Water Vapor Barrier Properties of Zein Films Plasticized with Oleic Acid

Water sorption, water vapor permeability, and tensile properties were evaluated for zein films plasticized with oleic acid. The effect of relative humidity on water vapor permeability and tensile properties of films was investigated. Samples were produced by two different methods: casting from a zein solution and stretching from a zein-fatty acid resin. Films were also coated with linseed oil. Results indicated that preparation method affected water sorption and permeability of zein films. Resin films showed lower water sorption than cast films, especially at high A_w values. Water vapor permeability was also lower for resin films. Coating with linseed oil further improved water vapor barrier ability of resin films. Permeability was affected by environmental relative humidity; higher relative humidity resulted in increased permeability. Environmental relative humidity also affected tensile properties of resin films. Toughness and elongation were improved when relative humidity increased from 50 to 85% rh. Tensile strength showed a maximum at 75% rh. Coating improved elongation and toughness of films. Maximum elongation and toughness were observed for coated samples at 85% rh. Zein resin films showed good tensile and water barrier properties that were maintained through environmental humidity levels from 50 to 98% rh.     

Thermal Behavior of Zein Sheets Plasticized with Oleic Acid

Zein, extracted from underutilized corn gluten meal may serve as an alternative starting material for fabrication of biodegradable packaging. Zein plasticized with oleic acid may be formed into flexible and water‐resistant sheets. Our objective was to investigate the effect of plasticization on thermal behavior of zein sheets employing differential scanning calorimetry (DSC). Zein sheets were rolled from a resin prepared by dispersing zein and oleic acid in aqueous alcohol followed by the recovery of the formed moldable compound by precipitation with water. Sheets were later replasticized with additional oleic acid to increase ductility. DSC thermograms were used to investigate zein‐oleic acid interactions in zein sheets. DSC endotherms were attributed to melting of free oleic acid or to the dissociation of zein‐oleic acid bonds. Plasticized sheets contained bound and free oleic acid. Further heat‐treatment of plasticized sheets apparently resulted in zein absorption of free oleic acid. However, high temperatures were believed to cause dissociation of some zein‐oleic acid bonds formed during replasticization.     

Tensile properties and water absorption of zein sheets plasticized with oleic and linoleic acids.

Corn zein has been investigated for fabrication of biodegradable packaging materials. Our objective was to investigate the effect of added plasticizers, oleic and linoleic acids, on tensile properties and water absorption of zein sheets. Moldable resins were precipitated from aqueous ethanol dispersions of zein and fatty acids and rolled into sheets of approximately 0.5 mm in thickness. To increase plasticization effects, zein-oleic acid sheets were replasticized by heating them in fatty acid baths. Plasticization resulted in flexible sheets of high clarity, low modulus, and high elongation and toughness, although low tensile strength. Water absorption of zein sheets was lowered by plasticization, attributed in part to reduced mass fraction of zein. Polymerization of linoleic acid may have sealed off pores on sheet surfaces, thus slowing water absorption.     

Physical,Mechanical, and Barrier Properties of Kafirin Films from Red and White Sorghum Milling Fractions

Fractions from the sorghum dry milling industry, including bran, are a potential source of kafirin. Free‐standing plasticized cast films were prepared from defatted kafirin preparations from red and white sorghum flour and bran fractions, and from commercial zein. All the kafirin preparations were able to form films. However, there were differences in film thickness, clarity, flexibility, surface texture, odor, and color between the different kafirin films. Bran kafirin films were highly colored, less flexible with a less smooth surface texture compared with films from flour, probably due to higher levels of contaminants in the bran kafirins. The strong color of the bran films could limit their use in certain coating applications. The kafirin films had much higher tensile strength and lower extensibility than zein film, probably because of the presence of β‐ and γ‐kafirins in the kafirin, giving high levels of disulfide cross‐linking in the kafirin films. The kafirin films had poorer water barrier properties than zein film, possibly due to greater thickness or to poorer flexibility, which may have caused microcracks.     

Effects of lamination and coating with drying oils on tensile and barrier properties of zein films.

Zein films plasticized with oleic acid have been considered potentially useful for biodegradable packaging applications. However, moisture was found to affect their tensile and gas barrier properties. We investigated the effects of two converting processes, fusion lamination and coating with drying oils, on tensile properties and gas permeability of zein films. Zein films were laminated to 4-ply sheets in a Carver press and coated with tung oil, linseed oil, or a mixture of tung and soybean oils. Tensile properties and permeability to water vapor, oxygen, and carbon dioxide were measured according to ASTM methods. Laminated films were clearer, tougher, and more flexible, and had a smoother finish than nontreated sheets. Lamination decreased O(2) and CO(2) permeability by filling in voids and pinholes in the film structure. Coating increased tensile strength and elongation and decreased water vapor permeability. Coatings acted as a composite layer preventing crack propagation and increasing film strength. They also formed a highly hydrophobic surface that prevented film wetting.     

Development of flexible antimicrobial packaging materials against Campylobacter jejuni by incorporation of gallic acid into zein-based films

In this study, antimicrobial films were developed against Campylobacter jejuni by incorporation of gallic acid (GA) into zein-based films. The zein and zein-wax composite films containing GA between 2.5 and 10 mg/cm(2) were effective on different C. jejuni strains in a concentration-dependent manner. Zein and zein-wax composite films showed different release profiles in distilled water but quite similar release profiles at solid agar medium. Depending on incorporated GA concentration, 60-80% of GA released from the films, while the remaining GA was bound or trapped by film matrix. The GA at 2.5 and 5 mg/cm(2) caused a considerable increase in elongation (57-280%) of all zein films and eliminated their classical flexibility problems. The zein-wax composite films were less flexible than zein films, but the films showed similar tensile strengths and Young's modulus. Scanning electron microscopy indicated different morphologies of zein and zein-wax composite films. This study clearly showed the good potential of zein and GA to develop flexible antimicrobial films against C. jejuni.     

Reducing the brittleness of zein films through chemical modification

Zein protein is a major coproduct of biofuel from corn. To reduce the brittleness of zein films, a new type of zein-based biomaterial, was synthesized by chemical modification of zein with lauryl chloride through an acylation reaction. The final products were confirmed by (1)H NMR, FT-IR analysis, and SDS-PAGE. Thermal analysis detected no microphase separation in the synthesized polymer matrix. As the content of lauryl moiety increased, the glass transition temperatures of modified zein decreased by as large as 25.8 °C due to the plasticization effect of the lauryl moiety. In addition, mechanical and surface properties of cast films from acylated zein were also investigated. The elongation at break of modified zein sheet was increased by about 7-fold at the high modification level with some loss of mechanical strength. The surfaces of modified zein films were as uniform as unmodified zein film but more hydrophobic, further suggesting that no microphase separation happened during the film formation process. This work indicated the potential of these new biomaterials in the development of biodegradable food packaging materials and delivery systems.     

Effects of hydrophilic plasticizers on mechanical, thermal, and surface properties of chitosan films

Chitosan films were plasticized with four hydrophilic compounds, namely, glycerol (GLY), ethylene glycol (EG), poly(ethylene glycol) (PEG), and propylene glycol (PG). Our objective was to investigate the effect of plasticizers on mechanical and surface properties of chitosan films. The stability of plasticized films was observed by storage for 3 and 20 weeks in an environmental chamber at 50 +/- 5% RH and 23 +/- 2 degrees C. Plasticization improves the chitosan ductility, and typical stress-strain curves of plasticized films have the features of ductile materials, except the film made with 5% PG that exhibits as a brittle polymer and shows an antiplasticization effect. In most cases, the elongation of plasticized films decreases with the storage time, which might be due to the recrystallization of chitosan and the loss of moisture and plasticizer from the film matrix. Although at the beginning the mechanical properties of films made with PG, at high plasticizer concentration, are comparable to those of films made with EG, GLY, and PEG, their stability is poor and they tend to become brittle materials. The surface properties, analyzed by contact angle measurement, reveal that plasticization increases film hydrophilicity. It is found that GLY and PEG are more suitable as chitosan plasticizers than EG and PG by taking into account their plasticization efficiency and storage stability. Furthermore, a plasticizer concentration of 20% (w/w) with GLY or PEG seemingly is sufficient to obtain flexible chitosan film with a good stability for 5 months of storage.     

Plasticizers for Zein: Their Effect on Tensile Properties and Water Absorption of Zein Films

Cast zein films are brittle at room conditions, so plasticizers are added to make them more flexible. The tensile properties of these films are known to be affected by the relative humidity (RH) of the ambient air. However, little is known about how the plasticizers are affected by RH. Cast zein films were plasticized with either glycerol (GLY), triethylene glycol (TEG), dibutyl tartrate (DBT), levulinic acid (LA), polyethylene glycol 300 (PEG), or oleic acid (OA). Mechanical properties and moisture content (MC) of the films were measured after one week of storage at 3, 20, 50, 70, 81, and 93% RH. The relative humidity of the films' storage had a great effect on the films' tensile properties. All the films' tensile strength and Young's modulus values decreased as RH increased. Films containing DBT, TEG, LA, or PEG showed an increase in the percent elongation with increasing RH. Films containing GLY, OA, or no plasticizer did not show any increase in percent elongation as RH increased. The changes seen in tensile properties with increasing RH are because of zein's hygroscopic nature. The absorbed water will further plasticize the zein. The type of plasticizer used determined the extent of the changes seen in the tensile properties of films stored at different RH values. Depending on the plasticizers used in the film, there were large differences in the amount of water absorbed. Films increasingly absorbed water depending on the plasticizer they contained in the order GLY > TEG > LA > PEG > NONE > DBT > OA. Films containing hygroscopic plasticizers like TEG absorbed too much water at high RH and became weak, but they absorbed enough water at lower RH values to not be brittle. While films containing the more hydrophobic plasticizer DBT were brittle at intermediate RH values, they had good mechanical properties at high RH values.     

Properties of zein films coated with drying oils

Zein films prepared by resin were coated with either flax oil or tung oil and cured by UV- or gamma-radiation. Coated zein films were then evaluated for tensile and water barrier properties. Film microstructure was examined by transmitted light microscope. Tensile strength, elongation, and toughness of oil-coated samples increased substantially with respect to uncoated films. Flax oil coated samples showed an increase in elongation of 300%. It was suggested that oil coatings fill in pinholes and cracks existing in zein films, affecting their mechanical properties. Water vapor permeability also decreased statistically for coated film (except for tung oil coated-UV treated films), suggesting water vapor transfer was controlled by film hydrophobicity and microstructure. Moreover, the liquid water transmission rate of coated films was at least 10 times slower than for control films. Examination of film microstructure revealed that flax oil coatings had uniform coverage and smooth finish, which explained their high elongation, low water vapor permeability. Tung oil coatings cured under UV light showed patterns of intertwined dark and light regions, which may be caused by cross-linking and drying at different times. The oriented structures were found when tung oil coatings were cured by gamma-radiation.     

Properties and Microstructure of Zein Sheets Plasticized with Palmitic and Stearic Acids

Interest in biodegradable materials for packaging and agricultural uses has grown in recent years. Plant proteins have been proposed as inexpensive, renewable, and abundant feedstock. Corn zein was investigated based on value-added considerations and on the unique thermoplastic and hydrophobic properties of zein. Films prepared from zein are known to be tough and resistant, but also hard and brittle, thus requiring the addition of plasticizers to improve flexibility. The objectives of this research were to study the tensile properties, water absorption, and microstructure of zein sheets plasticized with palmitic and stearic acids. Both palmitic and stearic acids showed similar effects as plasticizers of zein. Tensile strength of zein sheets increased with the addition of low levels of plasticizers. However, beyond a critical point, tensile strength decreased with further addition of fatty acids. Water absorption decreased continuously with increasing fatty acid content. Kinetic parameters indicated fatty acids decreased water absorption by decreasing the saturation level of zein sheets. Coating zein with flax oil decreased the rate of water absorption by sealing off surface pores. Scanning electron micrographs of zein sheets showed the development of layered structures as fatty acid content increased. Zein-fatty acid layers were believed to be responsible for the increased tensile strength of plasticized zein sheets and to have contributed to increased resistance to water absorption.     

Structural investigation of edible zein films/coatings and directly determining their thickness by FT-Raman spectroscopy

Near-infrared Fourier transform Raman (FT-Raman) spectroscopy was employed to study the molecular structure of edible zein films/coatings, which were fabricated directly from zein protein. The secondary structure of zein protein was mainly in alpha-helix and remained unaltered during film formation as evidenced by the vibrational modes of amide I at 1656 cm(-1) and amide III at 1274 cm(-1). Raman results indicated that hydrophobic interaction played an important role in the formation of zein film and disulfide bonding might be responsible for the structural stability of zein protein during film formation. To enhance its antimicrobial property, an antimicrobial zein film was manufactured by incorporating zein protein with benzoic acid whose structure was then characterized by FT-Raman. It showed that physical entrapment or hydrophobic interaction was crucial to the incorporation of benzoic acid with zein protein, and the secondary structure of the antimicrobial film was still maintained in alpha-helical form. In addition, FT-Raman exhibits its preference in directly determining the thickness of zein films/coatings. By correlating the Raman intensity ratio of nu(1003) to nu(84) (I(1003/84)) versus the thickness of zein film, a linear relationship with high coefficient (R(2) = 0.9927) was obtained, which was then used pragmatically to determine the thickness of zein coatings on apple. It showed that the FT-Raman result (thickness = 0.27 +/- 0.01 mm) was consistent with that of classical micrometric measurement (thickness = 0.28 +/- 0.02 mm). Consequently, FT-Raman provides a direct, simple, and reagent-free method to characterize the structure and the thickness of zein films/coatings.     

电场处理改善玉米醇溶蛋白膜理化性质

传统浇铸法制备的玉米醇溶蛋白薄膜表面粗糙,机械性质及耐水性较差。为了改善玉米醇溶蛋白理化性质,在传统浇铸法膜制备过程中引入平行匀强电场(1~5 A/m2)处理蛋白成膜液。经过电场处理后,玉米醇溶蛋白表面光滑、形状完整。试验结果表明:电场处理可改善薄膜力学性质、表面疏水性、水蒸气透过率等性质;随着电流密度的增大,薄膜拉伸强度、断裂伸长率、水蒸气透过率、吸水率以及表面接触角呈现规律性增加或者减小;电场处理下薄膜热特性改变,与未处理组相比变性温度略有提高,最大增幅为19.5℃。当电流密度为4 A/m2时,薄膜理化性质较佳:拉伸强度、断裂伸长率分别为73.09 MPa和9.68%,吸水率降低至14.87%,水蒸气透过率为2.55×10-8 g·m/(m2·h·Pa),静态接触角为62.18°,变性温度提高到118.39℃,热稳定性提高,薄膜表面光滑。电场可诱导成膜液中分子有序性排列,提高薄膜均一性;通过调节电流密度可得到具有一定力学强度和亲/疏水性的薄膜。试验结果为制备具有特定功能性的纯玉米醇溶蛋白薄膜材料提供了理论依据。     

Development of highly ordered nanofillers in zein nanocomposites for improved tensile and barrier properties

It has been a long-lasting challenge to prepare highly ordered biopolymer nanocomposites to optimize or tune the desired mechanical and barrier properties of the nanocomposite film. In this study, we developed a simple and cost-effective method to synthesize highly ordered zein nanocomposites. The method involved the synthesis of magnetic iron oxide (Fe(3)O(4)) nanofiller and the preparation of a highly ordered structure by in situ nanofiller reorientation under an external magnetic field. The successful preparation of Fe(3)O(4) magnetic nanoplatelets together with exfoliated and highly ordered zein resin nanocomposites was confirmed by scanning electron microscopy, X-ray diffraction, and a vibrating sample magnetometer. As a result, in comparison to zein resin film, the exfoliated zein nanocomposites (Fe-Zein) showed dramatic improvement on mechanical and barrier properties. The tensile strength, elongation, and Young's modulus of Fe-Zein were increased by 218, 48, and 264%, respectively, while the water vapor and oxygen permeability decreased by 68 and 29%. More importantly, the highly ordered zein nanocomposites (Fe-Zein-Mag) showed additional improvement on the mechanical and gas barrier properties. In comparison to Fe-Zein, the tensile strength and elongation of Fe-Zein-Mag were increased by 10 and 48%, respectively, and a 30% decrease in Young's Modulus was observed, indicating the Fe-Zein-Mag film was more elastic. Besides, the water vapor and oxygen permeability of Fe-Zein-Mag were also decreased by an additional 48 and 17%, respectively.     

Chemistry and physical properties of melt-processed and solution-cross-linked corn zein

Corn zein was cross-linked with glutaraldehyde (GDA) using glacial acetic acid (HAc) as catalyst. The objectives are to evaluate the swelling characteristics of GDA cross-linked zein gels in water, ethanol, and their combinations. Similar formulations, upon solvent evaporation, form films. The mechanical properties of the films are compared to compression molded tensile bars from GDA melt-processed zein as a second objective. Chemistry of the cross-linking reaction was based on the aldehyde binding characteristics defined by use of fluorescence spectroscopy; sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) to demonstrate the cross-linking reaction; FTIR to observe absorption differences of the cross-linked product; differential scanning calorimetry, dynamic mechanical analysis and thermogravimetric analysis to assess thermal properties; and the use of Instron Universal Testing Machine to evaluate mechanical properties. A reaction mechanism for acid catalyzed GDA cross-linking of zein is proposed. Thermal and mechanical properties of tensile bars cut from either film or formed by compression molding were similar, where both showed increased tensile strengths, ductility and stiffness when compared with unmodified controls. Samples that were reacted with 8% GDA by weight based on weight of zein from either process retained their integrity when tensile bars from each were subjected to boiling water for 10 min or soaking in either water or HAc for 24 h. The melt-processed, cross-linked zein is a more environmentally friendly method that would eliminate the need for HAc recovery.     

Mechanical and water barrier properties of glutenin films influenced by storage time

The goal of this work was to study the effect of storage time on the functional properties of glutenin films plasticized with selected hydrophilic low molecular weight compounds: glycerol (GL), triethanolamine (TEA), and sorbitol (S). Glutenins were extracted from wheat gluten, and films were cast from film-forming solutions. The glutenin-based films were homogeneous, flexible, translucent, and easy to handle. Films were stored in an environmental chamber at 50 +/- 5% realtive humidity and 23 +/- 2 degrees C. Optical, mechanical, and water vapor permeability properties were monitored at regular intervals for 16 weeks. Films plasticized with GL and TEA had similar mechanical and water vapor barrier properties during the first few days of fabrication. Films plasticized with S were stronger, with better water vapor barrier properties. Mechanical and water vapor permeability properties of films plasticized with GL changed dramatically over time, whereas the properties of films plasticized with TEA and S remained stable during storage. Color properties of films plasticized with GL, TEA, and S did not change within the time period studied.     

Water sorption properties of extruded zein films

Water interactions in extruded zein films were investigated through moisture sorption isotherms. Sorption isotherms of zein products were affected by composition and structure morphology. Zein powder showed moisture sorption hysteresis, which was not observed in extruded samples. Extruded samples held less moisture than zein powder, while films containing oleic acid showed further reduction in moisture uptake. Brunauer, Emmett, and Teller (BET) and Guggenheim, Anderson, and De Boer (GAB) models fitted well the moisture sorption isotherms of zein products. Monolayer values estimated by BET and GAB models were consistent with predictions based on zein structural models. Water vapor permeability (WVP) of zein films was affected by the relative humidity of testing environment. Higher relative humidity resulted in higher WVP.     

Antioxidants modulate molecular mobility, oxygen permeability, and microstructure in zein films

The effect of octyl gallate and propyl gallate on the molecular mobility, oxygen permeability, and microstructure of zein/glycerol films was studied. Films were cast from 70% ethanol/water containing 20% (w/w) glycerol and different amounts of the antioxidants propyl gallate or octyl gallate. The oxygen permeability and local mobility of these films were measured using phosphorescence from the dispersed triplet probe erythrosin B. Although both antioxidants increased the local mobility of the zein matrix to about the same extent, octyl gallate and to a lesser extent propyl gallate dramatically increased the permeability of the film to oxygen. Atomic force microscopy imaging indicated that propyl gallate induced aggregation of zein complexes, which could lead to a more condensed film. These results indicate that the addition of specific functional ingredients, such as antioxidants, to edible films may significantly affect the physical properties and structure and, thus, functional properties in ways that influence their eventual use.     

Compositional and moisture content effects on the biodegradability of zein/ethylcellulose films

The effect of moisture content and film composition on biodegradability is the focus of this study. Flexible films were first characterized for the effect on water sorption isotherms of relative humidity, temperature, zein content, and the addition of the plasticizers stearic acid, poly(ethylene glycol), or etoxylated ricine oil. Zein/ethylcellulose (EC) mixture films had a behavior between that for pure zein and EC films, which had the lowest water sorption. For films with plasticizer, the lowest water sorption at 25 degrees C was observed for those with stearic acid. Biodegradability of zein/EC films, evaluated using bacterial cultures selected for their zein proteolytic activity and isolated from a local solid waste landfill and a lagoon, showed no plasticizer effect even though its effect on moisture content was significant. Large differences were observed at different film zein concentration with the highest biodegradability for 100% zein. However, biodegradability did not mimic the water sorption behavior of zein/EC mixture films.     

Water Barrier Properties of Zein‐Oleic Acid Films

Free‐standing films were prepared from zein formulations containing 30, 40, 50, 60, 70, and 80% oleic acid (OA). Zein/OA formulations were also used as coating films for rodent diet bars. Water vapor permeability (WVP) of films and moisture loss rate (MLR) of coated rodent diet bars were measured at 4 and 25°C. Temperature affected the water barrier properties of films and coatings. At 4°C, WVP of films decreased with OA concentration while it increased at 25°C. WVP behavior was attributed to oleic acid phase changes due to temperature. At 4°C, OA is a crystalline solid that limits water diffusion through the films. At 25°C, liquid OA increased the system free volume and allowed for water diffusion. The effect was more pronounced the higher the OA concentration in films. Differential scanning calorimetry (DSC) of zein/OA films showed endothermic peaks at 12–18°C, confirming the melting of OA in that temperature range. MLR of coated rodent diet bars was also affected by temperature and OA concentration in coating formulations. In this case, formulations containing 40, 50, and 60% OA were better moisture barriers than coatings with higher OA content at both 4 and 25°C. Moisture losses were reduced at 4°C due to OA solidification.