Abstract
This study investigated the effect of overdrying potato starches on surimi products. The chemical composition of protein and chemical interactions, gel solubility, and protein conformation of the mixture of surimi gel protein, respectively, with 8% native potato starch and with 8% overdrying potato starch were investigated. The results show that the starch increased the insoluble protein content. In terms of the chemical interactions, the overdrying potato starch increased the amount of hydrogen bond and nondisulfide covalent bond and decreased the amount of ionic bond, which might stabilize the network structure of protein gel. The analysis of Raman Spectroscopy shows that more α-helices turn into random coin structure after the starch was added, which is conducive to higher strength and a better water retention ability of the surimi product.
1. Introduction
Starch is an important ingredient in surimi seafood products since it would affect textural and physical characteristics of surimi fish protein gels. For instance, it can improve surimi gel strength, modify texture, reduce cost [1], and improve freeze-thaw stability [2]. The starch could replace a portion of the fish protein while maintaining desired gel properties due to its water-holding ability [3].
For the mechanism of improving the quality of surimi gel with the addition of starch, there are three theory models which have been widely recognized: the cavity model raised by Couso et al. [4] at 1998; the filling extrusion model raised by Yang and Park [5]; and the pack and bundle effect model raised by Kong et al. [6]. In general, these three models were from the physical point of view, explaining the relationship between the starch and gel properties of surimi. However, the influence of starch on surimi gel protein function from the chemical perspective has not been reported yet.
In this study, compared with pure minced surimi gel, the effects of starch on surimi gel protein were investigated under the addition of 8% native and overdrying potato starch, in order to provide a theoretical basis for the mechanism of interaction between starch and protein in surimi.
2. Materials and Methods
2.1. Materials
Native and overdrying potato starches were obtained from France ROQUETTE. AA grade of silver carp surimi was purchased from Hong hu Hong ye Aquatic Food Co., Ltd. Polyvinyl chloride (PVC) plastic casing were purchased from Longhai Ri sheng plastic color printing packaging Co. Ltd.
2.2. Samples Preparation
Frozen surimi was thawed at 4°C overnight and then diced and chopped for 3 min. 3% salt was added to the surimi and it was cut for 2 min until the surimi paste was fully decentralized. Then 8% potato starches were added and the mixture was mixed at a low speed for 3 min until the slurry was uniform (adjust the final moisture content to 78%). The final intestinal samples were made according to simulated industrial formula. 8% fat, 5% ice egg white, 4% soy protein, 0.6% sugar, and 0.6% of the MSG were added to the slurry, and then the mixture was mixed for 2 min. This fish paste was chopped and mixed through the exhaust into a diameter of 30 mm plastic casing, made of approximately 20 cm in length of intestine. The intestines were placed at 40°C water bath for 30 min, the gelation; then at 90°C for 30 min; then in ice water to cool the samples rapidly. The samples were stored at 4°C overnight.
2.3. Determination of Protein Composition
According to Parker’s method [7], the composition of protein was determined as follows.
Determination of water soluble protein content: 100 mL low phosphate buffer (0.05 mol/L KCl-0.01 mol/L NaH2PO4-0.03 mol/L Na2HPO4) was added to 10 g of chopped surimi gel samples. The mixture was mixed and homogenized for 2 min and then stirred for 3 h and centrifuged at 4°C, 5000 r/min for 10 min. Protein content in the supernatant was measured by BCA method.
Determination of salt soluble protein content: 100 mL high phosphate buffer solution (0.5 mol/L KCl-0.01 mol/L NaH2PO4-0.03 mol/L Na2HPO4) was added to 10 g chopped surimi gel samples. The mixture was mixed and homogenized for 2 min and then stirred for 3 h and centrifuged at 4°C, 5000 r/min for 10 min. Protein content in the supernatant was measured by BCA method.
Determination of insoluble protein content: the measured total protein in surimi gel (crude protein) was subtracted by the content of water soluble and salt soluble protein content.
2.4. Determination of Chemical Forces
4 g minced fish sausage sample was, respectively, added to 20 mL of 0.05 mol/L NaCl (S1), 0.6 mol/L NaCl (S2), 0.6 mol/L NaCl + 1.5 mol/L urea (S3), 0.6 mol/L NaCl + 8 mol/L urea (S4), and 0.6 mol/L NaCl + 8 mol/L urea + 0.5 mol/L beta mercaptoethanol (S5). The mixture was mixed and homogenized for 2 min. The homogeneous liquid was centrifuged for 15 min [8]. The protein in the supernatant was determined by Lowry method. The differences in protein dissolved in S2 and S1 solutions said the contribution of ionic bond, the differences in S3 and S2 solutions indicated a hydrogen bond contribution, the differences dissolved in S4 and S3 solutions indicated the contribution of hydrophobic interactions, and the difference of protein content dissolved in S4 and S5 solutions indicated disulfide bond contribution [9]. All results were expressed as the percentage of protein content accounting for the total amount of protein.
2.5. Determination of Solubility of Gel
20 mL 20 mmol/L was added to 1 g chopped surimi gel sample, pH 8.0 in Tris-HCl buffer solution, and mixed and homogenized for 2 min. The buffer contains 1% (w/v) SDS, 8 mol/Lurea and 2% (v/v) β-mercaptoethanol. The mixed solution was heated at 100°C for 2 min and centrifuged at 10,00 for 30 min after stirring at room temperature for 4 h. After centrifugation, 50% (m/v) trichloroacetic acid (TCA) was added in 10 mL supernatant and the protein was precipitated. Mixing liquid was stored at 4°C for 18 h and then centrifuged at 10,00 for 30 min. Precipitation dissolved in 30 mol/L NaOH 0.5 mL after washing with 10% (m/v) concentration of TCA. Lowry method was used to determine protein content. The dissolution rate was expressed as the percentage of total protein content of the protein in the solvent [10]. Total protein content was measured by the amount of protein dissolved in 0.5 mol/L NaOH of surimi gel.
2.6. FT-Raman Spectroscopic Analysis
The sliced gel samples were stick to a layer of foil glass slide and placed on the object loading table of Raman Spectroscopy. The scanning range was from 300 to 3800 cm−1 [11]. Income spectrum for baseline correction with a benzene ring in 1003 ± 1 cm−1 ring vibration (due to the little microenvironmental impact) is the normalized spectra within standard [12]. Peak Fit software was used for original spectrum curve fitting of amide I band, and the overlap in different bands was completely resolved. The percent of the protein secondary structure was calculated by the integral area.
2.7. Statistical Analysis
All measurements were conducted at least three times. The least significant difference (LSD) at 5% was applied to define the significant difference. All analyses were performed using SPSS software v 19.0.
3. Results and Discussion
3.1. Protein Composition
Due to the presence of starch in the starch-surimi system, the total protein (TP) contents would be different among samples of CON (control, i.e., pure surimi gel), NPS (surimi gel with native potato starch), and LMPS (surimi gel with overdrying potato starch). Therefore, the total protein contents of surimi gel samples were first determined. The water soluble protein (WSP), salt soluble protein (SSP), and insoluble protein content (ISP) are expressed as the fraction accounting for the total amount of protein (Table 1). It showed that potato starch had a significant effect on surimi gel protein composition (). Soluble and salt soluble protein contents of LMPS were significantly lower than those of CON and NPS, while the insoluble protein content of LMPS was significantly higher. It is of note that the surimi gel formation is essentially a process of salt soluble protein crosslink and then gradually converted into the insoluble protein [11]. Therefore, we proposed that the presence of overdrying potato starch could promote the crosslink formation of salt soluble protein in the surimi gel, thus enhancing the quality of the gel.
3.2. Chemical Forces
The network structure in the surimi gel system is mainly maintained through the interactions among and within protein molecules, such as chemical bonds, ionic bonds, hydrogen bond, hydrophobic interaction, and covalent bonding [13] (Table 2). It can be seen from Table 2 that the potato starch has a significant effect on the formation of chemical interactions in the surimi gel proteins ().
There is a large amount of ionic bonds in the frozen surimi, thus salt ions are generally needed to break the ionic bond in order to disperse the protein, and then the dispersed protein could form the gel with elastic structure after heat treatment [14]. We found that the ionic bond was reduced in our starch-surimi gel systems, which may be because the presence of the starch blocks the formation of ionic bond between and within proteins. It could be concluded that the potato starch is beneficial to the elasticity of the surimi gel. Hydrogen bonds in surimi gel play an important role in the stability of the bound water and increase the strength of surimi gel during the cooling process [13]. Table 2 shows that the potato starch especially the overdrying starch significantly increased the amount of hydrogen bonds in the surimi protein ().
The hydrophobic sites in surimi protein would be exposed in the water environment after heat treatment. In order to maintain the stability of the thermodynamic system, the hydrophobic interaction is enhanced, resulting in the aggregation of protein to form a gel network [13]. In this study, the hydrophobic interactions between proteins significantly decreased () after the adding of potato starch. The interaction between the starch and the surrounding water may change the water status around the proteins, thus affecting the hydrophobic interactions within the surimi gel systems.
When the heating temperature is higher than 40°C, disulfide bonds are thought to be the main covalent bonds that can promote the formation of protein gel [15]. Compared with the pure surimi system, the amount of disulfide bonds was significantly reduced in native starch-surimi system (). However, the amount of disulfide bonds after adding the overdrying potato starch was higher than that in the native starch-surimi system. It indicates that such negative effect on the formation of disulfide bonds induced by the potato starch might be affected by the structure of starch granules.
3.3. Solubility of Gel
The solvent containing SDS, urea, and beta mercaptoethanol is usually used to dissolve nondisulfide covalent bonds in protein [16]. Therefore, the solubility of surimi gel refers to the formation of nondisulfide covalent bond that is also involved in one of the main chemical interactions to form surimi gel network structure [11]. As seen in Figure 1, the presence of starch especially overdrying potato starch significantly reduced the surimi gel dissolution rate (), indicating that overdrying starch significantly increased the amount of nondisulfide covalent bonds. Presumably, starch granules absorbed water in the heating process; therefore the contents of protein within the continuous phase and endogenous glutamine transfer enzyme (TGase) were increased.
3.4. Protein Conformation
Raman Spectroscopy (Figure 2) can be used to investigate intermolecular interactions among protein molecules: it could provide relative intensity information about amino acid side chains, peptides, and the vibration frequency of the polysaccharide backbone.
The Raman spectrum band within 1600~1700 cm−1 is called the amide I band, which gives information about the protein secondary structure. Specifically, the spectra of the bands of 1650~1660, 1665~1680, and 1660~1665 cm−1 ranges are, respectively, corresponding to α-helix, β-sheet, and random coil of the protein. The band peaks of CON, NPS, and LMPS were located in 1653 cm−1 and 1667 cm−1, in 1654 cm−1 and 1662 cm−1, and in 1656 cm−1 and 1664 cm−1. It can be seen that the addition of two kinds of starch makes a blue shift of the characteristic peaks of the helix structure, indicating a decrease of α-helix [17]. Liu et al. claimed the reason why the amount of α-helix in silver carp surimi decreased with the formation of the gel structure was the transition of α-helix into random coil structure [11] (Table 3). Table 3 shows the quantitative analysis of the protein secondary structure in the amide I band, which reports that the overdrying potato starch has a significant influence on the transition of α-helix into random coil structure: more α-helices were transformed into random coils, promoting the aggregation and interaction of protein molecules; therefore the starch seems to render the network of surimi gel more stable.
In terms of the range of 3100~3500 cm−1, the Raman spectrum peak refers to the stretching movement of O-H, which is related to the intermolecular vibration of water molecules bonded with hydrogen bond [18]. In this study, the O-H stretching band peaks of CON, NPS, and LMPS were located in 3280, 3277, and 3274 cm−1, and the corresponding strength values were 0.76, 0.84, and 0.89, respectively. The decrease in the wave number of these peaks implies a stronger hydrogen bond with water, and a higher strength value reveals a larger amount of bound water molecules [12, 19]. Hence, potato starch could improve water binding and retention ability of the surimi gel, which is conducive to the formation of the gel matrix during the heating process. The effect of overdrying potato starch seems more notable.
4. Conclusions
The overdrying potato starch is proved to improve properties of surimi gel. It promoted the formation of insoluble protein, decreased the amount of ionic bonds, and increased that of nondisulfide covalent bonds and hydrophobic interaction, which would facilitate the formation of gel network structure. The data about protein conformation also confirms these positive effects of potato starch on the water binding and retention ability of surimi gel. Currently, because of the complex food system and pretreatment, it is difficult to use small interactions such as π-π stack to explain the improvement of the properties of surimi gel.
Additional Points
Practical Applications. The overdrying potato starch can improve properties of surimi gel, as it promoted the formation of insoluble protein, decreased the amount of ionic bonds, and increased that of nondisulfide covalent bonds and hydrophobic interaction. Thus, potato starch, especially overdrying potato starch, can be used to improve the quality of surimi products.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This research was financially supported by the “Six Talent Peak” high-level talent project of Jiangsu Province (2015-NY-008), the Teaching and Researching Joint Innovation Funds of Jiangsu Province (Grant no. BY2015019-05), and the program of “Collaborative Innovation Center of Food Safety and Quality Control in Jiangsu Province.”