Zein (Figure 1), the main prolamin in corn, was first discovered by Gorham in 1821 in the product zea. It was classified by Osborne 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. [1] 

Composition of Zein

Zein is classified into four different fractions, based on differences in solubility and molar mass; these fractions are called α-, β-, γ- and δ-zein. The α-zein fraction is obtained in greater quantities in the commercial extraction process (approximately 80% of the total prolamine present in corn) and presents molar mass in the range of 21~25 kD. The β-zein, which constitutes 1%~5% of zein, is made of a polypeptide of 14 kDa.The γ-zein, which constitutes 10 %~20 % of zein, is made of two polypeptides of 16 and 27 kDa. The δ-zein accounts for 1%~5% of zein and has a molecular weight of 10 kDa. In general, zein contains more hydrophobic amino acids, such as glutamine, leucine, proline, and alanine, so it disperses easily in nonpolar media (such as organic solvents) and has low solubility in polar solvents, especially in pure water. β- and γ-zein contain less leucine and much higher cysteine compared to α-zein, which makes β- and γ-zein more hydrophilic, and the reducing agent is required during extraction.[2]

General Properties of Zein

The solubility and chemical reactivity of zein are determined by the presence of the following functional groups: amines, amides, hydroxyls, carboxylates and phenols. The presence of these different types of groups enables zein to be physically and chemically modified to improve its functional properties. Hydrophilicity/hydrophobicity can be controlled by the addition/grafting of groups with suitable hydrophilicity. Cross-linking between zein molecules can be induced by chemicals, such as formaldehyde, glutaraldehyde, citric acid, etc. Due to the presence of hydrocarbon groups in zein’s side chains, zein is water-insoluble, but it is soluble in mixtures of water with aliphatic alcohols, for instance, ethanol and isopropanol, and also in other organic solvents containing hydroxyls, carbonyls, amines and other polar groups. Zein is a completely amorphous polymer, which shows plasticizing viscoelasticity and a glass transition temperature (Tg) at about 165℃, although its Tg decreases significantly in response to an increasing degree of plasticization.Table 1 summarizes some general properties of α-zein.[3-4]

Main applications

Potential applications of zein include uses as biodegradable plastics, ?bers, adhesives, coatings, ceramics, inks, cosmetics, textiles and chewing gum, etc. When synthetic materials became cheaper in the 1950’s, zein products were not cost-effective and lost use. Currently, much of the zein from CGM is used for food, the medical and pharmaceutical fields.

Food related applications

Being mostly nonpolar in nature, zein films have been explored for coatings in numerous food applications. Applying it to the surface of food can increase its luster and extend its shelf life. With the help of surface hydrophobicity, it can be used as an oil simulant, not only to replace some cream in making ice cream, but also to replace salad oil in making mayonnaise. Compared with ordinary oils, zein contains less calories, which can effectively prevent obesity, hypertension and diabetes caused by high calorie diet, and meets the requirements of healthy diet. Unlike other non-wheat proteins, zein could form a viscoelastic mass/network when mixed with water above its Tg[5]. Mixing zein with starch at 40℃ can form a viscoelastic dough similar to the wheat dough.[6] It is a promising gluten substitute in the gluten-free system due to its similar viscoelastic properties to gluten,wihch is mainly composed of α-zein.

In addition, zein can be food packages buried agent. Subramanian et al.[7] investigated the adsorption characteristics of zein on hydrophobic and hydrophilic surfaces, targeted at understand the orientation changes associated with this protein structure localized at the surface.This studies indicate the possibilities of using zein as a protective, impermeable coating for food packaging, wherein different conditions, such as hydrophilic and hydrophobic surfaces, may be required to store edible materials.

Furthermore, zein can be food emulsifying agent by its surface activity. Julius etal. demonstrate the fabrication of surfactant free, biodegradable and edible oil-in-water Pickering emulsions based on fully natural renewable resources using relatively monodisperse colloidal zein particles as the emulsi?er. As an additional advantage with respect to applicability and scalability, the study showed that zein colloidal particles, as opposed to many other natural particle-stabilizers, display intrinsic surface activity.[8] 

Biomedical Applications

It was pointed out that the presence of hydrophilic-hydrophobic groups in zein chains is a very important aspect for obtaining material with different hydrophobicities by mixing with other moieties (polymeric or not), but also for obtaining derivatives with different properties. Micro and nanoparticles of zein have been studied as carriers of nonpolar drugs, due to their excellent biocompatibility, biodegradability and high surface contact. Zein can interact efficiently with hydrophobic and hydrophilic drugs, acting as delivery vehicles. Because of its high percentages of hydrophobic amino acid residues, zein is insoluble under physiological conditions and capable of the sustained release of encapsulated compounds.[9] 

Liu developed a zein-based nanoparticle drug delivery system for co-delivering gefitinib and curcumin. The aim is to overcome the application challenges of gefitinib and curcumin, enhance the solubility of hydrophobic drugs, and improve the combined therapeutic efficacy of the two drugs. Zein-based nanoparticles were prepared using the antisolvent precipitation method. The stability of the nanoparticles was enhanced by encapsulating water-soluble polysaccharide pectin on their surface through pH adjustment, forming core-shell composite nanoparticles.[10] Microspheres of zein have been produced that contain Ciprofloxacin, an antibiotic. They showed that these antibiotic-laden zein spheres inhibited bacterial growth compared to control spheres.

In summary, a number of potential zein formulations have been recommended for poorly watersoluble drugs, including micro/nanoparticles, SDs, nanofibres and modified processes, with the effort to improve the product quality from zein as well as enhancing drug therapeutic effects. Generally, zein is used to build formulations that result in nanosized particles that modulate drug release. Future advances in zein formulations are expected to translate a perfect nanosystem into clinical practice in the near future.[11]

References

[1] Anderson TJ, Lamsal BP. REVIEW: Zein Extraction from Corn, Corn Products, and Coproducts and Modifications for Various Applications: A Review [J]. Cereal Chemistry, 2011, 88: 159-173.

[2]Zhang Y, Xu M, Zhang X, Hu Y, Luan G. Application of zein in gluten-free foods: A comprehensive review. Food Res Int. 2022,160:111722. 

[3] Corradini E, Curti PS, Meniqueti AB, Martins AF, Rubira AF, Muniz EC. Recent advances in food-packing, pharmaceutical and biomedical applications of zein and zein-based materials. Int J Mol Sci. 2014,15(12):22438-22470. 

[4]Magoshi, J.; Nakamura, S.; Murakami, K.I. Structure and physical-properties of seed proteins.1. Glass-transition and crystallization of zein protein from corn. J. Appl. Polymer Sci. 1992, 45, 2043–2048.

[5] Lawton, J. W. Viscoelasticity of zein-starch doughs. Cereal Chemistry,1992, 69(4), 351–355.

[6] Schober, T. J., Bean, S. R., Boyle, D. L., & Park, S. H.  Improved viscoelastic zein–starch doughs for leavened gluten-free breads: Their rheology and microstructure. Journal of Cereal Science, 2008, 48(3), 755–767.

[7]Subramanian, S.; Sampath, S. Adsorption of zein on surfaces with controlled wettability and thermal stability of adsorbed zein films. Biomacromolecules 2007, 8, 2120–2128.

[8]De Folter J W J , Van Ruijven M W M , Velikov K P .Oil-in-water Pickering emulsions stabilized by colloidal particles from the water-insoluble protein zein[J].Soft Matter, 2012, 8(25):6807-6815..

[9]Hurtado-Lopez, P., Murdan, S. Zein microspheres as drug/antigen carriers: A study of their degradation and erosion, in the presence and absence of enzymes. J. Microencapsul. 2006, 23, 303–314.

[10]Liu Pl.Zein-Based Nanoparticles for Simultaneous Delivery of Gefitinib and Curcumin for Enhanced Antineoplastic Activity[D].Hubei University ,2024.DOI:10.27130/d.cnki.ghubu.2024.001022.(MA thesis)

[11]Tran PHL, Duan W, Lee BJ, Tran TTD. The use of zein in the controlled release of poorly water-soluble drugs. Int J Pharm. 2019,566:557-564.