Milling methods

Wet milling

Corn refining is a two-step process that involves the wet milling and processing of corn. Shelled yellow dent corn, which has been removed from the cob during harvesting, is cleaned and steeped (softened) in tanks with a water-based solution, releasing starch. Steepwater is drawn off and the corn is milled, breaking the germ loose. Mechanical and solvent processes during germ separation extract oil from the germ. The corn leaves the germ separator in a water suspension for further grinding to release starch and gluten from the kernel. The starch and gluten slurry is piped to starch separators. The fibre and germ residue (spent flake) are traditionally sold as animal feed. Protein, which is lower in density than starch, is removed in a centrifuge. Starch is collected and washed to produce a high-quality material that can be onward processed into specialty starches or converted into corn syrups (Pomeranz, 1987). Similarly, wheat starch can be produced from whole wheat by wet milling (Lineback and Rasper, 1988).

Dry milling

In dry milling, cereal components are separated by rolling; for example, wheat is converted to separate endosperm, bran and germ (Pomeranz, 1987). The mechanical properties of germ, bran and endosperm vary differently with moisture content such that germ and endosperm are easily segregated.

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