Dry and wet milling of corn

1. Dry and wet milling of corn NEHA RANA CCS HAU, HISAR

2. Structure and composition C o m p o n e n t s o f Yellow Dent Corn Starch 61.0 % Corn Oil 3.8 % Protein 8.0 % Fiber 11.2 % Moisture 16.0 % The mature corn is composed of four major parts: Endosperm 82% Germ 12% Pericarp 5% Tip cap 1%

3. Corn dry milling Dry milling process consists of the following steps: Cleaning Conditioning Degerming Drying Cooling Grading Grinding

4. Cleaning When corn is received at the mill, it is cleaned by both dry and wet process. Cleaning steps are sieving, separating particles by shape and density and removing ferromagnetic metals by permanent magnets. Conditioning The cleaned corn is conditioned, which basically means that water is added and the moisture allowed to equilibrate within the kernels . A moisture content of 21% is considered optimal.

5. Degermination The purpose of degerming is to remove hull, tip cap and germ and leave the endosprum into large grits. Drying The degermer product are to be dried to 15-18% moisture content for proper grinding and sifting. Drying is performed by conventional rotary steam tube dryers. Cooling Counter-flow or cross flow rotary cooler can be used for cooling the dried products.

6. Grading Recovery of various primary products is the next step. The through-stock is sifted or classified by particle size and enters into a conventional long reduction system having the function of removing bran and germ while releasing a maximum amount of clean large grits. Milling The milling operation consists of the steps of grinding, sifting, classifying, purifying, aspirating and in some case, final drying. The normal flow is through break rolls and then to sifters. The break rolls are followed by reduction roll which grind the endosperm to the desired particle size.

7. wet milling process Corn

8. Cleaning Clean the shelled corn to ensure that they are free from dust and foreign bodies. Steeping The corn is soaked in water, called steepwater , at 50˚C for between 20 and 30 hours, during which time it doubles in size. Sulphur dioxide is added to the water to prevent excessive bacterial growth. As the corn swells and softens, the mildly acidic steepwater starts to loosen the gluten bonds with the corn, and to release the starch. The corn goes on to be milled.

9. Milling and separation The corn is coarsely milled in the cracking mills to separate the germ from the rest of the components (including starch, fibre and gluten). The corn flows to the germ separators to separate out the corn germ. The corn germ , which contains about 85% of the corn’s oil, is removed from the slurry and washed. It is then dried and sold for further processing to recover the oil.

10. Fine grinding and screening After the fine grinding, which releases the starch and gluten from the fibre, the slurry flows over fixed concave screens which catch the fibre but allow the starch and gluten to pass through. The starch-gluten suspension is sent to the starch separators. The collected fibre is dried for use in animal feed.

11. Separating the starch and gluten The starch-gluten suspension passes through a centrifuge where the gluten, which is less dense than starch, is easily spun out. The gluten is dried and used in animal feed. The starch, which still has a small percentage of protein remaining, is washed to remove the last traces of protein and leave a 99.5% pure starch. The starch can either be dried and sold as corn starch, or it can be modified to turn into other products, such as corn sweeteners, corn syrups, dextrose and fructose .