alpha-amylase enzymes are
added and the mixture is heated at 110–150
C for an hour. This causes the
liquefaction of starch. When liquefaction is completed, the mixture is cooled down
and glucoamylases are added to produce dextrose. In dry-grind milling plants, often
the glucoamylases are directly added into the fermentor. The process is known as
“simultaneous saccharification and fermentation” (SSF) (Fig. 1.6).
In the fermentation process, yeasts convert glucose into ethanol and carbon
dioxide. The process is completed in about 40–50 h. During fermentation, the
mash is continuously mixed and it is cooled down. The beer obtained from fermentation is transferred to the distillation columns where ethanol is separated from the
stillage (Singh et al. 2001). The stillage contains protein, oil, and fiber and are dried
to obtain dried grains with solubles (DDGS) or just distillers dried grains (DDG).
DDGS contain the process syrup combined with the solids, while DDG don’t
contain it.
12
P. Bartocci et al.
solution of 95% (v/v) ethanol is obtained. Further concentration to absolute ethanol
is finally achieved by molecular sieves or azeotropic distillation (using benzene or
cyclohexane) (Chiaramonti 2007).
1.4.2 Bioethanol Production from Starch
As reported in Table 1.3, grains (corn, wheat, or barley) mainly provide starch. This
is made up of long chains of glucose units. The amylose structure contains 1000
monomeric units, while the amylopectin structure contains 1000–6000 units. Starch
is the most utilized for ethanol production in North America and Europe. To produce
ethanol it is necessary to hydrolyze the starch into monomers. The hydrolytic
reaction is catalyzed by glucoamylase enzyme. D-glucose, which is an isomer of
glucose, is obtained as final hydrolysis product. Enzymatic hydrolysis is then
followed by fermentation, distillation, and dehydration to yield anhydrous ethanol
(Kumar et al. 2010).
There are two distinct methods for processing corn: wet milling and dry milling.
Dry mills are usually smaller in size and are built primarily to produce only ethanol.
Wet mill facilities also produce a list of high-valued co-products such as highfructose corn syrup, corn oil, and corn gluten.
Corn dry-milling process is carried out in five steps (Vohra et al. 2014):
(i) Biomass handling (milling)
(ii) Liquefaction
(iii) Hydrolysis (saccharification)
(iv) Fermentation
(v) Distillation and recovery
In dry-grind process, the corns are milled to a powder and heated with water at
85
C (Kojima and Johnson 2005). Then hot water and alpha-amylase enzymes are
added and the mixture is heated at 110–150
C for an hour. This causes the
liquefaction of starch. When liquefaction is completed, the mixture is cooled down
and glucoamylases are added to produce dextrose. In dry-grind milling plants, often
the glucoamylases are directly added into the fermentor. The process is known as
“simultaneous saccharification and fermentation” (SSF) (Fig. 1.6).
In the fermentation process, yeasts convert glucose into ethanol and carbon
dioxide. The process is completed in about 40–50 h. During fermentation, the
mash is continuously mixed and it is cooled down. The beer obtained from fermentation is transferred to the distillation columns where ethanol is separated from the
stillage (Singh et al. 2001). The stillage contains protein, oil, and fiber and are dried
to obtain dried grains with solubles (DDGS) or just distillers dried grains (DDG).
DDGS contain the process syrup combined with the solids, while DDG don’t
contain it.
12
P. Bartocci et al.