The United States produces about 300-325 million tons of corn per year, accounting for about 40-50% of the world's annual production. About 50% of US corn is used directly as animal feed, about 25% is used for ethanol production and the rest for export and producion of food, feed, fiber and industrial products.

There are four basic methods for processing corn: Alkaline Processing, Dry Milling, Wet Milling and Dry-Grind (for ethanol production). Corn wet milling is a process that utilizes mechanical unit operations -- size reduction (milling), aqueous extraction and separation (filtration, centrifugation) -- for separating and purifying the major corn components such as oil, starch, protein, fiber. Corn refiners take the process further and convert the starch into a myriad of food and industrial products. Wet milling generates large amounts of protein co-products : about 5-6% of the corn becomes corn gluten meal (CGM, 60% protein) and 24% of corn dry weight is corn gluten feed (CGF, 21% protein).

Much of the fuel ethanol produced today in the US is by the "Dry-Grind" process. This also generates co-products (DDG or DDGS) that contain all the non-starch components of corn, amounting to about 28% of the corn processed. Most of these protein co-products are currently marketed as animal feed at low prices. Another one-third of the corn becomes carbon dioxide which is another zero- or low-value coproduct.

The high demand for corn starch and starch-based products -- sweeteners, syrups, ethanol, bioproducts, organic acids, food ingredients -- will create a disposal problem for the growing quantities of protein co-products. In addition, the corn processing industry must also consider increasingly tight environmental regulations and limitations of international markets for CGF and CGM.

There are two possible alternate higher-value outlets for corn protein:
(1) As an industrial polymer, e.g.,biodegradable plastics and films and
(2) As a functional ingredient in (human) food products.

Protein forms about 9% of the dry weight of corn. It is composed mainly of zein (a highly hydrophobic protein, soluble in isopropanol or ethanol), glutelin (soluble in aqueous alkaline solutions), albumins and globulins. Zein comprises about 40% of the total kernel protein on a dry basis.

1. Zein as an industrial biopolymer

Zein was first discovered by Gorham in 1821. Osborne developed the first patented process for extraction of zein from corn gluten meal (CGM) using 95% ethanol. Swallen et al. were granted a series of patents on zein production using different alcohols of varying concentrations and additives. Zein became commercially available in 1938 and quickly found application in coatings, fibers, films, plastics, adhesives and inks. Zein (often combined with vegetable oils and glycerin as plasticizers) is used as a waxing or glaze, to enhance shelf life of pharmaceutical tablets, nuts and candies by acting as a water and oxygen barrier. This was part of the “chemurgy” movement where crop/farm by-products were utilized by industry. Zein fibers with the commercial name of Vicara was produced by the Virginia-Carolina Chemical Corporation in 1948. It was marketed for its claims of washability, the warmth of wool and resistance to moths and mildew.

Zein reached a peak production of 7 million kg per year in 1956 but with the development of cheaper synthetic materials, the market for zein dropped significantly by 1960. Today there is one known manufacturer of zein in USA and one in Japan. The cost of purified zein is $20-70 per kg depending on the grade and purity. However, this price makes zein an uneconomical material for large industrial uses such as biodegradable plastics, since petroleum-based polymer resins sell for much lower prices ($0.35-0.70 per kg).

Even though there are several hundred patents on applications of zein and renewed interest due to its biodegradability and potential nanotechnology applications, its current high price is still a limitation. The potential market for zein should now be good, considering:
(1) Increased demand for true 100% biodegradable packaging and films
(2) The low margins in some sectors of the corn processing industry that are eager to find higher-value outlets for their co-products,
(3) Valuable lessons learned from previous attempts at corn-based "biodegradable" plastics : these were largely starch-based and did not fare well in the market for various reasons.

Assuming that "biodegradability" and being "renewable" and "green" can enhance the value of zein-based films and packaging, this market could use a substantial portion of the zein in the 5 billion bushels (1 bushel = 25 kg) of corn presently processed into food, feed and industrial products in USA today. However, the cost of zein must be reduced and the quality improved over what is currently available.

The COPE Project

We have developed methods for producing low-cost zein for the food and industrial markets. Our COPE (Corn Oil and Protein Extraction) process uses ethanol for the initial extraction of zein from corn or corn co-products. This is then combined with membrane technology for the separation, isolation and purification of zein from the ethanolic zein solution. Membrane technology offers the possibility of selective low-energy low-cost separation of zein and for recycle of the ethanol solvent without the substantial evaporation costs that now limit other zein extraction processes. Additional economies are possible if COPE-zein production is carried out in dry-grind ethanol plants, since they already have the two key raw materials (corn and ehanol).

Further refinements of the COPE process have been developed to extract additional high-value coproducts such as a "healthy" corn oil loaded with nutrients such as carotenoids, vitamins and phytosterols, and nutraceuticals such as xanthophylls (lutein and zeaxanthin).

Our preliminary economic analysis shows a net revenue of $2-4/bushel corn, depending on the coproduct(s). This translates into an additional income of $75-150 million per year for a plant producing 100 million gallons of ethanol/year (380 million liters/year) without any additional raw materials being used (we "borrow" in-house ground corn and ethanol). Considering that US corn-based ethanol capacity is over 10 billion gallons per year, the potential impact of our research is significant.

This technology has resulted in several US and Canadian patents and has been licensed through the University of Illinois to Prairie Gold Inc., Bloomington, Illinois for commercialization.

2. Zein as a functional food ingredient

The main obstacle to utilization of zein in human food products is their poor "functionality". Functional properties refer to those attributes that provide the desired physical or sensory properties to the food. For example, proteins contribute not only nutritional quality to a food, but also to textural properties such as gelation (important in meats, cheeses, gels, etc.), adhesion (meats, bakery, pasta), emulsification (Deli meats, soups, cakes), foaming/whipping (cakes, frozen desserts) and moisture sorption (intermediate moisture, shelf-stable foods).

Poor functionality of corn proteins is generally related to their insolubility in aqueous systems; proteins generally have to be in solution or fine suspension in order to exert their desirable functional properties in foods.

In the mid-1980s, our group at the University of Illinois set about to improve the water solubility of corn proteins and some of its functional properties. The approach was enzymatic modification combined with membrane technology to produce specific protein fractions or peptides from corn gluten meal. With zein (one of the two major proteins in corn), this involved a unique two phase sequential process: an organic phase enzyme reaction followed by an aqueous phase enzyme reaction.

What are the results?

A successful combination of enzymatic modification and fractionation by membrane technology has created two new corn protein products: a water-soluble zein and an enzyme treated corn gluten meal (in which the glutelin fraction has been modified). Functional properties were dramatically improved by this treatment. For example, for zein:
  • Protein solubility in water increased from 0% to 99% over the pH range expected in foods.
  • Protein solubility could be partially controlled by membrane pore size: solubility decreased with larger membrane pore size.
  • Clarity, solubility and foaming properties increased with Degree of Hydrolysis. Ultrafiltration increased foam volume and decreased foam stability.
  • Moisture sorption properties of Illinois-process zein fractions were vastly superior to unmodified proteins, and better than soy protein hydrolysates produced in a similar manner.
Similar improvements were also observed for the enzyme treated CGM. This development should enhance the utilization of corn proteins for food products.

Major funding for the above corn projects have come from the Illinois Corn Marketing Board . the Illinois Department of Commerce and Economic Opportunity, US Department of Agriculture NRICGP program, several membrane companies and MAFMA. Partnerships and collaborations were established with engineering companies, membrane manufacturers, food companies and ingredient manufacturers to research, develop and market the various processes.
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Updated July 2009 by mcheryan@illinois.edu