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.     

Oxygen permeability of films made from CO2-precipitated casein and modified casein

Oxygen permeabilities (OP) of CO(2)-casein (CO(2)CN), calcium caseinate (CaCN), and acylated casein (AcCN) films were determined as functions of % relative humidity (% RH), temperature, and plasticizer type. Tensile properties and water vapor permeabilities (WVP) were also measured. Plasticizers were glycerol (GLY) or a 3:1 ratio of GLY:poly(propylene glycol) (PPG), a hydrophobic plasticizer. OP of the CO(2)CN:GLY film was almost twice that of films containing either plasticizer at 35% RH, but its OP approached that of the other films at 70% RH. OP and WVP of films plasticized with GLY were greater than that for films plasticized with PPG. Plasticizer type had little impact on the tensile strength of CO(2)CN films while tensile strength of CaCN-GLY:PPG (3:1) films approximately doubled. Results show that structural dissimilarities in the films contribute to differences in OP only under conditions of low RH where the plasticizing effects of water are not significant.     

Effect of salts and plasticizers on stability of shellac film

The aim of this study was to increase the stability of shellac because of the polymerization. A few approaches have been applied in this study. Shellac film was prepared in two salt forms, that is, ammonium and 2-methyl-2-amino 1-propanol salts, and a comparison was made with shellac film in free acid form. The other approach was by the application of plasticizers. These plasticizers were diethyl phathalate, triacetin, and polyethylene glycol 400 (PEG 400). Plasticized shellac and unplasticized shellac films in free acid form were then compared. All shellac films were kept in stability chamber at 40 degrees C, 75% RH for a period of 3 months. The studied parameters such as insoluble solid, acid value, mechanical properties, and water vapor permeability were detected every month. Analysis of variance (ANOVA) technique was used to analyze data. The applications of salt forms proved statistically significant (p < 0.01) to reduce the polymerization process whereas certain plasticizers could enhance the stability. PEG 400 was the only plasticizer that could show the increase in stability. The improvement of stability might be a result of the interference of a larger molecule of PEG 400 causing the difficulty in interaction among carboxyl or hydroxyl groups of shellac and the effect of lower loss of plasticizer.     

Plasticizer effect on oxygen permeability of beta-lactoglobulin films

Plasticizer effect on oxygen permeability (OP) of beta-lactoglobulin (beta-Lg) films was studied. Propylene glycol (PG), glycerol (Gly), sorbitol (Sor), sucrose (Suc), and polyethylene glycol at MW 200 and 400 (PEG 200 and PEG 400, respectively) were studied due to their differences in composition, shape, and size. Suc-plasticized beta-Lg films gave the best oxygen barrier (OP < 0.05 cm3 x microm/m2 x day x kPa). Gly- and PG-plasticized films had similar OP values, and both had higher OP than Sor-plasticized films. PEG 200- and PEG 400-plasticized films were the poorest oxygen barriers. Empirical equations including plasticizer efficiencies for OP were employed to elucidate the relationships between OP of plasticized beta-Lg films and plasticizer type and content. Plasticizer efficiency ratios between mechanical and OP properties of beta-Lg films show the relative efficiency of plasticizers in modifying mechanical and OP properties. A large ratio is desirable.     

Effect of molecular weight and concentration of polyethylene glycol on physicochemical properties and stability of shellac film

The effects of molecular weight and concentration of plasticizer on physicochemical properties and stability of shellac films were investigated. Type of plasticizer was previously reported to have some effects on the stability of shellac films, and polyethylene glycol (PEG) was the plasticizer of choice for plasticizing shellac films. In this study, different molecular weights of PEG (200, 400 and 4000) were chosen at a concentration of 10% w/w of shellac films. Shellac in alcohol was prepared in a free film. The stability of shellac film was then performed at 75% RH, 40 °C for 3 months. The comparison was made between the film with and without plasticizer. Shellac films were then determined for acid value, insoluble solid, mechanical properties and water vapor permeability coefficient. It was reported that different molecular weights of PEG had some influence on physicochemical properties of the shellac films. Among different molecular weights of PEG, PEG 400 showed a suitable molecular weight that could protect the shellac chain at the carboxylic and hydroxyl groups. Therefore, the molecular weight of plasticizer played a crucial role for the protective ability at active sites. Further study was performed to investigate the effect of concentrations of PEG 400 on the stability. The results demonstrated that PEG 400 at a concentration of 10% (w/w) could prevent the polymerization process for only 4 months and a significant change of all parameters was then reported. However, a higher concentration, 20% (w/w) of PEG 400, could prolong the stability of shellac for 6 months of study. Therefore, the drawback of shellac as a natural polymer in pharmaceutical and food industries could be tackled by the appropriate size and concentration of plasticizer.     

Properties and Microstructure of Plasticized Zein Films

A new method for preparation of zein films involving plasticization of zein with oleic acid to form an intermediate moldable resin was presented. The resin was stretched over rigid frames to form thin membranes that were set in flexible films. The objective of the study was to investigate the effect of film preparation method on film properties. Tensile properties, microstructure, and thermal behavior of zein films plasticized with oleic acid were investigated for films prepared by conventional casting from ethanol solutions and by stretching of plasticized resins. Cast films were stiff and brittle, whereas resin films showed more flexibility and toughness. Differential scanning calorimetry thermograms of cast films indicated phase separations were generated when heated that were not observed for resin films. Microstructure images showed a higher degree of structure development and orientation in resin than in cast films. Glass-transition temperatures of resin films were measured at -94 and 104.4° C, indicating the film remained flexible through a wide temperature range. Resin film flexibility and toughness were attributed to effective plasticization that led to fiber formation and orientation.     

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.     

Solution and film properties of sodium caseinate/glycerol and sodium caseinate/polyethylene glycol edible coating systems.

The aim of this study is to determine the effects of plasticizer hydrogen bonding capability and chain length on the molecular structure of sodium caseinate (NaCAS), in NaCAS/glycerol and NaCAS/polyethylene glycol 400 (PEG) systems. Both solution and film phases were investigated. Glycerol and PEG reduced the viscosity of aqueous NaCAS, with the latter having a greater effect. This was explained in terms of protein/plasticizer aggregate size and changes to the conformation of the caseinate chain. In the film phase, glycerol caused more pronounced changes to the film tensile strength compared with PEG. However, the effect of glycerol on film water vapor permeability was smaller. These observations are attributed to the differences in plasticizer size and hydrogen bonding strength that controls the protein-plasticizer and protein-protein interactions in the films. Glass transition calculations from the tensile strength data indicate that the distribution of bonding interactions is more homogeneous in NaCAS/PEG films than in NaCAS/glycerol films.     

Molecular determinants of the influence of hydrophilic plasticizers on the mechanical properties of cast wheat gluten films

The influence of a set of hydrophilic plasticizers varying in their chain length (ethyleneglycol and longer molecules) on the tensile strength and elongation at break of cast gluten films was studied. When considered on a molar basis (moles of plasticizer per mole of amino acid), the effect of the different plasticizers depended on their respective molecular weights for plasticizer/amino acid ratios in the range from 0.10 to 0.40. However, above a ratio of 0.40-0.50 mol/mol of amino acid, these differences were abolished and both stress and strain reached a plateau value, with all plasticizers studied. In fact, when a homologous series of molecules was considered, the ability for plasticizer to decrease stress and increase strain was closely related to the number of hydrogen bonds the molecule was able to share with the protein network. Ethyleneglycol's efficiency was, however, lower than expected from its hydrogen-bonding potential; a comparison with other diols demonstrated that this was due to the small size of this molecule. The particular effect of glycerol concentration on the films' mechanical properties suggested that other molecular features of the plasticizer, such as the number and position of hydroxide groups in the molecule, were involved in the plasticization mechanism.     

Preparation and characterization of films from pea protein

The conditions for protein film preparation from an alkaline dispersion of a pea protein isolate were investigated in the presence of polyols as plasticizers. Mechanical and barrier properties of resulting films were studied as a function of protein dispersion conditions, protein and plasticizer concentrations and ratios, chain length of the plasticizer, and pH and composition of the alkaline medium. Neither the mode of protein hydration nor the pea isolate origin had a significant effect on the mechanical properties of pea protein films. However, increasing the plasticizer chain length induced slightly higher surface hydrophobicity but poor mechanical properties. Addition of monoglycerides to film-forming solution allowed a significant improvement of the films during aging. Both tensile strength and surface hydrophobicity increased when ammonium hydroxide was used as protein dispersing agent instead of sodium hydroxide.     

氨基酸对明胶膜结构及性能的影响

为了探索氨基酸作为明胶膜增塑剂的潜力,本试验研究了不同氨基酸(脯氨酸、亮氨酸、谷氨酸、丝氨酸和甲硫氨酸)对明胶膜厚度、机械性能、水蒸气透过率、透光率、颜色以及结构等的影响。结果表明,与对照组相比,除了甲硫氨酸外,其余的氨基酸都能增加明胶膜柔韧性;除了脯氨酸外,其余的氨基酸都增加了明胶膜的黄度值,并降低了明胶膜透光率和透明度;而添加脯氨酸的明胶膜的柔韧性最好、具有更好的表观特征,表面光滑紧致,同时具有更高的透明度、透光率和柔韧性。根据傅里叶变换红外光谱和二级结构分析发现,添加脯氨酸、丝氨酸和甲硫氨酸能通过氢键与明胶相互作用,降低明胶的无规则卷曲结构含量,增加β-折叠和β-转角结构含量。综合分析认为,脯氨酸可以作为明胶膜的天然增塑剂。本研究结果为增塑剂在明胶膜中的应用提供了新视角。     

Gas chromatographic-mass spectrometric determination of adipate-based polymeric plasticizers in foods

A method for the quantitative determination of adipate-based polymeric plasticizers in foods is described. The procedure involves extraction from the food and transmethylation of the polymeric plasticizer to form dimethyladipate (DMA). The derivative is cleaned up by size-exclusion chromatography and determined by capillary gas chromatography-mass spectrometry with selected ion monitoring. The use of a deuterated internal standard at the extraction stage enables quantitation by stable isotope dilution. A detection limit of 0.1 mg/kg of the polymeric plasticizer in foods and a relative standard deviation of 4% have been achieved routinely. The method has been applied successfully to the analysis of cheese, sandwiches, meat, biscuits, and cake that have been in contact with polymeric plasticized poly(vinyl chloride) films.     

Textural Stability of Intermediate‐Moisture Extrudates: Effects of Formulation

Effects of formulation on the textural stability of intermediate‐moisture, flour‐based, “jerky”‐type extrudates were assessed. Potato‐based extrudates containing various particulate‐meat concentrations and different plasticizers (sucrose, fructose, glycerol, and glucose) were produced and subjected to accelerated storage for three weeks. The elastic modulus of the samples was measured before storage and then weekly. The relative fluidity and moisture mobility of the specimens were assessed by dynamic mechanical spectrometry (DMS), electron spin resonance (ESR), and nuclear magnetic resonance (NMR). Samples were also evaluated by fluorometry and X‐ray diffraction to determine the extent of browning reaction and degree of molecular ordering, respectively. While elastic modulus increased appreciably during storage, firming was progressively reduced by entrained meat content and also by plasticizers, especially glycerol; plasticized and meat‐containing samples had correspondingly lower tan δ peak temperatures as measured by DMS. Textural results were also in keeping with fluidity and local viscosity as assessed by ESR measurements. NMR T1 relaxation values, reflecting moisture mobility, increased during storage. Diffraction spectra were consistent with published observations of hydrated starch, suggesting that water may have been released due to increased association of proteinaceous constituents. Fluorescence measurements confirmed moderate Maillard browning in all samples and significant chlorogenic browning in glucose‐containing samples, although these effects were unrelated to degree of firming. It was concluded that textural stability was optimized by interruption of the matrix by dispersed meat or by plasticization by low molecular weight constituents.     

Impact of Thiocyanate Salts on Physical,Thermal, and Rheological Properties of Zein Films

A new class of zein additives was investigated, thiocyanate salts. Ammonium, potassium, guanidine (GTC), and magnesium thiocyanate salts were added to solutions of zein in with various amounts of tri(ethylene glycol) (TEG), cast as films, and then tested to determine the impact that each salt had on properties. The presence of these salts affected solution rheology and intrinsic viscosity, demonstrating that the salts interacted with the protein. It was found that these salts acted as plasticizers, as they lowered the glass transition temperature of zein when evaluated with differential scanning calorimetry. In zein films in which TEG was present, these salts increased elongation and reduced tensile strength. However, unlike traditional plasticizers (such as TEG), when the salts were used as the only additive, elongation was not increased and tensile strength was not decreased. Of the salts tested, GTC in combination with TEG was found to increase elongation the most. The impact of salts on elongation was greatly affected by the relative humidity in which the samples were stored.     

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.     

腰果酚基乙酸酯增塑剂的合成及其增塑聚氯乙烯性能

为了进一步利用农林剩余物资源替代石化原料,该研究以腰果酚为原料,通过对其酚羟基进行酯化改性,制备腰果酚基乙酸酯(cardanol acetate,CA)增塑剂。采用核磁共振氢谱(1H nuclear magnetic resonance,1H NMR)和核磁共振碳谱(13C nuclear magnetic resonance,13C NMR)对产物的结构进行表征。通过动态力学性能(dynamic thermo mechanical analysis,DMA),拉伸性能测试,热重分析(thermogravimetric analysis,TGA),以及与聚氯乙烯(polyvinyl chloride,PVC)共混样品的傅里叶红外分析(fourier transform infrared spectroscopy,FT-IR)等方法,评价腰果酚基乙酸酯作为辅助增塑剂应用于软质聚氯乙烯的增塑效果,并与商业增塑剂对苯二甲二辛酯(bis(2-ethylhexyl)benzene-1,4-dicarboxylate,DOTP)进行对比。研究结果表明,m(DOTP)∶m(CA)=4∶6为较佳配伍比例,共混体系的玻璃化转变温度由41.52℃降低至35.93℃,断裂伸长率由244.75%增加到了302.13%,热稳定性及相容性均得到有效改善,因此腰果酚基乙酸酯可用作聚氯乙烯的优良辅助增塑剂。研究结果为腰果酚在增塑剂领域的应用提供了参考。     

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.     

Effect of plasticizer type and amount on hydroxypropyl methylcellulose-beeswax edible film properties and postharvest quality of coated plums (cv. Angeleno)

The effect of the composition of hydroxypropyl methylcellulose (HPMC)-beeswax (BW) edible coatings on stand-alone film properties and on postharvest quality of coated 'Angeleno' plums was studied. Glycerol (G) and mannitol (M) were tested as plasticizers at two different plasticizer/HPMC ratios (100:1 and 300:1 molar basis). BW content was 20 or 40% (dry basis). An increase in G content increased film flexibility and vapor permeability (WVP), whereas an increase in M content enhanced film brittleness without affecting WVP. An increase in BW content reduced film flexibility and reduced WVP of only G-plasticized films. Coatings reduced plum softening and bleeding, but were not effective in reducing plum weight loss. At low plasticizer content, coatings reduced texture loss effectively. Low BW also lowered plum bleeding. Plasticizer type affected only ethanol and acetaldehyde contents without affecting the remaining quality parameters. Therefore, HPMC-BW coatings have the potential to extend the shelf life of plums. However, this effect depends on coating composition. Differences between coating and film performance indicate that data from stand-alone films may be used as a preliminary screening, but coating performance should be analyzed on coated fruit.     

Molecular structure, physicochemical characterization, and in vitro degradation of barley protein films

Barley protein films were prepared by thermopressing using glycerol as a plasticizer. The combined effects of heating temperature and amount of plasticizer interacted to determine protein conformation and, subsequently, the properties of the film matrix. The film barrier and mechanical properties were systematically investigated using Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), SDS-PAGE, and protein solubility tests. These experiments demonstrated that heat treatment induced barley protein unfolding and then protein aggregation and the formation of covalent disulfide bonds to enhance film strength. Increasing the amount of plasticizer reduced protein denaturation and limited protein interactions, resulting in significantly improved film flexibility at the cost of reduced film moisture barrier property and tensile strength. In vitro degradation experiments demonstrated that barley films were resistant in gastric conditions, yet can still be completely degraded by intestinal enzymes, and they possess low cytotoxicity to Caco-2 cells. The prepared barley films have potential for development as delivery systems for gastric-sensitive bioactive compounds to the intestine for release.     

Physicochemical, antimicrobial, and cytotoxic characteristics of a chitosan film cross-linked by a naturally occurring cross-linking agent, aglycone geniposidic acid

The purpose of this study was to evaluate the characteristics of a chitosan film cross-linked by a naturally occurring compound, aglycone geniposidic acid (aGSA). This newly developed aGSA-cross-linked chitosan film may be used as an edible film. The chitosan film without cross-linking (fresh) and the glutaraldehyde-cross-linked chitosan film were used as controls. The characteristics of test chitosan films evaluated were their degree of cross-linking, swelling ratio, mechanical properties, water vapor permeability, antimicrobial capability, cytotoxicity, and enzymatic degradability. It was found that cross-linking of chitosan films by aGSA (at a concentration up to 0.8 mM) significantly increased its ultimate tensile strength but reduced its strain at fracture and swelling ratio. There was no significant difference in the antimicrobial capability between the cross-linked chitosan films and their fresh counterpart. However, the aGSA-cross-linked chitosan film had a lower cytotoxicity, a slower degradation rate, and a relatively lower water vapor permeability as compared to the glutaraldehyde-cross-linked film. These results suggested that the aGSA-cross-linked chitosan film may be a promising material as an edible film.