Food Chemistry 179 (2015) 222–231

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The effect of curdlan on the rheological properties of restructured ribbonfish (Trichiurus spp.) meat gel Chunhua Wu a,1, Chunhong Yuan b,1, Shiguo Chen a, Donghong Liu a, Xingqian Ye a, Yaqin Hu a,⇑ a Zhejiang University, College of Biosystems Engineering and Food Science, Fuli Institute of Food Science, Zhejiang Key Laboratory for Agro-Food Processing, Zhejiang R & D Center for Food Technology and Equipment, Hangzhou 310058, China b Faculty of Fisheries, Kagoshima University, 4-50-20 Shimoarata, Kagoshima 890-0056, Japan

a r t i c l e

i n f o

Article history: Received 6 August 2014 Received in revised form 8 January 2015 Accepted 28 January 2015 Available online 4 February 2015 Keywords: Fish meat gel Thermal reversible gel Thermal irreversible gel Curdlan Rheological properties Combinative gelation models

a b s t r a c t The influence of curdlan at different levels, as well as the method of addition, on the viscoelastic characteristics of ribbonfish meat gel was investigated. From a small amplitude oscillatory shear analysis (SAOA), a variety of viscoelastic parameters were established and identified to measure the intensity of the interactions between curdlan and protein in the fish meat gel network structure. The results of water holding capacity, texture, sensory property and microstructure analyses were strongly in agreement with the rheology data, suggesting that SAOA might be an appropriate method for the industrial assessment of the quality of fish meat gel. Additionally, the recombination mechanism of the complex system formed by the fish protein and curdlan was also clarified. Compared with the irreversible curdlan gel samples, the addition of reversible curdlan gel to the fish meat gel formed a much denser cross-linked interpenetrating structure, which led to a more stable and ordered three-dimensional gel complex. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Fish meat gel, a deboned and washed fish paste, is used for the preparation of Japanese traditional gels called ‘Kamaboko’, and it has been used for the production of restructured seafood analogues and other gel-based food products (Park, 2005). Due to its convenient processing, high protein, low fat, ready-to-eat and unique texture properties, fish meat gel-based restructured seafood products are very popular globally (Iglesias-Otero, Borderías, & Tovar, 2010). In recent years however, with the overfishing of Alaska pollock (the main source of fish meat gel) and the increasing demand for fish meat gel production, the development and utilisation of lower-quality raw fish meat gels, with lower gel functionality and darker coloured material, are likely to increase (Børresen, 2008; Campo & Tovar, 2008; Shiku, Yuca Hamaguchi, Benjakul, Visessanguan, & Tanaka, 2004). Ribbonfish (Trichiurus spp.) is one of the largest migratory commercially used species on the east coast of the Pacific Ocean, with an annual output of 850,000 tons (Chiou, Chen, Wang, & Chen, 2006; Shih, Hsu, & Ni, 2011). The approximate fat and protein contents of ribbonfish are reported to be 5.9% and 18%, respectively (Hsieh, Tsai, & Jiang, 2002). The utilisation of ribbonfish is an

⇑ Corresponding author. 1

The two authors contributed equally to this manuscript.

http://dx.doi.org/10.1016/j.foodchem.2015.01.125 0308-8146/Ó 2015 Elsevier Ltd. All rights reserved.

effective way to meet the demand for raw material for fish meat gel due to its availability and low cost. However, fish meat gel prepared from ribbonfish has been largely channelled into traditional restructured product applications, due to its relatively weak gelation properties (Dileep, Shamasundar, Binsi, Badii, & Howell, 2005; Hsieh et al., 2002; Jiang, Hsieh, Ho, & Chung, 2000). Therefore, improvement of the gel-forming ability is key to the utilisation of ribbonfish as a source material for fish meat gel. A large number of researchers have investigated the effects of various hydrocolloid polysaccharides as gel binders, texture stabilisers and fat substitutes on the properties of restructured fish meat gel products (Debusca, Tahergorabi, Beamer, Matak, & Jaczynski, 2014; Pietrowski, Tahergorabi, Matak, Tou, & Jaczynski, 2011; Ramírez, Uresti, Velazquez, & Vázquez, 2011; Saha & Bhattacharya, 2010; Xiong et al., 2009). Curdlan is a neutral bacterial exopolysaccharide produced by an agrobacterium species under nitrogen-limited conditions (Gao et al., 2008). The linear molecule chain of curdlan consists entirely of repeating D-glucose units linked by b-(1 ? 3)-glucosidic bonds (Zhang & Edgar, 2014). Following its discovery by Harada, Masada, Fujimori, and Maeda (1966), it has received a great deal of attention in food and non-food fields due to its unique physicochemical properties in comparison with other commonly used polysaccharides, such as cellulose and starch (Wong, Ngiam, Kasapis, & Huang, 2010), and has also been approved by the US Food and Drug Administration (FDA) as a food additive (Food & Administration, 1996).

C. Wu et al. / Food Chemistry 179 (2015) 222–231

Although insoluble in water, curdlan has the ability to form two types of gels based on heating temperature: a low-set gel and a high-set gel. The low-set curdlan gel is a thermo-reversible gel that is formed when an aqueous dispersion is heated to between 55 and 80 °C. Its behaviour is similar to that of agar–agar and gelatin. However, when this aqueous dispersion is heated above 80 °C, it can form a thermo-irreversible gel; it is much more stable not only at low temperatures (freezing), but also at high temperatures (Tada, Matsumoto, & Masuda, 1999). In the low-set gel, there is cross-linking between the curdlan micelles, which are occupied by molecules of a single-helix through hydrogen bonds, whereas, in the high-set gel, the curdlan micelles are cross-linked by a triple-stranded helix through hydrophobic interactions (Kanzawa, Harada, Koreeda, Harada, & Okuyama, 1989). Due to the variation in the gelation mechanisms of the low- and high-set gels, the textural and functional properties of the two types of gels have been demonstrated to be quite different and have been used as key ingredients in various types of foods, such as bean curd, noodle, jellies, ice cream, and low-fat meat products (Chen et al., 2010; Saha & Bhattacharya, 2010; Wang, Chen, Sun, Wang, & Fang, 2010). Additionally, curdlan also provides a nutritional benefit as a dietary fibre and has the potential to be produced at a higher quality for related foods. Until now, there has been limited research on the application of curdlan in fish meat gel-based seafood products. Chen and Xue (2009) have described how the addition of curdlan can produce the most beneficial effects on the gelation properties of horse mackerel surimi compare to other hydrocolloids (tamarind gum, konjac, carrageenan, agar and alginate). When silver carp mince was mixed with curdlan at a concentration of 1–3%, the gel strength and hardness of the resulting gels increased (Yinghong, Yokoyama, Hayakawa, & Saito, 2003). Furthermore, in our previous work, curdlan in combination with microbial transglutaminase would enhance the gel strength, water holding capacity and the whiteness of the heated fish meat gel (Hu et al., 2014). However, no specific information on the methods of adding curdlan gel to processed fish meat gel products or the mechanisms of the restructured fish meat gel–curdlan complex system has been reported. Rheological evaluation is a useful technique for gathering information on the textural and structural characteristics of fish meat sol-based gels, and it can aid food developers in controlling the chemical interactions of food components to produce particular food structures with desired textural attributes (Park, 2005; Tabilo-Munizaga & Barbosa-Cánovas, 2005). A number of studies have investigated the viscoelastic behaviour of fish meat gel/food hydrocolloid mixed systems using rheological measurement and found that all weak viscoelastic gels responded with G0 (storage modulus)  G00 (loss modulus), indicating a solid-like behaviour of the systems. Information regarding the mechanisms of fish meat sol–gel transitions and the interaction between protein and hydrocolloids in the gel system can also be obtained (Campo-Deaño & Tovar, 2009; Chen, 2007; Iglesias-Otero et al., 2010). The purpose of this study was to evaluate the influence of curdlan on the rheological characteristic of ribbonfish (Trichiurus spp.) fish meat gels. The methods for the addition of curdlan are discussed for industrial applications. The recombination mechanism of the fish meat gel– curdlan complex system was also clarified.

2. Materials and methods 2.1. Materials Frozen ribbonfish meat was provided by the Maruha Nichiro Corporation (Tokyo, Japan). The fish meat was stored at 80 °C for future use. Food grade curdlan was provided by the Ajinomoto Group Corporation (Tokyo, Japan).

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2.2. Preparation of fish meat gel To prepare the gel samples, frozen fish meat was sealed in plastic bags, thawed using tap water, cut into small pieces and homogenised in a vacuum cut mixer for 5 min with a 2.5% edible salt (sodium chloride) (w/w, based on fish meat gel weight) solution to solubilise the myofibrillar proteins. Curdlan was added into the fish meat (as described in Section 2.3), and the mixed samples were continuously chopped at 4 °C for 30 min, and subsequently packed into stainless steel rings (diameter 3.0 cm). The final moisture of the fish meat gel–curdlan samples was adjusted to 80% using ice-water. The steel rings were sealed before the two-stage heat treatment, 35 °C for 60 min, then 90 °C for 30 min. The treated gels were cooled with iced water for 10 min. Fish meat gels without curdlan were used as controls (CS). 2.3. Methods for adding curdlan Curdlan gel was added into the fish meat gels using two different thermal methods to prepare either reversible or irreversible gels. To produce thermal reversible gels, curdlan powder was slowly added into cool deionised water while stirring constantly, and after forming an aqueous suspension, the appropriate amount of 55–60 °C deionised water was added. When the gel swelled fully, stirring was stopped and the gel was held in a thermostatic water bath at 45 °C for 7 min to obtain a thermal reversible gel. The final curdlan concentrations (based on curdlan power) added into fish meat gel were 2%, 4% and 6% (w/w, based on the fish meat gel) and were labelled RC1, RC2 and RC3, respectively. To produce thermal irreversible gels, curdlan gels were prepared using the same method as for the thermal reversible gels, except that in the last step, curdlan gels were held in a thermostatic water bath at 90 °C for 15 min. The thermal reversible/irreversible gels were crushed into small particles before adding to the fish meat gels. Thermal irreversible curdlan gel concentrations in the fish meat gel were also adjusted to 2%, 4% and 6% (w/w, based on the fish meat gel), and labelled IRC1, IRC2 and IRC3, respectively. 2.4. Texture analysis Texture analysis of the fish meat gels was conducted on a texture analyser (Model TA-XT2i, Stable Micro Systems, Surrey, England). Gels held at 4 °C were equilibrated at room temperature and cut into cylinders (2.5 cm in diameter and 2.5 cm in height) prior to the test. The breaking force (maximum penetration force, g) and deformation (penetration depth, cm) were measured using the texture analyzer equipped with a spherical probe (P/5) (5 mm diameter; 1 mm/s depression speed). The gel strength (g/ cm) was expressed as:

Gel strength ¼ Breaking force  Deformation distance

ð1Þ

Five specimens per treatment were determined. 2.5. Determination of water holding capacity Water holding capacities (WHC) of fish meat gel samples were determined according to the method of Xiong et al. (2009) with a slight modification. Briefly, fish meat gel samples were cut into small pieces (2 g, m1) and placed in a centrifuge tube (diameter 10 mm) with enough filter paper. The samples were centrifuged at 5000 rad/min for 10 min at room temperature. After centrifugation, samples were carefully separated from the papers and then immediately reweighed (m2). WHC was calculated as follows:

WHC% ¼

m2  100 m1

ð2Þ

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Each test was analyzed in triplicate and the averages were calculated. 2.6. Scanning electron microscopy (SEM) Samples for SEM were prepared and analysed according to the procedure described by Hu et al. (2014) with slight modification. Cubic samples (3  3  3mm3) obtained from fish meat gels were fixed in 2.5% (v/v) glutaraldehyde and 0.1 M phosphate buffer (pH 7.0) for 24 h at 4 °C. The fixed specimen were washed three times with 0.1 M phosphate buffer (pH 7.0) (once for 15 min), and then post-fixed with 1% osmium tetroxide for 1 h and washed. After being dehydrated by a graded series of ethanol (50%, 70%, 80%, 90%, 95% and 100%) for approximately 15–20 min for each step, the samples were transferred to the mixture of alcohol and iso-amyl acetate (v:v = 1:1) for approximately 30 min and then were transferred to pure iso-amyl acetate for about 1 h. As a final step, the treated samples were dehydrated in a Hitachi Model HCP-2 critical point dryer with liquid CO2. After coating the samples with gold–palladium, the microstructure could be observed by SEM and with the scanned images amplified to 1000 times. 2.7. Small amplitude oscillatory (SAOA) shear measurements Small deformation shear oscillatory testing was performed using a MCR 302 rotational rheometer (Anton Paar GmbH, Inc., Graz, Austria). The measurements were carried out using a parallel-plate geometry (25 mm in diameter). Gel samples were cut into disk-shaped slices of 1 mm thick and 25 mm in diameter by a cylindrical slicer. Samples were allowed to rest at room temperature for 30 min prior to testing to ensure both thermal and mechanical equilibrium at the time of measurement. A thin film of silicone oil was gently applied to the edge of each exposed sample in order to prevent moisture losses. The linear viscoelastic region (LVR) of fish meat gel samples were measured at a constant frequency of 1 Hz at 25 °C, and stress (r) varied from 10 to 3000 Pa. The critical shear stress value (rmax) and shear strain values (cmax) were determined from amplitude sweep profiles of the gels at which the complex modulus (G⁄) value were just beginning to show a noticeable deviation from the previously constant values. The range of tolerable deviation (lower than ±10%) was corroborated using creep and recovery tests. At least three repetitions of each measurement were made. Both creep and recovery of the samples was determined (Initial load r0 30 Pa for 600 s, unloaded recovery 600 s). Creep measurements were made over the linear viscoelastic range on each sample. The creep and recovery results can be described in terms of the sheer compliance function, J(t) = c(t)/r0. Compliance curves constructed at variable linear stress levels overlapped, which allowed the structural properties of samples as a function of their curdlan content to be examined and compared. J(t) data were used to obtain the relaxation modulus, G(t), which in turn allowed gel strength (S) and the relaxation exponent (n) to be determined. All measurements were measured at 25 °C. Frequency sweeps were conducted over the range 0.1–10 Hz at 0.5% strain. The complex modulus (G⁄), storage modulus (G0 ), loss modulus (G00 ) and loss factor (tan d) were all determined as functions of frequency. Data were obtained in such a way as to ensure that the resulting r in the sample would always fall within the linear viscoelastic region. 2.8. Sensory evaluation Sensory evaluation of surimi samples was conducted for colour, taste, texture, odour and overall liking by a panel of 15 semitrained tasters. A nine-point hedonic scale (1, dislike extremely;

2, dislike very much; 3, dislike moderately; 4, dislike slightly; 5, neither like nor dislike; 6, like slightly; 7, like moderately; 8, like very much; 9, like extremely) was used for sensory evaluation. The samples were served on a white paper plate at room temperature. Panelists were instructed to rinse their mouths with water before starting and between sample evaluations. 2.9. Statistical analysis Data analysis was performed using SPSS software (SPSS 20.0 for windows, SPSS Inc., Chicago, IL). The differences of means were established by their least significant difference. The level of significance was set at p < 0.05. 3. Results and discussion 3.1. WHC analysis WHC is an important parameter that is critical to the stability of fish meat gel-based restructured seafood; a high WHC is essential to bind adequate water to preserve moisture during heating processes, e.g., cooking (Wang et al., 2010). The effect of curdlan on the WHC of restructured fish meat gel is shown in Table 1. The amount of reversible curdlan gel (RC) significantly affected WHC (p < 0.05), and with the supplementary curdlan, the WHC of all gels exhibited an increasing trend, indicating that a strong network gel structure was established between the salt-soluble myofibrillar proteins (SSP) and the curdlan complex system. This increase is similar to the results observed in SSP and food hydrocolloid composite gel systems with starch, carboxymethylcellulose, locust bean gum, xanthan gum, konjac glucomannan and sodium alginate (Chen & Xue, 2009; Ramírez et al., 2011; Xiong et al., 2009). These hydrocolloids acted as an effective gelling agent for muscular protein network structures and improved the WHC of mixed systems. Regarding the addition of the thermal irreversible curdlan gel (IRC), the WHC ranged from 82% to 88% for all samples, with no significant differences (p > 0.05) among them, and the lowest WHC was in the IRC3 samples. It is possible that the higher concentration of curdlan in the IRC 3 retarded the gel formation of the fish meat. As a result, a lower WHC was obtained. These results illustrate that the combinations of fish meat gel and RC and fish meat gel and IRC may be different and that the water absorbed by the IRC-fish meat gel was not as tightly bonded as that in the RC, which strengthened the fish meat gel network. It is well known that the ability of the fish meat gel–curdlan complex gel to stabilise moisture is dependent on the conformation of SSP and curdlan in the system. Both SSP and curdlan, when heated, can form complex three-dimensional network structures resulting from intramolecular and intermolecular interaction forces, such as hydration, hydrogen bonds, hydrophobic interactions and Van der Waals forces (Chen et al., 2010). Though the mechanism of water binding in the protein– curdlan system is not yet fully understood, several previous results have provided insights into the mechanisms. The addition of curdlan could introduce more hydrogen bonds, hydrophobic interactions, etc., thus enhancing the water binding capacity in gels (Funami, Unami, Yada, & Nakao, 1999; Funami, Yada, & Nakao, 1998; Tada et al., 1999; Zhang & Edgar, 2014), when compared with the control group. With the addition of the RC gel, fully swollen curdlan molecules may mix evenly with proteins during the chopping process and reform into a three dimensional gel network structure through heating (90 °C, 15 min), increasing the crosslinking of the protein–network and resulting in more water molecules locked in the fish meat gel network system. The IRC particles only acted as ‘‘filler’’ in the fish meat gel network structure because the IRC’s thermo-stability acted as a barrier to the formation of a

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Table 1 Effect of curdlan on water holding capacity (WHC) and gel strength of fish meat gel and linear interval limit values: shear stress (rmax), shear strain (cmax) and complex modulus (G⁄) for fish meat gel samples, from stress sweep test at 1 Hz and 25 °C. Sample CS RC1 RC2 RC3 IRC1 IRC2 IRC3 a–g

WHC (%)

Gel strength (g/cm) d

84.782 ± 2.554 88.110 ± 1.616c 92.861 ± 3.727b 95.231 ± 2.710a 86.458 ± 4.051d 87.792 ± 3.900cd 85.338 ± 3.662d

d

82.728 ± 4.683 116.723 ± 5.444b 198.9138 ± 3.709a 205.278 ± 5.391a 91.908 ± 3.88c 117.010 ± 4.883b 75.700 ± 5.878e

cmax ± SD (%)

rmax (Pa) f

132.000 ± 4.000 143.33 ± 3.512ef 223.670 ± 6.506b 291.332 ± 3.215a 184.932 ± 10.631d 212.013 ± 9.000c 147.334 ± 4.512e

c

9.073 ± 0.305 8.132 ± 0.275d 8.727 ± 0.270c 6.52 ± 0.220e 14.414 ± 0.500a 12.730 ± 0.451b 14.610 ± 0.500a

(G⁄ ± SD)  103 (Pa)

Tan d

1.457 ± 0.006f 1.767 ± 0.015d 2.620 ± 0.010b 2.987 ± 0.015a 1.60 ± 0.010e 2.217 ± 0.061c 1.010 ± 0.010g

0.230 ± 0.013a 0.196 ± 0.004b 0.182 ± 0.003c 0.156 ± 0.003c 0.214 ± 0.003b 0.203 ± 0.006bc 0.238 ± 0.008a

Different letters in the same column indicate significant differences (p < 0.05).

three-dimensional gel structure. This phenomenon will be verified in the following section. 3.2. Texture analysis Gel strength is a major component in measuring the texture characteristics of fish meat gel-based seafood products. Table 1 shows the gel strength of the fish meat gels mixed with the RC or the IRC at the levels of 0%, 2%, 4%, and 6% (w/w). The addition of the RC significantly affected the textural properties of the mixtures compared with the control (p < 0.05). The highest values were obtained from the gels with 6% RC. There were significant differences in the gel strength with the RC at the 0%, 2%, and 4% levels (p < 0.05), but no obvious differences were observed between the 4% and 6% curdlan concentrations (p > 0.05). An obvious change was observed with the increase in the IRC (p < 0.05), compared with the CS, but the 6% IRC had a detrimental effect on the gel strength of the mixture compared with the control. Additionally, with equal curdlan concentrations, the fish meat gel mixed with the RC showed a higher gel strength than that of the fish meat gel–IRC mixed system. As previously mentioned, curdlan, in general, is considered to be a filler in fish meat gel, like the dietary fibre polysaccharides, used to improve the mechanical properties of fish products (Ramírez et al., 2011). When curdlan particles absorb water and expand during heating, it gives pressure to the gel complex, resulting in an increasing gel strength. Unlike other polysaccharides, curdlan can form strong 3-D gel networks via heating; this property offers a performance advantage to restructured fish meat gel and enhances its gel strength (Funami et al., 1998). However, excessive curdlan particles may prevent the cross-linking of the SSP network and disturb the formation of an orderly gel matrix, thereby decreasing gel strength. Therefore, an appropriate RC content is of great importance for restructured fish meat gel. Although texture profile analysis provides information that quantifies specific characteristics that can be directly related to the overall acceptance or hedonic ratings of food products, small deformation oscillatory measurements have been considered a necessary and useful tool to aid food developers in controlling the chemical interactions of food components to produce particular food structures with desired textural attributes, especially in food gels that present complex viscoelastic behaviour (Park, 2005; Tunick, 2010). Therefore, it was necessary to conduct a thorough rheological study to identify the properties of these gel networks. 3.3. Small deformation oscillatory analysis Small deformation oscillatory measurements were based on the assumption that the applied strain and stress were varied harmonically with time in the linear viscoelasticity regime (LVR). These were non-destructive tests that provided information on polymer

bonding characteristics and can be applied to biopolymers, such as protein networks, in food (Tunick, 2010). To determine whether or not a strain was within the LVR, a material is usually subjected to a strain sweep by shearing the material in a strain range at a fixed frequency. 3.4. Strain sweep analysis An example of the typical curve obtained for raw fish meat gel (without curdlan) strain sweep is shown in Fig. 1. The G0 values were almost independent of the strain within the LVR, and a further increase in applied stress resulted in a decrease in the G0 due to rupturing myofibrillar protein–protein interactions (Tabilo-Munizaga & Barbosa-Cánovas, 2005). As shown in Fig. 1, the G0 was larger than the G00 in the LVR, indicating that the sample behaviour was more elastic than viscous and the sample gel was a network structure (Mezger, 2006). The critical values of shear stress (rmax) and shear strain (cmax), where the gel network structure began to break down and led to a sharp decrease in the moduli, were obtained from an automatic analysis and were corroborated using creep and recovery tests because compliance curves generated at different strain levels overlap when data are collected in the LVR range. The influence of the curdlan concentrations on stress and strain amplitudes (rmax, cmax), the complex modulus (G⁄) and the tan d within the LVR range is examined (Table 1). In general, viscoelastic moduli increased with the curdlan concentration, as did the gel strength, expressed as the complex modulus (G⁄) (see Table 1). With the addition of the RC into the fish meat gel, both amplitudes (rmax, cmax) and G⁄ values significantly increased with increasing levels of curdlan (p < 0.05). This indicated that the RC molecules translated to a triple-helical structure and formed a high-set gel with hydrogen bonds and hydrophobic interactions in the protein matrix gap, increasing cross-linking within the three-dimensional fish meat gel network and resulting in a more fine, uniform structure with numerous small pores, which would most likely result in a higher absorptive capacity and better gel performance (Chen, 2007; Funami et al., 1998; Kanzawa et al., 1989; Tada et al., 1999; Zhang & Edgar, 2014). The G⁄ values of the RC samples were significantly higher than those of the IRC samples, but the tan d of those samples were lower at the same curdlan concentrations, indicating that the connectivity in the protein matrix was noticeably enhanced in the RC samples, endowing more rigidity and a more compact network in those samples. Moreover, a statistically appreciable difference in the overall rigidity of their networks (G⁄) among the IRC samples can be observed (p < 0.05) in Table 1. IRC1 and IRC2 showed higher G⁄ and rmax than IRC3, which is consistent with their higher gel strength in the texture analysis (Table 1). The tan d of the IRC samples ranged from 0.203 to 0.238, indicating a weak gel and that the connection between the IRC particles and the protein matrix may, through hydrogen bonding, form a more porous and weaker

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1000

CK

G',G"(pa)

G' G"

100

10 0.1

1

10

100

1000

strain Fig. 1. Typical Strain sweep of at angular frequency of 1 Hz of raw fish meat gel.

network with smaller junctions (Solo-de-Zaldívar, Tovar, Borderías, & Herranz, 2014). These results agreed with the WHC and texture analyses.

3.5. Frequency sweep analysis The study of the frequency dependence of G0 and G00 , within the limit of the LVR range, allowed further investigation of valuable structural information of fish meat gel (Tabilo-Munizaga & Barbosa-Cánovas, 2005), and the results can be used to provide an indication of the type of gel formed in the sample. According to the relationship between viscoelastic modulus (G0 and G00 ) and frequency, protein gels can be classified as entangled networks (of biopolymers, frequency-dependent), chemical gels (crosslinked, frequency-independent), or physical gels (non-covalent linkages, a slight frequency-dependent) (Rao, 2010; Tunick, 2010). The influence of curdlan concentration on the mechanical spectra of the restructured fish meat gels is presented in Fig. 2a. Similar to the response of numerous other physical gels (Rao, 2010), all the samples showed an approximately flat mechanical spectrum within the available frequency range studied. Both G0 and G00 developed similarly, and the G0 values were considerably greater in magnitude than the G00 in all samples, suggesting a predominantly elastic, solid-like behaviour. This type of behaviour was expected for a 3D gel cross-linked gel network (Iglesias-Otero et al., 2010; Rao, 2010). As illustrated in Fig. 2a, with identical curdlan concentrations, the relative difference between G0 and G00 was greater in RC1, RC2, and RC3 fish meat gels than those in IRC1, IRC2 and IRC3 samples, which was consistent with the data of G⁄ and low tan d from the stress sweep (Table 1). Moreover, the slopes of the frequency sweeps curve of the RC fish meat gel samples were smaller than those in the IRC samples (which were almost parallel to the CS sample). This indicates that the gel network structure of the RC fish meat gel samples was different from that of the IRC samples and exhibited more rigidity than that of the IRC sample gel system. It is well known that curdlan can form two types of heat-induced gels – low-set gel (RC) and high-set gel (IRC). The low-set gel can transform to a high-set gel when heated to approximately 80 °C, however, the high-set gel is thermo-irreversible, which means that it is stable at low temperatures such as freezing, and also at high

temperatures as in retorting (Tada et al., 1999). Thus, it can be speculated that RC would transform to a thermal-irreversible gel in processed fish meat gel, similar to the deacetylated glucomannan gelation in a strong (true) gel, and that IRC was heat-stable within meat products, served as a simple filler, analogous to packing material, in the protein network gels (Rao, 2010; Solo-deZaldívar et al., 2014). These results were in agreement with Funami et al. (1998,1999) who noted that curdlan formed a heatstable, thermo-irreversible gel within meat products which held the water within its gel structures strongly. All samples showed a frequency dependence of G0 and G00 corresponding to straight lines in the log–log plot (Fig. 2a); therefore, the relationship between G0 (or G00 ) and angular frequency x can be fitted to power law equations:

G0 ¼ G00  xn

0

G00 ¼ G000  xn G00

ð3Þ 00

ð4Þ G000

where and are the deformation of G0 and G00 and related to the strength of intermolecular interactions of the gel network and the exponents n00 and n000 denote the rate of change of G00 and G000 with increasing x, respectively, related to the stability of the protein matrix (Ferry, 1980). Table 2 shows the fitted parameter values for Eqs. (3) and (4). The difference of (G00  G000 ) can be used as a new measure of gel strength (Campo & Tovar, 2008), and the value of (G00  G000 increased with the augmentation of curdlan content in restructured fish meat gel samples, which was consistent with the previous texture properties and complex modulus (G⁄) analysis. The increase was more marked in the fish meat gel samples containing the RC than in those containing the IRC (Table 2). This was also consistent with the previous results of the gel strength analysis and the complex modulus G⁄ (Table 1). The (G00  G000 in the RC fish meat gels were larger than those in the IRC samples, at the same concentration, due to more intermolecular interactions in the protein matrix in the RC fish meat gel samples, leading to a more stable fish meat gel. Additionally, the difference of (n00  n000 can also be used to distinguish the different classes of protein gels; the lower the value, the better the gel. Good gels always exhibit the same proportional change in G0 and G00 with frequency over a wide range (Park, 2005). Based on the values in Table 2, the variation in the trend of (n00  n000 was in agreement with the changes of

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C. Wu et al. / Food Chemistry 179 (2015) 222–231 8000

6000

RC

0%

2%

4%

6%

0%

IRC

2%

4%

6%

6000

G'(Pa)

G'(Pa)

4000

4000

2000

2000 1

10

1

100

10

100

1200

2500

RC

0%

2%

4%

0%

IRC

6%

2%

4%

6%

2000 1500

G"(Pa)

800

G"(Pa)

1000

400 500

1

10

100

1

10

100

Fig. 2. (a) Mechanical spectra curves of fish meat gel samples which contain 2%, 4%, and 6% curdlan. (b) Creep–recovery curves of fish meat gel samples which contain 2%, 4%, and 6% curdlan.

(G00  G000 , which again verified a stronger gel network structure for the RC fish meat gel samples. Additionally, the values of (n00  n000 of RC3 were the lowest among the RC fish meat gel samples, revealing that the proportion of curdlan for making a denser packed network in the gel samples must be lower than 6%. Similar results have been found in other food gels (Campo & Tovar, 2008; Campo-Deaño & Tovar, 2009).

3.6. Creep and recovery analysis The transient characteristics of fish meat gels can be conducted over a longer timeframe using creep–recovery experiments, rather than oscillatory tests. The creep–recovery test will provide useful information relating to the long-term properties of the gel network structure (Mezger, 2006). Creep–recovery curves for fish meat gel samples with different curdlan contents are presented in Fig. 2b. The creep and recovery compliance J(t) values varied between samples, but the overall pattern of responses was similar. The CS and IRC3 samples presented higher J(t) values over the entire time, and the samples with higher RC concentrations exhibited lower J(t) values during the creep and recovery test. Compared with the IRC fish meat gel samples, the RC fish meat gel samples tended to be more rigid (from their low J(t) values). These results were consistent with those of the stress sweep tests (Table 1), where the RC fish meat gel samples exhibited high critical G⁄ values. The increase in J(t) was strongest over

the curdlan content range 2–4%, and the trend was more uniform in the RC samples compared with the IRC samples. Moreover, the J(t) values from the creep values can be used to calculate additional parameters related to gel strength (S) and the relaxation exponent (n) using the equation of Winter and Chambon (Campo & Tovar, 2008):

GðtÞ ¼ S  t n

ð5Þ

where G(t) is the relaxation modulus, the reciprocal of J(t); S (Pa sn) is the gel strength parameter (which depended on the cross-linking density and the molecular chain flexibility); and n is related to the molecular structure and connectivity of the gel (i.e., the degree of connectivity in the gel) (Ferry, 1980). Table 2 displays the obtained S and n values. As expected, the S values evolved similarly to the texture profile analysis, (G00  G000 ) and complex modulus (G⁄) of restructured fish meat gel samples, namely, S increased with increasing curdlan concentration and the RC samples had higher S values than the IRC samples with equal curdlan contents, suggesting that the gel structure of the RC fish meat gel samples was more firm and stable. For the exponent n, the IRC3 sample had the largest value and the RC3 sample had the smallest value. The lower the n values, the higher the density of physical cross-links, which increases the extension of the junction zones in non-covalently cross-linked networks (Campo-Deaño & Tovar, 2009). This suggests that a less orderly structure and lower cross-linking density were present in the IRC3 sample gel, and this was confirmed by the maximum J(t)

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C. Wu et al. / Food Chemistry 179 (2015) 222–231 0.00030

RC 0% 4%

0.00025

2% 6%

J (1/Pa)

0.00020

0.00015

0.00010

0.00005

0.00000 0

250

500

750

1000

1250

Time (s) 0.00030

IRC

CK 4%

0.00025

2% 6%

J (1/Pa)

0.00020

0.00015

0.00010

0.00005

0.00000 0

250

500

Time (s)

750

1000

1250

Fig. 2 (continued)

Table 2 Power law parameters of Eqs. (3) and (4) from frequency sweep test and Eq. (5) creep–recovery test, at 25 °C for fish meat gel samples. Sample

(G00 ± SD)  103 (Pa sn)

n00 ± SD

(G000 ± SD)  103 (Pa sn)

n000 ± SD

(S ± SD)  104 (Pa sn)

(n ± SD)

CS RC1 RC2 RC3 IRC1 IRC2 IRC3

2.174 ± 0.159 3.015 ± 0.468 4.278 ± 0.416 4.916 ± 0.193 2.679 ± 0.283 3.272 ± 0.251 1.810 ± 0.295

0.094 ± 0.000 0.070 ± 0.001 0.080 ± 0.000 0.081 ± 0.001 0.080 ± 0.000 0.100 ± 0.001 0.117 ± 0.001

0.352 ± 0.017 1.06 ± 0.140 1.137 ± 0.02 1.490 ± 0.040 0.422 ± 0.011 0.627 ± 0.09 0.287 ± 0.015

0.038 ± 0.001 0.032 ± 0.001 0.054 ± 0.001 0.062 ± 0.001 0.075 ± 0.001 0.068 ± 0.001 0.061 ± 0.002

2.162 ± 0.024 2.747 ± 0.043 3.104 ± 0.040 3.950 ± 0.049 2.570 ± 0.036 2.850 ± 0.146 1.234 ± 0.018

0.217 ± 0.003 0.211 ± 0.004 0.179 ± 0.003 0.179 ± 0.003 0.239 ± 0.004 0.212 ± 0.010 0.349 ± 0.004

values obtained for the sample during recovery. In addition, although the RC3 sample possessed the largest S value, the n value of the RC3 sample was similar to that of the RC2 sample, indicating that the densities of non-covalently cross-links between the RC2 sample and the RC3 sample were not significantly different. This confirmed that 4% RC was an appropriate supplemental amount for restructured fish meat gel. From the results of texture analysis and the small deformation oscillatory test, it can be inferred that the RC fish meat gel samples

were more stable, with more ordered three-dimensional networks than the IRC fish meat gel samples, and that the connection modes of the RC and IRC between with fish meat gels were diverse. According to Ziegler and Foegeding (1990), there are five possible models for the spatial partitioning of a gelling protein and a coingredient: (i) model A, the filler remains soluble in the interstitial fluid of the gel matrix; (ii) model B, the filler exists as dispersed particles of liquid or as a secondary gel network due to the thermodynamic incompatibility causing phase separation to occur;

C. Wu et al. / Food Chemistry 179 (2015) 222–231

Fig. 3. Suggested mechanism for the combination behavior between curdlan and SSP.

(iii) model C, the filler may associate with the protein gel in a random fashion via nonspecific interactions in the gel matrix; (iv) model D, the co-ingredients may aggregate with the protein matrix and copolymerise to form a single, heterogeneous network; and (v) model E, the co-ingredient forms gel networks by itself and entwined with the protein matrix to form an interpenetrating polymer gel network (IPN). Because the G00 increased during the frequency sweep, the non-cross-linked parts of the polymer network had more relative motion and more energy lost to internal friction, indicating that the myofibrillar protein and curdlan were ‘‘biopolymer incompatible’’ at a molecular level with almost no

229

covalent interactions between these two biopolymer (IglesiasOtero et al., 2010; Park, 2005). Besides, as discussed above, RC would transform to a thermal-irreversible 3D gel network which was independent of protein gel matrix in processed fish meat gel, and IRC was heat-stable within meat products, serving as a simple filler in the protein network gels. Thus, the type of mixed fish meat gel may be Mode B, C or E. Based on the results of the texture and rheological analyses, it can be assumed that the thermal irreversible curdlan particles (high thermal stability) existed as filler particles dispersed in the restructured fish meat gel matrix after reheating, and considering the thermodynamic incompatibility of IRC and SSP, the combinative gelation model for IRC-added fish meat gel may be type B. The RC particles can reform heat-induced triple-helical domains that may be entwined with a protein 3-D network gel structure in some parts, which is defined as model E (illustrated in Fig. 3). In this model, the association of curdlan with the fish meat gel protein would intensify the network of fish meat gel proteins and hence produce a larger and better fish meat gel structure with improved structural rigidity, gel strength and WHC. These results were similar to those of other food hydrocolloids, such as starch, konjac glucomannan and hydroxypropyl methylcellulose mixed with SSP in restructured seafood (Chen, Ferng, Chen, Sun, & Lee, 2005; Ramírez et al., 2011).

3.7. Microstructure analysis To verify the assumption in the combination model of curdlanfish meat gel protein, the microstructure of the samples was observed by scanning electron microscopy (SEM). Fig. 4 shows the three-dimensional structure of a typical set of fish meat gels formulated with or without curdlan. The control fish meat gel

Fig. 4. SEM of fish meat gel sample ((a) raw fish meat gel; (b–d) RC1, RC2 and RC3, respectively; e.g: IRC1, IRC2 and IRC3, respectively).

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C. Wu et al. / Food Chemistry 179 (2015) 222–231

Table 3 Effect of curdlan on sensory evaluation value of fish meat gel samples. Sample CS RC1 RC2 RC3 IRC1 IRC2 IRC3 a–e

Colour

Taste a

6.333 ± 0.252 7.167 ± 0.287b 7.433 ± 0.404b 7.600 ± 0.173b 7.133 ± 0.351b 7.433 ± 0.404b 7.400 ± 0.173b

Texture a

5.267 ± 0.252 5.900 ± 0.100b 6.000 ± 0.100b 6.133 ± 0.115b 5.833 ± 0.289b 5.933 ± 0.153b 5.767 ± 0.252b

Odour a

4.233 ± 0.143 6.000 ± 0.200c 7.067 ± 0.404e 7.167 ± 0.153e 5.300 ± 173b 6.533 ± 0.252d 4.433 ± 0.404a

Overall liking a

5.933 ± 0.404 6.500 ± 0.500b 6.967 ± 0.153b 7.067 ± 0.208b 6.700 ± 0.200b 6.833 ± 0.252b 6.633 ± 0.115b

4.933 ± 0.404b 6.267 ± 0.208c 7.300 ± 0.436d 7.167 ± 0.305d 5.300 ± 173b 6.467 ± 0.404c 4.267 ± 0.289a

Different letters in the same column indicate significant differences (p < 0.05).

(Fig. 4a) appeared as a rough protein gel matrix with less uniform but larger cavities than those with curdlan added (Fig. 4b–f). In the case of RC particles, in general, the fish meat gel samples demonstrated well-formed and homogeneous networks containing few embedded holes and appeared similar to true gels under microscopy. Though the curdlan particles could not directly interact with SSP and acted as a ‘‘filler’’ in the system, the RC particles in the fish meat gel reformed thermal irreversible three-dimensional gel structures during the heat-induced gel process, resulting in the ‘‘simple’’ filled gels changing to ‘‘complex’’ filled gels and the curdlan gel network interpenetrated with the protein gel matrix in some areas (defined as model E). Additionally, with an increase in RC content, interconnections between the three-dimensional protein network and the curdlan in the fish meat gel also increased, leading to a more compact and denser restructured fish meat gel network (Fig. 4b, c and d, respectively). Higher levels of RC were also correlated with a higher WHC, stronger gel strength, and a more preferable viscoelasticity for restructured fish meat gels. In the case of the IRC particles, due to its thermal stability, curdlan particles served as a filler to partly occupy the holes that may link together through hydrogen bonding within the protein gel matrix (Fig. 4e, f and g, respectively), and they showed some discontinuous networks and a binding behaviour similar to model B. The addition of starch, konjac glucomannan and hydroxypropyl methylcellulose powder in fish meat gel samples produced similar results (Chen et al., 2005; Ramírez et al., 2011). The less compact fish meat gel network in the IRC samples (Fig. 4) may explain why WHC, mechanical properties (Table 1), and S (creep–recovery test) (Table 2) were lower than in samples of the RC gel. This could be partially responsible for the non-significant differences in WHC values among the IRC samples (Table 1). Moreover, with the lower gelation ability of the IRC, the superfluous content of IRC made the protein network became less homogeneous and generated large holes in the fish meat gel samples, and the relative concentration of protein in samples with added IRC was lower (Fig. 4g). This can help us to understand the lower physicochemical properties obtained in the IRC3 sample. 3.8. Sensory analysis Sensory evaluation can assist food scientists in instructively gaining a distinct understanding of the consequences of reformulation meat gel processes. The sensory evaluation profiles of restructured meat gel samples of each group are presented in Table 3. The addition of RC at the selected levels yielded gels with the higher texture scores, compared with that of IRC and control samples (p < 0.05). This was coincidental with the increased gel strength in the fish meat gels with addition of RC gel (Table 1). However, colour, taste and odour were not significantly different among the meat gel samples enriched with curdlan (p > 0.05), even though physical characteristics were different among these restructured meat gel samples. Similar to texture score, the overall likeness score is higher in RC samples than those in IRC samples

(p < 0.05). Among all the gel samples with added RC, those containing 4% RC gel had the highest overall likeness score, followed by those with 6% RC gel. Restructured fish meat gel with 6% IRC gel exhibited a lower overall likeness score to the control gel (p > 0.05). Therefore, the addition of RC gel at an appropriate ratio (4–6%) in this study positively affected the sensory properties of restructured ribbonfish meat gel. 4. Conclusions The influence of curdlan on the viscoelastic properties of ribbonfish (Trichiurus spp.) meat gel was evaluated. Various viscoelastic parameters, such as the complex modulus (G⁄), the difference (G00  G000 ) and S, that can be used as index of gel strength were established and confirmed by textural analysis. Due to the interaction of the RCs (thermal reversible curdlan gel) or the IRCs (thermal irreversible curdlan gel) with the ribbonfish meat protein gel matrix during the process of thermal gelation, the simple filled fish meat gels turned into complex filled gels, resulting in various changes to WHC and the textural and rheological properties of the final reconstructed ribbonfish meat gel products. The combinative gelation models of the RC or the IRC with SSP in fish meat gel samples were diverse. Compared with the CS and IRC samples, the addition of higher levels of the RC in the fish meat gels significantly increased the cross-linking density of the complex fish meat gel networks, leading to a more stable and ordered three-dimensional network complex gel structure and improving the gel properties (its rheology was more like that of a true gel), which was verified by mechanical spectra (higher viscoelastic modulus), creep–recovery analysis (lower n) and microstructure analysis. The addition of the RC at an appropriate level (in the range of 4–6% for ribbonfish according to this study) effectively strengthens the gelation properties and sensory property of restructured fish meat gel. Therefore, application of the RC as a gel binder and as a dietary fibre source in processed fish meat gel-based products appears feasible. However, further research is needed to determine the relevant sensory evaluation parameters and the synergic effect of the RC with other ingredients (e.g., starch, soy isolate protein) in related fish meat gel products. Acknowledgments The authors would like to thank the Key Project of National Science and Technology Ministry (2012BAD38B09) for the financial support. References Børresen, T. (2008). Improving seafood products for the consumer. Elsevier. Campo-Deaño, L., & Tovar, C. (2009). The effect of egg albumen on the viscoelasticity of crab sticks made from Alaska Pollock and Pacific Whiting surimi. Food Hydrocolloids, 23(7), 1641–1646. Campo, L., & Tovar, C. (2008). Influence of the starch content in the viscoelastic properties of surimi gels. Journal of Food Engineering, 84(1), 140–147.

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