Preparation and characterization of edible films from fish water-soluble proteins

Transcription

1 FISHERIES SCIENCE 2000; 66: Original Article Preparation and characterization of edible films from fish water-soluble proteins Kiyomi IWATA, 1 Shoichiro ISHIZAKI, 1 Akihiro HANDA 2 AND Munehiko TANAKA 1, * 1 Department of Food Science and Technology, Tokyo University of Fisheries, Minato, Tokyo and 2 R & D Division, Q.P. Corporation, Fuchu, Tokyo , Japan SUMMARY: As a means of effective utilization of fish water-soluble proteins (FWSP), which are mostly discarded into the waste water of seafood processing plants, the development and characterization of edible films from FWSP of blue marlin meat were investigated. The film-forming solutions were prepared from 3% FWSP solutions at ph 10 with 1.5% glycerol as a plasticizer, followed by heating at 70 C for 15 min. Edible films were successfully prepared by drying the film-forming solutions at 25 C for 20 h. It was revealed that FWSP had to be denatured somehow to unfold the protein structure, and the interaction of FWSP molecules, particularly through disulfide linkages, was attributed to the formation of films. Transparent edible films thus formed had better flexibility and lower water vapor permeability compared with most of the other protein films. KEY WORDS: blue marlin, edible film, fish water-soluble proteins, strength, tensile, water vapor permeability. INTRODUCTION In the food industry, chemically synthesized polymeric films are mostly used for packaging, because they are easily and inexpensively produced from uniform raw materials and are flexible as well as durable. A serious disadvantage of these films is that they are not biodegradable. On the contrary, edible films prepared from proteins, polysaccharides, and/or lipids have substantial possibilities for enhancing the stability and quality of foods and reducing food-packaging requirements. 1 3 For instance, edible films can be used for versatile food products to reduce loss of moisture, to restrict absorption of oxygen, to lessen migration of lipids, to improve mechanical handling properties, to provide physical protection, or to offer an alternative to the commercial packaging materials. 1,4,5 Components used for the preparation of edible films can be classified into three categories: hydrocolloids (such as proteins, polysaccharides, alginates), lipids (such as fatty acids, acylglycerols, waxes) and composites (made by combining substances from other categories). 6 Hydrocolloid films have good barrier properties to oxygen, carbon dioxide, and lipids, but not to water vapor. Most hydrocolloid films also *Corresponding author: Tel: Fax: mune@tokyo-u-fish.ac.jp Received 5 July possess superb mechanical properties, which are quite useful for fragile food products. It has been generally accepted that the mechanical and barrier properties of protein films are superior to those of polysaccharide films. 7 This may be because proteins consisting of about 20 different amino acids have a specific structure which confers a wider variety of functional properties, compared with polysaccharides which are mostly homopolymers. Furthermore, the chemical treatment to modify functional properties can be performed more easily on protein-based raw materials than on poysaccharide-based materials. 8 Proteins in their native states generally exist as either fibrous or globular forms. Fibrous proteins, which are water insoluble, are fully stretched and are closely associated with each other in parallel structures. Among fibrous proteins, collagen has been widely used as a source of edible films. Cuq and colleagues in France have been intensively conducting a series of studies on the development of edible films from fish myofibrillar proteins. 7,9 12 They used glycerol, sorbitol, or sucrose as a plasticizer and evaluated mechanical properties, water solubility, and water vapor permeability (WVP) of edible films prepared. The storage of film-forming solutions of fish myofibrillar proteins at 25 C for 6 h was required before casting in order to degrade myosin heavy chain. On the contrary, globular proteins generally have to be denatured in order to form more extended structures

2 Edible films from fish proteins FISHERIES SCIENCE 373 which are necessary for the film preparation. As a result, hydrogen, electrostatic, hydrophobic, and covalent bondings are consequently formed between extended protein molecules. The protein protein interaction which generates films is influenced by the sequence of amino acid residues and the degree of structure extension. Thus, edible films with a good WVP have been prepared from various types of globular proteins such as sodium caseinate, soy protein isolate, egg albumin, and wheat gliadin In contrast, large quantities of water are necessary for the processing of seafood products and a considerable amount of fish water-soluble proteins (FWSP) are discarded into the waste water. For instance, 5000 tons (dry weight) of FWSP were estimated to be discarded annually in the waste water of surimi processing plants in Japan. 17 Therefore, it is essential to recover and utilize these proteins as foodstuffs to increase their value and reduce the cost of waste water treatment. Although several methods of recovering FWSP from waste water of seafood processing plants have been investigated, the recovered proteins are mostly used as animal feeds and fertilizers because of the lack of techniques to use them as foodstuffs. This is mainly due to the fact that recovered proteins generally have poor functional properties. Furthermore, FWSP have not received as much study as other proteins for their film-forming potential. In the present study, the preparation and characterization of edible films from such underutilized FWSP were investigated. MATERIALS AND METHODS Preparation of fish water-soluble proteins Fish water-soluble proteins used in this study was prepared from the flesh of blue marlin Makaira mazara, which was obtained as a frozen block from Misaki, Kanagawa Prefecture, Japan. Fish water-soluble proteins were extracted from blue marlin flesh by homogenizing with 5 volumes of 0.1 M Tris-HCl buffer (ph 7.6). After the homogenates were centrifuged, the supernatants were collected and dialyzed against cold distilled water. Proteins in the dialyzate were freeze-dried and used as FWSP. 21 Preparation of film-forming solutions Freeze-dried FWSP powders were dissolved in distilled water at the different levels of protein content (2 4%) and the ph values of FWSP solutions were adjusted between 3 and 12. After glycerol (Wako Pure Chemical Ind. Ltd, Osaka, Japan) was added at 50% (w/w) of FWSP as a plasticizer, a light vacuum was applied to each solution to remove bubbles and the deaerated solutions were heated at temperature ranging between 55 and 90 C for up to 60 min. Film casting and drying The prepared film-forming solutions, while still warm, were cast by pipetting 4 ml onto rimmed silicone resin plate (50 50 cm) setting on a level surface and dried in a ventilated oven at 25 C for 20 h. After the water was evaporated, resulting FWSP films were manually peeled off. Transparent and easily handled films were thus formed. Six films were prepared for each solution. Film thickness Film thickness was measured using a micrometer (Dial Pipe Gauge, Peacock Co., Tokyo, Japan) to the nearest mm at five random locations around the film. Precision of the thickness measurements was ±5%. Mechanical property measurements Prior to the testing of mechanical properties, the films were conditioned for 48 h at relative humidity (RH) of 50 ± 5% and 25 ± 0.5 C (Environmental Chamber Model H110K-30DM; Seiwa Riko Co. Ltd, Tokyo, Japan). Tensile strength and percentage elongation at break were determined using a Texture Analyzer (TA.XT2 Stable Micro Systems, UK), operated according to the American Society for Testing and Materials (ASTM) standard method D ,22 ). Two rectangular strips (width, 20 mm; length, 45 mm) were cut from each FWSP film to measure mechanical properties. Initial grip separation and cross-head speed were set at 30 mm and 0.5 mm/s, respectively. Tensile strength (MPa) was calculated by dividing the maximum load (N) necessary to pull the sample film apart by the cross-sectional area (m 2 ). Average thickness of the film strip was used to estimate the cross-sectional area of the sample. Percentage elongation at break was calculated by dividing film elongation at the moment of rupture by the initial grip length of samples mutiplied by 100%. A total of eight samples were tested for each film type. Determination of surface hydrophobicity Surface hydrophobicity of film-forming solutions was determined by a hydrophobic fluorescence probe method using 8-anilino-1-naphthalene sulfonate (ANS). 23 Filmforming solutions with protein concentration of 0.01% were prepared by diluting with distilled water. Four hundred microliters of 0.04% ANS (Sigma Chemical Co., St. Louis, MO, USA) were added to 4 ml of the diluted film-forming solutions. After keeping at room

3 374 FISHERIES SCIENCE K Iwata et al. temperature for 15 min, fluorescence intensities of ANSprotein conjugates were measured using a spectrofluorometer (Model RF-1500; Shimadzu Co., Kyoto, Japan) at excitation wavelength of 365 nm and emission of 470 nm. Surface hydrophobicity was expressed as the fluorescence intensity relative to that of unheated film-forming solution (ph 10). Statistical analysis Microsoft Excel 5.0 (Microsoft Co., Redmond, WA, USA) was used for all statistical analyses. The data were analyzed with analysis of variance, and means were compared using Student s t-test. Differences were considered to be significant at P < Determination of sulfhydryl groups Total sulfhydryl (SH) groups of film-forming solutions were determined using Ellman s reagent (5,5 -dithiobis- 2-nitrobenzoic acid, DTNB). 24 Five millilitres of the diluted film-forming solutions (0.05% protein) were mixed with ml of 0.01 M DTNB and left at 4 C for 60 min. After centrifuging at 1200 g for 10 min, the absorbance of supernatant was read at 412 nm. A molar extinction coefficient of /M per cm was used for calculating mmoles of SH per g of protein. Sodium dodecylsulfate polyacrylamide gel electrophoresis Sodium dodecylsulfate polyacrylamide gel electrophoresis (SDS-PAGE) was performed according to Laemmli. 25 Film-forming solutions before and after heating as well as FWSP films were dissolved in 2% SDS-8 M urea- 20 mm Tris-HCl (ph 8.8) in the presence and absence of 2-mercaptoethanol (2-ME). A 4% stacking gel and a 5 15% gradient separating gel were employed. Gels were stained with 0.05% Coomassie brilliant blue R-250 (Sigma Chemical Co.) in isopropyl alcohol/acetic acid/water (25 : 10 : 65%, v:v:v) and were destained in isopropyl alcohol/acetic acid/water (10:7:83%, v:v:v). The gels were scanned with a dual wavelength flying spot scanning densitometer (Type CS-9300 PC; Shimadzu Co.). The standard protein mixture (Sigma Chemical Co.) ranged in molecular mass from 14.2 to 205 kda. RESULTS AND DISCUSSION The natural ph of FWSP solutions was around 6.7. The ph of film-forming solutions with 3% protein concentration was adjusted between ph 3 and 12 before heating at 70 C for 15 min. Figure 1 illustrates the influence of ph of film-forming solutions on the mechanical properties of edible FWSP films. In the ph ranging between 6.5 and 9, the film-forming solutions became opaque and FWSP were precipitated out during the subsequent heating process. Therefore, films could not be formed in this ph range. In contrast, film-forming solutions with ph below 2.5 or above 12.5 were not able to develop films because of the high viscosities. Therefore, FWSP films were obtained at ph between 3 and 6 or between 9.5 and 12. The similar behaviors were also observed for edible myofibrillar protein films from sardine. 7 In the ph range between 9.5 and 12, tensile strength decreased and elongation at break increased with increasing ph values of film-forming solutions. Usually Water vapor permeability measurements Water vapor permeability values were measured using a modified ASTM method 22 reported by Gontard et al. 26 Fish water-soluble proteins film was sealed in a glass permeation cup containing silica gel (0% RH) with silicone vacuum grease and a rubber band to hold the film in place. The cups were placed at 30 C in a desiccator cabinet with distilled water. The cups were weighed at 1 h intervals over a 12 h period and WVP (g/m per s per Pa) of the film was calculated as follows: WVP = (w x)/ A t (P 2 - P 1 ), 13 where w is the weight gain of the cup (g), x is the film thickness (m), A is the area of exposed film (m 2 ), t is the time of gain (s), and (P 2 P 1 ) is the vapor pressure differential across the film (Pa). This entire procedure was repeated twice, for a total of 14 tests one each film type. Fig. 1 Effect of ph of film-forming solutions on the mechanical properties of fish water-soluble protein films. ( ) Tensile strength (MPa); ( ) elongation at break (%). Means with different letters within each property are significantly different (P < 0.05).

4 Edible films from fish proteins FISHERIES SCIENCE 375 solutions. Elongation at break increased with increasing protein concentration. When the protein concentration of FWSP solution was 1.5%, the film formed was too thin to be peeled off. On the contrary, the formation of films from FWSP solutions with more than 4.5% protein was inhibited due to the high viscosities. These findings support those of Brault et al. 27 who studied the formation of casein films. Protein concentration for the remainder of this study was fixed at the level of 3%. The effect of heating temperature on the mechanical properties of FWSP films was examined (Fig. 3). Filmforming solutions of FWSP were prepared by adjusting the protein concentration and ph at 3% and 10, respectively, and were heated at the temperature range between 55 and 90 C for 15 min. Fish water-soluble proteins films could not be formed at the heating temperature below 50 C, which suggests that denaturation of proteins by heating is a prerequisite for developing a film from FWSP solution. This was expected, since the denaturation temperature of major fish sarcoplasmic proteins are between 50 and 55 C. 28 The maximum tensile strength was afforded by heating at 70 C, but it was assumed from the overall tendencies of tensile strength and elongation values in relation to heating temperature (Fig. 3) that practically identical films were formed from FWSP solutions by heating above 60 C. Subsequent film formation was accomplished by 70 C heating. Figure 4 shows the effect of heating time at 70 C on the mechanical properties of FWSP films. Edible films were not formed from unheated and 1 min heated filmtensile strength and percentage elongation at break of a film are inversely related. 1,2 In contrast, both tensile strength and elongation at break were minimum at ph 3. As a result, it was revealed that FWSP films have tensile strength between 3 and 5.5 MPa and elongation at break between 40 and 75%. Tensile strength of FWSP film was lower than that of polymeric materials such as polyester film (178 MPa), polyvinylidene chloride film (93 MPa), cellulose acetate film (66 MPa), myofibrillar protein film (17 MPa), and whey protein isolate film (14 MPa), but was higher than that of soy protein film (2 MPa) and wheat gluten protein film (1 MPa). 7 However, FWSP film had a higher elongation value than polyvinylidene chloride film (30%), cellulose acetate film (30%), myofibrillar protein film (23%), whey protein isolate film (31%), and soy protein film (36%). 7 Therefore, it can be concluded that, in general, tensile strength of FWSP films is slightly below and their percentage elongation is slightly above values determined with already reported protein films, suggesting that FWSP films are less mechanically resistant and more stretchable. Subsequent film formation was carried out using film-forming solutions at ph 10. The effect of FWSP concentration on the mechanical properties of FWSP films is presented in Fig. 2. Fish water-soluble proteins solutions with protein concentration between 2 and 4% (ph 10) were heated at 70 C for 15 min and films were prepared as described above. No significant difference (P > 0.05) was observed in the tensile strength of films formed from 2.5 to 4% FWSP Fig. 2 Effect of protein concentration of film-forming solutions on the mechanical properties of fish water-soluble protein films. ( ) Tensile strength (MPa); ( ) elongation at break (%). Means with different letters within each property are significantly different (P < 0.05). Fig. 3 Effect of heating temperature on the mechanical properties of fish water-soluble protein films. ( ) Tensile strength (MPa); ( ) elongation at break (%). Means with different letters within each property are significantly different (P < 0.05).

5 376 FISHERIES SCIENCE K Iwata et al. forming FWSP solutions, which could be due to the insufficient denaturation of FWSP. Films formed by heating at 70 C for more than 3 min had similar tensile strength and elongation values. An important aspect of protein texturization is the modification of globular proteins such as FWSP to produce fibrous or fiber-like structures. From the above results, the sequence of FWSP film formation can be presumed as follows. Fish water-soluble proteins are dissolved to form a film-forming solution, then denatured by heating or other means. While water evaporates, the filmforming solution increases its density and becomes insoluble due to the interaction of protein molecules. This presumption was further verified by determining surface hydrophobicity and SH groups of FWSP film-forming solutions and by SDS-PAGE patterns of FWSP films. Figure 5 shows the effect of heating time on the surface hydrophobicities of FWSP film-forming solutions determined by the ANS method. It is obvious that the surface hydrophobicities of film-forming solutions at ph 4 or 10 increased markedly with heating time up to 3 or 5 min and decreased gradually thereafter, while those at ph 12 did not change at all by the heat treatment. The increased surface hydrophobicity could be due to increased flexibility of the denatured protein molecules resulting in increased exposure of hydrophobic groups to ANS. Changes of surface SH groups of film-forming solutions during heat treatment are given in Fig. 6. The amount of SH groups increased significantly within 3 min of heating then decreased gradually when FWSP solutions were adjusted to ph 10. This tendency was quite similar to that of surface hydrophobicity shown in Fig. 5, which suggests that protein molecules in filmforming solutions (ph 10) are unfolded by heating at 70 C. On the contrary, before adjusting ph of the filmforming solutions, their surface SH group contents at ph 4 and 12 were large and almost equivalent to that at ph 10 heated for 3 min. Edible films were able to be formed from FWSP solutions at ph 12 without heating process but not from those at ph 4 and 10, indicating that FWSP solutions were denatured and unfolded by adjusting ph to 12. Surface SH groups of FWSP solutions could be attributed to the formation of new disulfide (SS) bonds. The contribution of SS bond formation between protein molecules to the edible film development was evaluated by SDS-PAGE. Figure 7 shows SDS-PAGE patterns with and without 2-ME, a SS bond-cleaving agent, of 3% filmforming solutions (ph 10) before and after heating at 70 C for 15 min along with edible FWSP films. From SDS-PAGE (with 2-ME), the overall patterns were not changed substantially during the film-forming procedure, but high molecular weight protein bands were noted on the films. In contrast, the film-forming solutions (ph 10) and film samples treated without 2-ME showed two distinct bands corresponding to around 50 and 70 kda. Fig. 4 Effect of heating time at 70 C on the mechanical properties of fish water-soluble protein films. ( ) Tensile strength (MPa); ( ) elongation at break (%). Means with different letters within each property are significantly different (P < 0.05). Fig. 5 Changes of surface hydrophobicity of film-forming solutions during heating at 70 C measured by ANS method. ( ) ph 4; ( ) ph 10; ( ) ph 12.

6 Edible films from fish proteins FISHERIES SCIENCE 377 Table 1 Films Water vapor permeability of protein films Water vapor permeability (g/m per s per Pa) Fish water-soluble proteins ph a ph a ph b Myofibrillar protein (ph 3) Soybean (ph 8.5) Rice bran ph ph Casein ph ph Polyvinylidene chloride Low-density polyethylene 2, a,b Values of fish water-soluble proteins films with different letters are significantly different at P < 0.05 using Student s t-test. Fig. 6 Changes of surface sulfhydryl groups of film-forming solutions during heating at 70 C measured by DTNB method. ( ) ph 4; ( ) ph 10; ( ) ph 12. Fig. 7 Sodium dodecylsulfate polyacrylamide gel electrophoresis patterns of film-forming solutions (ph 10) and fish water-soluble protein films with (+) and without ( ) the presence of 2-mercaptoethanol (2-ME). (a) Fish water-soluble proteins solution without ph adjustment. (b) Film-forming solution (ph 10) before heat treatment at 70 C. (c) After heat treatment at 70 C for 15 min. (d) Fish water-soluble proteins films. M1, High molecular weight standards; M2, low molecular weight standards. These bands were not detected in FWSP extracts, which suggests the formation of SS bonds takes place even during the ph adjustment. From these results together with those given in Figs 5 and 6, it can be concluded that the SS bond formation plays an important role in the development of edible FWSP films. It is well known that the enhanced polymerization of proteins by protein protein interactions results in edible films which are stronger but less flexible and less permeable to water vapors and gases. 1 Generally, protein films are good oxygen barriers but poor moisture barriers, since proteins contain a preponderance of hydrophilic groups. Table 1 lists WVP of FWSP films prepared at ph 4, 10, and 12 together with those of other typical films previously reported. No differences in WVP of FWSP films were observed between ph 4 and 10 (P > 0.05), but WVP of the films prepared at ph 12 showed slightly higher value (P < 0.05). Differences in ph of film-forming solutions did not apparently influence WVP, which is in agreement with the results reported previously. 1 3 Edible FWSP films prepared in this study had slightly better water vapor barrier capacity than protein films from soybean, 29 rice bran, 30 and casein, 31 while fish myofibrillar protein-based film 7 was more resistant to the transmission of water vapor than FWSP films (Table 1). Furthermore, WVP of edible FWSP films determined in this study was higher by approximately 102 than lowdensity polyethylene film 7 and slightly lower than polyvinylidene chloride film 30 as compared to those of typical polymeric packaging films (Table 1). Further improvement of those properties of FWSP films will be accomplished through the optimization of other parameters such as the amount and type of plasticizers. ACKNOWLEDGMENT The authors would like to thank Mr Kazushige Usui of Kanagawa Prefectural Research Institute for supplying frozen blue marlin meats.

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