2018 Volume 24 Issue 6 Pages 999-1006
The effects of “setting-heating-freezing” procedure (SHF) and “setting-freezing-reheating” procedure (SFR) on the quality changes of surimi gels with or without starch after frozen storage were evaluated and compared. The samples were frozen and stored at −20°C for 28 days. Surimi gels without starch subjected to SFR showed larger ice crystals after freezing, greater microstructural damage after thawing, and higher total drip loss. In contrast, starch-surimi gels subjected to SFR showed less structural damage, smaller void size, and reduced total drip loss, attributed to the un-gelatinized starch granules before freezing and gelatinization after reheating. Changes in physical properties were related to changes in gel microstructure and starch granules, and a high gel strength was maintained in starch-surimi gels subjected to SFR after frozen storage. These results indicate that the SFR procedure can effectively reduce the quality changes in frozen starch-surimi gels.
Surimi-based products are processed products made from surimi, salt, and other additives (Park and Morrissey, 2000), and are now popular worldwide because of their characteristic structural properties and considerable nutritional value (Ducept et al., 2012). Additionally, the consumption of surimi-based products in both developed and developing countries is increasing continually (Endoo and Yongsawatdigul, 2014).
Cryopreservation is a common method for preserving commercial surimi-based products before they are marketed (Okada, 1966; Yang, 1997); however, the product quality often changes during frozen storage. Castro et al. (1997) reported that the water holding capacity of sardine mince gels decreased after frozen storage. Montero et al. (1997) reported that microstructural changes and chemical aggregation were observed in frozen sardine mince gels. Jia et al. (2018a) reported the quality changes of commercial surimi-based products after frozen storage. Thus, it is necessary to minimize the damage to surimi-based products during frozen storage in order to satisfy the demand for high quality products.
The “cooking-freezing-reheating” method is a popular system used to control food quality and increase shelf life (Creed, 2001; García-Arias et al., 2003). It comprises two heating steps: a pre-heating step before frozen storage and a reheating step after storage (Skjöldebrand et al., 1984). To increase their elastic characteristics, the production of surimi-based products often requires two heating steps: a pre-heating step, below 40°C, followed by a high temperature heating step (Okazaki and Kimura, 2014); thus, the “cooking-freezing-reheating” procedure can be applied to the production of surimi-based products. Moreover, in our previous study, we found that drip loss in starch-surimi gels was increased when the starch granules absorbed water before freezing and when the expanded starch granules were destroyed during frozen storage (Jia et al., 2018b). Consequently, controlling the water absorption capacity of starch granules before freezing is essential for improving water retention. The setting process is under a low temperature and is lower than that required for the gelatinization of starch; it prevents starch granules from absorbing water and expanding (Yang and Park, 1998). Thus, it is expected that the procedure of “freezing the setting gel and reheating after thawing” will reduce quality changes, especially drip loss, of starch-surimi gels.
Therefore, the objective of this study was to investigate the effects of the conventional “setting-heating-freezing” procedure and the modified “setting-freezing-reheating” procedure on the quality changes of surimi gels with or without starch after freezing and thawing.
Materials AA grade frozen Alaska Pollock surimi was purchased from the Glacier Fish Company (Seattle, WA, USA). The frozen surimi was cut into ca. 500 g blocks, vacuum sealed, and stored at −30°C until used. Potato starch was purchased from Wako Pure Chemical Industries, Ltd. (Osaka, Japan).
Surimi gel preparation Frozen surimi was thawed at −2°C and cut into small pieces (approximately 1 cm3). The surimi cubes were then chopped at 1,500 rpm for 1 min using a vacuum cutter (UMC-5, Stephan Machinery Corp, Hameln, Germany). Over the next 8 min, 3% (w/w) NaCl in iced water with (5% (w/w)) or without (0% (w/w)) potato starch was added during continued chopping at 1,500 rpm. The moisture content of the surimi gels with or without starch was adjusted to 77% using iced water. During the chopping process, the temperature was kept below 10°C using a temperature-circulating chiller (Model C-503, Sibata Scientific Technology Ltd., Tokyo, Japan). After chopping, the salted surimi paste was shaped in the casing tube and sealed. Setting gels were prepared at 30°C for 60 min, and then removed from casing tubes, cut into cylindrical pieces (25 mm in height, 23 mm in diameter), and vacuum-packed into a plastic bag.
Two procedures, SHF (setting-heating-freezing) and SFR (setting-freezing-reheating), were performed in this study, as shown in Fig. 1. For SHF, two-step heated gels were prepared from the packed setting gels with continued heating at 90°C for 30 min and cooling in iced water for 30 min. The samples were then frozen in a −20°C freezer and stored for 1 or 28 days. The temperature in the central portion of the samples was determined using a thermometer (AM-8000K, Anritsu Meter Co., Ltd., Tokyo, Japan) with temperature sensors, and the freezing rate was −0.3049°C/min. After frozen storage, the samples were thawed at 4°C.
The procedures of “Setting-heating-freezing” (SHF) and “Setting-freezing-reheating” (SFR).
For SFR, packed setting gels were directly frozen using the conditions used for the samples subjected to SHF. The samples were then reheated at 90°C for 30 min and cooled in iced water for 30 min after storage and thawing. Quality assessment was performed after thawing SHF samples and reheating SFR samples. Control gels were defined as gels without starch, and starch-surimi gels were defined as gels containing starch.
Microstructure observation Samples obtained after freezing and those obtained after thawing or reheating were first subjected to fixation. The fixative used for the frozen and thawed or reheated samples was 10% formalin, which was prepared by diluting formaldehyde (37%) (Wako Pure Chemical Industries, Ltd.) 10-fold with ethanol or deionized water (Hagiwara et al., 2002; Kono et al., 2012). Vials containing the fixative for the frozen samples were placed in the same freezer as the samples (−20°C) one day before fixation. On the day of fixation, the frozen samples were cut into 5 mm cubes on dry ice, immediately immersed into the pre-frozen fixative, and kept at −20°C for 4 weeks. For thawed or reheated samples, small pieces were cut and immersed in freshly prepared 10% formalin solution for 24 hours. After fixation, the samples were embedded in paraffin using a rotary machine (RH-12DM, Sakura Finetek Japan Co., Ltd., Tokyo, Japan), and sections were prepared using a microtome (LS-113, Yamato Kohki Industrial Co., Ltd., Saitama, Japan). Control gels were stained with 1% eosin to distinguish between the protein matrix and ice crystals or voids in the structure (Kim et al., 1987). Starch-surimi gels were stained by the periodic acid-Schiff stain to distinguish among starch granules, the protein matrix, and ice crystals or voids in the structure. Sections were then observed using a microscope (BZ-9000, Leica Microsystems Corp., Wetzlar, Germany), and the equivalent diameter and cross-sectional area of the ice crystals and voids were determined from the micrographs using image-analysis software (WinROOF, Ver. 6.5.3, Mitani Co., Tokyo, Japan).
Determination of drip loss Thawing drip and expressible drip were determined as indicators of drip loss.
Thawing drip was determined using the method of mass difference. The sample (Ws) and drip (Wd) absorbed on the filter paper were weighed. Thawing drip was presented as a ratio of drip weight and sample weight, as shown in the following equation:
 |
Expressible drip was determined after collecting the thawing drip according to the method of Shimizu et al. (1960) and Ng (1987) with some modifications; samples were cut into 2 mm-thick slices, weighed (W1 g), and placed on a double-layer filter paper, and covered with another double-layer filter paper. Samples were compressed using a creep meter (RE-3305B, Yamaden Ltd., Tokyo, Japan) at a pressure of 10 Kg for 20 s and then weighed (W2 g). The expressible drip was calculated using the following equation:
 |
Determination of physical properties A puncture test in the central portion of samples was performed using the creep meter with a plunger (spherical, 5 mm diameter) at 1 mm s−1 depression speed. The breaking strength (g) and breaking deformation (mm) were used as indicators of physical properties.
Statistical analysis Experiments were performed in at least quadruplicate and results are presented as mean ± standard deviation. One-way analysis of variance (ANOVA) and Tukey's test (P < 0.05) were performed using SPSS statistical software version 19.0 (SPSS Inc., Chicago, IL, USA).
Drip loss The drip loss of surimi gels before and after frozen storage was evaluated, and the results are shown in Fig. 2.
Drip loss of control gels and starch-surimi gels after frozen storage. Different letters indicate significant difference (P < 0.05) among samples.
Before freezing (0 day), samples in SHF were two-step heated gels, and samples in SFR were setting gels. Somewhat higher expressible drip was observed in setting gels of SFR than in two-step heated gels of SHF for both control and starch-surimi gels (P < 0.05). For two-step heated gels, after low temperature heating followed by high temperature heating, a protein network structure was formed by the generation of noncovalent bonds between myosin heavy chains (Lanier et al., 2013); such protein networks are capable of holding water (Kimura et al., 1991). More water can be bound in stronger protein network structures, resulting in a higher water holding capacity.
In contrast, weaker structures show reduced water holding capacity, resulting in higher drip loss (Chaijan et al., 2006). In the case of setting gels, during the setting process, myosin heavy chains are polymerized by the formation of non-sulfide covalent cross-linking; however, the structure is incompact without high temperature heating (Benjakul et al., 2004; Kimura et al., 1991). Thus, the setting gels showed low water holding capacity and higher expressible drip. Additionally, for two-step heated gels of starch-surimi gels in SHF, starch granules absorbed water and expanded in the gel structure after high-temperature heating (due to gelatinization) and protected the absorbed water, thereby resulting in a lower expressible drip loss than setting gels in SFR.
After frozen storage, the total drip loss of control gels and starch-surimi gels increased significantly when the frozen storage time was increased (P < 0.05). During the freezing process, ice nucleation occurred when the temperature reached the freezing point, and with continued freezing, ice crystals were formed (Takai et al., 1997). After thawing, thawing drip occurred because of the melting ice crystals and the drips were released from the gel structure. As the storage time increased, the growth of ice crystals occurred, with increasing ice crystal size (Martino et al., 1998), resulting in more drip loss from the structure after freezing. In addition, as the storage time increased, the gel structure suffered a certain degree of damage, decreasing the water holding capacity of the gels, and subsequently increasing the expressible drip.
Notably, in control gels, the total drip loss after freezing and thawing was significantly higher in gels produced using SFR than in those produced using SHF under identical storage conditions (P < 0.05). This is likely because before freezing, setting gels in SFR had lower water holding capacity because of the less compact network structure formed after low temperature heating; this resulted in a higher drip loss after frozen storage. In contrast to two-step heated gels, proteins in setting gels were partly thermally denatured after low temperature heating. These partially denatured proteins were unstable during the frozen storage, which may have led to further denaturation (Farouk et al., 2004) and formation of weaker network structures after reheating, resulting in a higher drip loss.
Furthermore, the released water could not be completely reabsorbed when samples were reheated, and therefore, a higher drip loss was observed in control gels produced using SFR. This result indicated that the SFR procedure could not increase the water holding capacity of control gels, but rather increased their drip loss.
In contrast, the drip loss in starch-surimi gels was significantly lower in the gels produced using SFR than in those produced using SHF (P < 0.05). Starch, as an important additive, was dispersed evenly in the surimi paste during chopping, absorbed water, and swelled in the surimi matrix during thermal processing upon reaching its gelatinization temperature (Lee et al., 1992). The starch granules in two-step heated gels in SHF were well swollen as the gelatinization temperature for potato starch in the surimi gels is about 56–66°C (Leach, 1965). The expanded starch granules were destroyed by the pressure of ice crystals formed during frozen storage, resulting in drip loss from the gel structure after thawing. However, in the case of setting gels in SFR, the setting temperature (30°C) was lower than the gelatinization temperature of starch; thus, the starch granules could not absorb water and swell in the gel structure. Although drip loss occurred after frozen storage and thawing, upon subsequent reheating (90°C), the temperature reached the gelatinization temperature of starch, and the starch granules could absorb the released water, thereby decreasing the drip loss. This result indicated that the SFR procedure can effectively improve the water holding capacity of starch-surimi gels after frozen storage.
Microstructure observation The microstructural changes of control gels and starch-surimi gels before and after freezing are shown in Fig. 3. The protein in control gels (Fig. 3a) is stained pink, the protein in starch-surimi gels (Fig. 3b) is stained light pink, and the starch granules are stained purple. Before freezing, protein texture changes in control gels after setting and two-step heating were observed due to the thermal denaturation of proteins. In the case of starch-surimi gels, the starch granules kept their original granule size after setting. After two-step heating, the temperature reached the gelatinization temperature, and the starch granules absorbed water, expanded, and were enlarged (Wu et al., 1985).
Changes in microstructure of control gels (a) and starch-surimi gels (b) before and after freezing. Bar, 100 µm
After freezing, ice crystals were formed, and are visible as white spaces in the figures. After thawing or reheating, the ice crystals melted, and the drips were released from the gels, resulting in voids in the structure, which are visible as white spaces in the figures. Table 1 shows the equivalent diameter and cross-sectional area of ice crystals after freezing and voids after thawing or reheating. When we compared the microstructure of control gels between the two procedures (Fig. 3a, Table 1), a significantly larger size of ice crystals and voids was observed in control gels produced using SFR than in those produced using SHF. This is likely because the compact network structure, formed in two-step heated gels produced by SHF before freezing, resulted in high water holding capacity and smaller ice crystal size than that formed in two-step heated gels produced by SFR. After thawing or reheating, voids were formed, and the gel structure could not be restored to the condition before freezing because of the destruction by ice crystals. Greater pressure on the gel matrix, caused by larger ice crystals, resulted in a larger final void size when the ice crystals melted and released the pressure (Bevilacqua and Zaritzky, 1980). Therefore, a larger void size was observed in gels subjected to SFR than in those subjected to SHF. However, different results were observed in starch-surimi gels, as shown in Fig. 3b and Table 1. After freezing, the ice crystal size was similar in samples subjected to SHF and SFR. However, after reheating, the void size in samples subjected to SFR was significantly smaller than that of samples subjected to SHF. Additionally, starch granules subjected to SHF absorbed water and expanded before freezing, attributable to the occurrence of starch gelatinization during the 90°C heating process. These expanded starch granules appeared to be damaged after freezing by the pressure of ice crystals and changed to columnar shape after freezing, as reported by Jia et al. (2018b) and Yamamoto (1966). After thawing, starch granules could not be restored to their original shape before freezing and they were unevenly distributed around the voids, which was different from the starch granules in gels subjected to SFR. The starch granules in samples subjected to SFR could not be gelatinized and they did not swell in the gel structure during the setting process; these un-gelatinized starch granules maintained their original shape after freezing. During reheating, when the temperature reached the gelatinization temperature, starch granules absorbed the released water in the structure and expanded to a globular shape, thereby decreasing the void size. Furthermore, all the above results correspond to the results of drip loss, which indicates that the SFR procedure can effectively reduce the structural changes in starch-surimi gels after frozen storage; it is, however, ineffective for the control gels.
| SHF-after freezing | SHF-after thawing | SFR-after freezing | SFR-after reheating | ||
|---|---|---|---|---|---|
| Control gel | Equivalent diameter (µm) | 153.0 ± 22.0b | 73.0 ± 2.0c | 371.6 ± 31.5a | 131.9 ± 8.2b |
| Cross-sectional area (µm2) | 15,005 ± 1561b | 4,181 ± 227c | 95,833 ± 1,586a | 13,704 ± 1,689b | |
| Starch-surimi gel | Equivalent diameter (µm) | 107.5 ± 4.0a | 99.3 ± 4.1a | 97.3 ± 3.8a | 39.3 ± 2.4b |
| Cross-sectional area (µm2) | 9,079 ± 673a | 7,758 ± 636a | 7,445 ± 578a | 1,216 ± 146b |
Physical properties Breaking strength and breaking deformation are important quality parameters for surimi gels (Lanier et al., 2013). Figure 4 shows the breaking strength and deformation of control gels and starch-surimi gels before and after frozen storage.
Physical properties of control gels (a) and starch-surimi gels (b) after frozen storage.
Different small letters indicate significant difference (P < 0.05) in breaking strength. Different capital letters indicate significant difference (P < 0.05) in breaking deformation.
Before freezing, in both control and starch-surimi gels, breaking strength was significantly higher in the samples subjected to SHF than in those subjected to SFR. This was due to the compact network structure of the two-step heated gels in SHF. After storage and reheating, breaking strength in SFR increased and reached the level of that in the two-step heated gels.
In the case of control gels (Fig. 4a), the breaking strength decreased significantly after frozen storage in the gels subjected to SHF (P < 0.05). Moreover, with increased storage time, breaking strength in the gels subjected to SFR also decreased (P < 0.05). The decrease in breaking strength of control gels was considered to be due to the structural damage caused by ice crystals and the increased voids in the gel structure during frozen storage.
However, in the case of starch-surimi gels (Fig. 4b), gels subjected to SHF showed a significant increase in breaking strength (P < 0.05). The change in breaking strength of starch-surimi gels was affected not only by structural damage, as observed for control gels, but also by starch granules during frozen storage. The retrogradation of starch granules after freezing and thawing was thought to be due to the increased breaking strength (Yu et al., 2010). In starch-surimi gels subjected to SFR, after storage and reheating, the breaking strength was lower than that in those subjected to SHF under the same storage conditions (P < 0.05). Moreover, the breaking strength in samples subjected to SFR decreased with increasing storage time (P < 0.05). On one hand, it was suspected that there was no starch retrogradation during frozen storage, as retrogradation refers to the molecular rearrangement of gelatinized starch (Morris, 1990). The starch granules during frozen storage in SFR were un-gelatinized; thus, breaking strength could not increase by retrogradation. It was considered that the gelation of setting gels in SFR after reheating was affected by the structural damage caused by ice crystals, thereby decreasing the breaking strength (Benjakul et al., 2005). Additionally, although the breaking strength of samples subjected to SFR was decreased after frozen storage, the values were higher than those of control gels under the same conditions. This was because the starch granules absorbed water after reheating, and the pressure of these expanded starch granules on the gel matrix resulted in an increased protein concentration, thereby increasing the breaking strength. The results for breaking deformation showed a similar tendency to those for breaking strength, which is a typical characteristic of surimi gels (Benjakul et al., 2003). The above results indicated that the physical properties of surimi gels were correlated with the structural changes and drip loss after thawing or reheating. Furthermore, SFR could reduce the physical property changes in frozen starch-surimi gels.
In this study, two-step heated gels were used to investigate the new “setting-freezing-reheating” procedure. Because the setting process can change the salted surimi paste from sol to gel, it not only improves the gel strength, but also contributes to the shape of the gel. From the perspective of industrial production, compared to unshaped salted surimi paste (without setting), samples set to a specific shape are more convenient for cryopreservation, transportation, and reheating after thawing. However, some surimi-based products are manufactured by direct heating at a high temperature without setting; therefore, the study on the application of this new procedure on direct heated products is an important area for industrial production that requires further research.
In this study, the quality changes in control and starch-surimi gels produced using SHF (setting-heating-freezing) and SFR (setting-freezing-reheating) were compared after freezing. Control gels showed larger ice crystals after freezing, larger void sizes after reheating, and higher drip loss when produced using SFR. In contrast, in the starch-surimi gels produced using SHF, the incorporated starch granules absorbed water and expanded before freezing, and were destroyed after freezing and thawing, resulting in a high drip loss and a large void size. In starch-surimi gels produced using SFR, un-gelatinized starch granules in setting gels were gelatinized after reheating, resulting in the absorption of the drip loss released after freezing and thawing, leading to a lower drip loss and a smaller void size than in those produced by SHF. Additionally, changes in the physical properties of gels were correlated with the changes in microstructure and starch granules. These results indicate that SFR can effectively reduce the quality changes in frozen starch-surimi gels. These findings may provide a new proposal to produce high quality surimi-based products by combining manufacturing and freezing procedures and may have important implications for the frozen food industry.
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.
