Changes in physicochemical properties and protein

LWT - Food Science and Technology 84 (2017) 562e571

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Changes in physicochemical properties and protein structure of surimi enhanced with camellia tea oil Xuxia Zhou a, Shan Jiang a, Dandan Zhao a, Jianyou Zhang a, Saiqi Gu a, Zhiyan Pan b, Yuting Ding a, * a b

Department of Food Science and Technology, Zhejiang University of Technology, Hangzhou 310014, China Department of Environmental Engineering, Zhejiang University of Technology, Hangzhou 310014, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 December 2016 Received in revised form 13 March 2017 Accepted 13 March 2017 Available online 16 March 2017

The objective of this study was to determine the effects of different concentrations of camellia tea oil on surimi gel physicochemical properties and protein secondary structure. With the increase of camellia tea oil concentration (0e8 g/100 g of surimi), surimi gel hardness, whiteness, WHC, overall acceptability, storage modulus (G0 ) and the indexes of ionic bonds and hydrophobic interactions were increased significantly (P < 0.05). Cryo-scanning electron microscopy (Cryo-SEM) showed that the oil occupied the void spaces of the protein matrix and formed a firmer structure. The Raman spectroscopy study showed that there was a decreased trend in a-helix content and increased trend in b-sheet content in surimi protein as the oil content increased. Correlation analysis showed that the hardness was negatively correlated to the a-helix content (r ¼ 0.958, P < 0.01) and positively correlated to the b-sheet content (r ¼ 0.958, P < 0.01) and hydrophobic interactions (r ¼ 0.944, P < 0.01) of surimi gels. These results suggest that the presence of the oil could change the micro-environment and molecular structure of surimi proteins and further affect the physicochemical properties of surimi gels. In general, when the concentration of camellia tea oil was 8 g/100 g of surimi, the surimi gel showed the most favorable properties. © 2017 Published by Elsevier Ltd.

Keywords: Camellia tea oil Surimi gels Raman spectroscopy Protein structure

1. Introduction Surimi is a commercial preparation of fish myofibrillar protein. It is produced by solubilizing myofibrillar proteins during the comminuting and salting stages of manufacturing (Kong et al., 2016). Surimi is an inexpensive source of protein and is a useful ingredient for producing various kinds of processed foods due to the unique gelling properties of the myofibrillar protein. Surimibased products such as fish ball, fish sausage, breaded fish stick and paupiette have become increasingly popular due to the preferred textural properties and high nutritional value of surimi. Physicochemical properties including morphology, pasting properties, and gel properties are important criteria to evaluate the quality of surimi. Lipids play an important role in the texture, juiciness, color, and flavor of comminuted meat product, and removal of fat will result in meat products with a rubbery and dry

* Corresponding author. E-mail address: [email protected] (Y. Ding). http://dx.doi.org/10.1016/j.lwt.2017.03.026 0023-6438/© 2017 Published by Elsevier Ltd.

texture (Luruena-Martínez, Vivar-Quintana, & Revilla, 2004). However, in the process of frozen surimi manufacturing, fish fat is usually trimmed away to increase the concentration of myofibrillar protein and to extend the storage time. Therefore, to improve the physicochemical and gel properties of surimi, exogenous lipids are usually added during surimi product processing (Chojnicka, Sala, De Kruif, & Van de Velde, 2009; Debusca, Tahergorabi, Beamer, Partington, & Jaczynski, 2013; Pietrowski, Tahergorabi, & Jaczynski, 2012). Shi et al. (2014) reported that the addition of vegetable oil (soybean, peanut, corn, and rap oils) significantly increased the whiteness of surimi gels (P < 0.05). Alvarez, Xiong, Castillo, Payne, and Garrido (2012) found that canola-olive oils favored gel network formation and gel elasticity in pork frankfurters. Chojnicka et al. (2009) also observed that oil/fat could increase the brittleness and change the functional properties of fish protein gels. Myofibrillar protein is the primary functional ingredient of surimi-based products and is very important for the gelling properties of surimi (Kong et al., 2016). Thus, the influence of lipid on surimi gel properties may be related to changes in surimi protein

X. Zhou et al. / LWT - Food Science and Technology 84 (2017) 562e571

structures. The presence of lipids could alter the molecular structure of proteins, induce the exposure of hydrophobic groups, and further affect the formation and stability of emulsions and the textures of many food products (Meng, Chan, Rousseau, Eunice, & Li, 2005). Shao, Zou, Xu, Wu, and Zhou (2011) studied the protein structural changes in heated meat batters prepared with different lipids (pork fat and soybean oil) and found that there was a significant decrease (P < 0.05) in a-helix content accompanied by a significant increase (P < 0.05) in b-sheet structure. Shao, Deng, Zhou, Xu, and Liu (2015) also indicated that disulfide bonds, hydrophobic interactions, and hydrogen bonds were the main interactions at the emulsion interface of pork meat proteins and lipids (animal fat or soybean oil) by Raman spectroscopic analysis. Camellia tea oil, also known as the “olive oil of Asia”, is one of most important edible oils, and it is rich in oleic acid and contains many natural antioxidants with various biological activities (Long & Wang, 2008; Zhang, Jin, Wang, & Xue, 2013). It has been utilized in China for more than 1000 years (He & Gu, 1982). However, there has been no report on the effects of different concentrations of camellia tea oil on the properties of surimi and surimi-based products. Preliminary studies in our lab showed that camellia tea oil could improve the gel properties of surimi, improve the taste of surimi-based products and have additional nutritional value. A better understanding of the structural changes of surimi enhanced with different concentrations of camellia tea oil could be helpful for elucidating the role of lipids in the protein matrix structure and lead to the development of new surimi-based products. Therefore, the objective of this study was to determine the effects of different concentrations of camellia tea oil on the physicochemical properties (texture profile, color, water holding capacity, microstructural properties, sensory properties, rheological properties) and protein secondary structure of surimi gels using Raman spectroscopy. Additionally, the relationships between the physicochemical properties and structural changes of surimi were also analyzed by correlation analysis to provide guidelines for improving surimi gel properties. 2. Materials and methods 2.1. Materials Frozen surimi (White croaker, Grade A) was provided by Zhejiang Industrial Group Co., LTD. (Zhoushan, China) and stored at 80 C until needed. The moisture of the surimi was 76.72 g/ 100 g. Natural Camellia tea oil (Hangzhou Jiusheng biotechnology Co., Ltd, Hangzhou, Zhejiang, China) and salt (Zhejiang Lanhaixing Salt Industry Group Co., Ltd, Hangzhou, Zhejiang, China) were purchased from a local supermarket (Hangzhou, Zhejiang, China). Sodium tripolyphosphate, sodium pyrophosphate, and sodium hexametaphosphate were purchased from Shanghai Lingfeng Chemical Reagent Co., LTD. (Shanghai, China). 2.2. Preparation of surimi gels Frozen surimi was thawed at 4 C for 12 h and then cut into small pieces (about 1 1 1 cm3). Salt (2 g/100 g of surimi), compound phosphate (sodium tripolyphosphate: sodium pyrophosphate:sodium hexametaphosphate ¼ 2:2:1, 0.3 g/100 g of surimi), camellia tea oil, and ice water (total 20 g/100 g of surimi) were added into the surimi and mixed thoroughly for 5 min in a Philips blender (Zhuhai special economic zones, Philips domestic appliance Co., Ltd), and during the homogenization, the temperature was carefully controlled at 4e10 C. For different treatments, the final concentrations of camellia tea oil were 0, 2, 4, 6, 8, and 10 g/100 g of surimi, respectively. Oil was added to surimi paste by

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replacing chilled water (1:1, wt:wt) that is normally added during formulation of surimi paste. Surimi without camellia tea oil was used as a control. The pastes were then stuffed into plastic casing (22 mm in diameter), and both ends were sealed tightly. Subsequently, the samples were kept at 4 C for 4 h and then heated in water bath at 90 C for 20 min. The gels were cooled and stored at 4 C for further analysis. Three measurements were performance for each sample. 2.3. Proximate analysis Representative samples from each treatment were honogenized and analyzed, for percentage fat (ether-extractable), protein and hydroxyproline according to standard AOAC. (1990) procedures. Moisture content was determined by drying the sample at 105 C lez over 24 h until a constant weight was achieved (S anchez-Gonza et al., 2008). Each sampling was performed in triplicate and reported as g/100 g (wet weight basis). 2.4. Texture analysis Texture analysis was performed with a TA.XT Plus Texture Analyzer (Stable Micro System Company, UK). Samples stored at 4 C overnight were cut into cylinders (22 mm in diameter and 20 mm in height). TPA test conditions were as follows, probe: P/ 36R; pre-test speed: 2.00 mm/s; test speed: 1.00 mm/s; post-test speed: 2.00 mm/s; strain: 60%; triggering mode: automatic (power); trigger force 5.0 g; 200 points per second. Fifteen replicates of measurements were taken. Shear force was determined using a HDP/BSK-Warner probe. The parameters were as follows, pre-test speed: 2.00 mm/s; test speed: 2.00 mm/s; post-test speed: 2.00 mm/s; displacement: 20 mm; triggering mode: automatic (power); and trigger force 5.0 g. Fifteen replicates of measurements were taken. 2.5. Color evaluation Surimi samples were equilibrated at room temperature (about 25 C) for 1 h prior to the color measurement. The color values of surimi batters and gel samples were determined by a Color Quest XE colorimeter (HunterLab Co., Ltd, USA). Lightness (L*), redness (a*), and yellowness (b*) values were recorded respectively. The whiteness (W) was calculated using the equation: W ¼ 100-[(100L*)2þa*2þb*2]1/2. Fifteen replicates of measurements were taken. 2.6. Water holding capacity (WHC) WHC was carried out based on a method proposed by Chen, Gerelt, Jiang, Nishiumi, and Suzuki (2006) with a slight modification. Gel samples were cut into thin slices, and approximately 3 g samples were weighed and placed between two layers of filter paper. Subsequently, the samples were placed at the bottom of centrifuge tubes and centrifuged (CR21GII high-speed refrigerated centrifuge, Hitachi, Japan) at 10,000 rpm for 10 min. Gels were weighed again after centrifugation. WHC¼W2/W1 100%, where W1 is the initial weight of gels, g; W2 is the final weight of gels, g. Three replicates of the measurements were taken. 2.7. Cryo-scanning electron microscopy (Cryo-SEM) The microstructures of surimi gels with different concentrations of camellia tea oil were observed using Cryo-scanning electron microscopy (Cryo-SEM). The samples were loaded on the cryospecimen holder and cryo-fixed in slush nitrogen (196 C), then quickly transferred to the cryo-unit in the frozen state. The frozen

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samples were then fractured by striking them with a pre-cooled razor blade. The revealed fracture plane was sublimed at 90 C in a vacuum SEM chamber and examined in the cryo-stage SEM (Hitachi S-4800, Hitachi Corp., Japan) with temperature controller (GATAN AOTO2500, Hitachi Corp., Japan). Lastly, the samples were transferred on to the SEM stage at 140 C and visualised for distinct topography. 2.8. Sensory evaluation Sensory characteristics of surimi gels were evaluated by 30 panelists with previous experience in judging surimi gels. Surimi gels were cut into 5 mm thick sheet samples, and then were randomly distributed for evaluation. Color, taste and flavour, texture, elasticity, and overall acceptability were used to assess the samples using a 10-point scale ranging from 1 (dislike extremely) to 10 (like extremely). All samples were evaluated in the same condition. Fifteen replicates of measurements were taken. 2.9. Dynamic rheological properties For dynamic rheological test, surimi pastes containing different concentration of camellia tea oil were placed in a MCR302 Rheometer (Anton Paar Ltd., Austria) equipped with a 25 mm parallel steel plate. A gap of 0.5 mm was set and silicone oil was used to prevent water evaporation. The storage modulus (G0 ) was measured with a temperature increasing from 10 to 90 C at the heating rate of 2 C/min and frequency of 0.1 Hz. The rheogram presented in this study was based on mean values of three independent experiments. 2.10. Determination of soluble protein in surimi gels The following procedure was carried out based on a method mez-Guille n and Martí de Castro Pilar Montero proposed by Go (1997). Sample (2 g) of chopped gel was homogenized (IKA T 18 Ultra Turrax Digital Homogenizer, Staufen, Germany) for 1 min with 10 mL 0.05 mol/L NaCl solution (SA). In parallel, the same quantity of chopped gel was homogenized for 1 min with 10 mL 0.6 mol/L NaCl solution (SB), 10 mL 0.6 mol/L NaClþ1.5 mol/L Urea solution (SC), or 10 mL 0.6 mol/L NaClþ8 mol/L Urea solution (SD). The samples were kept at 4 C for 1 h and then centrifuged for 15 min at 10,000 g. Subsequently, the biuret method was used to determine the protein content of the supernatant (Shao et al., 2013). Results were the averages of three determinations. The index of ionic bands, index of hydrogen bonds, and index of hydrophobic interactions were expressed as follows: Index of ionic bands ¼ the content of protein in SB (mg/mL) - the content of protein in SA (mg/mL); index of hydrogen bonds ¼ the content of protein in SC (mg/mL) - the content of protein in SB (mg/ mL); index of hydrophobic interactions ¼ the content of protein in SD (mg/mL) - the content of protein in SC (mg/mL). 2.11. Raman spectrometric analysis The Raman spectra of the surimi gels were collected with a Raman spectrometer (HR 800 Lab RAM, Horiba Jobin Yvon, France) equipped with a 531.95 nm laser (frequency-doubled Nd:YAG, 20 mW) and a charge-coupled device (CCD) detector (multichannel, air cooled). The laser power was approximately 8 mW. The spectra were recorded in the range of 500e1800 cm1. Each spectrum was obtained under the following conditions: three scans, 30 s of exposure time, 2 cm1 resolution and a sampling speed of 120 cm1 with data collected every 1 cm1. The spectra were smoothed, baseline-corrected, and normalized against the

phenylalanine band at 1004 cm1 using Labspec software. 2.12. Statistical analysis One-way analysis of variance (ANOVA), principal component analysis (PCA) and correlation analysis were performed using a SPSS package (SPSS 19.0 for Windows, SPSS Inc., Chicago, IL, US). A significant difference was determined at the 0.05 probability level, and differences among the mean values of various treatments were measured by Duncan's multiple range test (P < 0.05). The data were reported as the mean values ± standard deviation (SD). The experiments were independently triplicated (n ¼ 3). 3. Results and discussion 3.1. Proximate composition of surimi gels The proximate compositions of surimi gels with the addition of different concentrations of camellia tea oil were determined and are shown in Table 1. Addition of oil changed the moisture content and fat content. As a result of moisture substitution by fat in surimi gels, the moisture content was inversely proportional to the fat content and surimi gel with 0.75 g/100 g fat had the highest level of moisture of 78.23 g/100 g. There was no significant difference in protein and collagen levels between all the products (P > 0.05). 3.2. Texture analysis of surimi gels Table 2 shows the effects of different concentrations of camellia tea oil on textural properties of surimi gels. Camellia tea oil increased the hardness, adhesiveness, springiness, cohesiveness, gumminess, chewiness, and resilience of surimi gels. The hardness of surimi gels increased with the increase of oil concentration until the oil content reached 8 g/100 g of surimi. These results are consistent with the report of Debusca et al. (2013), who found that fortification with u-3 oil (flax: algae: menhaden, 8:1:1) increased the hardness and shear force of surimi gels. Claus and Hunt (1991) showed that the hardness and chewiness of bologna sausage increased with the increase in fat content. Jin and Ma (2002) also observed that with the decrease of fat content, the hardness of meat gel decreased. However, there have also been inconsistent results. Luruena-Martínez et al. (2004) reported that the addition of olive oil produced a decrease in hardness of frankfurters. These differences might because the water content of the meat gels studied were different and also because the types of the oil added were different. Li, Carpenter, and Cheney (1998) found that water content also affected the gel properties of sausage and that with the increase of water content, the hardness, elasticity, and cohesiveness of smoked sausage decreased. The shear force of surimi gels was affected in the same way as hardness (Table 2). It increased with the increase of oil concentration until the oil content reached 8 g/100 g of surimi. This result is consistent with the previous study of Debusca et al. (2013). A possible explanation for this might be that external oil can occupy the void spaces of the protein gel matrix and change the structure of protein and thus increase the gel strength (Dickinson & Chen, 1999; Wu, Xiong, Chen, Tang, & Zhou, 2009). Increasing the oil concentration from 8 to 10 g/100 g of surimi did not change the shear force, indicating that the surimi protein wrapped oil was already saturated when the oil concentration was 8 g/100 g of surimi. 3.3. Whiteness of surimi gels The whiteness of both surimi paste and gels significantly


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Table 1 Effect of camellia tea oil on proximate composition, whiteness and WHC of surimi gels. Content of camellia tea oil (g/100 g of surimi)

proximate compositions

0 2 4 6 8 10

78.23 76.66 74.84 73.23 71.81 70.14

Moisture (g/100 g) ± ± ± ± ± ±

Whiteness

Fat (g/100 g)

0.06a 0.09b 0.16c 0.06d 0.16e 0.08f

0.75 2.36 4.04 5.81 7.46 9.13

± ± ± ± ± ±

0.03a 0.02b 0.10c 0.04d 0.05e 0.07f

Protein (g/100 g) 10.77 10.94 10.82 10.98 11.04 10.89

± ± ± ± ± ±

Collagen (g/100 g)

0.36a 0.25a 0.15a 0.13a 0.27a 0.18a

1.15 1.21 1.14 1.18 1.23 1.11

± ± ± ± ± ±

surimi paste

0.12a 0.09a 0.05a 0.08a 0.13a 0.14a

70.34 72.58 73.63 74.86 75.93 76.07

± ± ± ± ± ±

WHC (g/100 g)

surimi gels

0.12e 0.18d 0.14c 0.21b 0.11a 0.23a

73.75 75.41 76.68 77.97 79.02 79.24

± ± ± ± ± ±

0.31e 0.14d 0.25c 0.24b 0.35a 0.32a

71.43 77.36 77.86 79.17 83.39 84.95

± ± ± ± ± ±

1.27c 1.07b 0.70b 0.31b 1.41a 0.26a

Data were given as mean values ± standard deviation (proximate composition and WHC: n ¼ 3; Whiteness: n ¼ 15). Different letters within the same column indicate significant differences (P < 0.05) between mean values.

Table 2 Effect of camellia tea oil on texture profiles of surimi gels. Content of camellia tea oil (g/100 g of surimi)

Hardness (N)

0 2 4 6 8 10

31.80 33.69 35.89 37.59 39.84 39.48

± ± ± ± ± ±

0.60e 0.35d 0.23c 0.41b 0.71a 0.62a

Adhesiveness (N.s) 1.13 1.34 1.42 1.58 1.66 1.70

± ± ± ± ± ±

0.24b 0.30ab 0.28ab 0.35a 0.33a 0.41a

Springiness 0.82 0.83 0.85 0.83 0.85 0.84

± ± ± ± ± ±

0.01b 0.01b 0.01a 0.01b 0.01a 0.01ab

Cohesiveness 0.61 0.62 0.62 0.64 0.65 0.65

± ± ± ± ± ±

0.01b 0.01b 0.01b 0.01a 0.01a 0.02a

Gumminess (N) 20.72 21.23 23.24 24.36 24.90 24.39

± ± ± ± ± ±

0.61c 0.24c 0.32b 0.51a 0.41a 0.64a

Chewiness (N) 17.89 19.03 20.00 20.48 21.34 21.02

± ± ± ± ± ±

0.60e 0.24d 0.47c 0.51bc 0.20a 0.36ab

Resilience 0.27 0.28 0.28 0.31 0.32 0.32

± ± ± ± ± ±

0.01b 0.01b 0.01b 0.01a 0.01a 0.01a

Shear force (N) 3.82 3.88 4.12 4.52 5.21 5.18

± ± ± ± ± ±

0.16d 0.13d 0.04c 0.05b 0.14a 0.13a

Data were given as mean values ± standard deviation (n ¼ 15). Different letters within the same column indicate significant differences (P < 0.05) between mean values.

increased with the addition of camellia tea oil (P < 0.05, Table 1). These results agree with the results of several previous studies (Benjakul, Visessanguan, & Kwalumtharn, 2004; Shi et al., 2014). The increase of whiteness values might be due to oil droplet suspension in the surimi paste which could cause the gels to scatter light, leading to more light reflectance (Sell, Beamer, Jaczynski, & Matak, 2015). Compared with surimi paste, the whiteness value of surimi gels with the same oil concentration was significantly higher (P < 0.05), suggesting that heating could make the surimi form a more stable gel structure. Xu, Ge, Jiang, and Xia (2013) also reported that the whiteness of the surimi gels significantly increased after cooking.

3.5. Cryo-scanning electron microscopy (Cryo-SEM) Fig. 1 shows the microstructures of surimi gels with camellia tea oil in the concentrations of 0 g/100 g, 4 g/100 g and 8 g/100 g, respectively. A continuous protein matrix permeated by oil droplets can be observed, and with the increase of oil concentrations, the number of oil droplets was increased, which was also observed by Shi et al. (2014). Moreover, the holes in the network became smaller, which can be associated with the increase in hardness and shear force of surimi gels. The oil occupied the void spaces of the protein matrix and formed a firmer structure. These results were consistent with the textural results described above. 3.6. Sensory properties of surimi gels

3.4. WHC of surimi gels Addition of camellia tea oil significantly increased the WHC of surimi gel in a concentration-dependent manner, and it reached the highest level when the oil concentration reached 8 g/100 g of surimi (Table 1). One explanation for this is that the oil can combine with other ingredients to coat each piece of surimi, thereby preventing the loss of water. Pramualkijja, Pirak, and Kerdsup (2016) also reported that increasing rice bran oil content from 0 to 10 g/ 100 g resulted in an increase of WHC in beef soluble protein gels. However, Shi et al. (2014) reported that when the content of vegetable oils (soybean, peanut, corn, and rap oils) were increased, the WHC and the strength of surimi gels were decreased significantly. This might be due to the decrease in protein content of surimi/fish with the increase of vegetable oil concentration as observed in the studies by Shi et al. (2014). However, in the present study, surimi content was kept constant among all treatments. Another reason might be that oil occupied the void spaces of the protein matrix and formed a firmer structure which could trap more water and oil. The holding of fluids by the surimi gels network mainly depends on the strength of gels. Mao and Wu (2007) reported that when the cross-link density of the kamaboko gels from grass carp (Ctenopharyngodon idellus) was higher, the expressible water was lower, and the WHC of the gel was higher.

Fig. 2 presents the effects of camellia tea oil on the sensory characteristics of surimi gels. Changes (P < 0.05) in the sensory properties of surimi gels were found when oil was added into surimi gels, except for elasticity. For the color and texture, the scores of surimi gels significantly increased with the addition of camellia tea oil (P < 0.05), which were in agreement with the results from whiteness and texture profiles of surimi gels (Tables 1 and 2), respectively. Camellia tea oil could improve the taste and flavour of surimi gels. For overall acceptability, there was significant difference between control and surimi gels with camellia tea oil. In general, the panelists accepted surimi gels with 8 g/100 g camellia tea oil in term of color, taste and flavour, texture, and overall acceptability. 3.7. Dynamic rheological properties To study the effects of camellia tea oil on surimi gelation in relation to textural properties of surimi gels, dynamic rheology was conducted to measure the storage modulus (G0 ) (Fig. 3). The G0 value of surimi with the oil was higher than the control, and it increased with the increase of oil concentration (0e8 g/100 g of surimi). This suggests synergistic interaction between camellia tea oil and surimi proteins, which resulted in enhanced gelation.

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Fig. 1. Micrographs at 2000 (up) and 5000 (down) of surimi gels with different concentrations of camellia tea oil by Cryo-SEM: (A) control (without oil); (B) oil concentrations of 4 g/100 g surimi; (C) oil concentrations of 8 g/100 g surimi.

Fig. 2. Effect of camellia tea oil on the sensory characteristics of surimi gels at camellia tea oil concentrations of 0 g/100 g ( 8 g/100 g ( ) and 10 g/100 g ( ), respectively.

Several studies also showed similar conclusions that the G0 of myofibrillar protein was increased significantly with lard or peanut oil (Wu et al., 2009) and the G0 values of Alaska pollock surimi were markedly increased with u-3 oil (Pietrowski et al., 2012). The increase of G0 value of surimi protein might be because the oil

), 2 g/100 g (

), 4 g/100 g (

), 6 g/100 g (

),

globules filled out the void spaces within the gel matrix during thermal gelation (Debusca et al., 2013). Therefore, fortification with camellia tea oil enhanced the gelation as was supported by TPA and shear force data.


Fig. 3. Effect of camellia tea oil on the storage modulus (G0 ) of surimi gels at camellia tea oil concentrations of 0 g/100 g ( 8 g/100 g ( ) and 10 g/100 g ( ), respectively.

3.8. Soluble protein in gels Table 3 displays the changes in soluble protein of surimi gels, taken as a measure of ionic bonds, hydrogen bonds, and hydrophobic interactions. It can be seen that the addition of camellia tea oil results in significant changes (P < 0.05) in the soluble protein of surimi gels. With the increase of oil concentrations, the index of ionic bonds and hydrophobic interactions was increased, while the index of hydrogen bonds was decreased. Hydrophobic interactions, hydrogen bond rearrangement, and covalent bonds were reported to play important roles in the ncheznetwork forming of surimi gels (Meng et al., 2005; Sa Gonz alez et al., 2008). The changes in hydrophobic interactions might be related to the ability of the oil to affect protein-protein interactions and change the protein environment of surimi gels. It was supposed that external oil could interact with proteins in whey protein gel matrices via hydrophobic and physical interactions (Dickinson & Chen, 1999). The increase of hydrophobic interactions would further change the TPA properties of surimi gel as shown in Table 2. Ionic bonds play very important roles in the stability of tertiary and quaternary structures in proteins, and they are usually formed between two amino acid residues with opposite charges through attractive Coulombic forces (Zhang, 2013). One explanation for the increase in the index of ionic bonds is that the oil provides a hydrophobic environment, which causes unfolding of protein molecules, leading to exposure of the previously buried

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), 2 g/100 g (

), 4 g/100 g (

), 6 g/100 g (

residues and changes in the protein structure and electrostatic interactions of proteins (Meng et al., 2005). The changes in hydrogen bonds were consistent with those reported by Li (2011), who showed that the addition of fat decreased the content of hydrogen bonds in pork meat gels. The decrease in the index of hydrogen bonds in the presence of oil may be partly due to an increase in the oil-protein interactions and a decrease in the protein-protein interactions, meaning that there fewer hydrogen bonds between proteins. Another explanation is related to the fact that the increase in oil concentration is associated with a decrease in the water content of surimi gels, suggesting that the hydrogen bonds between water and water or protein would decrease. All of the above changes in the indexes of hydrophobic interactions, ionic bonds, and hydrogen bonds might lead to changes in the protein three-dimensional network structure and physicochemical properties of surimi gels (Zhang, 2013).

3.9. Raman spectrometric analysis 3.9.1. Protein secondary structure The amideI band of surimi gels exhibited maximum scattering from 1661.35 cm1 to 1665.13 cm1 when the oil concentration increased from 0 to 10 g/100 g of surimi, indicating changes in the structure of surimi proteins (Fig. 4A). The effects of camellia tea oil on secondary structures of surimi proteins were analyzed according to the method of Alix, Pedanou, and Berjot (1988), and the

Table 3 Effect of camellia tea oil on soluble protein of surimi gels. Content of camellia tea oil (g/100 g of surimi)

Index of ionic bonds (mg/mL)

0 2 4 6 8 10

1.131 1.564 1.755 1.776 2.220 2.156

± ± ± ± ± ±

0.029d 0.015c 0.107b 0.030b 0.042a 0.047a

),

Index of hydrogen bonds (mg/mL) 1.131 0.867 0.655 0.338 0.317 0.254

± ± ± ± ± ±

0.027a 0.015b 0.092c 0.045d 0.058d 0.054d

Index of hydrophobic interactions (mg/mL) 8.710 ± 0.090d 11.237 ± 0.601c 11.586 ± 0.647bc 12.495 ± 0.045ab 13.023 ± 0.581a 13.171 ± 0.540a

Data were given as mean values ± standard deviation (n ¼ 3). Different letters within the same column indicate significant differences (P < 0.05) between mean values.

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results, which were displayed as percentages of a-helix, b-sheet, bturn, and random coil, are shown in Fig. 4B. The results show that the percentage of a-helix tended to decrease while the percentages of b-sheet, b-turn, and random coil structures tended to increase with increases in oil content. The amide III band, which involves C-N stretching and N-H inplane bending vibrations of the peptide bond, can also provide information about protein secondary structure (Shao et al., 2011). Fig. 4A shows that the intensity of the band in the range of 1230e1350 cm1, which can be attributed to b-sheet formation, increased with the increase in oil content, and this result was consistent with the change in the amide I band. The changes in amide I and III bands were in line with the report nez-Colmenero, and Ruízof Herrero, Carmona, Pintado, Jime Capillas (2011), who found that the percentage of b-sheet structure of sodium caseinate tended to increase when it was adsorbed at the olive oil interface. Shao et al. (2015) also reported that pork meat protein a-helix content tended to decrease and the b-sheet content tended to increase when interacting with fat/soybean oil. The decrease in a-helix content and increase in b-sheet content might result from protein-lipid interactions or interactions among adsorbed protein molecules (Howell, Herman, & Li-Chan, 2001; vre, Subirade, & Paquin, 2007). Shao et al. (2011) reported Lee, Lefe that the increase in b-sheet structure was due to interactions of protein-lipid and protein-protein in the emulsion systems during the heating process. When oil is present in surimi paste, a greater proportion of surimi protein would be exposed to hydrophobic side chains and inserted into oil droplets, thereby changing the structure of surimi protein and generating stronger lipid-protein interactions and further affecting the properties of surimi gels. 3.9.2. Changes of local environment of surimi protein To monitor the effects of camellia tea oil on the microenvironment of proteins in surimi gels, the intensity of the 760 cm1 peak for tryptophan residues and the relative intensity ratio (I850/I830) for tyrosine doublet in Raman spectra were measured (Fig. 4C). It can be seen that the relative intensity of 760 cm1 tended to increase with the increase of oil concentration. This can be interpreted as the oil providing a hydrophobic environment in surimi. Li-Chan (1996) reported that when tryptophan residues were moved from a hydrophobic micro-environment to a polar aqueous solvent, the intensity of the band near 760 cm1 region decreased. Shao et al. (2015) also reported that there was an increase in the normalized intensity of the tryptophan band near 758 cm1 in fat cream layer or soybean oil cream layer compared to raw meat. The relative intensity ratio of the tyrosine doublet (I850/I830) of surimi gels decreased from 1.06 to 0.82 when the oil concentration was increased from 0 to 10 g/100 g of surimi. One explanation for this is that tyrosine might be buried in a more hydrophobic environment nchez-Gonza lez et al., 2008). Meng et al. when the oil is added (Sa (2005) also reported that the Raman intensity ratio of the tyrosine doublet in BSA decreased when it was adsorbed in the interface of mineral oil or corn oil. The changes in local environment of surimi protein indicate that the presence of oil could alter the micro-environment of proteins and induce the exposure of hydrophobic groups, further affecting the properties of surimi gels (Meng et al., 2005). This is in line with the above results that showed an increase in the index of hydrophobic interactions with increasing oil concentration (Table 3). Fig. 4. Effect of camellia tea oil on Raman spectra (500-1800 cm1) (A), protein secondary structure (B), normalized intensity of the 760 cm1 peak and relative intensity ratio of tyrosine doublet (I850/I830) (C) of surimi gels with different concentrations of camellia tea oil (0e10 g/100 g). A: 0 g/100 g ( ), 2 g/100 g ( ), 4 g/100 g ( ),

6 g/100 g ( ), 8 g/100 g ( ) and 10 g/100 g ( ); B: a-helix ( 1 turn ( ), random coil ( ); C: 760 cm1 ( ), I1 850 cm/I830 cm ( as the mean values ± standard deviation (n ¼ 3).

), b-sheet ( ), b). Data are given


569

3.9.3. PCA of Raman spectra PCA was performed to gain information regarding systematic spectral variations in relation to different oil treatments. Fig. 5A showed the PCA-score plot for Raman spectra of surimi gels in the 500-1800 cm1 range based on the concentration of camellia tea oil (0e10 g/100 g of surimi). PC 1 and PC 2 explained 55.24% and 17.39% of the spectral variation in the data set respectively. The first principal component (PC 1) was able to show the general protein structural changes in surimi gels with 0e10 g/100 g camellia tea oil. Fig. 5B shows the loading plot for PC 1, which revealed that surimi gels with 0e10 g/100 g camellia tea oil were positively correlated to the high intensity of bands at Trp (760 cm1), amideIII (1238 cm1), CH bending (1340 cm1) and amide-I(1665 cm1) regions; in contrast, there were negative correlations for the bands from Tyr (830 cm1 and 850 cm1) and C-C stretch (940 cm1). Among them, the bands at 1238 cm1 and 1665 cm1 revealed bsheet information while the band at 940 cm1 revealed a-helix information (Berhe, Engelsen, Hviid, & Lametsch, 2014). Therefore, Fig. 5B indicates that the secondary structure and local environment of surimi protein changed with the increasing oil concentration. Moreover, the increased b-sheet content and decreased ahelix content were consistent with the results in Fig. 4B, while the increased intensity of Trp and decreased intensity of Tyr were in line with the results in Fig. 4C. PC 2 scores discriminated surimi gels with and without 2 g/ 100 g camellia tea oil. The loading plot for PC 2 (Fig. 5C) showed that there was a positive contribution from bands at 1010 cm1 (Phe) and 1121 cm1 (CN stretch) and a negative contribution from bands at 1340 cm1 (CH bending) and 1450 cm1 (CH3, CH2 and CH bending), which might due to changes in protein or camellia tea oil or protein-lipid interactions involving CH groups (Meng et al., 2005). 3.10. Correlation analysis Generally speaking, changes in protein structure might play important roles in the textural properties of surimi gels. To test this, correlation analysis among hardness, springiness, hydrophobic interactions, a-helix, and b-sheet was conducted, and the results are shown in Table 4. It was found that the hardness of surimi gels was positively correlated to the b-sheet content (r ¼ 0.958, P < 0.01) and hydrophobic interactions (r ¼ 0.944, P < 0.01) and negatively correlated to the a-helix content (r ¼ 0.958, P < 0.01) of surimi proteins. This result was in line with Beattie, Bell, Farmer, Moss, and Patterson (2004) and Herrero, Carmona, Lopez-Lopez, and JimenezColmenero (2008), who reported that a higher amount of b-sheet was positively correlated to increased sensory toughness in beef and higher penetration force values in cooked meat batters. Kang et al. (2014) also found that increased b-sheet content and decreased a-helix content improved the hardness and cooking yield of frankfurters. Therefore, the increase in hardness of surimi gels when oil was added might be because the hydrophobic interactions between lipid and protein and b-sheet content of surimi gels were increased. The hydrophobic interactions of surimi gels were positively correlated to the b-sheet content of surimi protein (r ¼ 0.906, P < 0.05), and this may be because oil changes the protein local environment, leading to more hydrophobic groups exposed and subsequent changes in protein secondary structure, such as the formation of more b-sheets. 4. Conclusions This study demonstrated that camellia tea oil significantly changed the physicochemical properties and protein structures of surimi gels (P < 0.05). In general, the addition of 0e8 g/100 g of

Fig. 5. Principal Component Analysis (PCA) of Raman spectra (500-1800 cm1) from surimi gels with different concentrations of camellia tea oil (0e10 g/100 g). PC-score plot (PC 1 vs PC 2) signed according to the oil concentration (A) and PC-loading plot for PC 1 (B) and PC 2 (C). PC 1 and PC 2 explain 55.24% and 17.39% of the variation in the Raman spectra (n ¼ 12) respectively. A: 0 g/100 g (-), 2 g/100 g (C), 4 g/100 g (:), 6 g/100 g ( ), 8 g/100 g ( ), 10 g/100 g ( ).

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Table 4 Correlation analysis among hardness, springiness, hydrophobic interactions, a-helix and b-sheet of surimi gels.

Springiness Hydrophobic interactions a-helix b-sheet a b

Hardness

Springiness

Hydrophobic interactions

a-helix

0.701 0.944a 0.958a 0.958a

0.684 0.580 0.580

0.906b 0.906b

1.000a

Correlation is significant at the 0.01 level. Correlation is significant at the 0.05 level.

camellia tea oil increased the hardness, shear force, whiteness, WHC, overall acceptability, and G0 of surimi gels in a concentrationdependent manner. It is possible that the protein wrapped oil may become saturated when the oil concentration is at 8 g/100 g, as all of the indexes changed very little beyond this concentration. Camellia tea oil significantly increased the index of ionic bonds and hydrophobic interactions and decreased the index of hydrogen bonds of surimi proteins, possibly resulting from changes in surimi protein network structure. Cryo-SEM demonstrated that camellia tea oil could occupy the void spaces of surimi protein matrix and formed a firmer structure. Raman spectroscopy revealed that the increased oil concentrations changed the secondary structure and local environment of surimi protein, leading to an increase in hydrophobic interactions and b-sheet content and a decrease in ahelix content. Correlation analysis showed that the hardness of surimi gels was negatively correlated to the a-helix content and positively correlated to the b-sheet content and hydrophobic interactions of surimi gels. Therefore, camellia tea oil could improve the properties of surimi gels by changing the protein environment and forming more b-sheets in surimi proteins. To get the best properties for the surimi gel, a concentration of 8 g/100 g camellia tea oil is recommended. Acknowledgments This work was supported by the National Science Foundation of China (31371799 and 31471613). References Alix, J. P., Pedanou, G., & Berjot, M. (1988). Fast determination of the quantitative secondary structure of proteins by using some parameters of the raman amide I band. Journal of Molecular Structure, 174, 159e164. Alvarez, D., Xiong, Y. L., Castillo, M., Payne, F. A., & Garrido, M. D. (2012). Textural and viscoelastic properties of pork frankfurters containing canola-olive oils, rice bran, and walnut. Meat Science, 92, 8e15. AOAC. (1990). Official methods of analysis. Washington, DC: Association of Official Analytical Chemists. Beattie, R. J., Bell, S. J., Farmer, L. J., Moss, B., & Patterson, D. (2004). Preliminary investigation of the application of Raman spectroscopy to the prediction of the sensory quality of beef silverside. Meat Science, 66, 903e913. Benjakul, S., Visessanguan, W., & Kwalumtharn, Y. (2004). The effect of whitening agents on the gel-forming ability and whiteness of surimi. International Journal of Food Science & Technology, 39, 773e781. Berhe, D. T., Engelsen, S. B., Hviid, M. S., & Lametsch, R. (2014). Raman spectroscopic study of effect of the cooking temperature and time on meat propeins. Food Research International, 66, 123e131. Chen, C. G., Gerelt, B. I., Jiang, S. T., Nishiumi, T., & Suzuki, A. (2006). Effects of high pressure on pH, water-binding capacity and textural properties of pork muscle gels containing various levels of sodium alginate. Asian-Australasian Journal of Animal Science, 19, 1658e1664. Chojnicka, A., Sala, G., De Kruif, C. G., & Van de Velde, F. (2009). The interactions between oil droplets and gel matrix affect the lubrication properties of sheared emulsion-filled gels. Food Hydrocolloids, 23, 1038e1046. Claus, J. R., & Hunt, M. C. (1991). Low-fat, high added-water bologna formulated with texture-modifying ingredients. Food Science, 56, 643e652. Debusca, A., Tahergorabi, R., Beamer, S. K., Partington, S., & Jaczynski, J. (2013). Interactions of dietary fibre and omega-3-rich oil with protein in surimi gels developed with salt substitute. Food Chemistry, 141, 201e208. Dickinson, E., & Chen, J. (1999). Heat-set whey protein emulsion gels: Role of active and interactive filler particles. Journal of Dispersion Science and Technology, 20, 197e213.

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