Factors Affecting Yield and Composition of Zein Extracted from Commercial Corn Gluten Meal

Twelve corn gluten meal samples obtained from six wet-milling plants were processed into zein. Zein was extracted using 88% aqueous isopropyl alcohol at pH 12.5, followed by chilling. Protein recovery ranged from 21.3 to 32.0%, and protein purity ranged from 82.1 to 87.6%. Protein recovery increased as the protein purity increased (r = 0.76) (P < 0.01). One of the major factors influencing extraction yield was protein composition; especially α-zein content, which ranged from 53.4 to 64% of the total protein in the corn gluten meal samples. The intensity of red color of the corn gluten meal was negatively correlated with protein recovery and zein purity (r = -0.66 and -0.72, respectively) (P < 0.02).     

Effects of Extrusion and Starch Removal Pretreatment on Zein Proteins Extracted from Corn Gluten Meal

In this study, the structure and selected properties of zeins extracted from corn gluten meal (CGM) pretreated by extrusion and removal of starch were investigated. The structure and properties of the zeins from pretreated CGM changed significantly. Pretreatments can decrease the extraction yields of zeins and change the granule shape and size of zein aggregates. The studies indicated that extrusion and removal of starch can significantly decrease the thermal enthalpy (ΔH₁ and ΔH₂) of zein from 1.94 ± 0.20 to 0.19 ± 0.10 and from 107.20 ± 0.80 to 78.62 ± 2.30 and J/g, respectively. The SDS‐PAGE results confirmed that the molecular weight of zeins from CGM was 24,000 and 27,000, and the molecular weight of zeins did not change with the pretreatment. On the other hand, the circular dichroism spectroscopy results showed that the processing of extrusion and removal of starch can change the secondary structure content of β‐sheets and β‐turns; these results indicated that extrusion and removal of starch can significantly break the secondary structure of zeins. Furthermore, extrusion and removal of starch can change the sulfhydryl content of zeins. The obtained results provided some fundamental information that is useful for further modification of CGM to improve its functional properties and industrial applications.     

Isolation and Characterization of Zein from Corn Distillers' Grains and Related Fractions

Corn distillers' grains with solubles (CDGS), the major coproduct of fermentation of corn to produce ethanol, were extracted with 0.1M NaOH, 0.1% dithiothreitol (DTT), and 0.5% SDS yielding 35% of the total nitrogen and ≈25% of the protein nitrogen. Gel electrophoresis revealed that the extractable proteins contained zein plus other proteins similar to the extractable proteins from corn flour. Although difficult to extract, the proteins isolated from the fermentation coproducts appeared undegraded and apparently survived gelatinization, fermentation, distillation, and drying during the production of ethanol. Extraction of CDGS with 60% ethanol at 60°C yielded 1.5–3.9% of crude zein. When the ethanol contained DTT, yields of crude zein were increased to 3.2–6.6%. Protein contents of the crude zeins were only 37–57%, indicating that lipids and pigments were coextracted with the ethanol. Gel electrophoresis showed that the protein fractions extracted by ethanol contained primarily α-zein whereas the proteins extracted by ethanol + DTT contained α- + β-zein. Further confirmation of the presence of zein in the crude prolamin preparations was obtained by amino acid analyses. The amino acid compositions of the crude zeins paralleled those of commercial zein and α-zein.     

Development of New Method for Extraction of α‐Zein from Corn Gluten Meal Using Different Solvents

A modified procedure for the extraction of α‐zein from corn gluten meal was developed and compared against a commercial extraction method. The modification involved raising the concentration of alcohol in solvent and removing the precipitate by centrifugation. Five organic solvent mixtures were compared using the modified extraction procedure developed along with the reductant sodium bisulfite and NaOH. The modified procedure precipitated most of the non‐α‐zein protein solids by increasing the concentration of alcohol. The supernatant had α‐zein‐rich fraction, resulting in higher yield of α‐zein than the commercial method when cold precipitated. The commercial extraction procedure had a zein yield of 23% and protein purity of 28% using 88% 2‐propanol solvent. The three best solvents, 70% 2‐propanol, 55% 2‐propanol, and 70% ethanol, yielded ≈35% of zein at protein purity of 44% using the modified extraction procedure. Zeins extracted using the novel method were lighter in color than the commercial method. Densitometry scans of SDS‐PAGE of α‐zein‐rich solids showed relatively large quantities of α‐zein with apparent molecular weights of 19,000 and 22,000 Da. The α‐zein‐rich solids also had small amounts of δ‐zein (10,000 Da) because it shares similar solubility properties to α‐zein. A solvent mixture with 70% 2‐propanol, 22.5% glycerol, and 7.5% water extracted significantly less zein (≈33%) compared to all other solvents and had α‐zein bands that differed in appearance and contained little to no δ‐zein.     

Solvent Extraction of Zein from Dry‐Milled Corn

Batch extraction of zein from dry‐milled whole corn with ethanol was optimum with 70% ethanol in water, an extraction time of 30–40 min, and temperature of 50°C. High yields (60% of the zein in corn) and high zein contents in the extracted solids (50%) were obtained at a solvent‐to‐solids ratio of 8 mL of 70% ethanol/g of corn. However, zein concentration in the extract was higher at lower ratios. Multiple extraction of the same corn with fresh ethanol resulted in a yield of 85% after four extractions, whereas multiple extractions of fresh corn with the same ethanol resulted in high (15 g/L) zein concentration in the extract. Optimum conditions for batch extraction of zein were 45°C, with 68% ethanol at a solvent‐to‐solids ratio of 7.8 mL/g for an extraction time of 55 min. Column extractions were also best at 50°C and 70% ethanol; a solvent ratio of 1 mL/g resulted in high zein concentrations in the extract (17 g/L) but yields were low (20%).     

Analysis of Headspace Volatiles of Corn Gluten Meal

Corn gluten meal is the high‐protein fraction from wet milling of corn. Although protein, minerals, and fat compositions have been reported, minor components that cause unpleasant flavor and taste are not known. The objective of this study was to determine the compounds present in headspace of corn gluten meal and wet cake (dryer feed). A solid‐phase microextraction device with a polyacrylate coating was used to collect volatiles for 1 hr at 25 and 100°C above a stirred water slurry of corn gluten meal or wet cake. The absorbed compounds were desorbed onto the inlet injector of a gas chromatograph equipped with a mass selective detector. Compound identification was done using a Wiley library and confirmed by retention time of pure compounds in the gas chromatograph. Twenty‐nine compounds were identified. Knowledge concerning volatiles of corn gluten meal may lead to improvement of flavor and taste, and increased utilization of this material, produced over 1 million metric tons per year in the United States.     

Corn Gluten Meal as a Thermoplastic Resin: Effect of Plasticizers and Water Content

Corn gluten meal (CGM) was studied to investigate the effect plasticizers and water have on its melt processing, and how this melting affects its mechanical properties. GCM containing varying amounts of water were mixed with 23% (w/w) plasticizers; (glycerol, triethylene glycol (TEG), dibutyl tartrate, and octanoic acid in a Haake bowl mixer at 80°C. The amount of water in the CGM affected the amount of torque produced in the Haake mixer. This increase in torque was correlated with how well the CGM melted in the mixer. SEM images of CGM melted in the mixer showed a more uniform homogenous structure when processed at its optimum moisture content. Glycerol, TEG, and dibutyl tartrate produced the greatest torque when the CGM contained <1% water. Octanoic acid produced the greatest torque when the CGM was processed at 8% moisture. CGM plasticized with TEG and octanoic acid were mixed at either their optimum moisture or at 9.6% moisture and then compression molded into tensile bars. The tensile strengths of the bars that were mixed at their optimum moisture content were significantly greater than the bars mixed at 9.6% moisture. The tensile properties of the CGM samples were affected by relative humidity (rh). The tensile strength decreased and elongation increased as relative humidity increased. CGM plasticized with TEG saw a greater changes in its tensile properties due to relative humidity than did octanoic acid plasticized CGM.     

Sensory Analysis of Brownies Fortified with Corn Gluten Meal

Corn gluten meal is a high‐protein product from wet milling of corn. Substitution of 15% of the flour weight by corn gluten meal increased protein content of brownies from 6.3 to 8.0%. Sensory evaluation of brownies with 0, 10, and 15% corn gluten meal, with and without an added masking agent, showed addition of corn gluten meal to brownies did not have any detrimental effect as judged by trained sensory panelists.     

Corn Gluten Meal Odorants and Volatiles After Treatment to Improve Flavor

Production of corn gluten meal (CGM), a high‐protein coproduct from wet milling of corn, is increasing as production of fuel ethanol from corn increases. Unpleasant taste and odor have limited the use of CGM in human food. Adjustment of pH and extraction with water have been reported to reduce the off‐flavor of CGM but the improvement is not enough for substantial addition of CGM to the human diet. More study of CGM is needed. In this study, volatile compounds released under different conditions of pH, water extraction, and temperature were identified and compared using solid‐phase microextraction‐gas chromatography‐mass spectrometry (SPME‐GC‐MS). The water‐extractable portion, which improves the taste of CGM by its absence, was dried and analyzed by SPME‐GC‐MS. In addition, materials extractable from CGM with methylene chloride were identified by gas chromatography‐mass spectrometry (GC‐MS). Further, the spontaneous generation of a CGM‐like odor accompanied by a change in physical appearance of the CGM sample was described. Flavors and odors known to be associated with the identified CGM compounds were listed. Some possible origins of the volatiles, from degradation of corn constituents or as fermentation products of the corn steeping process, were noted.     

Effect of Hydrophilic Plasticizers on Thermomechanical Properties of Corn Gluten Meal

The glass transition temperature and rheological moduli of plasticized corn gluten meal (CGM) were determined with dynamic mechanical thermal analysis (DMTA). The tested plasticizers were water, glycerol, polyethylene glycols (PEG) 300 and 600, glucose, urea, diethanolamine, and triethanolamine, at concentrations of 10–30% (dwb). The glass transition temperature (T_g) of CGM, measured at 188°C when unplasticized, was lowered by >100°C at 30% plasticizer content, except by PEG 600 and glucose, which showed limited compatibility with CGM proteins. The highest plasticizing efficiency, on a molar basis, was measured with PEG 300 and was attributed to the large number of hydrophilic groups and the high miscibility of this compound with CGM proteins. The change in T_g due to the plasticizing effect was modeled with the Gordon and Taylor equation, but a better fit of the experimental data was obtained with the Kwei equation.     

Isolation of Zein Using 100% Ethanol

Traditionally, zein is isolated and recovered from corn gluten meal (GCM) using aqueous alcohol as the solvent. Recovery of zein from this solvent is inconvenient and costly. Zein is insoluble in 100% ethanol at room temperature, but it is soluble at 120°C in ethanol. Absolute ethanol effectively extracted zein from CGM, distillers dried grains (DDG), and ground corn. Zein was extracted from CGM with absolute ethanol in a high‐pressure reactor at 130°C. After extracting at 130°C for 45 min, the solution was pumped out of the extractor and allowed to cool. Upon cooling, the zein precipitated from solution. The precipitate was removed from the solution and air‐dried, resulting in 14% recovery of the starting material. The recovered precipitate had an average protein content of >90% on a dry basis, accounting for ≈20% of the CGM protein and recovered ≈35% of its zein. No differences were seen in the amount of zein extracted from CGM samples that were hand‐collected off the dewatering screen and gently dried, versus commercial CGM samples. The commercial CGM did produce a greater amount of solubles. The extraction procedure also worked at temperatures as low as 90°C. The lower temperature did produce lower yields of extracted zein. The zein extracted at the lower temperatures was less brown, but zein extracted at either temperature was almost fully soluble in traditional zein solvents.     

Effects of Lysine on Growth of Tilapia Fed Diets Rich in Corn Gluten Meal

Tilapia is a warmwater fish with mild flavor. Nearly 8.6 million kg are produced domestically, and ≈22.7 million kg are imported. Corn gluten meal (60% protein fraction) is a product obtained from wet-milling of corn. Diets (36% protein) containing 36–44% corn gluten meal with different levels of lysine and fish meal were formulated and fed to tilapia in aquaria for 12 weeks. Weight gain (WG) of tilapia fed diets containing the highest level of lysine (7.4% protein) with 4% fish meal was equal to that of fish fed a commercial control diet. Diets with lower lysine resulted in lower WG. The feed conversion ratio (FCR) and protein efficiency ratio (PER) of tilapia fed experimental diets containing adequate levels of essential amino acids and fish meal were the same as for fish fed the commercial control diet (also containing fish meal). Fish fed diets containing lower lysine levels had less favorable FCR and PER. This study shows that corn gluten meal is utilized at high levels in tilapia diets, particularly if essential amino acids are provided in adequate amounts.     

Relationship Between Popcorn Composition and Expansion Volume and Discrimination of Corn Types by Using Zein Properties

The objective of this study was to determine the relationship between the amount and type of lipids, starch composition and structure, and storage proteins on popcorn expansion and to evaluate whether popcorns could be discriminated from other types of corn based on the protein elution parameters. Seven commercial Argentinean popcorn samples were used in the study and significant differences were observed in the popping volume of these popcorns. A significant negative correlation was observed between oleic acid and popping volume and a positive correlation was observed between linoleic acid and popping volume. Popcorn starch properties were significantly different from normal corn but no particular measured attribute of starch correlated with popping volume. α‐Zein proteins and glutelins significantly correlated with popcorn expansion volume with R² = 0.963 and 0.744, respectively. The elution patterns of corn proteins could also be used to discriminate between different types of corn including popcorn, dent, and flint corns.     

An Improved Process for Isolation of Corn Fiber Gum

Sequential alkaline extraction and alkaline hydrogen peroxide (AHP) bleaching have been used to prepare corn fiber gum in yields ranging from 21 to 40%, depending on the pH of the extraction medium. The pH was adjusted by using different ratios of NaOH and Ca(OH)₂ The whitest product was obtained after AHP bleaching of the extract obtained using the lowest pH value. In order for the product gum to give its characteristic clear and low viscosity solutions, it was necessary to remove starch from the corn fiber substrate using α-amylase. The water-insoluble hemicellulose A fraction, a minor component, was removed by neutralizing AHP-treated extracts before ethanol precipitation of the useful hemicellulose B (corn fiber gum) fraction. At ambient temperature, AHP bleaching was near optimal after ≈2 hr under the processing conditions used. High ratios of arabinose (39%) to xylose (50%) were present in the corn fiber gum extracted under various alkaline conditions, and the H₂O₂ processing step did not significantly alter these ratios. The same low levels of galactose (7%) and glucuronic acid (4%) were present regardless of the extraction conditions. Molecular mass of the corn fiber gum preparations ranged from 2.78 × 10⁵ for the material extracted with Ca(OH)₂ to 3.94 × 10⁵ for the material extracted with NaOH. Molecular mass was unaffected by the H₂O₂ present in the second processing step. As expected for a carbohydrate polymer with a rather low uronic acid content, solution viscosities were unaffected by the presence of salt.     

Corn Kernel Structural Integrity: Analysis Using Solvent and Heat Treatments

Most corn (Zea mays, L.) processing is accomplished by causing a structural change to the kernel. Associations between corn endosperm structural components were characterized using textural analysis after solvent and heat treating kernels. Intact Asgrow 405W and B73xMo17 kernels were incubated and treated at 20, 40, 55, and 90°C for 1, 24, and 48 hr in static air, in acetone, and in aqueous solutions of water, calcium chloride, sodium chloride, sodium bisulfite, lactic acid, lime, lye, ethanol urea, and sodium dodecyl sulfate (SDS). After treatment, kernels were compressed between flat platens. Acetone did not significantly soften endosperm structure. Ethanol reduced kernel fracturability by weakening cell‐to‐cell (wall) bonds, but ethanol did not effectively reduce kernel hardness. Water and aqueous solvents swelled and softened kernels by plasticizing structural components. Bisulfite and SDS softened kernels more than water only soaks because they denatured matrix proteins. Alkaline soaks reduced fracturability and softened the kernel by dissociating both cell‐to‐cell and intracellular (starch‐protein) bonds. Soaking for longer periods and at higher temperatures increased aqueous‐based solvent softening effect. Urea imbibition into the kernel and its softening effects were highly dependent on time and temperature of soak. Endosperm structural integrity is the governed by a combination of cell‐to‐cell bonds and intra‐cellular (starch‐protein) bonds. Reagents that denatured the endosperm matrix proteins and disrupted hydrogen bonds resulted in the greatest alterations to kernel structural integrity. Ultimately a better understanding of kernel structural integrity will lead to the development of improved hybrids and process technologies designed to facilitate desirable structural changes.     

Isolation and Characterization of Cellulose/Arabinoxylan Residual Mixtures from Corn Fiber Gum Processes

White, fluffy cellulose/arabinoxylan mixtures (CAX) were generated from the solid residues remaining after corn fiber gum (CFG) production. Most CAX were produced using variations of a process in which a single alkaline hydrogen peroxide (AHP) step was used for delignification and for CFG (arabinoxylan) extraction. The optimal ratio of H₂O₂ to corn fiber to water was 0.1:1:20. Holding this ratio constant, time and temperature conditions were systematically varied, and yields of CAX and CFG determined. Parallel processes were conducted without H₂O₂ to determine its effect on CAX and CFG yield. CAX prepared under identical conditions but without H₂O₂ retained nearly twice the levels of CFG sugars, as revealed from L‐arabinose, D‐xylose, and D,L‐galactose levels. Even the CAX prepared under extreme AHP conditions (1 hr, 100°C), however, contained 32.9% of these CFG sugars. This CAX was obtained in a 25.1% yield, whereas those produced under less vigorous conditions were obtained in higher yields, because they retained more CFG. CAX prepared in the presence of H₂O₂ hydrated very effectively, as indicated by their high swollen volumes and water absorbance values. This suggests potential food applications for CAX as a bulking agent. In addition, the open structure of the CAX matrix would render these residues suitable for chemical derivatization and enzymatic saccharification.     

Isolation and Characterization of a Soluble Branched Starch Fraction from Corn Masa Associated with Adhesiveness

A water‐soluble starch fraction isolated from corn masa and identified by HPSEC as predominantly fragmented amylopectin was highly correlated in amount to both masa adhesiveness (r = 0.890, P < 0.01) and cook time (r = 0.957, P < 0.01). The molecular weight of the component ranged from approximately 6.4 × 10⁵ to 1.2 × 10⁶, based on HPSEC column calibration with pullulan standards. Debranching with isoamylase illustrated that the structure of the soluble masa starch component was highly branched with a similar debranched profile to native amylopectin. Further analysis revealed that a minor amount of amylose was present in the second half of the broad HPSEC peak containing the fragmented amylopectin component. There was a high second‐order correlation (r = 0.998, P < 0.01) between the absorbance at the wavelength of maximum absorbance (λ_max) of the soluble fraction from masa (527–532 nm) and masa adhesiveness, indicating that a rapid assay for masa adhesiveness could easily be developed. Increasing the shear at the stone mill by reducing the gap setting between the stones, increased the amount of fragmented amylopectin. The high correlation between the amount of fragmented amylopectin and masa adhesiveness suggests that this fraction is the main determinant of masa adhesiveness. The amount of fragmented amylopectin can be controlled by cook time and gap between the stone plates of the mill.     

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

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.     

Water Solubility and Macromolecular Properties of Corn Meal Extrudates as Affected by Epichlorohydrin

Degermed corn meal adjusted to 18% moisture content (db) with epichlorohydrin (ECH) content at 0, 0.5, 1, or 2% (w/w) were extruded with a twin-screw laboratory extruder at a screw speed of 140 rpm. Compression and metering barrel zones were set at 100, 120, or 140°C. Water solubility (WS) of ground extrudates ranged from 7.6 ± 1.1% to 14.3 ± 1.3%. ECH content had a significant (P < 0.01) negative effect on WS, while barrel temperature and the interaction between barrel temperature and ECH content were not significant (P > 0.05). Presumably, ECH reduced WS of extrudates through cross-linking between hydroxyl groups on starch and protein molecules. Gel-permeation chromatography patterns for both 100 and 140°C barrel temperatures showed that high molecular weight carbohydrates in the extrudates decreased with increasing ECH content without a simultaneous increase in low molecular weight carbohydrates. This suggested that the decrease in high molecular weight fractions was due to insolubilization by cross-linking rather than degradation. SDS-PAGE revealed that two protein bands of ≈29 and 17.5 kDa disappeared, and a new band appeared at 45 kDa with increasing ECH content. This indicated that, most likely, ECH reacted with protein in addition to reacting with starch. However, glycoprotein and starch-protein complexes were not identified with electrophoresis.     

Developments in the Science of Zein,Kafirin, and Gluten Protein Bioplastic Materials

Despite much research, there are very few commercial prolamin bioplastics. The major reason, apart from their high cost, is that they have inferior functional properties compared with synthetic polymer plastics. The inferior functional properties are because the prolamins are complex, each consisting of several classes and subclasses, and the functional properties of their bioplastics are greatly affected by water. Prolamin bioplastics are produced by controlled protein aggregation from a solvent or by thermoplastic processing. Recent research indicates that aggregation occurs by polypeptide self‐assembly into nanostructures. Protein secondary structure in terms of α‐helical and β‐sheet structure seems to play a key but incompletely understood role in assembly. Also, there is inadequate knowledge as to how these nanostructures further assemble and organize into the various forms of prolamin bioplastics such as films, fibers, microparticles, and scaffolds. Many methods have been investigated to improve prolamin bioplastic functionality, including better solvation of the prolamins, plasticization, physical and chemical cross‐linking, derivatization, and blending with synthetic and natural polymers, and some success has been achieved. The most promising area of commercialization is the biomedical field, in which the relative hydrophilicity, compatibility, and biodegradability of, particularly, zein and kafirin are advantageous. With regard to biomedical applications, “supramolecular design” of prolamin bioplastics through control over inter‐ and intramolecular weak interactions and disulfide/sulfhydryl interchange appears to have considerable potential.