Agriculture Reference
In-Depth Information
Steeping
Steep water
Corn kernels
Corn steep liquor
or
Corn gluten feed
Evaporating
Degermination
Germ
Drying/Oil
extraction
Corn oil
Grinding
Fiber
Fiber
Starch gluten
separation
Gluten
Corn gluten meal
Glucoamylase +/or
Rhizozyme ~0.05%
Starch
HT α-amylase
~0.05%
Distillation/
deh
Liquefaction
SSF
Ethanol
ydration
Fig. 22.2
A block-schematic
diagram of maize wet-milling
processes and products.
10
7
Yeast to ~1×
cells per milliliter mash
ethanol are the same for both maize and wheat
wet-milling plants.
With wheat fl our as a starting material, the
major co-product of wheat wet-milling ethanol
plants is vital wheat gluten. Current industrial
processes can achieve 80%-85% recovery of
wheat protein in the form of wheat gluten, which
generally contains 80% protein, 3%-20% carbo-
hydrates (mostly starch), 5%-8% total lipids, 1%
pentosans, and 0.5%-1.5% ash on a dry-weight
basis (Sayaslan 2004). Compared with the dry-
grind process, ethanol yield from the wet-milling
process is slightly lower because of the loss of
some starch to the wheat gluten. However, if the
drying process retains gluten vitality, it can be
sold into the higher-value market of vital wheat
gluten ($4,000 t
−1
), which exceeds the value for
gluten meal from the maize wet-milling process.
This compensates for the lower ethanol yield
(Day et al., 2006).
The dry-grind process of wheat in conven-
tional ethanol plants is similar to that of the maize
dry-grind process, which includes grinding,
liquefaction, simultaneous saccharifi cation and
fermentation (SSF), distillation/dehydration,
and separation/concentration of spent grains.
Grinding
Wheat grain
HT
α
-amylase
~0.05%
Liquefaction
Glucoamylase +/or
Rhizoz
Yeast to ~1×10
7
yme ~0.05%
cells
per
milliliter
mash
SSF
CO
2
Distillation/
deh
Ethanol
ydration
Centrifuge/
concentration
DGS
Fig.
22.3
A block-schematic diagram of wheat dry-grind
processes and products.
The co-products are distiller's grain with solubles
(DGS) and carbon dioxide. Maize DGS contains
about 30% protein and is sold as a protein supple-
ment for animal feed. Sales from DGS account
for 15%-20% of the annual revenue of a maize
dry-grind ethanol plant. Wheat DGS normally
has a higher protein content than maize DGS
(Wu et al., 1984; Rasco et al., 1987).
