Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
8.2.3.1 Steeping and Grinding
Steeping is the key operation in preparing kernels for effective separation and
splitting of their components. 3 As previously addressed, water diffusion, chemical
reactions, and removal of solved compounds together with fermentative processes
(in particular, a lactic acid fermentation) produce tissue structures adequately softened
for mechanical separation operations. 4 Understeeping results in high-protein
starch and starch losses with by-products, whereas oversteeping damages starch
quality and gluten losses by excessive solubilization.
Cleaned corn/maize is soaked in process water from starch refining to which
0.1 to 0.2% SO 2 is added to produce steep acid. The steeping battery is managed in
a complex system of countercurrent flow where steep acid enters the battery section
with oldest corn (approximately 45% moisture) and leaves the section with fresh
corn as light steep water. The temperature applied during steeping is set by tolerance
of lactic acid bacteria and ranges from 49 to 53°C. Steeping time may vary from
30 to 50 h; however, effective steeping times are 40 to 42 h. After reaching the
correct point of treatment, steeped corn is drained (about 50% moisture) and fed to
degerminating mills (attrition mills) to tear open kernels and leave the intact germ
free for the following separation. In general, degermination takes place in two stages
of a first and second grind, with intermediate removal of germs via hydrocyclones
152 mm in diameter. Ground germ-free material passes 50-µm screen bends (120°,
grit screens) to separate finely split starch and gluten (40 to 50% of the stream) from
coarse material, which is fed to the third grind by refiner or impact mills for
completed disintegration. Separated starch-free germs are dried. They have a lipid
content of 50 to 58% and are high-grade material for vegetable oil production.
8.2.3.2 Fiber Separation
Fibers made flexible by steeping become resistant to further fragmentation by milling
and are removed by screening from the slurry consisting mainly of starch and gluten.
The material leaving the grit screens for the third grinding passes the fiber-washing
station, a system of five or six stages of screen bends in series. Screen bends used
in the fiber-washing station resemble the 120° screens used as grit screens. The
aperture of the screens is also 50 µm. While starch and gluten are transported to the
first wash screen, fibers leave the last screen for dewatering and drying. Coarse fiber
is discharged after optimum washing with maximum 15% bound and 5% free starch
(dry substance basis).
8.2.3.3 Starch–Gluten Separation
The stream coming from grit screens and the first section of the fiber-washing
screens is designated as mill starch and contains separated starch, insoluble proteins
(zein, glutelins), and soluble impurities, mainly proteins and minerals. Mill starch
produced with a density of 6 to 8° Bé first passes degritting hydrocyclones to remove
sand and fine debris that potentially may damage separators and hydrocyclone
stations used for starch–gluten separation and starch refinement. Debris formed by
the action of centrifugal forces and high-speed movement applied there can cause