Alkali Wet Milling of Corn
S. R. Eckhoff,* L. Du
University of Illinois

A wet milling process has been developed using sodium hydroxide as the main chemical for the disruption of the endosperm protein matrix. The process involves alkali debranning of the kernels to recover coarse fiber, grinding to reduce the diffusional paths, and steeping in dilute alkali for 1 hour. The total process time is less than 1.5 hours. Product yields are comparable to conventional wet milling, with the major loss of starch being with the gluten fraction. Current efforts are focused on product functionality and control of effluent streams.

Intermittent milling and dynamic steeping (IMDS) is a process by which the kernel is milled in stages following short periods of steeping. The result is faster hydration of the kernel and diffusion of sulfite into the endosperm. Laboratory tests show that the IMDS process increases total starch recovery in a 5-hour process compared to the conventional wet milling process. The process yields comparable products to conventional wet milling. The increased starch appears to come from decreased production of solubles during steeping and from less starch in the fiber.

Conventional steeping for corn wet milling is a diffusion-limited process in which sulfur dioxide is required to diffuse through interstitial water in the corn kernel in order to perform the necessary reactions in the endosperm protein matrix. Gaseous diffusion of sulfur dioxide is approximately 100 times faster than liquid diffusion. Gaseous SO₂ steep times of 6 to 12 hours gave starch fields comparable to 24 hours of conventional steeping. An engineering economic evaluation showed that building a wet mill with gaseous addition of SO₂ would save 7.9% of initial capital expenditures or $0.065 per bushel at a 25% ROI.

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