Review pubs.acs.org/Biomac
Biodegradable Soy Protein Isolate-Based Materials: A Review Fei Song,* Dao-Lu Tang, Xiu-Li Wang, and Yu-Zhong Wang* Center for Degradable and Flame-Retardant Polymeric Materials (ERCPM-MoE), College of Chemistry, State Key Laboratory of Polymer Materials Engineering, National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), Sichuan University, 29 Wangjiang Road, Chengdu 610064, China ABSTRACT: Recently, there is an increasing interest of using bio-based polymers instead of conventional petroleum-based polymers to fabricate biodegradable materials. Soy protein isolate (SPI), a protein with reproducible resource, good biocompatibility, biodegradability, and processability, has a significant potential in the food industry, agriculture, bioscience, and biotechnology. Up to now, several technologies have been applied to prepare SPI-based materials with equivalent or superior physical and mechanical properties compared with petroleum-based materials. The aim of this review is focused on discussion of the advantages and limitations of native SPI as well as the bulk and surface modification strategies for SPI. Moreover, some applications of SPI-based materials, especially for food preservation and packaging technology, were discussed.
Legume proteins are a second important class of vegetable proteins of which soy protein (SP) is the most important representative.11 Glycinin (7S) and β-conglycinin (11S) are characterized to be the two major fractions of SP, 12 in which there consist of 20 different amino acids, including lysine, leucine, phenylalanine, tyrosine, aspartic and glutamic acid, etc. Now, SP is commercially available in three different forms from soybean processing plants, namely, soy flour (SF, 54% protein), soy protein concentrate (SPC, 65−72% protein), and SPI (≥90% protein).4 As a natural material, the protein content of SPI is more than those of other SP products, which makes it hold a higher film-forming ability.13 Moreover, SPI-based films are clearer, smoother, and more flexible compared to other plant protein-based films, and they have impressive gas barrier properties compared with those films prepared from lipids and polysaccharides.14 When SPI films are not moist, their O2 permeability was 500, 260, 540, and 670 times lower than that of films based on low-density polyethylene, methylcellulose, starch, and pectin, respectively.9 However, SPI films do not show satisfactory mechanical properties or water vapor barrier properties in practical applications due to the inherent hydrophilicity as well as the strong molecular interaction of natural protein, and these properties become poorer under highly humid conditions.15 A variety of modification methods, including bulk and surface modifications, have been developed to improve the drawbacks of SPI-based materials. This review describes the modification strategies of SPI that have been employed to date and also tracks recent advances in these
1. INTRODUCTION Packaging is a necessary step for preserving the organoleptic, nutritional, and hygienic characteristics of food during storage and commercialization. The wide variety of packaging films can be divided into synthetic and edible or biodegradable films.1 The petroleum-derived plastics packaging materials have been widely used nowadays; however, most of them are nonbiodegradable, difficult to recycle into direct uses largely, and generate much heat and exhaust gases when burned, which have led to serious environmental pollution and ecological problems.2,3 In addition, the price of petroleum will rise further in future for its excessive consumption and nonreproducible factor, which will influence the development of petroleumderived materials. Biopolymers derived from natural resources, which attracted lots of attention in recent years, are considered as potential substitutes for existing petroleum-based synthetic polymers owing to their low cost, easy availability from reproducible resources, and biodegradability.4 Therefore, the development of biodegradable biopolymer-based materials can not only solve the “white pollution” problem but also ease the overdependence on petroleum resource. Among biopolymers, polysaccharides are constituted of a few or even one monomer, while proteins are based on several amino acids.5 The secondary, tertiary, and quaternary structures of proteins result in various interactions and bindings differing in position, type, and energy,6 and the mechanical properties of protein-based edible films are also better than those of polysaccharide and fat-based films.7 Therefore, numerous proteins such as corn zein, wheat gluten, soy, peanut, cottonseed, sunflower, rice bran, serum albumin, egg white, collagen, gelatin, myofibrils, casein, and whey proteins have been studied as potential film-forming agents.8−10 © 2011 American Chemical Society
Received: June 30, 2011 Revised: September 4, 2011 Published: September 13, 2011 3369
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Figure 1. SEM micrographs of SPI films prepared at pH 2 (a), pH 8 (b), and pH 11 (c). Reproduced with permission from ref 24. Copyright 2006 John Wiley and Sons.
strategies and highlights how they play a crucial role in modifying the properties of SPI-based materials.
SPI solutions was obviously lower than that of the films from unheated solutions.26 Heating SP films at 80 or 95 °C for various periods of time can result in the increased TS and Hunter b color values as well as the decreased E, moisture content, and WVP of resultant films.27 In order to obtain the information on protein−protein interactions in such heattreated films, the changes in solubility and disulfide bond content of protein were studied for heat-cured SPI film.28 The solubilities of films heated at 65 °C for 6, 18, and 24 h and 80 °C for 6 h were in the range of 42−50% of the total protein, while prolonged heat treatment at 80 °C significantly decreased solubility, less than 25% when the films were heated for 24 h. Moreover, from the results that more than 95% of protein in all the heat-treated samples was dissolved in the buffers containing urea as a hydrogen-bond-disrupting agent and 2-mercaptoethanol as a disulfide bond-disrupting agent dissolved, and that SDS-PAGE patterns indicated aggregation of proteins during film formation and in heat-treated films, it can be concluded that SPI was aggregated primarily through hydrogen bonds and intermolecular disulfide bonds. Reduced pressure during curing would increase the rate of film drying and affect the level of cross-linking compared to curing at atmospheric pressure. In the case of protein films, drying conditions may influence the properties of the material because proteins can change their structure as a function of processing parameters.29 As a result of heat-curing at reduced pressure, pressure together with temperature significantly affected the moisture content, TS, and total soluble matter (TSM) of SPI films.30 During the drying period, the relative humidity (RH) could affect the drying rate of film-forming solution and further influence the type and proportion of covalent (S−S bonds) and noncovalent (hydrophobic interactions, ionic and hydrogen bonds) interactions between protein chains.31 The optimal drying conditions to obtain SPI films with good mechanical properties and low solubility were reported to be 70 °C and 30% RH for the commercial SPI purchased from Solae Company in Brazil and 60 °C and 60% RH for the SPI prepared in Mauri’s group.31 2.4. Blending. SPI materials without secondary components cannot show satisfactory physicochemical and mechanical properties for industrial application.32 That is to say that blending is probably the most useful methodology to improve properties of SPI-based materials. Up to now, SPI has been blended with different plasticizers and biodegradable polymers to achieve the desired properties. 2.4.1. Plasticizers. A great attention has been paid to thermoplastic SPI because thermomechanical processing technique is a simple and effective way to prepare full biodegradable materials.33 In order to improve the processing property of SPI as well as to overcome the brittleness of pure SPI films, plasticizers have to be added into the SPI matrix due
2. BULK MODIFICATION OF SPI 2.1. Fractionation. With respect to structure−property relationships, SPI is a complex mixture of proteins with widely different molecular properties, and so film-forming ability of each SP fractions is different. The tendency of 11S protein to form disulfide bonds is higher than that of 7S protein.16,17 The formed film from 11S showed a 2−3 times higher tensile strength (TS) than that from 7S.18 To further understand the relationship between the molecular weight and other properties of protein films, Cho and Rhee19 fractionated SPI by molecular weight with an ultrafiltration unit and investigated the effect of molecular weight of SPI fraction on moisture barrier and physical properties of films. Molecular weight variation did not influence the water vapor barrier properties of films, while the Hunter b color value of fractionated protein films decreased with the molecular weight of SPI.19 2.2. pH Value Manipulation. Protein is a kind of polyelectrolyte, and the variety of pH values will influence the association and dissociation behaviors of protein in aqueous solution. SPI was coagulated rather than dispersed in water at the pH near its isoelectric point (pH 4.5), thereby not allowing for the casting of the protein dispersions. SPI was denatured and unfolded at pH away from the isoelectric region, exposing sulfhydryl and hydrophobic groups which can self-associate and form new bonds during film formation. At extreme acidic and alkaline conditions, the strong repulsive forces of highly negative (pH > 12) or positive (pH < 1) charges were present along protein chains, which prevented protein molecules from associating and forming films.20−23 Mauri and Anon24 investigated the changes in solubility and molecular properties of SPI films prepared at different pH values (2, 8, and 11). During film formation, proteins retained their native conformation at pH 8, while were partially or extensively denatured at pH 11 and 2. The proteins at extreme pH values allowed them to readily establish chain-to-chain associations by a combination of covalent and noncovalent interactions, and films obtained at pH 2 and 11 showed denser microstructures than those formed at pH 8 (seen in Figure 1).24 In addition, SPI films prepared from pH 6 to 11 were found to have significantly higher TS, higher percentage elongation at break (E), and lower water vapor permeability (WVP) than those from pH 1 to 3.20 2.3. Heating and Reduced Pressure Treatment. Heating protein films and coating, or film-forming protein solutions, noticeably affected the film properties. Thermal treatment of proteins at alkaline pH could promote formation of intra- and intermolecular cross-links between protein molecules,25 and the WVP of the films prepared from heated 3370
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to their ability to reduce internal hydrogen bonding between protein chains as well as to increase the molecular spacing of proteins.34 The most effective plasticizers will resemble most closely in the structures of the polymers they plasticize, and the best plasticizers for SPI are compounds containing hydroxyl groups, such as water and alcohols. SPC and SPI were reported to be compression molded at various moisture levels which could affect the property of resultant SP-based plastics.35 The E was found to be increased as the water content increased in the molding material, attributing to the plasticizing effect of water in SPI matrix. EG can also be used as a plasticizer, and the effects of its content on the structure, morphology, mechanical properties, and water resistance of SPI/EG sheets were studied.33 For SPI films, glycerol is the most widely used plasticizer because of its small size and hydrophilic nature, which make it compatible with SPI molecules.36 To better understand the influence of glycerol on the physicochemical properties of SPI films, protein films were cast from 7% (w/w) SPI solution with various glycerol contents. A synergistic effect of glycerol and protein was found to affect the WVP, and the addition of glycerol and RH strongly enhanced the moisture absorption rates and permeability of SPI films.37 Moreover, other hydrophilic plasticizers were also used to improve the processability and flexibility of SP, including triethylene glycol, poly(ethylene glycol), butanediols, urea, and acetamide.38−41 Besides the studies focused on how the concentration, size and shape of plasticizers affect the properties of protein-based films,42,43 the effect of using more than one plasticizer on the properties of SPI films needs to be investigated. Cho and Rhee44 studied the effects of plasticizers composition (glycerol, sorbitol, and 1:1 mixture of glycerol and sorbitol) on moisture sorption characteristics of SPI films at three levels of plasticizer concentration (0.3, 0.5, and 0.7 g plasticizer/g SPI). It was found that the moisture sorption content as well as initial adsorption rate was higher for the films with higher glycerol content (Figure 2). Moreover, films with lower glycerol content were more sensitive to RH variation compared to those with higher glycerol content (Figure 2), whereas sorbitol concentration affected the RH region in which the sharp decrease in TS value occurred. In addition, Wan et al.36 investigated the impact of using different individual plasticizers, such as propylene glycol, poly(ethylene glycol), sorbitol, and sucrose, as well as mixtures of one of these plasticizers and glycerol in the ratio of 25:75, 50:50, 75:25, and 0:100 on water barrier and mechanical properties of SPI films. In order to improve the properties of glycerol-containing SPI films, 2,2-diphenyl-2hydroxyethanoic acid (DPHEAc) and different amounts of salicylic acid (SAA) were added.45 Transmittance of the composite films was high and intermolecular hydrogen bonding was observed between SPI and SAA, which meant of a good compatibility between SPI and SAA. The resultant composite films showed higher TS and modulus with lower water uptake after the addition of DPHEAc and SAA. The presence of hydrophilic plasticizer leads to high wateradsorbing properties of SP films and thus a strong decrease in TS after absorbing moisture. Therefore, thiodiglycol, a relatively nontoxic compound from organic wastes found worldwide, was used as a hydrophobic plasticizer of SPI. Water uptake experiments indicated that the water resistance of the thiodiglycol-plasticized SPI films was higher than that of the glycerol-plasticized SPI films.46 Besides the hydrophobic lowmolecular-weight plasticizer, the addition of a moderated
Figure 2. Effects of plasticizers on moisture sorption curves of soy protein films at various relative humidities (RH): ○, glycerol:sorbitol = 100:0; ▽, glycerol:sorbitol = 50:50; □, glycerol:sorbitol = 0:100; − · −, RH 32%; − − −, RH 52%; , RH 75%. Reproduced with permission from ref 44. Copyright 2002 Elsevier.
hydrophobic polymer is a very interesting approach to endow materials with better mechanical and water-resistance properties. Polycaprolactone triol (PCL-T) is a type of biodegradable, synthetic aliphatic polyester. From the results reported by Schmidt et al.,47 the mechanical and thermal properties and the morphology of SPI films were found to be controlled by changing the amount of PCL-T, enabling the fabrication of rigid and flexible materials. But an apparent loss in thermal stability was found for this system. Therefore, thermogravimetry and infrared spectroscopy were applied to study the influence of PCL-T on the thermal degradation properties of SPI films under a nitrogen atmosphere. The Fourier transform infrared (FTIR) spectra of gas products evolved during the thermal degradation indicated the formation of OH, CO2, NH3, and other saturated compounds, suggesting that the reaction involved simultaneous scission of the C(O)−O polyester bonds and C−N, C(O)−NH, C(O)−NH2, and −NH2 bonds of the protein.48 ε-Caprolactone (CL) is the monomer of polycaprolactone (PCL) and can perform ring-opening polymerization at high temperature after initiating with −OH, −NH2, or −COOH.49−51 Both SPI and glycerol could offer the active groups for reaction, which can construct a premise to prepare a SPI plastic with lower glycerol concentrations and higher water resistance property via in situ reactions among SPI, CL, and glycerol during processing.52 When the CL content was low (
