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Physical and mechanical properties of peanut protein films
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Lebensm.-Wiss. u.-Technol. 37 (2004) 731–738
Physical and mechanical properties of peanut protein films Chin-Chi Liua, Angela M. Tellez-Garayb, M. Elena Castell-Perezc,* a
Department of Biological Sciences, Louisiana State University, Baton Rouge, LA 70803, USA b Department of Food Science, University of Guelph, Canada c Department of Biological and Agricultural Engineering, Texas A&M University, College Station, TX 77843-2117, USA Received 5 August 2003; accepted 10 February 2004
Abstract The properties of peanut protein films were modified using physical and chemical treatments, and their effects on color, mechanical strength, water solubility and barrier to water vapor and oxygen of the films were investigated. Physical treatments consisted of heat denaturation of film-forming solution for 30 min at 60 C, 70 C, 80 C and 90 C, ultraviolet irradiation of films for up to 24 h, and three ultrasound processes of film-forming solution. Chemical treatments consisted of addition of aldehydes and anhydrides. Heat curing at 70 C, ultraviolet irradiation for 24 h, ultrasound for 10 min in a water-bath, and formaldehyde and glutaraldehyde addition caused a significant increase in the tensile strength of the films. The water vapor permeability (WVP) and oxygen permeability (OP) of the films decreased after heat denaturation and aldehyde treatment. OP also decreased with UV treatment. Heat curing was the most effective treatment, making the films stronger, more resistant to water and less permeable to water vapor and oxygen. r 2004 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. Keywords: Permeability; Mechanical strength; Food packaging; Biopolymers
1. Introduction Proteins have been studied for decades for their ability to spontaneously form primary, secondary and higher order structures that can exhibit biological function and intermolecular protein organization in tissues and organs. Driven by the awareness that care should be taken out not to exhaust the world’s natural resources and to deteriorate the environment by using nondegradable and nonrecyclable materials, there is an enormous potential for the application of natural proteins as packaging materials and coatings of the future (Cuq, Gontard, & Guilbert, 1998). The interest in the study of plant and animal protein films has increased during the past decade, and research on the properties of such films has been outlined in recent literature (Krochta, Baldwin, & Nisperos-Carriedo, 1994; Callegarin, Debeaufort, Quezada-Gallo and Voiley, 1997; Gennadios, Hanna, & Kurth, 1997; Guilbert, Cuq, & Gontard, 1997; Krochta & De *Corresponding author. Tel.: +1-979-862-7645; fax: + 1-979-8478828. Email-address: [email protected] (M.E. Castell-Perez).
Mulder-Johnston, 1997; Cuq et al., 1998; Debeaufort, Quezada-Gallo, & Voilley, 1998, 2000; Rhim, Gennadios, Fu, Weller, & Hanna, 1999; Sothornvit & Krochta, 2000; Grevellec, Marquie, Ferry, Crespy, & Vialettes, 2001; Sperling, 2001; Shaw, Monahan, O’Riordan, & Sullivan, 2002). Peanuts are of interest as a potential component of biopolymeric films because of their high protein content. The whole seed of peanut contains 45% lipid and 22– 33% protein (Ahmed & Young, 1982; Woodroof, 1983). Peanuts are an agricultural product ranked eight among the primary crops produced in the United States. The growing areas of peanuts are concentrated into three regions: the Southeast (60%), Southwest (25%), and the Virginia–Carolina region (15%). Large product losses due to kernel contamination with aflatoxins, unpredictable weather and improper processing (i.e., roasting) result in reduction of the crop value and ultimate use as feed for animals. Finding alternative uses for protein from those seeds unsuitable for human consumption would increase the value-added aspect of this crop. Only a few studies have been devoted to the film forming ability of peanut protein (Aboagye & Stanley,
0023-6438/$30.00 r 2004 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.lwt.2004.02.012
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1985; Jangchud & Chinnan, 1997, 1999a, b). It is also known that the mechanical and water vapor barrier properties of protein films are inferior to those of synthetic films (Krochta & De Mulder-Johnston, 1997). The functional properties of proteins are highly dependent on structure heterogeneity, thermal sensibility and hydrophilic behavior of proteins. Previous studies report the beneficial effects of heat curing and other physical and chemical treatments on protein-based films (Miller, Chiang, & Krochta, 1997; Mohammed, Hill, & Mitchell, 2000). Therefore, the effect of modifying treatments on the properties of peanut protein films should be investigated. The main limitation to the use of peanut protein to make films, however, is cost; and the economic viability of these films should be estimated in the future. The overall objective of this work was to assess the potential of using peanuts for development of biopolymeric films with acceptable properties for future applications in packaging. The specific objectives were: (1) To evaluate the effect of heat denaturation and ultrasound of film-forming solutions and film ultraviolet radiation on the physical and mechanical properties of cast films. (2) To establish the effect of selected chemical treatments (anhydrides and aldehydes) on the properties of the films.
2. Materials and methods Peanut protein isolate (PPI) was prepared from defatted peanut flour (0.01 g/100 g fat) using a modification from the method of alkaline extraction and acid precipitation described by Kim, Kim, and Nam (1992). The defatted peanut flour (g/100 g, DM) (5.0.070.02 protein, 12.070.02 fat, 10.070.02 crude fiber and 30.070.02 carbohydrate) was obtained by extracting 12.0 fat flour (g/100 g, DM) (Golden Peanut Co., GA, USA) twice using the hexane extraction method. The defatted peanut flour was mixed with distilled water in the ratio of 1:10, adjusting the pH to nine with 1 N NaOH, and stirring with a magnetic stirrer at medium speed for 1 h. After filtering it through a polyester screen (mesh no. 120), the filtrate was centrifuged at 10,000 rpm for 30 min. The pH of the supernatant was adjusted to 4.5 with 1 N HCl to form a precipitate and then centrifuged at 10,000 rpm for 30 min. Precipitates were washed with 400 mL distilled water several times to remove carbohydrates. The protein isolate was then placed in a rapid freezer (2871 C) while still wet. The frozen sample was placed in a Labconco Freeze Dry 5 Model 75050 (Labconco, Kansas City, MO, USA) freeze dryer unit and processed at a pressure of 1 104 mmHg for 2 days. The dried sample was placed
in a desiccator until further testing. Protein content was determined by the total Kjeldahl nitrogen method (AOAC, 1975). The fat content was determined by using a Soxtec System HT (Pertorp, Silver Spring, MD, USA) extraction with petroleum ether (Liu, 2002).
3. Preparation of film (casting) A film-forming solution was prepared by adding distilled water to the PPI (average >95 g/100 g protein) for a final 75 g/100 g protein content in the film-forming solution. The pH of the solution was adjusted to nine by adding 1 N NaOH and glycerin (Fisher Scientific, Fair Lawn, NJ, USA) as a plasticizer. The ratio of plasticizer: solution was 90:1 on a mole:mole basis (0.58 molar plasticizer concentration). This ratio was the optimum result from a previous work (Liu, 2002). The total solid content of the film-forming solution was determined using the AOAC method 16.032 (AOAC, 1994). The solution was filtered through a polyester screen (mesh #120) to remove air bubbles, and poured onto a smooth high-density polyethylene (HDPE) casting plate. The film was formed by casting and drying at 90 C for 16 h. The dried film was then peeled off from the plate after cooling. Film samples were kept in Ziploct bags in a desiccator up to 24 h at room temperature and 65% relative humidity. Films that were not modified at all were considered the control samples. Modified films were prepared using several physical and chemical methods on the film-forming solution or on the film itself. Following is a description of the treatments.
4. Physical treatments 4.1. Film-forming solutions 4.1.1. Heat denaturation of film-forming solution The protein solution was prepared after solubilizing the protein and plasticizer (90/100 g glycerin) mixture to pH 9. The solution was then heated in a beaker at temperatures of 60 C, 70 C, 80 C and 90 C for 30 min to denature the protein. The solution was cooled down to room temperature followed by final degassing as it was filtered through a polyester screen (mesh #120). 4.1.2. Ultrasound The film-forming solution was prepared inside a 125 mL flask. Exposure times were 10 and 30 min based on preliminary work (Liu, 2002). Three different ultrasound treatments were applied to the solutions: (1) directly into the flask using a Sonifier 350 cell disruptor (Branson); (2) placing flask was placed inside a tank filled with water and (3) using a bransonic cleaner 2200 (Branson). The purpose of this experiment was to
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4.2. Films 4.2.1. Ultraviolet radiation The films peeled off from the casting plate were cut into 70 mm long 35 mm wide strips, and placed in a metal cabinet. An ultraviolet lamp (Germcidal 15, 253.7 nm, General Electric, Cleveland, OH, USA) was turned on about 1 h before testing to stabilize the intensity of ultraviolet light. Five different UV treatments were tested, with the different UV radiation dosages obtained by changing exposure time (0, 2, 4, 8, 16, 24 h). After 24 h of UV treatment, films showed signs of significant burning. Therefore, 24 h was selected as the maximum allowable UV exposure time. 4.3. Chemical treatments Acetic anhydride (0.01 of 100 mL film-forming solution), succinic anhydride (0.01 of 100 mL film-forming solution), 37/100 g formaldehyde solution (0.02 of 100 mL film-forming solution), or 25/100 g glutaraldehyde solution (0.03 of 100 mL film-forming solution) were added to the film-forming solution. When anhydrides and aldehydes were incorporated, the solutions were stirred for 1 h or 20 min, respectively, at room temperature to allow for interaction between the reagent and the protein (Liu, 2002). 4.4. Film properties measurement 4.4.1. Thickness Film thickness was measured using a digital micrometer XL-750 (Brunswick Instrument, Niles, IL, USA) to 70.00254 mm. Five thickness measurements were taken at random positions along the length of the 70 mm 35 mm strip. Film strips were placed between the jaws of the micrometer and the gap reduced until the first indication of contact. The mean of five replicates was used for each specimen. The average thickness of the films was 0.2570.07 mm. 4.4.2. Color The color values of the films (70 70 mm strips) were measured with a chroma meter LABSCAN XE (Hunterlab, VA, USA). Films were placed on a white standard plate (HunterLab Color Standard No. LX16437, L ¼ 91:40; a ¼ 1:12; b ¼ 1:00), and the Hunter-lab color scale L; a; b values was used. The total color difference (DE) and chroma (C) were calculated using standards methods (Liu, 2002).
4.4.3. Mechanical strength The mechanical properties of the films under large deformations (tension mode) were measured in accordance with ASTM D882-00 (ASTM, 2000). A Texture Analyzer (TATX2, Texture Technologies Corp., NY, USA) was used. All measurements and tests were performed at constant room temperature and 65% relative humidity (2271 C and 65%). Films were kept under these conditions for up to 24 h. Initial grip separation and crosshead speed were set to 35 mm and 12 mm/min, respectively. The tested film strips were 70 mm long and 35 mm wide. Five measurements were taken from each film specimen. Force and elongation values were recorded and the values of tensile strength, percentage elongation-atbreak, Young’s modulus and toughness were calculated from the resulting stress–strain curve (also called force– deformation curve). Tensile strength was calculated by dividing the maximum load on the film before failure by the initial cross-sectional area (thickness width). Percentage elongation-at-break was calculated by measuring the maximum extension of the film between the initial and final grip separation (35 mm). Young’s modulus was calculated from the slope of the initial linear region of the force–deformation curve (at very small strain). Toughness was obtained by estimating the area underneath the stress–strain curve before it ruptured (TellezGaray, 1999). Fig. 1 shows a typical force–deformation curve with the calculation of the values of the mechanical properties for the control films. 4.4.4. Barrier properties and water solubility Water vapor permeability (WVP) and oxygen permeability (OP) of films were determined using water (MAS 100) and oxygen (MAS 500) diffusion systems (MAS Technologies, Inc., MN, USA), according to the ASTM F1770 Standard Method (ASTM, 1997). The tests were 0.6 tensile strength = 0.49 MPa
Stress (MPa)
determine which ultrasound method would be more suitable for the tested films without actually damaging the structure of the film.
733
0.4
stress
strain
area = toughness = 7.49 J
0.2 Young's modulus = 1.8 MPa elongation = 118.3% 0.0 0
50
100
150
Strain (%)
Fig. 1. Typical (stress) force (strain)–deformation curve for calculation of mechanical properties of films (control sample at room temperature, tension test).
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performed under controlled relative humidity (100% for WVP, 50% for OP) and temperatures (30 C, 37.8 C and 50 C) with three replications. A value of 101,325 Pa (1 atm) was assumed for pressure for the OP (pure oxygen on one side of the film and pure nitrogen on the other side). Film water solubility was measured using a modified version of Stuchell and Krochta (1994). Film pieces (20 mm 20 mm) were dried at 70 C in an oven for 24 h and weighed to the nearest 0.0001 g to determine their initial dry weight. Films were then immersed in 20 mL of distilled water in 50 mL screw top centrifuge tubes. The tubes were capped and placed in a shaking water-bath for 24 h at 25 C. The solution and film piece were poured onto (Whatman #1) qualitative filter paper, rinsed with 10 mL distilled water, and dried (70 C, 24 h) to determine the final dry weight of the film. Triple measurements were done for each treatment replicate. Total water soluble matter was calculated from the ratio of the difference between the initial and the final dry weights to the initial dry weight (Liu, 2002).
Fig. 2. Effect of physical and chemical treatments on color characteristics of peanut protein films (US, ultrasound; F, formaldehyde; G, glutaraldehyde; SA, succinic anhydride; AA, acetic anhydride).
difference compared with the control films. This indicates that the modifying treatments did not affect the appearance (color) of the peanut protein films.
5. Statistical analysis
6.2. Physical treatments
Measurements of each property were in five replicates for color, mechanical, and triplicates for barrier properties and water solubility. The SAS Statistical software package release 6.12 (SAS, 1996) was used to calculate analysis of variance (ANOVA) on a completely randomized design using the General Linear Models Procedure. Duncan’s multiple range tests were used to determine the significance of the properties of modified films at Po0:05:
6.2.1. Film-forming solutions Heating the film-forming solution at 60 C did not significantly (P > 0:05) change the mechanical properties of the films compared with the control film (Table 1). Films produced from the 70 C treatment were stronger and tougher as indicated by their significantly higher values of Young’s modulus and toughness. The films produced from the solutions heated at temperatures above 70 C did not show any significant differences from the control films. Heating the film-forming solution at 60 C decreased the WVP from 105.31 g cm/ m2 day mmHg (control, unheated film-forming solution) to 95.65 g cm/m2 day mmHg (Table 2). A further increase in the heating treatment temperature (up to 80 C) caused a significant decrease in WVP, while heating at 90 C increased the WVP of the film. Similar results were observed for the OP values of the modified films. In summary, heating at 70 C for 30 min yields stronger films with higher barrier properties. It is suggested that a heating process can alter the three-dimensional structure of proteins and reveal the free sulfidryl (SH) groups and hydrophobic side chains (Miller et al., 1997; Mohammed et al., 2000). Upon drying of the film-forming solution, the disulfide crosslinking binding is reformed and exposed hydrophobic residues come closer to form the intermolecular hydrophobic interaction due to the evaporation of the solvent (Chavez, Luna, & Garrote, 1994). The increased mechanical strength of the film due to the heat treatment of the film-forming solution (70–80 C) is probably due to cross-linking of the protein, thus resulting in a tighter,
6. Results and discussion 6.1. Color Formaldehyde-treated films had the lightest color (higher L ¼ 76:3270:25), followed by the acetic anhydride-treated and heat cured films, with no significant differences (P > 0:05) between the two treatments (Fig. 2). The glutaraldehyde-treated film was significantly darker in color (lower L ¼ 59:1970:82). These films also had increased ‘‘a’’ (red) and ‘‘b’’ (yellow) color values. The color of the films changed from light red, with ‘‘a’’ values of about 5 for the control films to over 13 for the glutaraldehyde-treated films, and from light yellow with ‘‘b’’ values about 30 to over 35. The yellow/ brown coloration associated with protein–aldehyde interactions is due to the various intermediate or final products of the Maillard reaction (Damodaran, 1996). Hunter-lab color values of all other physical and chemically treated films did not show any significant
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Table 1 Effect of physical treatments on the mechanical strength of peanut protein filmsa Treatment conditions b
Tensile strength (MPa)
Elongation (%)
Young’s modulus (MPa)
Toughness (J)
Control Heat curing temperature ( C) 60 70 80 90
0.5570.08acf
140.73726.63abef
1.7970.31ace
13.5472.30acf
0.6470.03a 1.2170.11b 1.1170.18b 1.2770.23b
111.92732.86a 133.45720.65a 135.65739.54a 98.26732.25a
1.8270.23a 3.2470.27b 3.1370.36b 3.3470.46b
15.6874.07a 29.9577.05b 31.8675.24b 30.2575.86b
UV exposure time (h) 2 4 8 16 24
0.4570.03c 0.4370.04c 0.5870.09c 0.7470.07d 1.0170.15e
71.20721.38c 87.11713.37cd 112.91720.41bd 116.39715.99bd 160.34739.79e
1.3970.17c 1.3470.09c 1.7470.29c 2.3270.31d 3.3870.49d
6.8271.38d 5.3871.96d 11.6371.96d 15.0372.79c 28.9076.57e
113.83738.07f 112.56739.69f 125.15754.71f 117.12733.82f 128.06747.34f 40.2272.33g
1.6170.32ef 1.3370.46ef 4.0470.41g 1.3270.23eh 1.1870.12fh 0.8870.17e
11.5674.13f 8.7473.08f 27.1379.29g 11.5873.61f 10.8174.39f 1.7870.27h
Ultrasoundc (process #, time in min) 1, 10 0.5870.04f 1, 30 0.5070.21f 2, 10 1.1270.03g 2, 30 0.5670.04f 3, 10 0.4670.10f 3, 30 0.2870.05c
a Means of five replicates7standard deviation; any two means followed by the same letter in the same column with the same treatment are not significantly (P>0.05) different by Duncan’s multiple range tests. b Control means no treatments applied. c 1=direct probe method; 2=probe with water bath method; 3=cleaning bath method.
Table 2 Effect of physical treatments on the barrier properties and water solubility of peanut protein filmsa Treatment conditions
WVPb (g cm/m2 day mmHg)
OPb ( 106 g cm/m2 day mmHg)
Solubility (%)
Controlc Heat curing temperature ( C) 60 70 80 90
105.3172.98af
22.5270.43a
49.9971.18a
95.6575.65b 45.2173.77c 50.3271.09d 70.6372.28e
14.8870.65b 8.8470.26c 9.7770.54d 13.4770.48e
— 42.7471.04b — —
105.4172.65f 105.1376.77f 105.0171.09f 105.4570.03d 105.6872.61f
15.1870.69f 11.8470.69g 9.9770.46h 9.3670.13h 6.9670.60I
— — — — 43.5671.12b
—
53.1272.01c
UV exposure time (h) 2 4 8 16 24
Ultrasoundd (process #, time in min) 2, 10 — a
Means of five replicates7standard deviation; any two means followed by the same letter in the same column with the same treatment are not significantly (P > 0:05) different by Duncan’s multiple range tests. b WVP=water vapor permeability at 37.8 C, 100% RH; OP=oxygen permeability at 37.8 C, 50% RH. c Control means no treatments applied. d Probe with water bath method.
more compact protein network. The decrease in water solubility of the films produced from heated filmforming solutions (from 49.99% (control) to 42.74%, Table 2) corresponds with an increase in hydrophobic and disulfide bindings as suggested by Damodaran (1996).
Films subjected to ultrasound waves were difficult to peel off after drying. This made it impossible to obtain film samples large enough to perform the permeability tests (85 mm 85 mm). The ultrasound treatment had a detrimental effect on the mechanical properties of the films except when applying Process 2 for 10 min. This
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treatment caused a significant (P > 0:05) increase in the properties of the films (Table 1). Upon drying, the conformational change of the film-forming solution should increase the formation of disulfide cross-linking and hydrophobic interaction. However, ultrasound treatment increased the water solubility of the film from 49.99 % (control) to 53.12% (Table 2). This could be explained by the possible dissociation of the quaternary structure, releasing smaller peptides, and facilitating their solubilization in water. Thus, improvements in the mechanical properties of the films will come at the expense of decreased film hydrophobicity (Suslick, 1998). 6.2.2. Films Tensile strength of UV-treated films increased from 0.55 MPa (control) to 1.01 MPa for the high-dose treatment (24 h) (Table 1). The ability of the film to elongate decreased significantly (P > 0:05) when exposed to UV light for up to 16 h and increased (greater than control) when treated for 24 h. Young’s modulus of the film increased from 1.79 to 3.38 MPa while the toughness increased from 13.54 to 28.90 J after 24 h of UV exposure. UV treatment did not significantly affect the WVP of the films (P > 0:05). On the other hand, OP decreased significantly with UV exposure time (Table 2). Unlike heat curing, UV treatment was applied directly to the films and not to the film-forming solution. Irradiating protein in a solid state can also facilitate cross-linking by causing conformation changes from the formation of protein-free radicals to recombine and polymerize the polymer network (Rhim et al., 1999). Reduction of water solubility (down to 43.46%), increased mechanical strength and decreased permeability (oxygen) values suggest the formation of new cross-linking, resulting in a stronger (tenser) polymer network. The tenser polymer work will then reduce the diffusion of oxygen through the film matrix. On the other hand, it seems that more extensive cross-linking is required to substantially lower the WVP of the highly hydrophilic protein film (Parris, Coffin, Joubran, & Pessen, 1995).
6.2.3. Chemical treatments Treatment with chemicals as cross-linking agents is another means of protein modification. By changing the charge of protein chains with anhydride, protein aggregation and cross-linking of the protein was expected (Means & Feeney, 1973). The acylation of peanut protein film with acetic and succinic anhydride increased the water solubility of the films (to 52.15% and 57.24%, respectively). No significant changes were found on the mechanical strength and barrier properties of these films (Tables 3 and 4). This finding suggests that these modified films could be used in development of hot water-soluble edible or nonedible packaging materials from proteins. The ability of formaldehyde and glutaraldehyde to promote covalent intermolecular and intramolecular cross-linking of peanut protein film was effective to increase the mechanical and barrier properties of the films. Tensile strength and Young’s modulus values had a threefold increase compared to the control films (Table 3). The toughness of the films more than doubled. However, these films also had a significant (P > 0:05) decrease in WVP and a slight increase in OP (Table 4). This suggests that the condensation reaction of crosslinking was successfully induced in the peanut protein (Means & Feeney, 1973). Water solubility decreased to 36.05% and 35.07% for the formaldehyde and glutaraldehyde treatments, respectively (Table 4). This is most likely due to aldehydes reacting with free amino groups of the basic amino acids, which are among the primary water-bonding sites on the protein (Means & Feeney, 1973).
7. Conclusions Properties of peanut protein films can be substantially modified by the treatments investigated in this work, thus helping tailor such films to specific applications. Increased mechanical strength, reduced water solubility and increased barrier properties are all desirable improvements. In general, physical and mechanical
Table 3 Effect of chemical treatments on the mechanical strength of peanut protein filmsa Chemical added b
Control Fc Gc SAc AAc
Tensile strength (MPa)
Elongation (%)
Young’s modulus (Mpa)
Toughness (J)
0.5570.08a 1.8570.28b 1.9870.47b 0.4870.08a 0.4270.12a
140.73726.63a 115.69745.28ab 117.25724.95ab 88.23733.22ab 74.88721.08b
1.7970.31a 5.7670.56b 6.1870.78b 1.3270.55a 1.2570.27a
13.5472.30a 34.0976.85b 36.5875.99b 11.2873.69a 12.6571.52a
a Means of five replicates7standard deviation; any two means followed by the same letter in the same column with the same treatment are not significantly (P > 0:05) different by Duncan’s multiple range tests. b Control means no treatments applied. c F, formaldehyde; G, glutaraldehyde; SA, succinic anhydride; AA, acetic anhydride.
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Table 4 Effect of chemical treatments on the barrier properties and water solubility of peanut protein filmsa Chemical added c
Control Fd Gd SAd AAd
WVPb (g cm/m2 day mmHg)
OPb ( 106 g cm/m2 day mmHg)
Solubility (%)
105.3172.98a 92.6573.59b 85.6574.56c 106.5874.85a 107.5172.19a
22.5270.43a 24.7370.85bc 25.1670.94b 22.7970.67a 23.5570.98ac
49.9971.18a 36.0571.00b 35.0770.59b 57.2471.82c 52.1572.60c
a Means of five replicates7standard deviation; any two means followed by the same letter in the same column with the same treatment are not significantly (P > 0:05) different by Duncan’s multiple range tests. b WVP=water vapor permeability at 37.8 C, 100% RH; OP=oxygen permeability at 37.8 C, 50% RH. c Control means no treatments applied. d F, formaldehyde; G, glutaraldehyde; SA, succinic anhydride; AA, acetic anhydride.
properties of the modified films were significantly modified by induced cross-linking of the protein network. Heat curing was the most effective treatment, followed by UV exposure, and treatment with aldehyde. Anhydride treatments did not cause any significant change on the properties of the films except for an increase in their water solubility. In conclusion, peanut protein could be used as a component of new biopolymeric films for packaging and other applications.
References Aboagye, Y., & Stanley, D. W. (1985). Texturization of peanut proteins by surface film formation. 1. Influence of process parameters on film forming properties. Canadian Institute Food Science Technology Journal, 18, 12–20. Ahmed, E. H., & Young, C. T. (1982). Composition, nutrition, and flavor of peanuts. In H. E. Pattee, & C. T. Young (Eds.), Peanut science and technology (pp. 655–688). Yoakum, TX: American Peanut Research and Education Society, Inc. AOAC. (1975). Method 990.03. Official methods of analysis of the association of official analytical chemists (12th ea., ed.) (p. 15.). Washington, DC: W. Horwitz. AOAC. (1994). Method 16.032. Official methods of analysis of the association of official analytical chemists (12th ea., ed.). Washington, DC: W. Horwitz. ASTM. (1997). F1770-97el. Standard test method for evaluation of solubility, diffusivity, and permeability of flexible barrier materials to water vapor. In: Annual book of ASTM standards (pp. 394–1440). Philadelphia, PA: American Society for Testing and Materials. ASTM. (2000). D882-00. Standard test method for tensile properties of thin plastic sheeting. In: Annual book of ASTM standards (pp. 165–173). Philadelphia, PA: American Society for Testing and Materials. Callegarin, F., Debeaufort, F., Quezada-Gallo, J. A., & Voilley, A. (1997). Lipids and biopackaging. Journal of American Oil Chemists Society, 74, 1183–1192. Chavez, M. S., Luna, J. A., & Garrote, R. L. (1994). Crosslinking kinetics of thermally processed alginate gels. Journal of Food Science, 59, 1108–1110, 1114. Cuq, B., Gontard, N., & Guilbert, S. (1998). Proteins as agricultural polymers for packaging production. Cereal Chemistry, 75, 1–9.
Damodaran, S. (1996). Amino acids, peptides and proteins. In O. R. Fennema (Ed.), Food chemistry (3rd ed.) (pp. 321–429). New York, NY: Marcel Dekker. Debeaufort, F., Quezada-Gallo, J. A., & Voilley, A. (1998). Edible films and coatings: tomorrow’s packaging: A review. Critical Reviews in Food Science, 38(4), 299–313. Debeaufort, F., Quezada-Gallo, J. A, & Voilley, A. (2000). Edible barriers: A solution to control water migration in foods. In S. J. Risch (Ed.), Food packaging (pp. 9–16). Washington, DC: American Chemical Society. Gennadios, A., Hanna, M. A., & Kurth, L. B. (1997). Application of edible coatings on meats, poultry and seafoods: A review. Food Science and Technology/Lebensmittel-Wissenschaft und Technologie, 30, 337–350. Grevellec, J., Marquie, C., Ferry, L., Crespy, A., & Vialettes, V. (2001). Processability of cottonseed proteins into biodegradable materials. Biomacromolecules, 2, 1104–1109. Guilbert, S., Cuq, B., & Gontard, N. (1997). Edible and biodegradable food packaging. In P. Ackermann, M. Jagerstad, & T. Ohlsson (Eds.), Food and packaging materials: chemical interactions (pp. 159–168). Cambridge, England: The Royal Society of Chemistry. Jangchud, A. & Chinnan, M. S. (1997). Development of surface and deposition films from peanut protein. In G. Narsimhan, M. R. Okos & S. Lombardo (Eds.), Advances in food engineering (pp. 168–172). Proceedings of the Fourth Conference of Food Engineering, CoFE 1995, Chicago, IL, 1995. New York: American Institute of Chemical Engineers. Jangchud, A., & Chinnan, M. S. (1999a). Peanut protein film as affected by drying temperature and pH of film forming solution. Journal of Food Science, 64(1), 153–157. Jangchud, A., & Chinnan, M. S. (1999b). Properties of peanut protein film: Sorption isotherm and plasticizer effect. Food Science and Technology/Lebensmittel-Wissenschaft und Technologie, 32, 89–94. Kim, N., Kim, J. Y., & Nam, Y. J. (1992). Characteristics and functional properties of protein isolates from various peanut cultivars. Journal of Food Science, 57, 406–410. Krochta, J. M., Baldwin, E. A., & Nisperos-Carriedo, M. (1994). Edible coatings and films to improve food quality. Lancaster, PA: Technomic Publishing. Krochta, J. M., & De Mulder-Johnston, C. (1997). Edible and biodegradable polymer films. Food Technology, 51(2), 61–74. Liu, C. C. (2002). Mechanisms to enhance functionality of peanut-based biopolymer films (205pp). Ph.D. Dissertation, Texas A&M University, College Station, Texas. Means, G. E. & Feeney, R. E. (1973). Chemical modification of proteins (254pp). San Francisco, CA: Holden-Day. Miller, K. S, Chiang, M. T., & Krochta, J. M. (1997). Heat curing of whey protein films. Journal of Food Science, 62, 1189–1193.
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Mohammed, Z. H, Hill, S. E, & Mitchell, J. R. (2000). Covalent crosslinking in heated protein systems. Journal of Food Science, 65, 221–226. Parris, N., Coffin, D. R., Joubran, R. F., & Pessen, H. (1995). Composition factors affecting the water vapor permeability and tensile properties of hydrophilic films. Journal of Agricultural and Food Chemistry, 43, 1432–1435. Rhim, J. W., Gennadios, A., Fu, D., Weller, C. L., & Hanna, M. A. (1999). Properties of ultraviolet irradiated protein films. Food Science and Technology/Lebensmittel-Wissenschaft und Technologie, 32, 129–133. SAS. (1996). SAS/STAT users guide, version 6.2. Cary, NC: SAS Scientific Institute. Shaw, N. B., Monahan, F. J., O’Riordan, E. D., & Sullivan, M. (2002). Effect of soya oil and glycerol on physical properties of composite WPI films. Journal of Food Engineering, 51, 299–304.
Sothornvit, R., & Krochta, J. M. (2000). Plasticizer effect on oxygen permeability of betalactoglobulin (b-Lg) films. Journal of Agricultural and Food Chemistry, 48(12), 6298–6302. Sperling, L. H. (2001). Introduction to physical polymer science (3rd ed.) (720pp). New York: Wiley. Stuchell, Y. M., & Krochta, J. M. (1994). Enzymatic treatments and thermal effects on edible soy protein films. Journal of Food Science, 59, 1332–1337. Suslick, K. S. (1998). Ultrasound, its chemical, physical, and biological effects (336pp). New York, NY: VCH Publishers, Inc. Tellez-Garay, A. M. (1999). Characterization of peanut-soybean films for food packaging applications (124pp). M.Sc. Thesis, Texas A&M University, College Station, TX. Woodroof, J. G. (1983). Composition and nutrition values of peanut. In J. G. Woodroof (Ed.), Peanuts, production, processing (pp. 165–179). Westport, CT: AVI Publishing.
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Physical and mechanical properties of peanut protein films Chin-Chi Liua, Angela M. Tellez-Garayb, M. Elena Castell-Perezc,* a
Department of Biological Sciences, Louisiana State University, Baton Rouge, LA 70803, USA b Department of Food Science, University of Guelph, Canada c Department of Biological and Agricultural Engineering, Texas A&M University, College Station, TX 77843-2117, USA Received 5 August 2003; accepted 10 February 2004
Abstract The properties of peanut protein films were modified using physical and chemical treatments, and their effects on color, mechanical strength, water solubility and barrier to water vapor and oxygen of the films were investigated. Physical treatments consisted of heat denaturation of film-forming solution for 30 min at 60 C, 70 C, 80 C and 90 C, ultraviolet irradiation of films for up to 24 h, and three ultrasound processes of film-forming solution. Chemical treatments consisted of addition of aldehydes and anhydrides. Heat curing at 70 C, ultraviolet irradiation for 24 h, ultrasound for 10 min in a water-bath, and formaldehyde and glutaraldehyde addition caused a significant increase in the tensile strength of the films. The water vapor permeability (WVP) and oxygen permeability (OP) of the films decreased after heat denaturation and aldehyde treatment. OP also decreased with UV treatment. Heat curing was the most effective treatment, making the films stronger, more resistant to water and less permeable to water vapor and oxygen. r 2004 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. Keywords: Permeability; Mechanical strength; Food packaging; Biopolymers
1. Introduction Proteins have been studied for decades for their ability to spontaneously form primary, secondary and higher order structures that can exhibit biological function and intermolecular protein organization in tissues and organs. Driven by the awareness that care should be taken out not to exhaust the world’s natural resources and to deteriorate the environment by using nondegradable and nonrecyclable materials, there is an enormous potential for the application of natural proteins as packaging materials and coatings of the future (Cuq, Gontard, & Guilbert, 1998). The interest in the study of plant and animal protein films has increased during the past decade, and research on the properties of such films has been outlined in recent literature (Krochta, Baldwin, & Nisperos-Carriedo, 1994; Callegarin, Debeaufort, Quezada-Gallo and Voiley, 1997; Gennadios, Hanna, & Kurth, 1997; Guilbert, Cuq, & Gontard, 1997; Krochta & De *Corresponding author. Tel.: +1-979-862-7645; fax: + 1-979-8478828. Email-address: [email protected] (M.E. Castell-Perez).
Mulder-Johnston, 1997; Cuq et al., 1998; Debeaufort, Quezada-Gallo, & Voilley, 1998, 2000; Rhim, Gennadios, Fu, Weller, & Hanna, 1999; Sothornvit & Krochta, 2000; Grevellec, Marquie, Ferry, Crespy, & Vialettes, 2001; Sperling, 2001; Shaw, Monahan, O’Riordan, & Sullivan, 2002). Peanuts are of interest as a potential component of biopolymeric films because of their high protein content. The whole seed of peanut contains 45% lipid and 22– 33% protein (Ahmed & Young, 1982; Woodroof, 1983). Peanuts are an agricultural product ranked eight among the primary crops produced in the United States. The growing areas of peanuts are concentrated into three regions: the Southeast (60%), Southwest (25%), and the Virginia–Carolina region (15%). Large product losses due to kernel contamination with aflatoxins, unpredictable weather and improper processing (i.e., roasting) result in reduction of the crop value and ultimate use as feed for animals. Finding alternative uses for protein from those seeds unsuitable for human consumption would increase the value-added aspect of this crop. Only a few studies have been devoted to the film forming ability of peanut protein (Aboagye & Stanley,
0023-6438/$30.00 r 2004 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.lwt.2004.02.012
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1985; Jangchud & Chinnan, 1997, 1999a, b). It is also known that the mechanical and water vapor barrier properties of protein films are inferior to those of synthetic films (Krochta & De Mulder-Johnston, 1997). The functional properties of proteins are highly dependent on structure heterogeneity, thermal sensibility and hydrophilic behavior of proteins. Previous studies report the beneficial effects of heat curing and other physical and chemical treatments on protein-based films (Miller, Chiang, & Krochta, 1997; Mohammed, Hill, & Mitchell, 2000). Therefore, the effect of modifying treatments on the properties of peanut protein films should be investigated. The main limitation to the use of peanut protein to make films, however, is cost; and the economic viability of these films should be estimated in the future. The overall objective of this work was to assess the potential of using peanuts for development of biopolymeric films with acceptable properties for future applications in packaging. The specific objectives were: (1) To evaluate the effect of heat denaturation and ultrasound of film-forming solutions and film ultraviolet radiation on the physical and mechanical properties of cast films. (2) To establish the effect of selected chemical treatments (anhydrides and aldehydes) on the properties of the films.
2. Materials and methods Peanut protein isolate (PPI) was prepared from defatted peanut flour (0.01 g/100 g fat) using a modification from the method of alkaline extraction and acid precipitation described by Kim, Kim, and Nam (1992). The defatted peanut flour (g/100 g, DM) (5.0.070.02 protein, 12.070.02 fat, 10.070.02 crude fiber and 30.070.02 carbohydrate) was obtained by extracting 12.0 fat flour (g/100 g, DM) (Golden Peanut Co., GA, USA) twice using the hexane extraction method. The defatted peanut flour was mixed with distilled water in the ratio of 1:10, adjusting the pH to nine with 1 N NaOH, and stirring with a magnetic stirrer at medium speed for 1 h. After filtering it through a polyester screen (mesh no. 120), the filtrate was centrifuged at 10,000 rpm for 30 min. The pH of the supernatant was adjusted to 4.5 with 1 N HCl to form a precipitate and then centrifuged at 10,000 rpm for 30 min. Precipitates were washed with 400 mL distilled water several times to remove carbohydrates. The protein isolate was then placed in a rapid freezer (2871 C) while still wet. The frozen sample was placed in a Labconco Freeze Dry 5 Model 75050 (Labconco, Kansas City, MO, USA) freeze dryer unit and processed at a pressure of 1 104 mmHg for 2 days. The dried sample was placed
in a desiccator until further testing. Protein content was determined by the total Kjeldahl nitrogen method (AOAC, 1975). The fat content was determined by using a Soxtec System HT (Pertorp, Silver Spring, MD, USA) extraction with petroleum ether (Liu, 2002).
3. Preparation of film (casting) A film-forming solution was prepared by adding distilled water to the PPI (average >95 g/100 g protein) for a final 75 g/100 g protein content in the film-forming solution. The pH of the solution was adjusted to nine by adding 1 N NaOH and glycerin (Fisher Scientific, Fair Lawn, NJ, USA) as a plasticizer. The ratio of plasticizer: solution was 90:1 on a mole:mole basis (0.58 molar plasticizer concentration). This ratio was the optimum result from a previous work (Liu, 2002). The total solid content of the film-forming solution was determined using the AOAC method 16.032 (AOAC, 1994). The solution was filtered through a polyester screen (mesh #120) to remove air bubbles, and poured onto a smooth high-density polyethylene (HDPE) casting plate. The film was formed by casting and drying at 90 C for 16 h. The dried film was then peeled off from the plate after cooling. Film samples were kept in Ziploct bags in a desiccator up to 24 h at room temperature and 65% relative humidity. Films that were not modified at all were considered the control samples. Modified films were prepared using several physical and chemical methods on the film-forming solution or on the film itself. Following is a description of the treatments.
4. Physical treatments 4.1. Film-forming solutions 4.1.1. Heat denaturation of film-forming solution The protein solution was prepared after solubilizing the protein and plasticizer (90/100 g glycerin) mixture to pH 9. The solution was then heated in a beaker at temperatures of 60 C, 70 C, 80 C and 90 C for 30 min to denature the protein. The solution was cooled down to room temperature followed by final degassing as it was filtered through a polyester screen (mesh #120). 4.1.2. Ultrasound The film-forming solution was prepared inside a 125 mL flask. Exposure times were 10 and 30 min based on preliminary work (Liu, 2002). Three different ultrasound treatments were applied to the solutions: (1) directly into the flask using a Sonifier 350 cell disruptor (Branson); (2) placing flask was placed inside a tank filled with water and (3) using a bransonic cleaner 2200 (Branson). The purpose of this experiment was to
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4.2. Films 4.2.1. Ultraviolet radiation The films peeled off from the casting plate were cut into 70 mm long 35 mm wide strips, and placed in a metal cabinet. An ultraviolet lamp (Germcidal 15, 253.7 nm, General Electric, Cleveland, OH, USA) was turned on about 1 h before testing to stabilize the intensity of ultraviolet light. Five different UV treatments were tested, with the different UV radiation dosages obtained by changing exposure time (0, 2, 4, 8, 16, 24 h). After 24 h of UV treatment, films showed signs of significant burning. Therefore, 24 h was selected as the maximum allowable UV exposure time. 4.3. Chemical treatments Acetic anhydride (0.01 of 100 mL film-forming solution), succinic anhydride (0.01 of 100 mL film-forming solution), 37/100 g formaldehyde solution (0.02 of 100 mL film-forming solution), or 25/100 g glutaraldehyde solution (0.03 of 100 mL film-forming solution) were added to the film-forming solution. When anhydrides and aldehydes were incorporated, the solutions were stirred for 1 h or 20 min, respectively, at room temperature to allow for interaction between the reagent and the protein (Liu, 2002). 4.4. Film properties measurement 4.4.1. Thickness Film thickness was measured using a digital micrometer XL-750 (Brunswick Instrument, Niles, IL, USA) to 70.00254 mm. Five thickness measurements were taken at random positions along the length of the 70 mm 35 mm strip. Film strips were placed between the jaws of the micrometer and the gap reduced until the first indication of contact. The mean of five replicates was used for each specimen. The average thickness of the films was 0.2570.07 mm. 4.4.2. Color The color values of the films (70 70 mm strips) were measured with a chroma meter LABSCAN XE (Hunterlab, VA, USA). Films were placed on a white standard plate (HunterLab Color Standard No. LX16437, L ¼ 91:40; a ¼ 1:12; b ¼ 1:00), and the Hunter-lab color scale L; a; b values was used. The total color difference (DE) and chroma (C) were calculated using standards methods (Liu, 2002).
4.4.3. Mechanical strength The mechanical properties of the films under large deformations (tension mode) were measured in accordance with ASTM D882-00 (ASTM, 2000). A Texture Analyzer (TATX2, Texture Technologies Corp., NY, USA) was used. All measurements and tests were performed at constant room temperature and 65% relative humidity (2271 C and 65%). Films were kept under these conditions for up to 24 h. Initial grip separation and crosshead speed were set to 35 mm and 12 mm/min, respectively. The tested film strips were 70 mm long and 35 mm wide. Five measurements were taken from each film specimen. Force and elongation values were recorded and the values of tensile strength, percentage elongation-atbreak, Young’s modulus and toughness were calculated from the resulting stress–strain curve (also called force– deformation curve). Tensile strength was calculated by dividing the maximum load on the film before failure by the initial cross-sectional area (thickness width). Percentage elongation-at-break was calculated by measuring the maximum extension of the film between the initial and final grip separation (35 mm). Young’s modulus was calculated from the slope of the initial linear region of the force–deformation curve (at very small strain). Toughness was obtained by estimating the area underneath the stress–strain curve before it ruptured (TellezGaray, 1999). Fig. 1 shows a typical force–deformation curve with the calculation of the values of the mechanical properties for the control films. 4.4.4. Barrier properties and water solubility Water vapor permeability (WVP) and oxygen permeability (OP) of films were determined using water (MAS 100) and oxygen (MAS 500) diffusion systems (MAS Technologies, Inc., MN, USA), according to the ASTM F1770 Standard Method (ASTM, 1997). The tests were 0.6 tensile strength = 0.49 MPa
Stress (MPa)
determine which ultrasound method would be more suitable for the tested films without actually damaging the structure of the film.
733
0.4
stress
strain
area = toughness = 7.49 J
0.2 Young's modulus = 1.8 MPa elongation = 118.3% 0.0 0
50
100
150
Strain (%)
Fig. 1. Typical (stress) force (strain)–deformation curve for calculation of mechanical properties of films (control sample at room temperature, tension test).
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performed under controlled relative humidity (100% for WVP, 50% for OP) and temperatures (30 C, 37.8 C and 50 C) with three replications. A value of 101,325 Pa (1 atm) was assumed for pressure for the OP (pure oxygen on one side of the film and pure nitrogen on the other side). Film water solubility was measured using a modified version of Stuchell and Krochta (1994). Film pieces (20 mm 20 mm) were dried at 70 C in an oven for 24 h and weighed to the nearest 0.0001 g to determine their initial dry weight. Films were then immersed in 20 mL of distilled water in 50 mL screw top centrifuge tubes. The tubes were capped and placed in a shaking water-bath for 24 h at 25 C. The solution and film piece were poured onto (Whatman #1) qualitative filter paper, rinsed with 10 mL distilled water, and dried (70 C, 24 h) to determine the final dry weight of the film. Triple measurements were done for each treatment replicate. Total water soluble matter was calculated from the ratio of the difference between the initial and the final dry weights to the initial dry weight (Liu, 2002).
Fig. 2. Effect of physical and chemical treatments on color characteristics of peanut protein films (US, ultrasound; F, formaldehyde; G, glutaraldehyde; SA, succinic anhydride; AA, acetic anhydride).
difference compared with the control films. This indicates that the modifying treatments did not affect the appearance (color) of the peanut protein films.
5. Statistical analysis
6.2. Physical treatments
Measurements of each property were in five replicates for color, mechanical, and triplicates for barrier properties and water solubility. The SAS Statistical software package release 6.12 (SAS, 1996) was used to calculate analysis of variance (ANOVA) on a completely randomized design using the General Linear Models Procedure. Duncan’s multiple range tests were used to determine the significance of the properties of modified films at Po0:05:
6.2.1. Film-forming solutions Heating the film-forming solution at 60 C did not significantly (P > 0:05) change the mechanical properties of the films compared with the control film (Table 1). Films produced from the 70 C treatment were stronger and tougher as indicated by their significantly higher values of Young’s modulus and toughness. The films produced from the solutions heated at temperatures above 70 C did not show any significant differences from the control films. Heating the film-forming solution at 60 C decreased the WVP from 105.31 g cm/ m2 day mmHg (control, unheated film-forming solution) to 95.65 g cm/m2 day mmHg (Table 2). A further increase in the heating treatment temperature (up to 80 C) caused a significant decrease in WVP, while heating at 90 C increased the WVP of the film. Similar results were observed for the OP values of the modified films. In summary, heating at 70 C for 30 min yields stronger films with higher barrier properties. It is suggested that a heating process can alter the three-dimensional structure of proteins and reveal the free sulfidryl (SH) groups and hydrophobic side chains (Miller et al., 1997; Mohammed et al., 2000). Upon drying of the film-forming solution, the disulfide crosslinking binding is reformed and exposed hydrophobic residues come closer to form the intermolecular hydrophobic interaction due to the evaporation of the solvent (Chavez, Luna, & Garrote, 1994). The increased mechanical strength of the film due to the heat treatment of the film-forming solution (70–80 C) is probably due to cross-linking of the protein, thus resulting in a tighter,
6. Results and discussion 6.1. Color Formaldehyde-treated films had the lightest color (higher L ¼ 76:3270:25), followed by the acetic anhydride-treated and heat cured films, with no significant differences (P > 0:05) between the two treatments (Fig. 2). The glutaraldehyde-treated film was significantly darker in color (lower L ¼ 59:1970:82). These films also had increased ‘‘a’’ (red) and ‘‘b’’ (yellow) color values. The color of the films changed from light red, with ‘‘a’’ values of about 5 for the control films to over 13 for the glutaraldehyde-treated films, and from light yellow with ‘‘b’’ values about 30 to over 35. The yellow/ brown coloration associated with protein–aldehyde interactions is due to the various intermediate or final products of the Maillard reaction (Damodaran, 1996). Hunter-lab color values of all other physical and chemically treated films did not show any significant
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Table 1 Effect of physical treatments on the mechanical strength of peanut protein filmsa Treatment conditions b
Tensile strength (MPa)
Elongation (%)
Young’s modulus (MPa)
Toughness (J)
Control Heat curing temperature ( C) 60 70 80 90
0.5570.08acf
140.73726.63abef
1.7970.31ace
13.5472.30acf
0.6470.03a 1.2170.11b 1.1170.18b 1.2770.23b
111.92732.86a 133.45720.65a 135.65739.54a 98.26732.25a
1.8270.23a 3.2470.27b 3.1370.36b 3.3470.46b
15.6874.07a 29.9577.05b 31.8675.24b 30.2575.86b
UV exposure time (h) 2 4 8 16 24
0.4570.03c 0.4370.04c 0.5870.09c 0.7470.07d 1.0170.15e
71.20721.38c 87.11713.37cd 112.91720.41bd 116.39715.99bd 160.34739.79e
1.3970.17c 1.3470.09c 1.7470.29c 2.3270.31d 3.3870.49d
6.8271.38d 5.3871.96d 11.6371.96d 15.0372.79c 28.9076.57e
113.83738.07f 112.56739.69f 125.15754.71f 117.12733.82f 128.06747.34f 40.2272.33g
1.6170.32ef 1.3370.46ef 4.0470.41g 1.3270.23eh 1.1870.12fh 0.8870.17e
11.5674.13f 8.7473.08f 27.1379.29g 11.5873.61f 10.8174.39f 1.7870.27h
Ultrasoundc (process #, time in min) 1, 10 0.5870.04f 1, 30 0.5070.21f 2, 10 1.1270.03g 2, 30 0.5670.04f 3, 10 0.4670.10f 3, 30 0.2870.05c
a Means of five replicates7standard deviation; any two means followed by the same letter in the same column with the same treatment are not significantly (P>0.05) different by Duncan’s multiple range tests. b Control means no treatments applied. c 1=direct probe method; 2=probe with water bath method; 3=cleaning bath method.
Table 2 Effect of physical treatments on the barrier properties and water solubility of peanut protein filmsa Treatment conditions
WVPb (g cm/m2 day mmHg)
OPb ( 106 g cm/m2 day mmHg)
Solubility (%)
Controlc Heat curing temperature ( C) 60 70 80 90
105.3172.98af
22.5270.43a
49.9971.18a
95.6575.65b 45.2173.77c 50.3271.09d 70.6372.28e
14.8870.65b 8.8470.26c 9.7770.54d 13.4770.48e
— 42.7471.04b — —
105.4172.65f 105.1376.77f 105.0171.09f 105.4570.03d 105.6872.61f
15.1870.69f 11.8470.69g 9.9770.46h 9.3670.13h 6.9670.60I
— — — — 43.5671.12b
—
53.1272.01c
UV exposure time (h) 2 4 8 16 24
Ultrasoundd (process #, time in min) 2, 10 — a
Means of five replicates7standard deviation; any two means followed by the same letter in the same column with the same treatment are not significantly (P > 0:05) different by Duncan’s multiple range tests. b WVP=water vapor permeability at 37.8 C, 100% RH; OP=oxygen permeability at 37.8 C, 50% RH. c Control means no treatments applied. d Probe with water bath method.
more compact protein network. The decrease in water solubility of the films produced from heated filmforming solutions (from 49.99% (control) to 42.74%, Table 2) corresponds with an increase in hydrophobic and disulfide bindings as suggested by Damodaran (1996).
Films subjected to ultrasound waves were difficult to peel off after drying. This made it impossible to obtain film samples large enough to perform the permeability tests (85 mm 85 mm). The ultrasound treatment had a detrimental effect on the mechanical properties of the films except when applying Process 2 for 10 min. This
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treatment caused a significant (P > 0:05) increase in the properties of the films (Table 1). Upon drying, the conformational change of the film-forming solution should increase the formation of disulfide cross-linking and hydrophobic interaction. However, ultrasound treatment increased the water solubility of the film from 49.99 % (control) to 53.12% (Table 2). This could be explained by the possible dissociation of the quaternary structure, releasing smaller peptides, and facilitating their solubilization in water. Thus, improvements in the mechanical properties of the films will come at the expense of decreased film hydrophobicity (Suslick, 1998). 6.2.2. Films Tensile strength of UV-treated films increased from 0.55 MPa (control) to 1.01 MPa for the high-dose treatment (24 h) (Table 1). The ability of the film to elongate decreased significantly (P > 0:05) when exposed to UV light for up to 16 h and increased (greater than control) when treated for 24 h. Young’s modulus of the film increased from 1.79 to 3.38 MPa while the toughness increased from 13.54 to 28.90 J after 24 h of UV exposure. UV treatment did not significantly affect the WVP of the films (P > 0:05). On the other hand, OP decreased significantly with UV exposure time (Table 2). Unlike heat curing, UV treatment was applied directly to the films and not to the film-forming solution. Irradiating protein in a solid state can also facilitate cross-linking by causing conformation changes from the formation of protein-free radicals to recombine and polymerize the polymer network (Rhim et al., 1999). Reduction of water solubility (down to 43.46%), increased mechanical strength and decreased permeability (oxygen) values suggest the formation of new cross-linking, resulting in a stronger (tenser) polymer network. The tenser polymer work will then reduce the diffusion of oxygen through the film matrix. On the other hand, it seems that more extensive cross-linking is required to substantially lower the WVP of the highly hydrophilic protein film (Parris, Coffin, Joubran, & Pessen, 1995).
6.2.3. Chemical treatments Treatment with chemicals as cross-linking agents is another means of protein modification. By changing the charge of protein chains with anhydride, protein aggregation and cross-linking of the protein was expected (Means & Feeney, 1973). The acylation of peanut protein film with acetic and succinic anhydride increased the water solubility of the films (to 52.15% and 57.24%, respectively). No significant changes were found on the mechanical strength and barrier properties of these films (Tables 3 and 4). This finding suggests that these modified films could be used in development of hot water-soluble edible or nonedible packaging materials from proteins. The ability of formaldehyde and glutaraldehyde to promote covalent intermolecular and intramolecular cross-linking of peanut protein film was effective to increase the mechanical and barrier properties of the films. Tensile strength and Young’s modulus values had a threefold increase compared to the control films (Table 3). The toughness of the films more than doubled. However, these films also had a significant (P > 0:05) decrease in WVP and a slight increase in OP (Table 4). This suggests that the condensation reaction of crosslinking was successfully induced in the peanut protein (Means & Feeney, 1973). Water solubility decreased to 36.05% and 35.07% for the formaldehyde and glutaraldehyde treatments, respectively (Table 4). This is most likely due to aldehydes reacting with free amino groups of the basic amino acids, which are among the primary water-bonding sites on the protein (Means & Feeney, 1973).
7. Conclusions Properties of peanut protein films can be substantially modified by the treatments investigated in this work, thus helping tailor such films to specific applications. Increased mechanical strength, reduced water solubility and increased barrier properties are all desirable improvements. In general, physical and mechanical
Table 3 Effect of chemical treatments on the mechanical strength of peanut protein filmsa Chemical added b
Control Fc Gc SAc AAc
Tensile strength (MPa)
Elongation (%)
Young’s modulus (Mpa)
Toughness (J)
0.5570.08a 1.8570.28b 1.9870.47b 0.4870.08a 0.4270.12a
140.73726.63a 115.69745.28ab 117.25724.95ab 88.23733.22ab 74.88721.08b
1.7970.31a 5.7670.56b 6.1870.78b 1.3270.55a 1.2570.27a
13.5472.30a 34.0976.85b 36.5875.99b 11.2873.69a 12.6571.52a
a Means of five replicates7standard deviation; any two means followed by the same letter in the same column with the same treatment are not significantly (P > 0:05) different by Duncan’s multiple range tests. b Control means no treatments applied. c F, formaldehyde; G, glutaraldehyde; SA, succinic anhydride; AA, acetic anhydride.
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Table 4 Effect of chemical treatments on the barrier properties and water solubility of peanut protein filmsa Chemical added c
Control Fd Gd SAd AAd
WVPb (g cm/m2 day mmHg)
OPb ( 106 g cm/m2 day mmHg)
Solubility (%)
105.3172.98a 92.6573.59b 85.6574.56c 106.5874.85a 107.5172.19a
22.5270.43a 24.7370.85bc 25.1670.94b 22.7970.67a 23.5570.98ac
49.9971.18a 36.0571.00b 35.0770.59b 57.2471.82c 52.1572.60c
a Means of five replicates7standard deviation; any two means followed by the same letter in the same column with the same treatment are not significantly (P > 0:05) different by Duncan’s multiple range tests. b WVP=water vapor permeability at 37.8 C, 100% RH; OP=oxygen permeability at 37.8 C, 50% RH. c Control means no treatments applied. d F, formaldehyde; G, glutaraldehyde; SA, succinic anhydride; AA, acetic anhydride.
properties of the modified films were significantly modified by induced cross-linking of the protein network. Heat curing was the most effective treatment, followed by UV exposure, and treatment with aldehyde. Anhydride treatments did not cause any significant change on the properties of the films except for an increase in their water solubility. In conclusion, peanut protein could be used as a component of new biopolymeric films for packaging and other applications.
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