CORN WET MILLING

 

December 13, 2015

Issue #279

Corn wet milling is an extremely large but relatively little known industry. It is where corn is processed into vital products, of which the four principal ones are corn oil, starch, fructose, and animal feed. Facilities are located in dozens of corn-growing nations around the world. The best known producers are multinationals like ADM, Bunge, Cargill, Ingredion, and Tate & Lyle. In addition there are a great number of lesser known local companies in this industry.

The process used is to first steep the harvested corn kernels in a caustic solution. This allows the corn to be cracked so that the germ can be separated. The germ is the part of the kernel which contains corn oil. Most facilities ship their germ to yet other facilities which specialize in recovering the oil.

After separating the germ, the next step is to separate the starch. Corn starch is converted, in yet another process, into fructose. Fructose is a substitute for sucrose (which is produced from sugar cane and sugar beets). There is intense competition between the fructose and sucrose markets.

Screw presses have two important applications in the corn wet milling industry. One of these is in dewatering the residual fiber after the starch has been removed. This fiber is dried to low moisture content and then pelleted for sale as animal feed. To minimize the fuel required by the dryer, screw presses are used, ahead of the dryer, to remove as much water as possible. Our customers' specification usually is that the moisture content of the press cake should be in the range of 55% to 58%. Our high torque Series VP screw presses are well suited for this application.

The second screw press application in wet corn milling is removing water from the corn germ. In order to liquefy the fat in the germ, it is run through an oil Expeller® type screw press. Typically these are supplied by Anderson International. However, before the germ can be pressed in an Expeller®, it must be dewatered. That is where Vincent screw presses prove their value. Although the press cake moisture content comes out lower, 50% to 55%, the pressing action must be gentle in order not to squeeze out the oil. We use Series KP screw presses in corn germ applications.

Historically, the industry was dominated worldwide by Vetter screw presses. However, today most of these presses are very old machines, subject to frequent maintenance requirements and costly repair parts. This has opened the market to other press manufacturers.

An interesting fiber application for Vincent screw presses is as a pre-press, for a first pressing ahead of existing Vetter's. Low-torque, high capacity Series KP presses have proven effective in separating the "easy" water ahead of Vetter presses. This has resulted in a small, but valuable, increase in starch yield. More importantly, it has taken load off the Vetters so that a mill can continue operating at a high throughput rate even when presses are down for repairs.

There is yet another application for Vincent presses in corn wet milling. It is found in facilities which produce corn oil. The oil flowing from an Expeller® contains some fiber and moisture. These foots sink to the bottom of a settling tank. Their name, foots, comes from the fact that they are found at the foot of the tank. They are scooped, or dragged, from the bottom of the tank and then run through a Vincent screw press.

The Vincent press has proven effective in separating the oily water (as press liquor) from the fiber (press cake). This cake contains residual oil, so it is re-admitted to the Expeller® presses. Pressing News #70, from 1997, describes this application.

An alternative system being testing involves pumping oil from Expanders® and Expellers® to a simple prethickening screen to separate the foots. These fall into the screw press. It is hoped that reduced floor space and construction costs will result.