Deep grain processing

Deep processing separates a grain into its components rather than milling it whole. Maize wet milling is the archetype, and each step exists because of a specific physical or chemical obstacle.

Steeping: breaking one class of bond

Starch granules in the maize endosperm are embedded in a continuous protein matrix, chiefly zein, which is cross-linked by disulfide bonds. Mechanical milling alone cannot cleanly separate starch from that matrix; it fragments both together.

Kernels are therefore steeped for a day or two in warm water containing sulfur dioxide, which acts as a reducing agent and cleaves those disulfide bonds. The protein network loosens, releasing the starch granules intact and hydrating the kernel so it can be milled without shattering the germ.

Steepwater is not sterile and is not meant to be. Lactic acid bacteria proliferate in it naturally, producing lactic acid that lowers pH, assists protein solubilisation and suppresses spoilage — a spontaneous fermentation embedded in an industrial separation process. The concentrated steepwater is itself sold as a fermentation nutrient, and it is a common nitrogen source in industrial fermentation media.

Separation by density and by size

After coarse milling, the germ is separated first, because it is rich in oil and therefore the least dense fraction; hydrocyclones exploit that difference, and the germ goes to oil extraction. Fine milling and screening then remove the fibre.

What remains is a slurry of starch and gluten protein. These are separated by density too — starch granules are denser than protein — in centrifuges and hydrocyclones. The protein fraction becomes corn gluten meal, a high-protein feed ingredient; the starch is washed and either dried or sent to conversion.

Nothing here is chemistry: three of the four fractions are separated purely by exploiting density differences that the steeping step made accessible.

Conversion: three enzymes, three jobs

Starch is turned into sugar syrups by a sequence in which each enzyme does something the others cannot.

α-Amylase liquefies. It cuts internal α-1,4 bonds at random, collapsing the viscosity of gelatinised starch quickly. Industrial versions are thermostable enzymes from Bacillus species, allowing the step to run at temperatures that gelatinise the starch fully — a case where the process temperature was set first and the enzyme found to match it.

Glucoamylase saccharifies. It removes glucose units one at a time from chain ends, and unlike α-amylase it can also cut the α-1,6 branch points, so it can take the reaction essentially to completion. This yields high-purity glucose syrup.

Glucose isomerase converts glucose to fructose, which is sweeter. It is an equilibrium reaction reaching roughly 42 percent fructose, and higher-fructose syrups are made by chromatographically enriching a portion to around 90 percent and blending back to 55. The enzyme is immobilised on a carrier and used in packed columns continuously for months, which is one of the largest-scale applications of immobilised enzymes anywhere in industry.

Dry milling, by contrast, separates by fracture and sieving without steeping, giving grits, meal and flour — a different product set entirely, and the route used for fuel ethanol.