Physical properties of Alaska pollock surimi gel as affected by emulsified level and heating rates

Surimi paste was put into a polystyrene chamber between two titanium plate electrodes that faced each other at opposite ends of the chamber. The electrodes were 40 mm wide, 50 mm high, and 0.05 mm thick. This was done to test the electrical conductivity (EC) of the samples. The separation between the two electrodes was 30 mm. A consistent voltage of 50 volts was maintained across the electrodes until the desired temperature was achieved. According to Liu et al. (2007), the samples’ immediate Z, X, Rp, and Rs values (Z: impedance, C: capacitance, Rp: resistance parallel mode, Rs: resistance serries mode) were found using an liquidity coverage ratio (LCR) m (Hitester TESTER3532-50, HIOKI, Nagano, Japan) at a single point in time, when the temperature reached 5°C and the frequency was set to 20 kHz. The experiments were replicated five times. The EC, σ (S m−1), was determined based on electrode shape and resistance using the following equation:

where R is the electrical resistance of the food (Ω), L is the gap between the two electrodes (m), and A is the surface area of the electrodes in contact with surimi paste (m²).

A small amount of the emulsion was placed on a cooled slide glass (No. S2445, Matsunami glass, Bellingham, WA, USA) and the cover glass was spread thinly. The dispersion state of the fish oil was observed under fluorescence microscope BZ-9000E (Keyence, Osaka, Japan). The particle size distribution of the oil spheres on the microscopic image was measured and analyzed for fish oil particle diameter using image analysis software ImageJ developed by WayneRasband (ImageJ - http://imagej.nih.gov/ij/docs/index.html). The total area on the screen of particles of each size was calculated for each size, and the ratio to the area occupied by all oil balls was determined. The average particle size was determined to evaluate the effect of mixing conditions.

Breaking force (gf) (gel strength) and penetration distance (mm) (deformability) of surimi gels were determined using a texture analyzer (RE-3350B, Yamaden, Tokyo, Japan) as described by Rawdkuen et al. (2007). Six cylindrical samples (2.5 cm long) were prepared and equilibrated to room temperature (~25°C) for 1 h before analyses. A spherical probe (diameter 5 mm) penetrated into the center of the gel sample perpendicularly at a constant crosshead speed (60 mm/min). The force to puncture the gel (breaking force) and the distance from the point where the probe touched the gel to the point where the gel was punctured.

Expressible moisture was measured according to the method described by Rawdkuen & Benjakul (2008), with modifications. Gel samples were cut into thin slices (1.000 ± 0.025 g) (W1) and placed between 4 layers of filter paper (No. 4A paper on the inner side and No. 2 paper on the outer side). The filter papers were purchased from Advantec, Tokyo, Japan. Samples were then placed between 2 plastic plates (diameter: 4 cm) and a constant force (10 kg/cm²) was applied on top of the samples for 20 s using a rheometer (RE-3305, Yamaden, Tokyo, Japan) with 2 plastic plates attached to the crosshead (top) and the metal plate (bottom). Following this, the weight of the sample was measured again and recorded as W2. The expressible drip content was determined by applying the following equation and presenting the results as a percentage of the sample’s weight:

Samples were placed at room temperature (about 25°C) for 1 h prior to the color measurement. The color values of surimi gels samples were determined by Color Reader CR-13 (Konica Minolta, Marunouchi, Japan). Lightness (L*), redness (a*), and yellowness (b*) values were recorded respectively. The whiteness (W) was calculated using the equation:

Six replicates of measurements were taken.

Surimi pastes with varying emulsification levels, made using varied mixing speeds, were subjected to a dynamic rheological test. A rheometer (HAAKE MARS 60 Rheometer, Thermo Fisher Scientific, Yokohama, Japan) with a 35 mm parallel plate geometry was used to measure changes in storage (G′) or elastic modulus (G″). Once a linear response was observed in the viscoelastic area, an oscillation with a frequency of 1 Hz and a deformation of 1% was employed. The temperature sweep was recorded while the temperature increased from 5°C to 90°C using two distinct heating rates of 3 and 40°C/min. To reduce the evaporation of water from surimi pastes during measurement, a layer of paraffin oil was added to cover the samples (Gao et al., 2018).

Six samples were used for each measurement. The entire experiments were repeated twice. Mean values were subjected to analysis of variance (ANOVA) at a level of 95% significance using MS-Excel 2010 (Microsoft, Redmond, WA, USA). Error bars shown in figures indicate standard deviations of the values. Differences between the samples were compared statistically using Tukey’s method at a significant level of p < 0.05.

The EC of the research material is a vital attribute in ohmic heating technology. Table 3 exhibited a decline in the EC of emulsified surimi paste when fish oil was added. The mixing circumstances exerted a substantial influence on the EC, and the EC exhibited a positive association with the escalation in mixing velocity. EC is a complex connection between time and temperature, and it directly affects the rate of ohmic heating. Bozkurt & Icier (2010) state that the EC of food is influenced by not just the fat content, but also the size and distribution of the fat particles. The arrangement of fat globules in a meat product or mixture will impact the thickness of the samples and the growth in the size of the fat particles, resulting in a reduction in the movement of ions and consequently a decrease in EC (Okazaki et al., 2002). Changes in EC can lead to adjustments in heating conditions. To ensure the effective design of the ohmic heating system, it is crucial to gather and analyze data on the EC of foods, as well as their impact on key factors such as the frequency of the AC voltage, voltage gradient, and food matrix composition. Published data exists for high-frequency processes such as microwave, but it is not relevant for low-frequency ohmic heating processes. Only a limited number of articles (Fryer et al., 1993; Salengke & Sastry, 2007; Tulsiyan et al., 2008) have documented the electrical conductivities of actual food matrices, which consist of both liquid and solid components. It is commonly noted that the EC of liquids is greater than that of solids (Sastry & Palaniappan, 1992). The impact of the solids content in liquid foods is relatively less important but nonetheless noteworthy. Typically, when the proportion of solids increases (excluding salt), the EC decreases.

An investigation was conducted to examine the impact of mixing circumstances on the size of oil particles in surimi and fish oil. The microscopic observation photograph of several emulsions and the particle size distribution determined through image analysis are presented in Fig. 4 and Table 4, respectively. It was observed that the size of oil particles decreased as the mixing speed increased. Based on the microscopic examination depicted in the images of hand mixing and mixing at 300 rpm, numerous oil droplets of varying sizes and irregular shapes were seen. As the mixing speed increased, the oil particle size decreased dramatically, and the shape became more uniform. By employing intense agitation, the oil droplets achieved a size of approximately 3–5 μm. The oil particles were measured using ImageJ software, and the results were consistent with Table 4. The findings demonstrated that the velocity at which the components were mixed had a significant impact on the size of the droplets in microemulsions. Furthermore, high-speed mixing functioned as an emulsifying agent, hence enhancing the stability of the emulsion.

Fig. 4. Microscopic observation of fish oil droplets in emulsified surimi paste obtained by different mixing conditions.

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The impact of emulsifying fish oil on the breaking strength and breaking deformation of surimi gel generated by heating is illustrated in Fig. 5A and 5B. The emulsified level had a noticeable impact on the properties of the emulsified gel, with a statistically significant effect (p < 0.05). Insufficient emulsification of fish oil, either by hand or at a speed of 300 rpm, results in lower gelation ability compared to surimi without fish oil at the same protein concentration. The phrase “the product quality is reduced” is believed to pertain to the gel-forming capacity when fish oil is in a mixed state. However, if the emulsified state acquired with the gradual increase in speed during the awarding process, as used in this work, is attained, the inclusion of fish oil enhances the elasticity of the final product rather than diminishing it. Although the exact mechanism by which finely divided fish oil affects the ability of surimi to form a heated gel is not well understood, Jost et al. (1986) shown that it can decrease the size and uniformity of fat globules in whey protein gelation. Assuming that the creation of protein network structure is not obstructed, the increase in the ratio of protein adsorbed to the lipid-water interface, as the oil sphere surface area increases, leads to the immobilization of fat globules and the reinforcement of the gel structure. Thus, it appears that the gelation of surimi protein can be estimated in a similar manner.

Fig. 5. Breaking strength (A), breaking deformation (B), and EMC (C) of Alaska Pollock surimi gels heated ohmically at different heating rates. Bars represent the standard deviation (n = 6). Different letters on the bar including the control indicate a significant difference (p < 0.05). EMC, Effective moisture content; OH, ohmic heating; WB, water bath.

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The gel characteristics of emulsified surimi paste were similarly affected by the heating rate caused by OH. Intense agitation reduced the number of oil droplets, created a consistent and stable emulsified surimi paste, and enhanced the gel strength throughout both water-bath heating and OH processing. The presence of a stable protein barrier surrounding the emulsified oil particle would enhance the stability of the gel structure and increase its breaking strength, as compared to a surimi gel that does not have additional fish oil.

Fig. 5C shows the measurable amount of liquid that is lost from a heated gel when it is subjected to a pressure of 10 kg for a duration of 20 s. The findings indicated a consistent decrease in the effective moisture content (EMC) of the gel formed by heat exposure as the amount of fish oil added increased. In general, when the EMC decreases, the water holding capacity (WHC) of surimi gels increases, leading to better gel strength and a more compact protein network. In their study, Fukushima et al. (2007) observed that the WHC of emulsified surimi gel improved with increasing amounts of fish oil. Additionally, the gel structure prevented oil droplets from being released when subjected to a compression force of 1 MPa. The addition of fish oil to the emulsified and heated gel results in a decrease in water content as the emulsification of fish oil increases. Specifically, the use of non-emulsified heated gel and 10% emulsified heated gel resulted in a drop of expressible drips from 6.5% to 4.0% when subjected to ohmic heating at a rate of 3°C/min. Uniform improvement in water retention was observed in all fish surimi, resulting in increased stability of the water content in the heated gel due to fish oil emulsification. However, we have shown that the gel strength decreases as the fish oil level increases (Fig. 5A), which is significant and goes against the previous thinking. There are two primary factors that can explain this phenomenon. The introduction of fish oil gradually decreases the moisture content in the test sample, leading to a reduced equilibrium moisture content in the gel with high oil content. Additionally, the presence of fish oil droplets plays a crucial role in preventing water migration within the gel network, thereby ensuring the stability of the oil-water-protein complex system (Liu et al., 2016). The ohmic heating gel has a reduced EMC compared to the WB group, which is strongly linked to the desirable features of the gel (Fig. 5). This conclusion aligns with the outcomes of several prior studies (Nguyen et al., 2020; Okazaki et al., 2002; Zhou et al., 2017).

The inclusion of fish oil resulted in a significant increase in the whiteness of all the gels, except for hand mixing, as seen in Table 5 (p < 0.05). The emulsified gel exhibited increased whiteness when made with greater mixing speeds (p < 0.05). The samples that were treated with fish oil and mixed vigorously exhibited the highest level of whiteness among all the samples (p < 0.05). The researchers found that the addition of soybean oil to mixed surimi made from bigeye snapper (SSA and SA grade) and mackerel resulted in an increase in whiteness, as demonstrated in the study conducted by Benjakul et al. in 2004. The enhanced whiteness of surimi when mixed with vegetable oil can be due to the phenomenon of light scattering caused by the emulsion formed when oil is combined with surimi and water (Park, 2000). Shi et al. (2014) found that the whiteness of silver carp surimi gel increased as the oil concentration rose. The addition of corn oil to Alaska pollock surimi gels resulted in increased whiteness, as seen in a study by Pietrowski et al. (2011). Manual mixing of the surimi paste resulted in poor mixing of the oil particles, leading to a reduction in the whiteness. The object had a golden or light-yellow color. By using fish oil and ensuring enough mixing speed, the little oil droplets were able to spread evenly throughout the gel matrix, resulting in enhanced light scattering. Hence, the inclusion of fish oil, particularly when vigorously mixed, directly enhanced the whiteness of the surimi gel.

There was no discernible disparity in the level of whiteness seen when comparing various heating rates or heating methods. Put simply, the choice of heating method (either water bath or ohmic heating) or heating rate (either 3°C/min or 80°C/min) did not have a noticeable impact on the whiteness of emulsified surimi gel (p < 0.05).

Fig. 6 illustrates the rheological characteristics of surimi paste with and without the addition of 10% fish oil, under various mixing speeds. The value of G′ gradually grew and reached its maximum at approximately 30°C. The initial rise in G′ at low temperatures may be attributed to the molecular interactions between actomyosin and the construction of a protein network structure by hydrogen bonding between protein molecules. This results in the formation of a less stable three-dimensional gel network (Zhang et al., 2013). Following that, the value of G′ declined at a temperature of 40°C and then sharply dropped to its lowest point at approximately 50°C for all the samples. At elevated temperatures, the protein aggregate formed was destabilized due to the destruction of a significant number of hydrogen bonds (Liu et al., 2007). In this temperature range, the gel structure can be broken down by the degradation of myosin, which is facilitated by endogenous proteolytic enzymes. This process is commonly referred to as ‘Modori’ (Gani & Benjakul, 2018). With continued heating, the value of G′ grew until it reached 65°C and then stayed nearly constant up to 90°C for the control sample, hand mixing, and mixing at 300 rpm. In the cases where the remaining speed was 900 rpm and the mixing was vigorous, the value of G′ consistently grew until the temperature hit 90°C. This value was notably greater than the values observed in the previously described cases. The occurrence could be attributed to the development of a stable gel structure linked to increased protein aggregation (Liu et al., 2007). There was a strong correlation between the increasing G′ values and the breaking strength values of the samples. The evolution and trends of the G′ as a function of heating temperature exhibited consistent behavior across all samples. In general, the G′ value was greater in the emulsified sample that was mixed at high rates, suggesting the development of a more solid gel network. Disrupting the muscle protein filaments can occur when mixing speeds are varied, such as with hand mixing or slow mixing, which can hinder the entanglement process during gelation. This was demonstrated by the decrease in the value of G when the mixing speed was slow. This outcome aligned with the findings on the strength and deformation at the point of failure.

Fig. 6. Storage modulus (G′) of AP surimi (RA grades) gel added fish oil and mixing at different speed and heated at 3°C/min and 40°C/min during heating from 5°C to 90°C.

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