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Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. CONCLUSION
  5. REFERENCES

This article aims to review the present scenario of protein based natural polymer development, which has the ability to stand against synthetic polymer. Demand of natural polymers would increase in future considering their environmental safety aspect. Protein characteristics and their suitability for polymer development are discussed here, along with the polymer reinforcement techniques such as development of blends, chemical block copolymerization, and modification of existing protein material, which are used for the development of biopolymer from protein. The application of protein based polymer product range varies from food and nonfood packaging stuffs to healthcare sectors. POLYM. ENG. SCI., 55:485–498, 2015. © 2014 Society of Plastics Engineers


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. CONCLUSION
  5. REFERENCES

The primary goal of natural polymer based research is the development of a system that can mimic the structure and function of native nondegradable synthetic polymer at some extend, so that they can be replaced from extensive use thus making our environment safe. A synthetic polymer, which is made-up of petroleum products like polyethylene, polyvinyl chloride, polystyrene, are nondegradable and cause environmental damage because they do not break down for tens of hundreds of years [1] and persistence in the environment for a long time. The natural polymer over synthetic polymer offers a number of advantages such as complete degradation, increased soil fertility, low accumulation of bulky plastic materials in the environment, and reduction in the cost of waste management. Natural polymers can obtained from three kinds of renewable resources: (a) plants originated polymer such as starch [2], soy protein [3], and cellulose [4]; (b) from animals such as chitosan [5], keratin [6], silk [7]; and (c) by microbial fermentation such as polyhydroxyalkanoates (PHA) and polyhydroxybutyrate (PHB) [8]. Technological advancement in polymer engineering gives new polymer composites with novel characteristics for their desired application [9-11]. Cellulose [4], chitosan [12], starch [13], and PHB [4, 13] etc., and their blends are explored considerably by various group of scientists because of its intrinsic ability to perform very specific biochemical [14], mechanical, and structural roles [3, 15].

Among natural polymer proteins are one of the strong candidates, which can be used for the development of new blend and/or composite material. In natural state proteins are present as either globular or fibrous structure form. The globular proteins fold into complicated sphere-shaped structures held mutually by an arrangement of hydrogen, ionic, hydrophobic, and covalent (disulphide) bonds. The fibrous proteins are entirely extended and coupled strongly together in parallel constructions, commonly through hydrogen bond to form fibers. The chemical and physical characteristics of these proteins depend on the comparative amount of amino acid residues and its placement along the polypeptide chain. The use of such amino acid sequence with other natural and synthetic polymer can enrich polymer chemistry and science, which can be further exploited through chemical and protein engineering modifications to get novel polymer design from proteins. Blends of protein with nonprotein, natural, and synthetic molecules such as keratin-chitosan [16], gluten-methyl-cellulose [17], keratin-polypropylene, keratin-cellulose-polypropylene [18-20], and keratin-polyethylene [21] etc., are explored by several scientists and they have reported that the properties of the native protein film improved to some extend (i.e., film strength, flexibility, and water vapor permeability, etc.). Gluten [22], milk protein [23, 24], and soy protein [25] is used in development of edible film, while keratin is used to develop nanofiber [26], film [27], and composites [28] for material industries. The future application of protein based natural polymer seems to be intense in the field of biomaterials [14, 29], packaging material and in coatings industries [30, 31]. The present article provides a characterization of protein in its native state for the formation of bio-based polymer along with the polymer reinforcement technique for improving the characteristic/properties of peptide polymer to develop novel polymer designs. The applications of such protein blends and composites ranging to micro to macro structural level are comprehensively discussed.

Protein Characteristics and Its Suitability for Polymer Development

Proteins are prepared from the basic unit called amino acids. The protein's structure is broadly categories into four structural forms, which are called primary, secondary, tertiary, and quaternary structure. The primary structure of a protein is a linear polymer with a string of amino acids coupled by peptide bonds. Secondary structures of proteins are usually very regular in their conformation and in point of fact, they are the spatial arrangements of primary structures. “Alpha Helices” and “Beta Pleated Sheets” are two types of secondary structures and they are majorly stabilized by hydrogen bonds. The tertiary structure of a protein is the three-dimensional structure and is stabilized by the series of hydrophobic amino acid residues and by disulfide bonds formed among two cysteine amino acid. The tertiary peptide structure with less disulfide bonds form weak, rigid structures that are bendable, but still tough and can oppose rupture such as hair and wool. While the structures that contain more disulfide bonds lead to stronger, stiffer, and harder structures. Quaternary structure of protein is the arrangement of more than two chains of peptide, to form an entire unit. The interactions between the chains are not like from those in the tertiary structure, but are differentiated solely by being an intermountain range rather than an intrastring interaction. The quaternary structure occupies the bunch of abundant individual peptide chains into an ultimate shape. A range of bonding interactions, including salt bridges, hydrogen bonding, and disulfide bonds hold the variety of chains into a particular geometry. There are two foremost categories of proteins with quaternary structure, i.e., fibrous and globular protein. Fibrous proteins such as the keratins in hair and wool are composed of coiled alpha helical protein chains with other various coils analogs. Alternatively, globular proteins may have an arrangement of the above types of structures and are predominantly clumped into a shape of a globe. Major examples include insulin, hemoglobin, and most enzymes.

Keratin is a protein that contains disulfide bonds and has an array of characteristics that ranges from a structurally robust, impact-resistant material (horn) to a simple waterproof layer (turtle shell). Keratin is together mechanically efficient in tension (wool) and compression (hooves) [32]. Keratins are found in hair, wool, claws, nails, skin, fur, hooves, beaks, feathers, horns, scales, actin, and myosin protein found in muscle tissues. The main differences in various keratins arise from their sulfur content. If there are many cysteine disulfide crosslinks, then there is very little flexibility as in claws, hooves, horns, and nails. In wool, skin, and muscle proteins, there are fewer disulfide crosslinks, which allow some stretching, but returns to normal upon relaxation of tension.

Silk is another outstanding biological polymer but has much more complex structure. Actually, the final beta-pleated sheet structure of silk is the result of the interaction of many individual protein chains. Specifically, hydrogen bonding on amide groups on different chains is the basis of beta-pleated sheet in silk proteins. Silks are produced by some insects such as from spider and silkworm (Bombyx mori), but generally only the larvae silks have been used for textile manufacturing. Most silks have extraordinary mechanical properties and demonstrate a matchless combination of high tensile strength and extensibility. The arrangement of strength and extensibility gives silks a very high toughness, which equals that of commercial aromatic nylon filaments [33], which themselves are benchmarks of current polymer fiber technology. Spider silk has long been documented as the marvel fiber for its unique combination of high strength and break elongation. An earlier learning indicated spider silk has strength as high as 1.75 GPa at a breaking elongation of over 26% [34].

The quaternary structure of collagen consists of three left-handed helices twisted into a right-handed coil. Collagen is a cluster of naturally occurring proteins found in animals, because of its unusual characteristics, such as biodegradability and weak antigenicity [35], it is an important biomaterials for tissue engineering applications. Collagen fibers are commonly white, opaque, and viscoelastic matter acquire high tensile strength and less extensibility. This collagen has been used as biomaterials in drug delivery systems [36] and in tissue engineering [37]. A scientist reported that the Young's modulus of the rat tail collagen Type I vary between 3.7 and 11.5 GPa [38]. A series of studies has focused on the structural and tensile properties of collagen scaffolds for the purpose of designing functional biomaterials for clinical application [39-42].

The investigations in a range of proteins such as gluten [43], corn zein [44], soya [45], and milk [46] revealed that the proteins acquire the capability to form films, which can be used for packaging. Such proteins have nutritional value as well, so it can be also used for the development of edible film too. Recently the progress of degradable films from protein has drawn attention to a large extent due to protein's skill to form films and for its large quantity and renewable nature.

Polymer Enforcement Technique for Protein Material

Proteins/peptides are made up by several to thousands repeating unit of amino acid. The main chain of the peptide remains constant throughout the length. The side chain of peptide may consist of one of the 20 different functional groups acting on their reactive side group “R”. These side chains verify the nature and properties of the protein, and are accountable for the infinite variety of protein shapes, functions, sequences, and its nature [47]. Interaction phenomenon between natural and/or synthetic polymer with side chain groups of peptide allows to develop natural polymer based film with enhanced properties such as elasticity and toughness, etc. [48-50]. The different reinforcement approaches that are used for the development of protein based polymer can categorize in several ways that are chemical agents and radiation treatment [51], chemical block copolymerization [52-54], and preparation of blends [21, 55-57]. Using these approaches it is possible to create a biodegradable and high performance polymer. For protein reinforcement just four chemical targets account for the majority of crosslinking and chemical interaction, these targets are as follows:

  1. Primary amines ([BOND]NH2): This group exists at the N-terminus of each polypeptide chain and in the side chain of lysine (Lys) residues.
  2. Carboxyl ([BOND]COOH): This group exists in the C-terminus of each polypeptide chain and in the side chains of aspartic acid (Asp) and glutamic acid (Glu).
  3. Sulfhydryl ([BOND]SH): This group exists in the side chain of cysteine (Cys). Often, as part of a protein's secondary or tertiary structure, cysteine is joined together between their side chains via disulfide bonds ([BOND]S[BOND]S[BOND]), and
  4. Carbonyls (RCHO): These aldehyde groups can be created by oxidizing carbohydrate groups in glycoprotein.

Chemical and Physical Treatment

Interaction between natural and synthetic polymer and in-between natural polymer is created by the use of chemical agents and it is generally known as crosslinking agents. The aldehyde such as formaldehyde and glutaraldehyde has widely used as crosslinking agents to get better film characteristics. Formaldehyde is the common crosslinking agents and it interact with the amino acids of peptide chain such as tryptophan, tyrosine, histidine, arginine, and cysteine, amino acids. Glutaraldehyde is more specific than formaldehyde and it can interact with histidine, cysteine, tyrosine, and lysine. The effect of aldehyde as a crosslinker on the properties of glutenin rich films have been studied by Hernandez-Munoz et al. [58]. They mentioned that the value of water vapor permeability of gluten rich films declines by around 30% when crosslinking agents such as glutaraldehyde, glyoxal, and formaldehyde has incorporated. Their results also describe that the formaldehyde gives higher tensile strength values followed by glutaraldehyde and glyoxal. However, aldehydes also have a major disadvantage that is their toxicity. This must be taken into explanation when synthesizing biodegradable materials. Because of the toxicity of aldehyde many researchers have been trying to use the natural crosslinking agents to improve the film properties. The study on the effects of natural crosslinking agents (tannins and gallic acid) on the characteristics of thermo molded films generated from sunflower protein isolate, elucidate the incorporation of tannins and gallic acid in films gives higher tensile strength than for control films [59]. The Chestnut and Tara tannin gave the largest gain in tensile strength from 2.8 to 4.2 MPa and 4.4 MPa for 3.5% and 6% of tannin respectively, but inferior than those films obtained with an aldehyde. This may possibly because they act through weak connections rather than covalent bonds in the case of aldehyde. The addition of 1.5% glutaraldehyde increased the tensile strength of the film from 2.8 to 5.2 MPa with no loss of elongation [59].

Irradiation is a physical treatment, which is used to induce modification in protein molecule [60]. It has found to be an efficient method for the enhancement of barrier (water and gas) and mechanical (strength) properties of protein based edible films. Proteins are affected by irradiation treatment (Fig. 1) either by causing oxidation of amino acids, changes in amino acid conformation, split, or development of covalent bonds, and by the formation of protein free radicals [36]. In film forming solutions of protein, super-oxide and hydroxyl anion radicals are generated during the radiation. These anion radicals have the ability to modify the molecular properties of proteins. Lacroix et al. reported that γ-irradiation was efficient in inducing crosslinks in whey, casein, and soya protein edible films [61]. It is observed that the puncture strength of γ-irradiated whey protein film has increased by 20% to 50% as compared with pure calcium caseinate film, suggested favorable interactions between whey protein and calcium caseinate [61]. The soy protein isolates film's puncture strength have 37% higher than the nonirradiated film [61]. The effect of γ-irradiation on gluten film [62] and has been investigated and the result indicates that irradiation treatment increases its tensile strength from 2.68 to 3.99 MPa and decreases its water vapor permeability by 29% as compared to nonirradiated sample. Irradiation treatment may be a useful as a crosslinking agent to improve protein film properties.

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Figure 1. The NHS Diaxirine ester is a hetero-bifunctional crosslinker, which bind with primary amines of protein A and make diazirine-protein complex. This complex again interacts with protein B when UV light exposed to them. Source: http://www.piercenet.com/browse.cfm?fldID=F3324640-A85B-7AB2-CBB8-CFD7065F70C6.

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Chemical Block Copolymerization

Over the years, block copolymers have attracted a great deal of interest because they offer a unique platform to develop material for diverse applications such as for drug delivery, tissue engineering, and for food packaging. Block structures are placed into various formats such as homo-block polymers, hetero-block polymers, and hybrid block copolymers, which can made-up of both natural and synthetic materials (Fig. 2). Combining polypeptide blocks with synthetic materials create hybrid block copolymers with striking functional attributes that is the solubility, rubber elasticity, melt process ability of the synthetic block, and the structure formation, mutual recognition, and biodegradability from the peptide block [53, 63]. Furthermore, advances have been made in both the control of the assembly and function of homo-di-block copolypeptides. Hybrid block copolymers exhibited both photo-activity and electro-activity, suggesting applications in the field of biosensors, tissue engineering, and nanoelectronic [64, 65]. In addition, hybrid block copolymers containing pegylated peptides that respond to specific cellular signals, such as the adhesion and migration of endothelial cells, have been developed. The preparation of intelligent polymeric micelles of functional polyethylene glycol-poly amino acid (PEG-PAA) block [66] appear to be superior for both controlled drug release and targeted delivery features with reduced toxicity and improved efficacy significantly [66]. A few of examples of block copolymer with polypeptide are shown in Table 1.

Table 1. Instance applications of protein based block copolymers.
S. no.Block copolymerFeatureReferences
1.Peptide-synthetic hybrid blocks copolymers. (polyisoprene-b-poly(epsilon-benzyloxycarbonyl-L-lysine) PI-b-PZLys and polyisoprene-b-poly(L-lysine) PI-b-PLys block copolymers)Self assembled rod-coil copolymer nanostructures[67]
2.Poly (ethylene glycol)-poly (amino acid) block copolymersThe polymeric micelles feature a spherical 100 nm core shell structure in which anticancer drugs are loaded avoiding undesirable interactions in vivo[66]
3.PEG/Peptide block copolymersIts self assembly process[68]
4.Poly (ethylene oxide) -block-peptide block copolymersAs for drug delivery, nontoxic, nonimmunogenic, controlled-release systems for hydrophobic drugs[69]
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Figure 2. Block copolymerization of peptide and synthetic polymer [53]. Synthetic polymer N-Carboxyanhydrides (NCA) bind with protein molecule by interaction of primary amines ([BOND]NH2). By repeating of the above interaction long polymer chain formed.

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Preparation of Blends

A polymer blend is a mixture of materials in which at least two different polymers are blended together to create a new material with different physical properties. Some of the proteins such as keratin [6], gluten [70], milk protein [71], zein [72], soy [50], silk [73], and jatropha protein [74], etc., are well described by the researchers and conclusions drawn from them are clearly depicted that the composites made up of such proteins have a promising future for natural polymer development. Protein such as keratin and silk fibroin show high hydrophobic activity than cellulose agriculture fibers and they have been used for development of composite with synthetic polymer like polyethylene [6, 48, 75]. This kind of composites is chemically compatible with the hydrophobic polyethylene. Barone and Schmidt [21] have been reported glass transition temperature (Tg) and crystalline melting temperatures (Tm) of thermoplastic like polyethylene and polypropylene become improved when they are blended with keratin [21, 75]. The scanning electron microscope (SEM) image (Fig. 3) of keratin and low-density polyethylene (LDPE) blend demonstrate the fiber/polymer interface. A quantity of matrix deformation occurs together with the fibers as the fibers are pulled. A few of the fibers are fractured in the identical fracture plane as the polymer, which would point towards strong fiber/polymer interactions [21].

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Figure 3. SEM images of (a) 10 wt% and (b) 40 wt% 0.1 cm keratin feather fiber in LD133A LDPE. The scale bar is 30 μm (Reproduced from Ref. [21], with permission from Elsevier).

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Blends of keratin and chitosan have been developed, which is used as a biomaterial since these proteins/carbohydrate composite possess biocompatibility and various biological functions such as wound healing and antibacterial activity [16]. Chitosan gave strength and flexibility to the keratin film. Keratin-chitosan composite film had a softness judging from Young's modulus and composite film showed remarkably improver water insoluble characteristics. However, the film prepared from keratin mixed with 10 wt% chitosan have fairly flexible and strong judged from ultimate strength (27 ± 8 MPa), ultimate elongation (47 ± 2%), and Young's modulus (152 ± 76 MPa) [16]. Yoo and Krochta [46] studied the barrier, tensile, thermal, and transparency properties of whey protein and polysaccharide blend. They found that the blended films are intermediate to properties of the pure polysaccharide and whey protein film depends on the particular polysaccharide. In the case of methyl-cellulose or hydroxypropyl-methyl-cellulose with whey protein, the blended films reflect tensile strength, and lower oxygen permeability [46]. The blending technology gives an opportunity to develop blends of protein polymer, which have different physical and mechanical properties, such as soy protein-agar blend film [50], of keratin-PEO blend nanofibers [26], polylacticacid-keratin fibrous [76], keratin-chitosan composite film [16] etc. Some of the examples of protein blends are shown in Table 2.

Table 2. Blend of proteins with natural and synthetic polymer with their improved properties.
S. no.Blend nameImproved property of the film of nanofiberReferences
1.Carboxymethyl-cellulose/Soy proteinEnhanced tensile strength from 42.0 to 59.2 Mpa[3]
2.Keratin/Chitosan composite filmStrong and flexible film, judged from ultimate strength 27–34 MPa, ultimate elongation 4–9% in the presence of 10–30 wt% of chitosan.[16]
3.Keratin/PEO blend nanofibersKeratin/PEO blend at 30:70 ratio give relatively high tensile strength, judged from young modulus 31 MPa, stress at break 6 MPa and strain at break 46.3 MPa.[26]
4.Whey protein/cellulose blendFilms show the better water insoluble property. Over 98% of the recovery of blend film have been observed when treated at 100°C/30 min and over 99% recovery observed when treated at 37°C/24 h[77]
5.Soy protein/Agar blendThe tensile strength of the film raised from 4.1 to 24.6 MPa[50]

The SEM images of soy protein-agar blend film (Fig. 4) is a sign of that the casting film possessed homogeneous interfaces [50] and its Fourier transform infrared spectroscopy (FTIR) study (Fig. 5) reveal that the interactions existed between soy protein and agar by hydrogen bonding, and the active sites on the soy protein molecular chains may be oxygen atoms of carbonyl groups [50]. The interactions between soy protein and agar become stronger because of the enhanced blue-shift of these bands with the increase of agar. The X-ray diffraction (XRD) patterns of native agar and soy protein-agar blend films (Fig. 6) gives a peak at 18.38° and a slight shoulder at around 14°, indicating a slightly crystalline [50]. After the use of glycerol as plasticizer, the shoulder peak at around 14° weaken and a new weak peak at about 11.5° have been observed, which indicate that the structure of agar have changed and it lead to strong three-dimensional structure formation [50]. Figure 7 showed the SEM images of nanofibers produced electrospinning the keratin/PEO blend solutions. The solution containing varying wt% (90–10 wt%) of keratin, produced droplets, and bead-like defect structure, solutions with a lesser content of keratin could be electrospun without defects. Furthermore, SEM analysis exposed that the diameter distribution of keratin-rich nanofibers is narrow and the common diameter of the filaments reduced with the raise of the keratin content.

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Figure 4. SEM micrographs of the cross-sections of the molding soy protein/agar blend films (a) SA15M and (b) SA65M (Reproduced from Ref. [50], with permission from Elsevier).

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Figure 5. FTIR spectra of native agar, glycerol plasticized soy protein and agar, and the soy protein/agar blend films (Reproduced from Ref. [50], with permission from Elsevier). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Figure 6. XRD patterns of soy protein-agar blend films (Reproduced from Ref. [50], with permission from Elsevier). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Figure 7. Scanning electron micrographs of keratin/PEO blend nanofibers: (a) 90/10, (b) 70/30, (c) 50/50, (d) 30/70, and (e) 10/90 (Reproduced from Ref. [26], with permission from Elsevier).

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Application of Protein Based Material in Packaging

Protein based edible films are attractive for food application because of their high nutritional quality, excellent sensory properties, and good potential to adequately protect food products from their surrounding environment. Such films act as a carrier of antioxidant, flavor, and bacteriostats and can improve the quality of food products. Protein based films have received considerable attention in recent years because of its uses in edible and nonedible packaging materials. The proteins such as soybean [45], corn [44], wheat [44], peanut [78], and sunflower seed [79] etc., as well as gelatin from collagen [80] and milk proteins (casein and whey proteins) [81] are appropriate for the production of edible film because of its nutritional properties as aforementioned. Several globular proteins, including gluten [82], corn protein [44], soy protein [50], and whey protein [83] etc., have been investigated for their film properties. Protein based edible films have impressive gas barrier properties as compared with those prepared from lipids and polysaccharides [84]. When they are not moist, the O2 permeability of the soy protein film was 500, 260, 540, and 670 times lower than that of low density polyethylene, methyl-cellulose, starch, and pectin, respectively [84]. Several researchers studied the application of protein based edible films in food use [85, 86] and they reviewed the applications of protein films, such as soy protein film, casein emulsion film, whey protein film, and corn-zein films on nut and fruit products. The polymeric characteristics of the protein film have been used for edible food packaging application [45, 72, 81, 87, 88] but for nonfood packaging application the major problems are an advance of mechanical properties (such as toughness, strength, and elasticity, flexural, shear strength, tensile modulus, etc.). The step head blends of protein and nonprotein molecule have been prepared with improved mechanical properties [26, 89-92]. Massive chances still exist to create a new kind of blends with new characteristics, which could be used for both food and nonfood packaging.

Gluten Films

The cohesiveness and elasticity of gluten facilitate the film formation [43]. The gluten films are stronger and are also a good barrier to O2 and CO2 [22] but are highly permeable to water vapor and need to be made it impermeable for commercialization. Gluten has been used for coating dry roasted peanuts and fried chicken pieces [87]. Experimental results reveal that the casting films of gluten/methyl-cellulose blend containing (25 wt %) glycerol plasticizer are superior in mechanical properties of the molded composites. The wheat gluten/methyl-cellulose binary blend films show tailored mechanical and moisture barrier properties (17.3 to 33.4 wt % at 87% RH), tensile strength (1.7 to 44.0 MPa for different blend fraction), and water vapor permeability (17.86 to 30.38 10−11 gm−1 s−1 Pa−1 at 87% RH) [17]. Cationic waterborne polyurethanes (CWPU) have been prepared and blended with gluten in aqueous dispersion, this blend powders were thermally compressed molded into sheets with improved water resistance property [82]. The SEM image of gluten and the CWPU (Fig. 8) indicates that homogeneous morphology and good interfacial adhesion are beneficial and increase the impact strength of the materials. While the FTIR study (Fig. 9) demonstrate the compatibility is exist in between gluten and CWPU [82]. Parnas and coworkers [93] mentioned that the silica particles in the gluten matrix (gluten/organo-silica composites) led to mechanical strength i.e., tensile modulus 3.52 GPa, tensile strength 35.48 MPa, and elongation at break 1.03%, and similarly, coating the alumina particles with saline coupling agents (ICEOS) in the gluten matrix (gluten/Al2O3–ICEOS composite) gives the tensile modulus 3.13 GPa, tensile strength 40.49 MPa, and elongation at break 1.17% [93]. The fracture surface of composite as shows on the SEM image (Fig. 10) describe that the composites made from the two-step blending, provided better results for strength and elongation [93].

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Figure 8. SEM images of fractured gluten-CWPU blend sheets with different types and contents of CWPU (a) Pristine gluten, (b) WP-20-1, (c) WP-50-1, (d) WP-20-2 (Reproduced from Ref. [82], with permission from Wiley).

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Figure 9. FTIR spectra of powders of CWPU and two molded gluten/CWPU blends with PCL-1 diol and MDI (hard segment: 46%). (Reproduced from Ref. [82], with permission from Wiley).

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Figure 10. Fracture surfaces of composite prepared by the in situ blending at 1/1 silane/Al2O3 by mole (a) WG/Al2O3 in acetone, (b) WG/Al2O–TEOS in acetone catalyzed by TEA, (c) WG/Al2O3–MTMOS in acetone catalyzed by TEA, (d) WG/Al2O3–GTMOS in acetone catalyzed by TEA, (e) WG/Al2O3–ICEOS in acetone catalyzed by TEA (f) WG/Al–ICEOS in toluene catalyzed by Sn(Oct)2 (g) WG/Al2O3–TESBA in acetone catalyzed by TEA (Reproduced from Ref. [93], with permission from Elsevier).

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Milk Protein Films

Milk proteins (casein and whey) have excellent nutritional value and possess numerous functional properties (its solubility in water and ability to act as an emulsifier), which are important for the formation of edible films [49]. The report of Tien et al. [81] explain the effect of milk protein based edible coatings on the browning reaction of sliced apples and potatoes [81]. The results confirm that the film formulations become effective in delaying browning reactions by acting as O2 barriers on the surface of sliced apples and potatoes. Whey protein fractions (β-lactoglobulin and β-lactalbumin) and pure whey protein isolates has used successfully for film development [46] while caseinate films have used for coating in apricot, papaya, chicken eggs, apples, oranges, and for enzyme immobilization [94]. Whey protein concentrate have reported to be less permeable to water vapor as compared to caseinate based film and whey protein (fraction) based films. Also, the puncture strength of whey protein concentrates films was lowest and provided a good barrier to O2, aroma and oil at low to intermediate relative humidity [95]. Casein based films and biomaterials obtained from caseinate can find many applications in packaging [96-98], in edible films and coatings for fruits and vegetables [98-100], or in mulching films [101]. Some of the milk films formed with plasticizer are described in Table 3 with their mechanical strength.

Table 3. Comparison of tensile strength and elongation at break of the milk protein film formed in the presence of plasticizers.
FilmElongation at break (%)Tensile strength (MPa)References
Sodium caseinate/Glycerol (4:1)10.517.4–26.7[102]
Sodium caseinate/Glycerol (2:1)73.7–84.210.9–11.7
Sodium caseinate/PEG (4.54:1)5.310.9–16.35
Sodium caseinate/PEG (1.9:1)25.410.9–13.9
Whey protein/Glycerol (5.7/1)4.129.1[103, 104]
Whey protein/Glycerol (2.3/1)30.813.9
Whey protein/Sorbitol (2.3/1)1.614.0
Whey protein/Sorbitol (1/1)8.714.7

Soy Protein Films

Soy proteins are inexpensive, abundant, and biodegradable and have nutritional value as well. They have the potential to be developed as biodegradable and edible films. Protein based edible films can form bonds at diverse positions and offer high potential for forming numerous linkages. However, soy protein films still have low moisture barrier properties because of their hydrophilic property and the considerable amount of hydrophilic plasticizer used in film preparation. One broadly used method to enhance the water vapor barrier of films has been the integration of hydrophobic compounds such as lipids into the film forming solution. In addition, another way to get better the properties of soy protein film is to alter the protein network arrangement through crosslinking of the protein chains. The occurrence of reactive functional groups in the amino acid side chain of protein makes this crosslinking process achievable through enzymatic, chemical, or physical treatments. The exceptionally low O2 permeability values of soy protein isolates (SPI) films provide an opportunity for preserving foods from oxidative deterioration [45]. SPI film is used for delaying the oxidation and hydrolysis reaction of packaged lard [45]. Therefore, SPI films have potential as a packaging material, which will preserve the qualities of stored food ingredients. A succession of SPI and agar blend films containing 33% glycerol as plasticizer have fabricated and its tentative results exposed that hydrogen bonding interactions existed among soy protein and agar [50]. The tensile strength of the blend films enhanced with the incorporation of agar at different concentration. With the increase of agar, the tensile strength of the blend films raised from 4.1 to 24.6 MPa [50]. In another study, food grade carboxy-methyl-cellulose (CMC) and SPI blend fruitfully employed to fabricate novel edible films [3]. The XRD (Fig. 11) investigated reveals that SPI and CMC are extremely compatible, and the addition of glycerol reduces the crystallinity of CMC/SPI blends [3]. The FTIR study (Fig. 12) explains that complex Maillard reactions should happen between SPI and CMC. Additionally, in the SPI/CMC blends the free amino groups of SPI/CMC films decreased rapidly with the increasing CMC addition. The images of CMC/SPI film (Fig. 13), which made-up by a continuous casting method, demonstrate that it can be manufactured easily with no cracks and puncture. Overall the films have a good durability, and flexible as much as necessary to be rolled into forms for sensible applications [3].

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Figure 11. X-ray patterns of (a) pure SPI powder, (b–g) CS-5, CS-10, CS-15, CS-20, CS-25, and CS-30 CMC/SPI films, (h–l) CS-20-1, CS-20-2, CS-20-3, CS-20-4, and CS-20-5 CMC/SPI glycerol films (Reproduced from Ref. [3], with permission from Elsevier). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Figure 12. Maillard reactions tested by FTIR (a) FTIR spectra of CMC, (b) molecular structure of CMC with a degree of substitution x, (c) FTIR spectra of SPI, (d and e) a and b molecular structure of SPI, (f) FTIR spectra of CS-5, CS-10, CS-15, CS-20, CS-25, CS-30, CS-35, and CS-40 CMC/SPI blends, (g) a typical Maillard reaction between CMC and an amino acid (glutamine, Gln) (Reproduced from Ref. [3], with permission from Elsevier). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Figure 13. Photographs of CMC/SPI film in expanded and rolled states fabricated by a continuous casting method (Reproduced from Ref. [3], with permission from Elsevier). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Researchers have tried to advance the properties of soy protein films that have prospective uses in the food packaging industry [105-107]. In the United States it is estimated that, about 25,000–50,000 metric tons of soy proteins are used in paper coating industries [108]. It is found that SPI coated paper act as gas and oil barrier as well as having enough mechanical properties (strength, elasticity, etc.), for pull out the shelf life time of food commodities [109]. Rhim et al. [110] give a statement that the SPI coated paper boards grant higher water resistance than that of alginate coated paper boards. Brother and McKinney [111] reported plastics making by using soy protein and various crosslinker agents at melt state. Paetau et al. [112] reported the groundwork and processing environment for making biodegradable plastics from soy concentrate and soy isolate. Soy concentrate and isolate, as well as acid treated soy concentrate and soy isolate, were compression molded at various molding temperatures and moisture levels. Sulfuric acid, acetic acid, hydrochloric acid, and propionic acid have examined for their appropriateness for treating soy protein with regard to ending properties. The molded specimens have also tested for their percentage elongation, Young's modulus, tensile, yield strength, and water absorption. The plastics obtained by molded technique are rigid and brittle, with tensile values from 10 to 40 MPa, yield strength values from 30% to 167% weight [112]. Plastic specimens made from soy concentrate displayed similar tensile value, but greater water absorption as compared with plastics made from soy isolate.

Corn Zein Films

Zein, a component of corn, is a unique and complex material and has long been investigated for its uses for a variety of purposes other than food and feed such as coatings, inks, adhesives, and fibers, etc. [113]. Zein proteins have importance due to its ability to solubilize in binary solvent system containing water and a lower aliphatic alcohol, such as aqueous isopropanol and aqueous ethanol. Corn zein films and its coatings are used as O2 and moisture barrier for nuts, candies, and other foods [72]. They have relatively insoluble in water and forms strong glossy films resistant to grease and O2. Zein has natural resistance to bacterial attack [114], it forms tasteless coating and has stability in conditions of high humidity and high heat as well as corn zein coated paper proved more effective films for wrapping O2 sensitive foods and in regards to O2 barrier properties [72]. Performance of zein films as barrier packaging for tomatoes, cooked turkey, popcorn, and shell eggs has been evaluated by Ersus and coworkers [115] and they found that the zein film coating provided barrier effect and beneficial internal O2 composition for inhibiting microbial growth. It is valuable to conduct advanced researches on coating of intermediate moisture fruits with different coating materials or different combinations of edible films. Usage of different additives or antimicrobial agents, determination of film thickness, could be alternative research areas for improving film coating effect on intermediate moisture fruits.

Lim and Jane [116] reported that the injection molded corn starch-zein plastics at 6.6% water content and 11.5% glycerol exhibited good 4.5–5.3% elongation at break and 22 to 25 MPa tensile properties, in a molding temperature range of 150 to 160°C.

Rakotonirainy and Padua [117] used compression molding technique to obtain ply laminated zein sheets. The individual film components have been obtained by solution-casting and the pressing carried out in at 120°C. The lamination procedure induced melting and flow of the oleic acid-zein films, decreasing voids, and defects. As a result, mechanical and oxygen permeability properties improved. Pol et al. [118] took advantage of the thermoplastic properties and differences in molecular weight of soy protein and corn zein to produce a single and double coat laminates by compression molding. Compression molding can result in the development of a film based on protein with a range of barrier and mechanical properties that are dependent on the formulation and processing conditions used. Compression molding is an appropriate technology to inspect the thermoplastic properties of plasticized proteins as well as the properties of the resulting films. It can also serve as a step toward the use of a more continuous, high speed technology for film manufacture.

Gelatin Films

Gelatin is a transparent, flavorless solid colorless material, derived from collagen obtained from diverse animal by-products. It is commonly used in the pharmaceutical and food industries, and produced on a large scale at comparatively low price. On account of its functional properties it has been used for the production of biodegradable films [119]. Although like the mainstream of films based on protein, it has a partial barrier to water vapor [119]. The gelation, thermal, mechanical, and oxygen permeability properties of gelatin films have studied by Avena Bustillos et al. [120]. They described that the tensile strength, percent elongation, and puncture deformation were highest in mammalian gelatin films, followed by warm-water fish gelatin film and then by cold-water fish gelatin films [120]. Further the property of gelatin film can improve by the chemical treatment, for example films reticulated with formaldehyde and glyoxal have higher tensile strengths, opacity, lower water vapor permeability, and good color differences than the untreated films [121].

Silk Protein Films

Silk is a fibrous protein and have a range of properties like resistance towards oxidation, anti-bacterial, UV resistant, capability to absorb, and releases moisture. It can be copolymerized, blended, and crosslinked with other natural/synthetic materials with ease. The silk composites are useful to develop biomedical materials, functional membranes, fabrics, and fibers. Jiang et al. [122] developed robust, ultra-thin silk fibroin films, and characterized it by a high elastic modulus of 6–8 GPa (after treatment with methanol) and by the ultimate tensile strength up to 100 MPa. They also suggested that these films have probable applications in biocompatible implants, micro-scale bio-devices, synthetic coatings for artificial skin [122], and coating material for natural and artificial fibers, fabrics [123]. As well as such kind of silk protein materials are useful preparation of degradable shopping bags, wrapping film, and composting bags, etc. [124].

Application of Protein Based Material in Health Care

Biocompatible materials from proteins have been used to develop biological matrix or scaffolds for various biomedical applications including, tissue engineering, wound dressings, membrane filters, and drug delivery. Protein materials give a chance to develop new generation biomaterials because protein is capable of carrying out a variety of functions such as carriers for drug delivery, scaffold in tissue engineering. Protein such as silk [125], gelatin [89], and elastin [126] etc., shows good compatibility within the human body and use as a scaffold in tissue engineering with great success. Hu et al. [127] developed biomaterials from blends of silk fibroin and tropoelastin system. These blends film offers a new material system for cell support and tailored biomaterial properties to match mechanical needs such as modification, of mechanical features, with resilience from 68% to 97%, and elastic modulus between 2 to 9 Mpa, depending on the ratio of the two polymers [127]. Researchers developed homo-polymers, diblock copolymers, and triblock copolymers of the protein [128], and such block copolymers provide an extra level of control for drug delivery, and serve as novel scaffolds for tissue engineers because they provide good physical and biochemical support for both differentiated and progenitor cells [129].

The continuing developments in the protein based material have been carried out by researchers, which offer the viewpoint of substantial impact on clinical practice in surgery and regenerative medicine. Controlled drug delivery systems have become increasingly important mainly because of the awareness of the difficulties associated with a variety of old and new drugs. Biodegradable protein polymers can be used as drug delivery systems because of their biocompatibility and biodegradability [130]. The protein based natural polymers can be used in the form of micro particles, from which the incorporated drug is released to the environment in a controlled manner [131]. Advances in protein polymer science have led to the development of several novel drug delivery systems [132]. A proper consideration of surface and bulk properties can aid in the designing of protein based polymers for drug delivery applications [133]. Much of the development of novel protein based materials in controlled drug delivery is focused on the preparation and use of these responsive protein polymers with specifically designed macroscopic and microscopic structural and chemical features. The application of the protein polymers for drug delivery shows great promise when protein is immobilized in smart hydrogels [132]. It would then be possible to translate the chemical signal into the environmental signal and then into the mechanical signal, namely shrinking or swelling of smart gel. The shrinking or swelling of smart hydrogels globule in reaction by minute changes in temperature or pH can be utilized effectively to organize drug release [134]. By using such drug formulations incorporated into hydrogels, pharmaceutical companies will be able to increase the efficiency, cost effectiveness, and range of applications for existing therapeutics.

Almany and Seliktar [135] described a biosynthetic hybrid hydrogels scaffold [135] composed of a fibrinogen backbone and crosslinked with bifunctional polyethylene glycol side chains, which provides a distinct advantage over other hydrogel scaffold materials because its mechanical properties are highly malleable while the biological functionality is maintained by the backbone of the polymeric network. Koutsopoulos et al. [136] have demonstrated a gel known as a “nanofiber hydrogel scaffold,” which is composed of small protein fragments, can successfully carry and release drugs of different size, potentially enabling delivery of drugs such as insulin and herceptin [134, 137]. And they can control the rate of release by changing the density of the gel, allowing for continuous drug delivery over a specific period of time [136]. These hydrogels enables, over hours, days or even months, a gradual release of the drug from the gel, and the gel itself is eventually broken down into harmless amino acids the building blocks of proteins. Peptide hydrogels are ideally suited for drug delivery as they are pure, easy to design and use, nontoxic, nonimmunogenic, bio-absorbable, and can be locally applied to a particular tissue.

Challenges for the Development of Protein Based Biopolymer

In natural states, proteins generally exist in two form fibrous proteins and globular proteins [138]. The former form is water insoluble [139] and the later form is water soluble or soluble in aqueous solutions of acids or bases [140]. The chemical/physical properties of these proteins depend on the relative amounts of amino acid residues and their position along the protein polymer chain [83]. Films from peptide polymer are usually produced from dispersions of the protein as the solvent evaporates, and the solvent is generally limited to ethanol water mixtures, ethanol, or water. The challenge is to make possible to use some more solvents have a good quality and that can help to make a better crosslink in between protein molecules. Another challenge is to control the degree of bond formation during the chain interaction. Generally, acid, bases, heat, and/or solvents are usually used to denature the protein in order form film. Once the extended film, chain to chain interaction of peptide can attach throughout by ionic, hydrophobic, hydrogen, and by covalent bonding, and such interaction of peptide chain could be controlled by the deviation in degree of bond formation. Protein molecules are expected to have all kinds of feasibility to prepare natural polymer and it is predicted that the major challenges are to improve the physical and mechanical properties of protein polymers, so that they can mimic the function of native synthetic polymer at some extend. The polymer characteristics of the proteins have been successfully used for the formation of edible food packaging by various researchers [45, 72, 81, 87, 88]. But in nonfood packaging the major problems in the development of protein based polymer are an enhancement of protein film properties such as toughness, strength, and elasticity, flexural, shear strength, tensile modulus, etc. The step head blends of protein and nonprotein molecule have been prepared successfully with improved characteristics [26, 89-92]. Use of suitable binder/plasticizers or crosslinking agent is also a part of future challenge in respect to the enhancement of the adhesive or cohesive property of the protein based biopolymer.

CONCLUSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. CONCLUSION
  5. REFERENCES

Protein molecules are one of the biological materials among other natural polymers such as PHA, PLA, starch, and cellulose, etc., and it has been used for the development of natural polymers. Many of the proteins like silk, gelatin, keratin, soy protein, and casein, etc., have executed very interesting features of polymers such as flexural, shear strength, tensile modulus, as well as exceptional material properties including toughness, strength, and elasticity, for the creation of novel bio-polymer. However, in native form protein polymers are weak and not suitable for the product development, but this problem can be overcome by the polymer reinforcement technology. Polymer reinforcement technology offers an opportunity to change the physical and mechanical properties of protein polymer as per desired product. Such modification will be helpful to use them for various applications from micro (as biomaterial for drug delivery application) to macro scale (as material for packaging, for tissue engineering and for bio-composite). Massive chances still exist to create a new kind of blends of protein polymer with new characteristics, which could be used for both food and nonfood packaging.

REFERENCES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. CONCLUSION
  5. REFERENCES