Page 56 - Advances in Eco-Fuels for a Sustainable Environment

Ecofuel feedstocks and their prospects 33

are added, and the mixture is heated again at 110–150°C for about an hour. This causes
liquefaction of starch and reduces the bacterial level in the mix. This is again cooled to
85°C for 1h after a second addition of alpha-amylase enzymes. It is then cooled to
room temperature, with the addition of gluco-amylase enzymes to ensure conversion
of corn starch to dextrose. Gluco-amylase enzymes usually are directly added into the
fermenter using a process known as “simultaneous saccharification and fermentation”
(SSF). In the fermentation process, operated either in batch or continuously, yeasts
convert glucose into ethanol and CO 2 . It usually takes about two days to complete
the fermentation process, after which the resulting “beer” is transferred to distillation
columns where the ethanol is located. The residual stillage containing the remaining
protein, oil, and fiber is dried to a 27% protein product (DDGS) or just distillers dried
grains (DDG), used in animal feeds.
Wet mills instead fractionate corn first to separate corn oil, corn gluten meal
(CGM), and corn gluten feed (CGF) to capture value for food and animal feed, and
the starch is then hydrolyzed to sugars for ethanol production. Cellulosic biomass
is hydrolyzed with heat, acids, or enzymes to release sugars that can be fermented
to ethanol, with lignin and other remaining fractions burned to provide process heat
and electricity for the conversion, with excess electricity left. In the process, corn ker-
nels are split in three parts prior to fermentation: (1) the hulls, (2) the germs, and (3)
the endosperms. The primary process products are starch and derived products (HFCS
and ethanol), and corn oil and gluten, obtained via multiple steps. Shelled corn is
mechanically cleaned, removing unwanted material, then are next fed into steeping
tanks, where they remain for 1–2 days soaking in dilute sulfuric acid at 52°C. This
softens the kernels, breaking down proteins holding the starches, and removes other
soluble constituents. Process water from the first steep tank, rich in protein and con-
taining about 6% of the original dry weight of the grains, called “light steepwater,” is
sent to evaporators, where it is concentrated to 30%–55% solids. The resulting liquor
is usually sold as an animal feed. Germs are removed from the steeped corn in
degerminating mills, where kernels are broken to divide the germ and remaining
starch and gluten. The former is separated in liquid cyclones from the mixture,
washed, dewatered, dried, and further processed to extract corn oil. Starch and gluten
from the slurry are removed by further washing, grinding, and screening operations.
Discarded hulls are dried for animal feed. The starch is then separated from gluten by
centrifugation, and at this point the wet-mill process is quite similar to dry milling: the
pH of the starch slurry is adjusted to 5.8–6.2 with lime, and alpha-amylase is added to
convert the starch polymer into soluble short-chain dextrins (liquefaction). Calcium
(20–100ppm) may be added to enhance enzyme stability. The starch stream is almost
free of fiber or other components, and is therefore well suited to high temperatures and
short times of jet cooking enzyme liquefaction. Slurry from the liquefaction stage is
mixed with heat-sterilized steep water, providing both fermentation nutrients and pH
adjustment, and sent to saccharification, where added glucoamylase converts dextrins
o
to glucose at pH¼4.5 and 65 C. After saccharification, S. cerevisiae is added to fer-
ment sugars to ethanol and CO 2 . Total fermentation time varies from 20 to 60h,
depending on the degree of saccharification prior to final fermentation. The final prod-
uct will have an ethanol content of 8%–10% by volume [35].