3.1. Drip Loss of Frozen Raw Mackerel
In this study, frozen mackerel samples were used to prepare a HMR product. To maintain the quality of the product, we evaluated the drip loss of thawed mackerel using three different methods, as shown in
Figure 2. The thawing method significantly affected the drip loss of the frozen mackerel. HFD reduced drip loss significantly compared to WT and AT methods (
p < 0.05).
Conventional thawing methods with WT and AT required a longer time (60 and 90 min, respectively) than the HFD method (20 min) to achieve a defrost state. A short thawing time maintains fish quality and minimizes mechanical damage to cell membranes [
21,
22]. Drip loss in frozen mackerels is caused by the release of free water from the muscles. Water-soluble proteins leach out during thawing periods, lowering the quality of the product [
23]. Freezing-thawing processes may damage cells, lead to the denaturation of protein, and dehydrate muscles [
24]. Farag et al. [
25] showed that the high-frequency thawing method was more efficient in reducing drip and micronutrient losses than conventional thawing. The thawing process can worsen the texture of frozen fish. Genç et al. [
26] reported that thawing fish filets decreased the water holding capacity and increased hardness. Low drip loss indicates that HFD can prevent the loss of nutrients and maintain the yield rate during defrosting, even without water use. Thus, the HFD method for defrosting frozen mackerel was selected for use in the further steps of this study.
3.2. Optimal Heating Conditions
Two variables, overall acceptance and hardness, were selected to evaluate the optimal heating conditions using an SSR. The overall acceptance score represented the sensory properties of braised mackerel with radish, while hardness values represented radish conditions. The hardness value was measured only on radishes because the range of temperature and time in the optimization experiment was sufficient for heating the mackerel, but it varied in radishes. In this study, RSM was used to optimize the heating conditions to achieve a specific set of objectives. The output of the RSM was the optimum combination of time and temperature as independent variables and overall acceptance and hardness as dependent variables.
Table 3 shows the predictive models that reflect the relationship between overall acceptance or hardness and the heating conditions. These models can be used to estimate the score of overall acceptance and hardness of braised mackerel with radish when cooked at different times and temperatures at SSR. The predicted and actual values from the regression model and variable measurements are shown in
Table 4. The
R2 and lack of fit values were analyzed to evaluate the adequacy of the developed models and their predictive values. The results showed that the models were significant at a 95% confidence level (
p < 0.05). The
R2 values of both predictive models were close to 100% (i.e., perfect fit), indicating that the models explained 98.82 and 96.93% of the variability of the response data around its mean. The
p-values of lack of fit of both models were not significant (
p > 0.05), indicating that the models were fit and adequate for predicting the heating conditions of braised mackerel with radish.
The three-dimensional response surface graph of overall acceptance showed an increased score with increasing temperature and time but decreased when it reached the optimal condition of 180.625 °C for 9.0316 min (
Figure 3a). The hardness value of the radish decreased continuously with an increase in the heating time and temperature (
Figure 3b).
At temperatures of over 180 °C for more than 10 min, the braised mackerel with radish sauce was scorched and had a bitter taste. The radish texture also became mushy because of overcooking. Conversely, at lower temperatures and shorter heating times, the radish texture was hard and uncooked. The optimal heating conditions of braised mackerel with radish using SSR were 180.625 °C for 9.0316 min (
Table 5). The predicted values of response for overall acceptance and hardness were 8.49 and 740.74 g, respectively. The actual experimental values obtained from the temperature at 181 °C for 9 min were 8.55 ± 0.10 and 761.83 ± 78.33 g for overall acceptance and hardness, respectively. The differences in the recommended heating conditions were measured using RSM because the SSR setting option was not available. Therefore, the recommended temperature and time values were rounded off. The HMR product containing braised mackerel with radish was cooked under these optimal conditions and analyzed in subsequent experiments.
The utilization of SSR in this study provides a faster process for the production of braised mackerel with radish. In a single running process, mackerels were exposed to both roasting and stewing treatments. The heating time was associated with the moisture and hardness of the product. A longer heating process results in increased hardness owing to less moisture [
27]. Under optimal heating conditions, the product is neither overcooked nor undercooked. Heating mackerel under SSR provides a lower weight loss than an electric pan [
28]. Several studies have shown the superiority of SSR to other heating methods in the maintenance of moisture, prevention of lipid oxidation, and reduction of the time taken for heating [
8,
10,
29,
30].
3.3. Effect of Freezing Method on the Sensory Properties of Braised Mackerel with Radish
The HMR products containing braised mackerel with radish were assessed for their quality after freezing. Freezing of HMR products is important for promoting market distribution. Freezing inhibits microorganism growth and enzymatic activity to maintain the nutritional properties of food. However, ice crystal formation may have a detrimental effect on food texture and cause membrane disruption, leading to oxidation [
31]. A proper freezing method can ensure the delivery of quality products to the consumers. Improvements in the freezing phase are also linked to an increase in the freezing rate, which can be accomplished by more advanced refrigeration systems. The HMR products were frozen in two separate freezers according to their freezing rate: slow freezer and quick freezer. After freezing, all the tested HMR products were reheated using a microwave and evaluated by panelists. The sensory properties of the HMR products before and after freezing are shown in
Table 6.
The HMR products were reheated using a microwave to recapitulate the experience of the product as it is used by customers. The frozen HMR products did not differ significantly in color, aroma, flavor, and overall acceptance, but the freezing treatment significantly affected their texture score (
p < 0.05). Compared with the HMR products before freezing, the texture score slightly decreased following quick freezing (1.18%), but it significantly decreased following slow freezing (5.67%). Zhu et al. [
32] reported that frozen salmon showed physical changes in texture and weight loss after thawing due to ice crystal formation. Furthermore, Ottestad et al. [
33] observed that raw salmon changed color under frozen conditions but returned to their initial color after thawing.
Slow freezing has been known to result in the formation of large extracellular ice crystals, which may cause severe tissue damage in frozen foods [
34]. Alizadeh et al. [
22] reported that the use of a slow-freezing method for fish resulted in the development of large ice crystals and significantly destroyed the muscle fibers. Quick freezing can remove heat faster than slow freezing. The heat removal rate determines the crystal growth rate [
35]. In addition, there is less disruption to the cell walls in the quick freezing system owing to the rapid rate of heat removal and ice formation [
36]. Because quick freezing yields better results in terms of sensory properties, particularly in the texture score, we subjected the HMR products containing braised mackerel with radish to the quick freezing method to evaluate their shelf life throughout 90 days of storage.
3.4. Chemical Properties of Braised Mackerel with Radish
The HMR products containing braised mackerel with radish were prepared through a high-frequency thawing process for frozen mackerel, cooked in an SSR at optimal time and temperature, and frozen by the quick freezing system. The chemical properties, including pH, TBARS, and VBN values, were evaluated during 90 days of storage at −18 °C. A storage temperature of −18 °C was used because it is the temperature set on most freezers in the market. The pH, TBARS, and VBN values increased slightly but did not vary significantly over the storage time. These values represent the quality of the food during storage. The pH and VBN values can be used to describe the spoilage conditions of food, such as fish [
37], while the TBARS value describes lipid peroxidation in food [
38].
In this study, the pH values of the HMR product ranged from 5.97 ± 0.07 to 6.07 ± 0.03 within 90 days of frozen storage, comparable to the pH values of fresh fish, which varies from 5.5 to 6.6 [
39]. The pH values were consistent with the findings reported by El-Dengawy et al. [
40], in which frozen mackerel increased their pH values from 5.96–6.20 after 4 months of storage. Lipid oxidation of the HMR products was detected using TBARS analysis. The TBARS values of the HMR products ranged from 2.15 ± 0.11 to 2.40 ± 0.03 mg MDA/kg. The products were considered to be in a perfect condition because they had a value less than 3 mg MDA/kg [
39]. Freezing treatment improved the stability of the lipids of processed mackerel and extended the shelf life. Additionally, a heating method with low oxidation possibility can help preserve the initial quality of the product [
41].
SSR offers some value in processed foods, such as appropriate texture, and prevents oxidation during processing [
29,
42]. Low TBARS values indicate that the quality of HMR products is suitable, lacks rancidity, and is appetizing. In addition to lipid oxidation, high VBN levels also generate an unpleasant aroma. The amount of VBN in fish products is related to the activities of microorganisms and endogenous enzymes [
43]. The VBN values of HMR products ranged from 11.67 ± 0.51 to 12.53 ± 1.12 mg% during 90 days of storage. This explains why microorganism and enzyme activities were inhibited, which might have occurred due to heating and freezing processes. Our HMR products are rated as good quality due to a VBN value of less than 25 mg% [
39]. These findings show that the use of heating technology for preparing HMR product containing braised mackerel with radish can preserve the quality of the product by maintaining the rate of lipid oxidation and preventing changes in the levels of spoilage-related chemicals.
3.5. Nutritional Compositions of Braised Mackerel with Radish
The nutritional quality of HMR products containing braised mackerel with radish was evaluated based on proximate, fatty acid, and amino acid compositions (
Table 7,
Table 8 and
Table 9).
Table 7 shows the biochemical profile of the HMR product containing braised mackerel with radish. The results showed that moisture content (63.21%) was the highest compared to the others, followed by fat (13.42%) and protein (13.03%) contents. Moon et al. [
44] reported that the moisture content of raw mackerel was 65.5% and decreased to 49.2% after it was fried in a pan. Differences in the preparation process with respect to sauce addition and the utilization of SSR, which can help retain moisture during heating, led the HMR product having a higher moisture content than previously reported. Higher moisture content also resulted in lower protein and fat contents than those reported by Moon et al. [
44] (24.1 and 22.2%, respectively). Tirtawijaya et al. [
8] reported that proximate content, which includes sodium, carbohydrate, protein, and fat, increased during the heating process. The addition of sauce to the HMR product as a seasoning increased the sodium and carbohydrate contents. The increase in such contents is related to an increase in calories. Based on the nutritional intake standard for Koreans, the HMR product containing braised mackerel with radish meets 10% of calories, 24% of proteins, 17% of sodium, 2.7% of carbohydrates, 4.3% of sugars, 25.8% of fat, and 8.9% of cholesterol daily intake of 2000 kcal [
45].
Table 8 shows the amino acid profiles of the HMR product containing braised mackerel and radish. The total amino acid content was 12.94 g per 100 g HMR product, which was dominated (above 8%) by glutamic acid, aspartic acid, lysine, and leucine. These amino acids contribute to the taste and flavor of seafood [
46]. Glutamic acid is responsible for the umami flavor, while alanine and glycine contribute to sweetness in food [
47]. The total essential amino acid (EAA) content was 40%, including valine, leucine, isoleucine, methionine, phenylalanine, threonine, lysine, histidine, and tryptophan, which was lower than that of nonessential amino acids (NEAA). However, both EAAs and NEAAs play important roles in human health. Wu [
48] classified arginine, cystine, leucine, methionine, tryptophan, tyrosine, aspartate, glutamic acid, glycine, proline, and taurine as functional amino acids in human nutrition. According to Mohanty et al. [
49], amino acids play important roles in cell division, growth, tissue repair, and immune function. These results show that the HMR product containing braised mackerel and radish may provide amino acids to support human health.
The fatty acid profiles of the HMR product containing braised mackerel with radish are shown in
Table 9. The total fatty acid content was 1.1 g per 100 g of the product, consisting of total unsaturated fatty acids (0.79 g), which was higher than that of saturated fatty acids (0.36 g). The three major fatty acids in the HMR product were palmitic acid (19.88%), oleic acid (25.65%), and DHA (18.07%). These results are in accordance with those of Celik [
50], who reported that the levels of major fatty acids in mackerel captured in different seasons varied, including palmitic acid (19.36–26.59%), oleic acid (4.13–10.69%), and DHA (17.12–24.94%). Moreover, the levels of polyunsaturated fatty acids in mackerel, such as DHA and EPA, are the highest during winter. This report can be considered when selecting mackerel as a material for the development of HMR products. Various heating methods have been reported to affect the fatty acid content of mackerel. The frying pan method significantly decreased the DHA content of the mackerel, but the oven and microwave methods did not differ significantly [
44]. The HMR product containing braised mackerel with radish was cooked using the SSR method, which has comparable characteristics to an oven and a microwave, but it is markedly better because it prevents the oxidation of the product [
29]. Consequently, the DHA content of the HMR product can be preserved. DHA helps maintain memory and reduces cognitive impairment, and is used to treat cardiovascular diseases [
51]. The high DHA content in the HMR product (2.1 g per 100 g) is beneficial for health.
3.6. Evaluation of Storage Effect on the Quality of Braised Mackerel with Radish
The HMR product containing braised mackerel with radish was frozen using quick freezing and stored in a deep freezer at three different temperatures of −13, −18, and −23 °C. The quality of the test HMR products was evaluated based on their sensory and microbiological properties. Sensory properties were evaluated during 90 days of storage. The data on color, aroma, flavor, texture, and overall acceptance score obtained from 21 trained panelists are shown in
Table 10. HMR products at the lowest storage temperature of −23 °C maintained their color, aroma, and flavor for 90 days, but texture and overall acceptance score decreased significantly on days 60 and 90, respectively (
p < 0.05). At a temperature of −13 °C, color and flavor scores decreased significantly on day 60, while texture and overall acceptance scores decreased significantly on day 30 (
p < 0.05). Storage temperature did not affect the aroma scores of the HMR products. The sensory properties of foods are correlated with their physicochemical changes [
52]. The presence of oxygen causes lipid oxidation during frozen storage, resulting in the loss of nutrition, color, taste, and texture. Lipid oxidation products may cause protein oxidation and alter food texture [
53]. Changes in the chemical properties of the product over 90 days were not significantly different. This might be the reason why sensory properties had scores above 8 in all HMR products at all storage temperatures.
The perception of consumers can be used to estimate the shelf life of food products. Changes in sensory properties can determine the shelf life of hygienic or microbiologically stable foods [
54]. Overall, the sensory properties were excellent, with scores higher than 8 during 90 days of storage. The overall acceptance scores gradually decreased with increasing storage temperatures and times; hence, these scores were used to represent the estimated shelf life. The shelf life of the HMR products was estimated using a program simulation from MFDS based on the Arrhenius equation model. The overall acceptance was established to meet first order reaction kinetic.
Table 11 shows the linear regression of the first order reaction kinetic of overall acceptance. The shelf life was estimated at a storage temperature of −18 °C. A temperature of −18 °C is required for frozen food storage [
55]. The HMR product can be stored for up to 48 months at −18 °C with a _target overall acceptance score threshold of 5.
To assess the safety of the HMR products, we evaluated the microbiological conditions during 90 days of storage. Pathogenic bacteria, including the
coliform group,
Staphylococcus aureus, and
Salmonella spp., were not detected in the HMR products during 90 days of storage. The absence of these pathogenic bacteria indicated that the raw materials and development processes were hygienic. The TBC (3.11 ± 0.02 to 3.21 ± 0.02) did not change significantly with storage temperature and time (
Appendix A Table A1). All TBCs were less than 5 log CFU/g, indicating that the HMR products were safe [
56]. The heating treatment removed the risk of harmful microorganisms, and freezing maintained the quality of HMR products by preventing the growth of microorganisms. Alizadeh et al. [
22] reported that storage temperature influences enzymatic activity in salmon. Low temperature storage in a freezer preserves nutritional composition better than chilled storage. An increase in the activity of microorganisms affects food flavors and produces a foul odor [
57].
The estimation of shelf life was also evaluated based on the TBCs in HMR products. The TBCs were based on the zero order reaction kinetic. The linear regression of zero order kinetic of TBC is shown in
Table 11. HMR products can be stored for up to 236 months at −18 °C, with a _target score threshold of 5 log CFU/g. The results of the estimation of shelf life estimation based on sensory properties and TBCs were different. To determine the estimated shelf life, we used the shortest shelf life value between sensory properties (48 months) and TBC (236 months), which changed the HMR product quality. Based on these values, the shelf life of the HMR product containing braised mackerel with radish was 48 months. However, considering the temperature fluctuation during distribution and storage in the market, the shelf life value was multiplied by a safety factor of 0.8 [
20]. Therefore, the HMR product containing braised mackerel and radish had a shelf life of up to 40 months.