3.1. Effects of Different Roasting Methods on the Color of Large Yellow Croaker
Color is an important criterion for the sensory evaluation of fish and is also an important determinant of consumers’ desire to buy [
17]. L* represents the brightness of the color, a* represents the red–green value, and b* represents the yellow–blue value [
18].
The experimental results of the color-difference measurements are shown in
Table 2. Kong et al., reported a rapid whitening followed by a browning phase during thermal processing of pink salmon at 100 to 131.1 °C [
19]. The browning phase usually appears as processing temperature and time increase [
20]. In this study, as the temperature and roasting time increased, protein denaturation, lipid oxidation, and the Maillard reaction occurred in the fish [
21]. The fish in group A were grey-white, with the highest L* value and the lowest a* and b* values. Compared to group A, the L* values of the five types of large yellow croaker meat roasted in different ways showed a downward trend, and the a* value showed an upward trend. This is because, with the increase in temperature or the extension of roasting time, the water evaporation of fish meat was accelerated, resulting in water loss from the surface, and the reaction and refraction of light were weakened, resulting in a decrease in the L* value [
22]. With protein denaturation and lipid oxidation, the surface color of fish meat deepens to reddish-brown or golden yellow, resulting in an increase in the a* value. The L* values of large yellow croaker meat roasted using the three types of light waves or light wave–microwave combination were significantly lower than those of large yellow croaker meat roasted in the oven and using the far-infrared method (
p < 0.05). It is speculated that the reason is that there is intense movement of the water molecules inside the fish as they are excited by microwaves, while the light waves act on the surface of the fish. The constant roasting time was long, and the temperature was high, which led to the greatest water loss on the surface of the fish [
23]. As shown in
Table 2, the L* value of the fish roasted under the five methods was most reduced by the 70% light wave and 30% microwave roasting method, which showed a value of 28.34. The a* value of large yellow croaker meat roasted with 100% light wave increased the most, with a value of 5.90, and was significantly different to the a* value obtained using the other roasting methods (
p < 0.05). The b* value increased the most in the far-infrared-roasted meat, which showed a value of 9.51. This was because the Maillard reaction occurred during roasting. Far-infrared radiation had a certain penetration force and acted directly on the interior of the large yellow croaker meat [
24,
25]. When the temperature was the highest, the heat was uniform, and more brown substances were produced [
26,
27]. Therefore, the surface of the far-infrared-roasted fish was golden yellow and the b* value was the largest.
3.2. Effects of Different Roasting Methods on the Texture Characteristics of Roasted Large Yellow Croaker Fish
In this study, the elasticity, hardness, chewiness, and adhesiveness of roasted large yellow croaker were analyzed. Texture is a key quality used by the processed food industry to evaluate product quality and consumer acceptability, while hardness is an important factor in measuring differences in fish taste [
28]. The textural characteristics of the large yellow croaker meat obtained using the five roasting methods are listed in
Table 3. The elasticity, hardness, chewiness, and adhesiveness of group A (raw fish) are the highest. There were no significant differences in the effects of the five roasting methods on the elasticity of the meat of the large yellow croaker (
p > 0.05). Compared with control group A (raw fish), hardness and chewiness of the roasted large yellow croaker meat showed a downward trend, which was positively correlated with chewiness. The reason may be that after roasting, the protein is denatured, resulting in the destruction of the three-dimensional network structure, the loosening of the muscle fibers [
29], the decrease in the cell binding capacity, and the softening of the fish texture. The hardness of fish in groups B, C, D, E, and F accounted for 36.00%, 39.00%, 49.84%, 36.80%, and 41.64% of the raw fish, respectively. The hardness of fish roasted by method B (45% light wave and 55% microwave) decreased the most. The reason may be that microwave stimulates specific molecules to penetrate the sample, resulting in cell rupture, the excitation of reactive oxygen species [
30], and the stimulation of increased protein oxidation in meat products [
31]. This showed that the large yellow croaker roasted with 45% light wave and 55% microwave had great elasticity, soft meat, was more chewy, and tasted good.
3.3. Effects of Different Roasting Methods on Water Migration in Large Yellow Croaker
Moisture is an important factor that affects the texture and flavor of food. A reduction in immobilized water affects the chewiness and hardness of large yellow croaker meat and causes a deterioration in its sensory quality [
32,
33]. In meat science research, T
2 is commonly used to characterize relaxation time. The change in T
2 can reflect the fluidity of water molecules, which is related to the degree of freedom and binding force of hydrogen protons in water. A larger T
2 value indicates a higher degree of freedom of water, and a smaller T
2 value indicates that the water is combined closer to the substrate [
34,
35]. The T
2 inversion map of large yellow croaker meat subjected to different roasting methods is shown in
Figure 2a, and three peaks can be observed: T
21, T
22, and T
23. The range of T
21 is 0.01–10 ms, which is considered to be closely attached to the polar groups on the surface of the fish protein molecules and it combines with biological macromolecules, such as proteins and lipids, to form a hydration layer, that is, bound water. T
22 ranges from 10 to 100 ms and is considered to be the fixed water trapped in the myofibrillar structure, that is, the immobile water. T
23 ranges from 100 to 1000 ms and is considered to be the free water in the myofibrillar structure [
36]. Therefore, the total area of the low-field nuclear magnetic resonance (LF-NMR) inversion spectrum of large yellow croaker meat after different roasting methods tended to decrease, and the peak of the spectrum moved to the right, indicating that different roasting methods changed the water state of the meat, and most of the water in the large fish was fixed water [
37,
38].
The immobilized water and free water of the roasted fish changed significantly under different roasting methods (
p < 0.05). As shown in
Figure 2b, compared with control group A (raw fish meat), the T
21 values of the roasted fish were not significantly reduced (
p < 0.05). As shown in
Figure 2c, the bound water content is very low, so it is considered to have little effect on the water holding capacity [
39]. The T
22 relaxation time of large yellow croaker meat was significantly shortened after roasting. Therefore, the degree of freedom of water in the fish was reduced, and the binding force was increased, indicating that the stability of the fixed water and free water was destroyed during the roasting process, which led to the migration and loss of water [
40,
41]. This was in agreement with the evaluation of marinated salmon flesh by Feng et al. using a combination microwave oven at 200 °C [
42]. As shown in
Table 4, the T
21 value of light–microwave combined roasting in groups B and C increased significantly, indicating that during the roasting process, some of the immobile water was converted into bound water, which made the meat softer. The T
21 value of group F oven roasting decreased, indicating that the bound water content of the large yellow croaker decreased during the roasting process. The reason may be that the cells of fish meat were destroyed during high-temperature heating, and the bound water flowed out. The immobilized water and free water volatilized rapidly through hot air roasting, which increased the hardness of the fish meat [
43]. This shows that microwave and light wave roasting directly act on the small molecules inside the large yellow croaker fish meat and are evenly heated, which can make the fish meat fiber shrink evenly during the heating process, resulting in a certain elasticity of the fish meat, significantly improving the fluidity of the large yellow croaker’s immobilized water, which is conducive to the diffusion of internal water. Similar phenomena were discovered when cooking ridgetail white prawn (
Exopalaemon carinicauda) [
44].
To fully understand the dynamic changes and migration of the different aqueous phases, the values of A
21, A
22, and A
23 corresponding to the quantitative data of the T relaxation times T
21, T
22, and T
23, respectively, were calculated. As shown in
Figure 2c and
Table 5, the A
22 values were the main water component, and the A
22 values of the five roasting methods showed a downward trend, with significant differences (
p < 0.05). Compared with control group A, the largest decrease in A
22 value was seen for the 70% light wave- and 30% microwave-baked large yellow croaker meat, which decreased by 37.66%. The 45% light wave and 55% microwave roasting decreased the A
22 value by 39.84%. The A
23 values of the two roasted large yellow croaker fish decreased by 53.96% and 34.52%, respectively. This shows that the water molecules in the large yellow croaker meat after roasting in these two ways are lost. The reason may be because, during the 70% light wave and 30% microwave roasting process, the polar molecules inside the large yellow croaker meat absorbed the microwave energy and converted it into heat energy, which caused the fish meat to heat rapidly, and the whole sample was in a heated state. Owing to the water pressure difference between the fish tissue and the surrounding air, the water is evenly lost [
45]. However, the A
22 values of the other three roasting methods showed an opposite trend. This shows that far-infrared roasting, oven roasting, and 100% light wave roasting promote the conversion of fixed water to free water to a certain extent. The main reason is that the high temperature heating of the three roasting methods makes the protein denatured, resulting in the contraction of myofibrillar protein and myosin, causing the change in the water storage space structure, which in turn leads to the release of the water originally stored in myofibrils, which is converted into free water loss and reduces the rate of water migration. The texture of the fish meat baked using microwaves and light waves was clear, due to the effect of microwave heat on the interior of the material, which promotes the migration of water to the surface [
46]. The torn surface of the fish baked in the oven was uneven and partially broken. Because blast-drying acts on the outside of the material, the water on the surface of the material can evaporate with time, so that the transfer of water and heat is more uniform, and the structure of the muscle fiber bundle is greatly improved. Therefore, the hardness of the fish baked using microwaves and light waves was low.
Magnetic resonance imaging (MRI) can be used to visualize the water distribution and internal structure of food samples and visually display changes induced by various food-processing methods [
47]. Higher signal intensities at different points on the cross-section of the sample indicate greater proton enrichment, and these pixels are combined and pseudo-colored to obtain the magnetic resonance image [
48]. Red represents a high proton density and blue represents a low proton density in 2-weighted images in the corresponding curve related to T strength. In
Figure 2d, the changes in water distribution and internal structural information of raw fish and the five different roasted fish samples are displayed in the form of pseudo-colored images. The MRI results of all fish samples are red and blue. The signal of the fish meat roasted with microwaves and light waves was the highest among the five different roasting methods. Compared with the other three roasting methods, it takes longer to remove water from the fish meat, which may be related to its high bound-water content.
3.5. Effects of Different Roasting Methods on Sensory Evaluation of Large Yellow Croaker Fish
Sensory quality is an indicator that can be used to directly characterize the flavor of fried fish using different roasting methods. After roasting, fish meat shows significant improvements in appearance, smell, and taste [
60]. The sensory evaluation of fish fried using the five different methods was carried out based on five aspects: shape, smell, color, taste, and acceptance. The results are shown in
Figure 5.
Compared with raw large yellow croaker fish, among the five different roasted large yellow croaker fish samples, the overall sensory score of the far-infrared-roasted large yellow croaker fish was the highest, at 83.1 points. The lowest score was 53.1 for 100% light wave-baked fish. As shown in
Figure 5, the color, taste, and smell of the large yellow croaker meat significantly improved after roasting, and acceptance also improved. Group E (far-infrared-baked large yellow croaker meat) had the highest odor, color, and shape scores. The three types of light wave–microwave combination-baked fish showed large differences in color and smell, but the taste attribute scores were similar between these groups. The taste scores were in the order of light wave and microwave roasting > far-infrared roasting > oven roasting; the smell scores were in the order of far-infrared roasting > oven roasting > light wave and microwave roasting; and the overall acceptance scores were in the order of far-infrared roasting > light wave and microwave roasting > oven roasting.
In general, the sensory evaluation showed that the surface color of far-infrared-baked large yellow croaker fish was golden, the shape was complete, and the muscle was tender, delicious, and highly acceptable. Fish roasted with 45% light wave and 55% microwave had a golden color and the best taste. The morphology of large yellow croaker baked using 100% light wave was soft, the color was dim, and the smell was slightly fishy, but the meat was full of juice and was dense. This is also consistent with the above research results on texture, color difference, moisture, and flavor, and provides a reference for the optimization of the big yellow croaker baking process.