The Wet Milling Process of CornWhereas the separation achieved on dry milling is purely mechanical the separation of constituent in wet milling achieved by a combination of chemical and mechanical means.
Wet milling begins with steeping cleaned corn or 30-48 hours with process water recycle from downstream operations.
Sulfur dioxide is added to the water to achieve a level of 0.1-0.2%, and the solution is heated to about 50 degree C.
These conditions are unsuitable for putrefying microorganisms but are conducive to growth of indigenous Lactobacillus.
The low of water through the battery of steep tanks is countercurrent so that the freshest corn contacts the weakest sulfur dioxide concentration.
During steeping, the kernels absorb solution and swell, activating enzymes native to the kernel to assist in breaking down the structure; the bisulfate ion reduces disulfide binds in the protein matrix, increasing protein solubility and diminishing interactions between starch and protein; the lactic acid and/or exogenous enzymes produced by the lactobacilli help soften the endosperm.
After contacting 8-16 different of corn over the 30-48 hour period, about 5 gallon of light steep per bushel of corn processed is withdrawn and evaporated to about 50% solids.
The steep liquor may be sold for use in fermentation media, but this market is small compared to total steep liquor production.
It is common to combine steep liquor with feed fraction (gluten feed). The remaining 3-4 gal/bu of water used in wet milling is absorbed by the corn and carried downstream and must be dried from the resulting corn factions.
After steeping, the softened corn is ready for grinding and fractioning.
The steeped corn is sluiced to disk attrition mills, where the objective is to free the rubbery germ without breaking it into pieces.
The ground slurry is pumped to hydroclones (liquid cyclones) to separate the lighter weight germs.
Multiple passes may be used to achieved clean separations.
The germs are dried and processed for oil and meal. The heavier underflow from hydroclones is screened and larger particles are finely reground with an impact mill to free the starch, protein and fiber from each other.
Next, the fiber is separated, washed over a series of screens and dewatered.
The remaining stream of starch and protein (term “millstarch”) is pump to disk nozzle type centrifuge, where heavier starch is separated from the gluten.
The gluten is dewatered using additional centrifuges and vacuum filters.
Some proteins (3-5%) and impurities remain in the starch slurry, which are removed by starch washing hydroclones.
The Wet Milling Process of Corn
