by Jeanne Yacoubou, MS
VRG Research Director
Introduction
Growing 21st century interest and investment in renewable energy sources and biodegradable products mean that the food industry, as well as the pharmaceutical, textile, paper, and biomedical industries, is becoming greener, too. A major reason for this shift is the corn protein, zein, (pronounced “ZEE-in”). A major zein scientist told The VRG that zein is a “plastic protein with huge potential.”
Zein is considered Generally Recognized as Safe (GRAS) by the Food and Drug Administration (FDA), and so can be used in food. Some of its characteristics that make it appealing in many different food applications described in this article include its ability to: form films and coatings; resist grease; act as a water vapor barrier; act as an oxygen barrier; resist microbial contamination; and be non-allergenic. Zein is considered renewable and biodegradable. Zein is of poor nutritional value as a protein source.
History and Uses of Zein
Zein was first identified and isolated from corn in 1821. Commercial production didn’t begin until the late 1930s and lasted only twenty years, until it was replaced mostly by cheaper petrochemical-based substances (for example, plastics, polyurethane varnishes, and nylon that typically cost less than $1/lb). At its height, zein was used in the manufacture of fiber, adhesives, and binders (for example, in ink).
The first major commercial use for zein, however, was as a coating. During World War II, when shellac (produced by an insect) was in short supply, zein was used as its replacement: in lacquers, varnishes, and coatings. Zein was commonly used as a floor coating for steamship engine rooms because of its durability and resistance to grease. Sometimes, zein was mixed with rosin (derived from trees or plants). A zein-rosin floor coating had improved resistance and remained glossy longer than shellac-covered floors.
However, zein would sometimes form a gel and not be usable as a coating. Despite chemical modifications that made zein remain in liquid form and not form a gel, 1950s consumers showed a stronger preference for shellac in spite of the fact that the shellac cost substantially more than zein at the time.
A quality assurance manager at a shellac company told The VRG that the 2010 lac resin (i.e., the raw material of shellac) harvest was particularly bad due to climate conditions and has doubled the price of shellac, (to ~$7/lb.) . Currently, zein is even more expensive than shellac per pound (~$10-$20/lb.), according to a zein company, but not so considering that only half as much zein is used as shellac for the same application. Scientists at several companies told The VRG that increasing supply of zein due to corn extraction improvements is predicted to make zein more price-competitive.
Zein was also frequently used in the mid-20th century to coat pharmaceutical tablets because of its ability to form films and resist microbes. It is still widely used today for this purpose, usually labeled as “vegetable protein” on a pill bottle. Several zein scientists in private industry and in government told The VRG that zein should continue to play a major role in the pharmaceutical industry.
During its peak, zein was also used to coat foods, such as fortified rice. A senior scientist at a major bioethanol company told The VRG that today there is interest in zein as a rice coating for use in rice-containing premixes in order to make cooking times for all ingredients in the premix more uniform.
Zein was also used to coat nuts (to prevent rancidity), fruit (to prevent mold growth and decrease dryness), and candy (as a protective film) in the 1940s-1950s. At the time, zein was believed to be equal or superior to shellac in terms of gloss and moisture resistance in candy. Today shellac or vegetable wax is more common in fruit, nuts and candy. More recently, there has been some work done with zein as a coating for tomatoes, because it delays color change, maintains firmness, and reduces weight loss during storage.
Grease resistance is one of the most important characteristics of zein, making it very useful in the food packaging industry. Zein coatings were, and still are, put on cartons of doughnuts, crackers, pies, and cookies. Often the coating consisted of zein, rosin, and other substances.
The VRG spoke with Dr. Nicholas Parris who has done much work with zein as a food packaging coating. Waxed paper boxes (such as those used for frozen foods) are not presently recyclable, because it is difficult to separate the paraffin wax (a petrochemical) from the paper fiber without damaging machinery. Parris was able to accomplish this on a test scale easily and inexpensively ($1/lb.) when zein was used along with corn lipid (i.e., fat) molecules known as free fatty acids (FFA). According to Parris, the lipid in the zein-lipid mixture replaced the paraffin and other petrochemical-based substances used to make wax paper and wax-coated packaging. In the mixture, the zein resisted grease and the fatty acids repelled water. Boxes coated in this fashion would not only be biodegradable; they would be also recyclable.
According to two zein companies, zein is expected to become common as a chewing gum base in the near future. One company is planning to build an Iowa plant (near the cornfields) for this purpose. Another new use for zein may be as a food coating to reduce fat absorption in high-fat foods.
Improvements in Zein Extraction from Corn
Zein is considered a “value-added” co-product of bioethanol production. This means that zein, if it is a marketable co-product of a process intended to make biofuel, could substantially lower the production cost of that biofuel. This is desired by (1) bioethanol companies because it increases their profits; and (2) all driving consumers paying for gasoline.
One of the reasons that zein is expensive is that it is traditionally extracted from corn gluten meal, an expensive byproduct of the corn industry. The corn gluten meal is produced by a wet-milling process that separates the oil, starch, protein, and fiber components of corn. In this process only 5% of the corn becomes corn gluten meal. In this process, sulfur dioxide, a very potent chemical, is used. According to the United States Department of Agriculture (USDA), approximately 36% of total bioethanol production comes from the wet-milling operation.
Approximately sixty-four percent of fuel ethanol, however, is produced by the “dry grind” method. This method yields dried distiller’s grains, (DDGs), all the non-starch components of corn, equivalent to 28% of the corn processed. In bioethanol production today, there is approximately twice as much DDGs produced per year in the US as compared to corn gluten meal. Finding and perfecting a method to extract zein from DDGs is therefore more cost efficient for large scale production
POET, the largest bioethanol company in this country, which uses a dry grind process, does not use sulfur dioxide to separate out the zein in its “no-cook” (i.e., without heat) ethanol production. DDGs are usually sold as cheap animal feed. Now, DDGs are being re-routed and used to extract value-added co-products such as zein. Two zein scientists told the VRG about research being done to incorporate DDGs, which become nutritional sources of protein after zein is removed, into breads.
Much research is being done to perfect the extraction of zein from DDGs. A method to extract the zein using ethanol at a bioethanol plant is more cost effective than producing zein from corn gluten meal at an urban plant simply because two needed materials (corn and ethanol) are present. After some ethanol produced in the plant as fuel is diverted to extract the zein, this ethanol is separated and recycled back into the stream of production, eventually becoming part of the total ethanol yield at the plant.
Many refinements have occurred in the dry grind process to extract zein, including techniques used to decolorize and deodorize it by physical separation methods, making the final product purer and more abundant than before. Another technique, the COPE (corn oil and protein extraction) process developed by the University of Illinois, has been licensed to Prairie Gold, a Midwest company. The University of Nebraska is also developing its own method of zein extraction. A few bioethanol companies are actively conducting further research as well.
Conclusion
The current trend toward biofuels and its concomitant production of value-added, corn-based ingredients, all of which are biodegradable and annually renewable, means that “old’ uses for zein may become popular again. Zein as a replacement for shellac is just one example, used as a coating on produce, nuts, and candy.
Zein could also be used as a coating on disposable diapers, bed sheets or tablecloths. Zein may be used to replace a host of petrochemical-based products in the form of soda bottles, plastic bags, foam cups, etc., and is considered better than corn starch-based products. New food uses for zein include zein as a chewing gum base. New agricultural uses include zein as a mulch or fertilizer coating or as an edible hay bale wrapper. New biomedical uses include zein as a component in tissue scaffolding needed for skin and bone regeneration.
Consumer demand and economic necessity will drive the new markets for corn-based products such as zein as well as biofuels.
For more information on shellac, see http://www.vrg.org/ingredients/index.php#lac-resin
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