2020 Volume 26 Issue 2 Pages 185-194
The effect of fish oil supplementation on the quality of frozen and unfrozen sasa-kamaboko was evaluated. Alaska pollock surimi was emulsified with 0% (control), 2.5%, or 5% fish oil, and after shaping and heating, the resultant sasa-kamaboko samples were stored at −20 °C for 2 or 4 weeks. Compared to the control, emulsification of fish oil into sasa-kamaboko was found to significantly (p < 0.05) improve the water-holding capacity, whiteness, and breaking strength before freezing and after frozen storage. Microscopic observation revealed that the fish oil particles were well distributed even after frozen storage, and there was only slight structural damage. It was indicated that fish oil supplementation affected the formation and distribution of ice crystals, which had positive effects on the quality of thawed sasa-kamaboko. Our findings may be used to improve the manufacture process of high-quality, frozen surimi-based products.
Surimi is an intermediately processed seafood product used in the formulation/ fabrication of a variety of finished surimi-based products, such as kamaboko, chikuwa, fish sausage, and crab-analogues (Okazaki et al., 2014). Sasa-kamaboko is a traditional, bamboo leaf-shaped surimi-based product that is highly popular as a snack food in the north-east part of Japan. The smooth and soft texture of sasa-kamaboko is produced during the heating process, and the characteristic flavor and appetizing surface color are achieved by baking at a higher temperature (Ooizumi et al., 2013). Recently, some manufacturers have developed new types of value-added seafood products with a focus on functionality and prime quality using fish oil. Fish oil is an excellent source of long-chain polyunsaturated fatty acids, such as eicosapentaenoic acid (EPA, 20:5 n-3) and docosahexaenoic acid (DHA, 22:6 n-3), which have been shown to have health benefits (He K., 2009). Miyata et al. (2018) found that a diet of sasa-kamaboko containing fish oil lowered liver lipid levels in a mouse model with fatty liver disease, and thus may potentially prevent and reduce the risk of developing fatty liver disease. However, after production, this kind of surimi-based product is usually transported and sold at temperatures of 10 °C or lower and requires to be consumed within 7 days, greatly limiting the sales potential and volume. Thus, an alternative preservation method is needed to ensure the safety and quality of products before marketing and consumption.
Freezing is one of the most commonly used preservation methods that can maintain quality, prevent spoilage, extend the shelf-life, and ensure the safety of foods (Coombs et al., 2017). Frozen foods are also convenient for consumer selection and consumption. Although freezing and frozen storage may be effective for surimi-based products, certain products can be easily damaged by such methods, thus deteriorating their quality. Especially, sasa-kamaboko is rarely distributed in a frozen state. Negative impacts may include increased drip loss and structural and textural deterioration upon thawing, leading to undesirable sensory attributes and lower commercial product quality (Jia et al., 2018a), causing economic losses within the industry. Thus, improvements in both the freezing-tolerance and nutritional value of surimi-based products contribute to the added value of products, and may be achieved by developing new techniques and new types of products.
Previously, we found that emulsification with fish oil can positively affect the quality of model types of heat-induced surimi gels, e.g., by improving the gel-forming ability, physical properties, water-holding capacity (WHC), and emulsifying stability (Okazaki et al., 2002; Okazaki et al., 2006; Fukushima et al., 2007; Gao et al., 2018a). Moreover, it was confirmed that the emulsified surimi gels following frozen storage showed a higher WHC, decreased frozen damage, and smaller ice crystals than surimi gels without emulsification (Niu et al., 2016). It was, therefore, hypothesized that fish oil supplementation may also have positive impacts on frozen/thawed commercial surimi-based products by preventing quality deterioration. The present study was conducted to confirm the potential implication of fish oil emulsification on the quality changes in sasa-kamaboko, a kind of surimi-based product, during frozen storage.
(1) Sample preparation The sasa-kamaboko samples were produced at a local professional company in Miyagi Prefecture, Japan. The formulation and processing procedures are described in Table 1 and Figure 1, respectively. Refined fish oil [DHA-22K, produced by Maruha Nichiro Corporation, containing more than 99% triglyceride with DHA and EPA contents of 19.0% and 3.5%, respectively]. The saturated fatty acid and unsaturated fatty acid contents in the fish oil were 34.7% and 65.3%, respectively. The fish oil was sealed with N2 and stored in plastic bottles at −0 °C until use. The sasa-kamaboko samples were prepared with fish oil to surimi content ratios of 2.5% and 5.0%. The ratio of protein to water content was maintained at a constant level (18.0%) in all of the samples (shown in Table 2). Thawed Alaska pollock (Theragra chalcogramma) surimi (A grade, DAP, USA) was cut into blocks and placed in a refrigerated Stephan cutter mixer with vacuum (UM44E, Stephan food processing machinery, Germany). The surimi cubes were ground at low speed (1,500 rpm). Salt and ice water were then added to the chopped surimi, which was further ground at low to high speed (1,500 rpm – 3,000 rpm). Subsequently, the surimi was vigorously mixed with 2.5% or 5.0% fish oil under vacuum following the manufacturer's production manual and the knowledge acquired in previous experiments with model emulsified surimi gels. We increased the mixing speed from low speed (1 500 rpm for 20 s) to high speed (3,000 rpm for 40 s) to effectively emulsify the fish oil. The control sample (without added fish oil) was prepared following the same procedures. All chopping procedures were performed at temperatures below 10 °C. The surimi mixture was then chopped with seasonings (dried egg white, sugar, vitamin C, and amino acids), starch, and ice water. The total amount of ice water added was 5 000 g for each kind of sasa-kamaboko. After shaping, the surimi paste was heated by far infrared rays, gas-baked to obtain the final product, and then cooled. The above-mentioned process was performed in the same conditions and using the same equipment as those used for preparing commercial products. The samples were transferred to the laboratory within 24 h of production by cold storage transportation, and individually vacuum-packaged in polyethylene bags (A7-2030, Starplastic, Japan) and frozen in an air-blast freezer (KQF-5AL, Air Operation Technologies, Inc., Japan) at a rate of 0.37 ± 0.01 °C/min. Quality evaluation before freezing was performed on the day of packaging. Samples (0% (control), 2.5%, or 5.0% of fish oil) were evaluated after frozen storage at −20 °C for 2 and 4 weeks. After frozen storage, the samples were thawed in a refrigerator at 4 °C overnight before quality evaluations were performed.
| Item | Surimi | Water | Seasonings | Fish oil | Starch | Salt |
|---|---|---|---|---|---|---|
| Control (0) | 10 000 | 5 000 | 1 518 | 0 | 300 | 250 |
| Emulsified (2.5%) | 10 000 | 5 000 | 1 518 | 250 | 300 | 250 |
| Emulsified (5.0%) | 10 000 | 5 000 | 1 518 | 500 | 300 | 250 |
Experimental scheme to prepare the frozen sasa-kamboko supplemented with fish oil.
| Item | Crude protein | Moisture | Crude oil | Protein/Moisture |
|---|---|---|---|---|
| Control (0) | 13.0 | 71.9 | 0.1 | 18.0 |
| Emulsified (2.5%) | 12.8 | 70.9 | 0.9 | 18.0 |
| Emulsified (5.0%) | 12.6 | 69.9 | 1.7 | 18.0 |
(2) WHC measurement The WHC of the sasa-kamaboko was determined from the drip loss, including the thawing drip and expressible drip, and water retention rate.
a) Thawing drip The thawing drip was calculated as a percentage of the weight of thawed drip (W2) from the initial weight of the sample before freezing (W1). The thawing drip was calculated using the following equation:
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b) Expressible drip Expressible drip was measured according to the method described by Ng (1987) with slight modifications. Thawed samples (after removing the thawing drip) were cut into thin slices weighing (Wa) ∼1 g (range: 0.975–1.025 g) and placed between four Whatman filter papers (No. 2A on the outer sides and No. 4A on the inner sides; Whatman International Ltd., UK). The samples were compressed at 10 kg/cm2 for 20 s using a rheometer (RE-3305, Yamaden Ltd., Japan), and re-weighed (Wb). The expressible drip was calculated using the following equation:
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c) Drip loss Drip loss was calculated using the following equation:
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d) Water retention rate The water retention rate was calculated as the ratio of remained water after thawing and expressing (Wc) compared with the initial total moisture content in the sample before freezing (Wd). The water retention rate was calculated using the following equation:
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3) Moisture content in sasa-kamaboko The moisture content of the sasa-kamaboko samples was determined according to the method described by the Association of Official Analytical Chemists (1990).
(4) Microscopic observation after frozen storage
a) Microstructure observation in the frozen state The microstructure was evaluated using an optical microscope. The microstructure of samples in the frozen state was observed after frozen storage for 4 weeks, and the samples were cut into 5 mm cubes and directly fixed in 10% formalin-ethanol solution for 5 weeks at −20 °C (Martino and Zaritzky, 1988; Kageyama and Watanebe, 1988; Hagiwara et al., 2002). After fixation, the samples were embedded in a gelatin solution followed by frozen section compound (FSC22, Leica Microsystems Corp., Germany) and then immersed in n-hexane cooled by dry ice. After adhering adhesive film (Cryofilm type II C(9), SECTION-LAB Co., Ltd., Japan) to the surface of the embedded samples, they were sliced into 10-µm-thick sections using a cryostat (CM-1500, Leica Microsystems Corp., Germany) at −20 °C. All of the prepared films were placed on the bottom of glass slides, stained with 1% eosin solution, mounted (Kawamoto, 2003; Suzuki, 2008; Jia et al., 2018b), and then observed using a microscope (BZ-9000, Leica Microsystems Corp., Germany). The equivalent diameter of the void portion of frozen samples was analyzed using a commercial image processing and analysis software WinROOF (Ver. 6.5.3, MITANI Corp., Japan) (Jia et al., 2018b).
b) Observation of the microstructure and fish oil particles in thawed samples The observation of thawed samples after frozen storage was performed following the method of Koji et al. (1976) with slight modifications (Gao et al., 2018b). Samples were cut into thin slices (∼2.0 mm thickness) and fixed in a 4% paraformaldehyde phosphate buffer solution in the refrigerator for 12 h, followed by fixation in 2% osmium tetroxide for more than 24 h. The fixed samples were then cooled and sliced into 2-µm-thick sections using a microtome (LS-113, Yamato Kohki Industrial Co., Ltd., Japan). An adhesive film (Cryofilm type II C(9), SECTION-LAB Co., Ltd., Japan) was attached to the surface of the sections. The sections were dewaxed in an alcohol solution, stained with 1% eosin solution for 60 s, and then washed in distilled water for 5 min. The sections were mounted onto glass slides with two drops of dimethylbenzene solution, and then observed using a microscope (BZ-9000, Leica Microsystems Corp., Germany).
(5) Color measurement The color of the samples was measured according to the CIE L*/ lightness, a*/ redness, b*/ yellowness system, using a color reader (Minolta Cr-13, Minolta Co., Ltd., Japan). Each measurement was repeated six times on the center surface of each thawed sample and the whiteness was calculated using the following equation:
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(6) Measurement of physical properties Physical properties of the samples were assessed using breaking strength (gf) and breaking deformation (mm). These parameters were measured via a puncture-test using a rheometer (Rheoner 2, Yamaden Co., Ltd., Japan). Thawed samples (12.5 mm in height) were carefully positioned on a plate and measurements were conducted using a 5 mm spherical plunger on the center of the thawed samples at a stable speed of 1 mm/s (Jia et al., 2019). Measurements were repeated four times per sample.
(7) Statistical analysis For each treatment, measurements were obtained in quadruplicate (except for color measurement, where six replicates were used) and expressed as means ± standard deviations. The statistical analyses were performed using a two-way analysis of variance (ANOVA) followed by Tukey's multiple comparison test. Differences were regarded to be statistically significant at p-values < 0.05. All the data were analyzed using SPSS software for Windows version 17.0 (SPSS Inc., Chicago, IL, USA).
(1) Effect of fish oil supplementation on the WHC of sasa-kamaboko during frozen storage The effect of fish oil emulsification on the drip loss of sasa-kamaboko before freezing and after frozen storage is shown in Figure 2, wherein the expressible drip is denoted by the black regions. The filter paper pressing method has been widely used to measure WHC owing to its simplicity and reliability. Akahane et al. (1988) reported that the expressing method may be a reliable and easy standard to reflect the stability of water that exists in kamaboko.
Effect of fish oil supplementation on the drip loss of frozen/thawed sasa-kamaboko. Mean values and standard deviation are show. (n = 4). Different uppercase letters indicate significant differences upon different contents of fish oil within the same period (p < 0.05). Within the same contents of fish oil, different lowercase letters indicate significant differences due to the storage period (p < 0.05).
A comparison of the expressible drip of emulsified sasa-kamaboko and control samples before freezing revealed a notable decrease in samples emulsified with 2.5% or 5.0% fish oil before freezing (p < 0.05), even though the protein concentration in the matrix, which is strongly related to the gel-forming ability and WHC of surimi gels, decreased with an increase in fish oil supplementation. A previous study also demonstrated that the emulsification of fish oil into surimi positively affected the WHC of heat-induced model surimi gels (Okazaki et al., 2002), and these improvements were enhanced with the increase in fish oil amount (Fukushima et al., 2007). Gani et al. (2018) showed that the expressible moisture content decreased with the increase in virgin coconut oil to 15% and uniform oil droplets were distributed inside the gel network of heat-induced croaker surimi gels. Zhou et al. (2017) also found that the addition of 8% camellia tea oil significantly increased the WHC.
After frozen storage, the drip loss (including thawing drip and expressible drip) of the emulsified samples was also significantly lower than that of the control samples, and the drip loss decreased with an increase in fish oil content for all frozen storage periods (p < 0.05). A previous study reported that a relatively higher storage temperature (e.g., −20 °C or higher) and longer storage period of heat-induced surimi gels cause severe quality deterioration after frozen storage (Jia et al., 2019). Therefore, in this study, a temperature of −20 °C was selected for the frozen storage of products, and for the clear observation of quality differences among samples over a short storage period. Notably, the commercial industry often uses this condition for the storage of such products.
In heat-induced surimi gels, water is entrapped in the protein network after heating, which forms ice crystals during freezing. After thawing the samples, the free water that could not be reabsorbed to its origin would be released as the thawing drip (Xiong, 1997; Benjakul and Visessanguan, 2011). The thawing drip data of emulsified samples was lower than that of the control samples, and it decreased with an increase in fish oil content for all frozen storage periods. The thawing drip of all samples was higher after 4 weeks compared to 2 weeks of frozen storage. The level of the growth rates of thawing drip was suppressed by increased fish oil content and was 7.9%, 7.6%, and 2.3% in the control, 2.5% and 5.0% emulsified samples, respectively. In contrast, expressible drip showed a slightly different trend. The amount of expressible drip from the control and 2.5% emulsified samples hardly changed during frozen storage for 4 weeks, but that from the 5.0% sample increased with storage time. However, the increase in the total drip loss of the 5.0% emulsified sample was not obvious during the period between 2 and 4 weeks of frozen storage, while the total drip loss of the control samples increased significantly with the increase of frozen storage period (p < 0.05).
The observed phenomena may be partly related to the differences in moisture content between the control and emulsified sasa-kamaboko samples, since the moisture content of the control samples was slightly higher than that of the emulsified samples, as the fish oil diluted the sample compositions (as shown in Table 2). Therefore, evaluation of the water retention rate was further performed by comparing the ratio of remained moisture content with the initial total moisture content before freezing, which was used as the WHC parameter (Figure 3). The same tendencies presented in Figure 2 were confirmed, in which the emulsified samples were compared to the control samples for the same storage periods. In addition, the 5.0% emulsified sample showed the highest and relatively most stable water retention rate over the entire storage period, and had less obvious decreases in the water retention rate between 2 weeks and 4 weeks of frozen storage (p < 0.05) compared to the other samples. As previously mentioned, the emulsification of fish oil had positive effects on the WHC of sasa-kamaboko not only before freezing, but also after frozen storage.
Effect of fish oil supplementation on the water retention rate of frozen sasa-kamaboko. Mean values and standard deviation are shown (n = 4).
(2) Microscopic observation after frozen storage Microstructural observation is often used to evaluate the quality of frozen meat products (Zhan et al., 2018). To further confirm the effect of fish oil supplementation on the improvement of WHC, observations of the microstructure of the samples were performed after 4 weeks frozen storage, both before (i.e., in the frozen state) and after thawing. These observations involved the microstructure in the frozen state and fish oil particles inside the thawed samples. Light microscopy is a known experimental evaluation method for the quality of frozen products (Kiani and Sun, 2011).
a) Microstructure observation in the frozen state The microscopic observations of all sasa-kamaboko samples after frozen storage are shown in Figure 4. The pink regions in the images indicate the position of the protein matrix after staining of the protein and its structures, and the white regions mainly reflect the position of ice crystals in the frozen state.
Microscopic images of emulsified sasa-kamaboko after 4 weeks frozen storage in frozen state. The scale bar is 100 µm.
After 4 weeks of frozen storage, the white regions of the emulsified samples appeared to be smaller than those of the control samples, and the average diameters of the control samples, 2.5%, and 5.0% fish oil-emulsified samples were 167.5 ± 9.1 (n = 97), 104.7 ± 4.0 (n = 148), and 90.9 ± 11.0 µm (n = 235), respectively. In addition, the white regions were observed to be smaller and more uniform in the emulsified samples with a higher content (5.0%) of fish oil.
During frozen storage, the decline in the WHC is related to structural changes following ice crystal formation, resulting in excessive drip loss after thawing. The final size of ice crystals is a function of the rates of nucleation and ice crystal growth and also of the final temperature (Mehl et al., 1993; Xiong, 1997; Martino et al., 1998). In this experiment, although all of the samples were prepared to be frozen and stored under the same conditions, larger voids were detected in the control sasa-kamaboko samples, and the WHC of these samples was lower than that of the emulsified samples after frozen storage. It was also previously confirmed that the emulsification of fish oil into smaller ice crystals prevents the growth of ice crystals after f rozen s torage compared to cont rol gels (without emulsification) (Niu et al., 2016). Jia et al. (2019) reported that large ice crystals may damage the microstructure of heat-induced surimi gels and cause water leakage from the gel network. These strongly suggest the possibility that the free water content was reduced in the emulsified heat-induced surimi gels in relation to the addition and emulsification of oils and fats, attributable to their physiochemical effects on oil-protein and protein-protein interactions (Gao et al., 2018a; Zhou et al., 2016). Wu et al. (2009) reported that emulsified sausages have a compact gel structure filled with protein-coated fat globules that interact with the protein matrix through disulfide bonds. Yang et al. (2016) also suggested that in sausages, free water and molten fat flow from a three-dimensional gelled network due to protein hydrogen bonding, rearrangement of disulfide bonds, and exposure of hydrophobic groups. Thus, the stability changes in the water contained in the emulsified gels may affect the formation of smaller ice crystals, which may be one of the reasons for the reduction in thawing drip. The larger ice crystal formation and greater thawing drip in the model control surimi gels were also demonstrated (Niu et al., 2016).
b) Observation of fish oil particles and microstructure after thawing The results of microscopic observation of the thawed samples after 4 weeks of frozen storage are shown in Figure 5. The black regions within the sample images mainly indicate the fish oil particles after fixing and staining with osmium tetroxide solution, while the white regions mainly indicate structural damage caused during freezing and frozen storage. The fish oil-emulsified sasa-kamaboko presented small and well-distributed fish oil particles (< 5.0 µm in equivalent diameter) after frozen storage and thawing, as reported for model surimi gels (Gao et al., 2018b). This shows that the fish oil emulsified with surimi under strong mixing conditions could be stabilized and distributed around the structure not only after heating (Fukushima et al., 2007; Okazaki et al., 2006; Gao et al., 2018a), but also after frozen storage and thawing.
Microscopic images of emulsified sasa-kamaboko after 4 weeks frozen storage and thawing. The scale bar is 40 µm.
Thanasukarn et al. (2004) reported that the formation of thick interfacial protein layers around palm oil droplets provides good protection to the emulsion system during freezing. According to Figure 5, the structural damage in both the emulsified samples after thawing was less than that in the control; the control sample displayed severe frozen damage, presumably because of the formation of large and irregular ice crystals.
A previous study involving the emulsification of fish oil into surimi showed some positive effects on the characteristics of heat-induced surimi gels, and further demonstrated that these effects may be related to the protein membranes surrounding fine oil particles formed by emulsification (Okazaki et al., 2006; Gao et al., 2018a). Zhou et al. (2019) also illustrated that the formation of a more condensed three-dimensional network structure by the addition of fat significantly improved the WHC, and that water is restricted by the physicochemical interactions between protein-coated lipid globules and the protein matrix. Therefore, it was speculated that another reason for the lesser frozen damage, mainly caused by smaller ice crystal formation, in the emulsified sasa-kamaboko was that the formation of protein membranes surrounding fish oil particles by emulsification strengthened the gel structure to be more compact, thereby restricting the formation and growth of ice crystals. Furthermore, the water contained in a compact gel structure is considered to be effectively incorporated into the gel by capillary action and is difficult to release from the gel network, as previously reported for emulsion-type sausages (Tornberg, 2005). It was also revealed that controlling the formation and growth of ice crystals would create smaller, homogeneous, and regular ice crystal distribution, resulting in a better microstructure and an improved quality of frozen meat (Zhan et al., 2018). The frozen kamaboko is not consumed in the frozen state (the way ice creams are), but is eaten without heating after thawing. Therefore, drip loss, which occurs particularly after thawing, is a very serious problem, and the maintenance of small ice crystals is essential for the quality improvement of frozen kamaboko.
As described above, in the emulsified sasa-kamaboko, the frozen damage of the gel structure and the drip loss after thawing were also reduced compared to the control. It was speculated that these effects were due to a combination of results: improvement in the WHC of surimi gels by the emulsification itself, and strengthening of the gel structure by the protein membranes surrounding the fish oil particles with emulsification.
(3) Effect of fish oil supplementation on the change in color and physical properties Color is one of the important parameters of surimi-based products. It is fundamental to attracting consumer selection from first impressions and has a great impact on product preferences. In particular, the white color of the products tends to be emphasized in sasa-kamaboko. Formulations involving the addition of oil or fat can improve the color of products via the light scattering effect when oil is commuted with the protein and water during processing (Park, 2013; Mi et al., 2017). Differences were observed in the color of the control and emulsified sasa-kamaboko samples, as revealed by the parameters of L*, a*, and b*, and whiteness calculated from the L*, a*, and b* values (Table 3). All emulsified samples showed higher whiteness values than the control samples (p < 0.05), and the samples emulsified with 5.0% fish oil showed the highest whiteness values. Fukushima et al. (2007) found that the Hunter whiteness of heat-induced gels increased with the addition of fish oil. Zhou et al. (2017) also reported that the whiteness of surimi gels significantly improved with the increase in camellia tea oil concentration. Significant changes in the L* and whiteness values were observed in the control samples with the extension of the frozen storage period. Data showed that the decline in whiteness was possibly linked to the decrease in the L* value in the sasa-kamaboko samples during storage. The whiteness of the control sample was lower and significantly more unstable than that of the fish oil-emulsified sasa-kamaboko samples (p < 0.05). This finding may be attributed to a lower light-scattering effect resulting from structural modifications caused by the growth of ice crystals in the gel matrix (Otero et al, 2017). Ma et al. (2015) also reported that large ice crystals could induce disorganization of the protein network in frozen shrimp and a decrease in L* values after frozen storage. In addition, the relatively stable whiteness of the samples supplemented with fish oil during frozen storage may be related to less frozen damage inside the samples. A less damaged structure could retain more water in the emulsified samples. On the other hand, in the control sample, heavy frozen damage and lower whiteness values were observed. Fish oil supplementation was found to directly improve and maintain the whiteness of emulsified sasa-kamaboko before freezing and during frozen storage.
| Item | Sample | L* | a* | b* | Whiteness | ||
|---|---|---|---|---|---|---|---|
| Before freezing | Control (0) | 75.1 ± 0.9Ca | −2.7 ± 0.1Ab | 3.6 ± 0.3Ca | 74.7 ± 0.8Ca | ||
| Emulsified (2.5%) | 78.5 ± 0.3Ba | −1.1 ± 0.2Bb | 8.9 ± 0.3Ba | 76.7 ± 0.4Ba | |||
| Emulsified (5.0%) | 81.4 ± 0.5Aab | −0.8 ± 0.2Cb | 9.8 ± 0.6Aa | 79.0 ± 0.6Aa | |||
| 2 weeks | Control (0) | 73.9 ± 0.6Cb | −2.9 ± 0.1Aab | 2.9 ± 0.4Cb | 73.6 ± 0.6Cb | ||
| Emulsified (2.5%) | 78.3 ± 0.5Ba | −1.3 ± 0.2Ba | 8.3 ± 0.3Bb | 76.8 ± 0.5Ba | |||
| Emulsified (5.0%) | 82.0 ± 0.5Aa | −1.2 ± 0.1Ba | 8.7 ± 0.2Ab | 79.9 ± 0.5Aa | |||
| 4 weeks | Control (0) | 73.0 ± 0.6Cc | −3.0 ± 0.0Aa | 2.7 ± 0.1Cb | 72.7 ± 0.6Cc | ||
| Emulsified (2.5%) | 77.4 ± 0.4Bb | −1.3 ± 0.2Ba | 8.1 ± 0.4Bb | 76.0 ± 0.5Ba | |||
| Emulsified (5.0%) | 81.1 ± 0.4Ab | −1.2 ± 0.1Ba | 8.5 ± 0.2Ab | 79.2 ± 0.4Aa |
Emulsified sasa-kamaboko showed increased a* and b* values with increasing contents of added fish oil (p < 0.05). A higher a* value indicates more red coloration. A higher b* value indicates more yellow coloration, and these values increased from 3.6 in the control to 9.8 in the emulsified 5.0% sasa-kamaboko (Table 3). This large increase in yellowness (5.0% emulsification) may be a result of the large number of fish oil particles and uniform distribution of the small fish oil droplets throughout the surimi matrix. Fish oils are known to be yellow in color because they typically contain carotenoids that are yellow-orange (Jaswir et al., 2011). As shown here, a* noticeably higher and more stable L* value and whiteness were observed in the emulsified sample, which relates to the light scattering effect by emulsification and less frozen damage during frozen storage.
The results of the changes in physical properties of sasa-kamaboko during frozen storage are shown in Table 4. Before freezing, a significant increase in the breaking strength was observed with the addition of fish oil (p < 0.05). It is known that surimi gels with lower protein concentrations show poorer thermal gelation characteristics and, as a result, show poorer textural properties (Rawdkuen et al., 2009). Although the protein concentration of the surimi matrix slightly decreased with an increase in the added fish oil, the emulsified samples had higher breaking strength and deformation, which may reflect the effect of fish oil emulsification on the improvement of gel-forming ability. The emulsified samples showed the texture characteristic of higher breaking strength and deformation, which may reflect the effect of fish oil emulsification on the improvement of gel-forming ability. The texture properties of emulsified surimi gels should be investigated in detail, including from the viewpoint of commercial products. As previously mentioned, the positive effects of fish oil emulsification on the gel-forming ability of emulsified heat-induced surimi gels may be related to the formation of protein membranes surrounding fine oil particles in the emulsified surimi gels (Gao et al., 2018a).
| Item | Sample | Breaking strength (gf) | Deformation (mm) |
|---|---|---|---|
| Before freezing | Control (0) | 518.5 ± 27.0Bc | 11.9 ± 0.9Aa |
| Emulsified (2.5%) | 754.6 ± 19.7Aa | 12.6 ± 0.8Aa | |
| Emulsified (5.0%) | 789.0 ± 9.3Aa | 12.3 ± 0.4Aa | |
| 2 weeks | Control (0) | 650.8 ± 67.2Bb | 10.0 ± 0.6Ab |
| Emulsified (2.5%) | 731.4 ± 88.9Ba | 9.7 ± 0.5Ab | |
| Emulsified (2.5%) | 731.4 ± 88.9Ba | 9.7 ± 0.5Ab | |
| 4 weeks | Control (0) | 674.0 ± 53.5Aa | 8.4 ± 0.2Bc |
| Emulsified (2.5%) | 709.2 ± 166.7Aa | 9.2 ± 0.2Bb | |
| Emulsified (5.0%) | 742.3 ± 62.9Aa | 9.7 ± 0.7Ab |
Evaluation of the breaking strength after 4 weeks frozen storage revealed that the breaking strength of the control sasa-kamaboko increased significantly while that of the emulsified samples remained stable, which may reflect the extensive freezing damage caused to the control samples. The larger ice crystal size and higher drip loss of the control samples may have resulted in changes in protein concentration, which may have led to the formation of a dense layer of protein and an increase in the breaking strength (Jia et al., 2018b). In contrast, the breaking strength of the emulsified samples decreased slightly during frozen storage; however, the changes were not as obvious as for the control samples. Jia et al. (2019) suggested that the breaking strength of two-step heated surimi gels decreased at the initial stage of frozen storage, which was accompanied by a decrease in S-S bonding in the gels. Although the detailed mechanisms of this phenomenon have not yet been clarified, similar changes were observed in the breaking strength of emulsified sasa-kamaboko during frozen storage. On the other hand, all samples showed decreased deformation after frozen storage, which may influence the elasticity of thawed sasa-kamaboko. However, the deformation of emulsified samples did not decrease significantly during 2 to 4 weeks of frozen storage, whereas that of the control samples decreased significantly (p < 0.05). These results suggested that the changes in the physical properties of frozen sasa-kamaboko are slightly suppressed by emulsification.
This study confirmed that the new type of emulsified sasa-kamaboko could improve the freezing-tolerance and maintain the quality of sasa-kamaboko products after freezing and frozen storage. In the emulsified sasa-kamaboko, the ice crystal size was found to be smaller and more uniform, and the frozen damage of the tissue structure and the drip loss after thawing were also reduced compared to the control. It was speculated that these effects were due to the combined results of 1) the improved WHC of surimi gels by the emulsification itself and 2) mitigation of ice crystal growth by strengthening of the gel structure formed with protein membranes surrounding fish oil particles generated by emulsification. There were also notable differences in whiteness and physical properties between emulsified and control sasa-kamaboko samples. The results indicate that emulsification with fish oil, used for fortifying the nutritive value and functionality, is an effective method to minimize frozen damage of frozen sasa-kamaboko. The results of the present study recommend the application of emulsification technology in the manufacturing of surimi-based products.
Acknowledgements This study was supported by the project “A Scheme to Revitalize Agriculture and Fisheries in Disaster Areas through Deploying Highly Advanced Technology” from the Ministry of Agriculture, Forestry and Fisheries, Japan. We also appreciated the valuable support from MARUHA NICHIRO, Japan.
