In contrast to the dry-grind process (where the whole corn kernel is ground and enters the fermenter) in wet milling, the kernel is first fractionated into separate components and only the starch enters the fermenter. Corn wet milling plants are often referred to as biorefineries, comparable to petroleum refineries, because wet milling fractionates corn into its components and then processes the components into more valuable products [20]. Wet milling separates the kernel into germ (oil), gluten (protein), fiber, and starch fractions, yielding corn oil, animal feed, and a variety of products derived from starch.

Steeping

The wet-milling process starts with steeping, which enables isolation of the different kernel fractions [20]. Corn is screened to remove foreign material, and soaked in dilute (0.12-0.20%) sulfurous acid at 52°C for (typically) 30-36 h. Lactobacillus and related bacteria growing in the steep water produce lactic acid and other metabolites that further acidify the medium. Steeping occurs in a series of large stainless steel tanks, with steep water recirculated countercurrently from tanks holding “older” corn that is nearing the end of the steeping process to “newer” corn that is beginning the steep. The effect of recycling the steep liquid is the progression of the corn up a SO2 concentration gradient. The combined action of sulfurous acid and lactic acid, and probably also direct effects of microbes growing in the steep, prepare the kernels for processing into fractions. Steeping softens and swells the kernels, disrupts disulfide bonds between the protein and starch in the endosperm, and releases the starch granules into solution.