Zein Extraction from Corn, Corn Products, and Coproducts and Modifications for Various Applications: A Review

Cereal Chem. 88(2):159–173 Corn can be fractioned to produce starch, fiber, oil, and protein in relatively pure forms. The corn kernel contains 9–12% protein, but half of this is an industrially useful protein called zein. Dry milled corn (DMC), corn gluten meal (CGM), and distiller’s dried grains with solubles (DDGS) are all coproducts from corn that contain zein and are used for zein extraction. Because it is insoluble in water, zein has found uses in many products such as coatings, plastics, textiles, and adhesives. Newer applications are taking advantage of zein’s biological properties for supporting growing cells, delivering drugs, producing degradable sutures, and producing biodegradable plastics. This review covers zein characteristics and nomenclature, past and current practices in processing and extraction of zein from corn products and coproducts, and the modifications of zein for various applications. ZEIN EXTRACTION AND APPLICATIONS: AN OVERVIEW Maize or corn is a major cereal grain throughout the world; it also is one of the most dominant crops in the Unites States (Anonymous 2010). Yellow dent variety of corn has become the most utilized. It varies greatly from the sweet corn for human consumption. The endosperm and germ contain the bulk of the dry mass of the kernel at 81.9 and 11.9%, respectively (Earle et al 1946). The bran and tip cap constitute the remaining portion of the kernel. Starch is the main component of the endosperm at 86.4% of its mass (db). The starch can be extracted in pure form for various food and industrial uses. Starch has been used mostly in the food sweetener market. Oils extracted primarily from the germ can be utilized in cooking oils or in other food products. Proteins are located mainly in endosperm and germ. Different types of proteins are found in the two main constituents: albumins and globulins centralized primarily in the germ, and prolamintype proteins found mostly in the endosperm. These prolamin proteins provide nitrogen for growing corn kernels during germination. Zein, the main prolamin in corn, was first discovered by Gorham in 1821 in the product zea, otherwise known as “Indian corn” (Gorham 1821). It was classified by Osborne (1924) as a prolamin and shown to be extractable in aqueous alcohol such as ethanol. As production of zein was commercialized in 1939, many potential uses for zein were identified. Because of zein’s insolubility in water, resistance to grease, and glossy appearance, it was ideal for adhesives, plastics, and fiber applications. As the protein structure and properties of zein have become known, there has been a surge in zein-related research. However, commercial production of zein has been low with mainly two companies producing it: Freeman Industries (Tuckahoe, NY) now owned by Flo Chemical Corp. (Ashburnham, MA) and Showa Sangyo (Tokyo, Japan). Recently POET Inc. (Sioux Falls, SD) and Prairie Gold Inc. (Bloomington, IL) have introduced zein prepared using different processes. The POET product called Inviz is extracted from POET's Dakota Gold HP distillers’ grains. COPE-zein from Prairie Gold Inc. is extracted from ground corn before the dry-grind process. Zein has normally sold for $10–40/kg with higher purities commanding higher prices. Until new extraction methods or new products, such as the two listed here, can prove themselves as economically viable, zein will not likely be able to compete with synthetic plastics that have a very low market price of ≈$2/kg. Zein is a protein that is found only in corn; however, there are proteins that share prolamin characteristics similar to that of the zein found in corn. Other cereals such as wheat, barley, rye, and sorghum each contain prolamins with similar characteristics to zein. The extracted prolamin proteins from these cereals each have industrial importance, but zein is favored because of higher yields and the large volume of corn coproducts available for extraction. Corn is processed using four different methods and zein extracted from these products/coproducts could differ in properties and end uses. The four corn processing methods are wetmilling, dry-milling, dry-grind processing, and alkaline treatment. Corn wet-milling produces a protein-rich coproduct called corn gluten meal (CGM) from which zein has been extracted commercially. Dry-milled corn (DMC) separates fibrous material from grits. Dry-grind ethanol process is grinding of corn and the subsequent saccharification and fermentation of glucose to ethanol, leaving behind the coproduct distillers’ dried grains with solubles (DDGS). Because of the conversion of starch to sugars and subsequently ethanol, fractions such as cellulosic materials and protein are concentrated in DDGS. Alkaline-treated corn has been mainly utilized for human consumption and has little basis for zein extraction. Zein has been extracted from all these coproducts, but commercial zein is normally produced from CGM. Most zein extractions have been based on aqueous alcohol extractions, but many other solvents can solubilize zein. Zein extraction schemes have been optimized for different corn products and coproducts because of differences in protein concentrations and processing conditions. Zein extractions are a complex balance of yield, quality, and purity. Yields refer to the amount of zein extracted. Until recently, commercial zein has been composed primarily of α-zein, and purity is the amount of protein contained in extracted zein (Pomes 1971). Commercial zein tends to be of high quality and purity, but yields are low. The production and properties of zein have been reviewed in the past. Two recent excellent reviews are by Shukla and Cheryan (2001) and Lawton (2002). The review by Shukla and Cheryan (2001) has an overview of zein properties, extractions, and applications. The review by Lawton (2002) includes data on extractions and zein-solvent interactions, but it mainly 1 Graduate student, Department of Food Science and Human Nutrition, Iowa State University, Ames, IA 50011. 2 Assistant professor, Department of Food Science and Human Nutrition, Iowa State University, Ames, IA 50011. 3 Corresponding author. Phone: (515) 294-8681. Fax: (515) 294-8181. E-mail: