Study on Processing Technology of Frozen American Red Fish Fillet

Raw material pretreatment → Freezing, freezing → Thawing (adding antioxidants) → Scale removal → Washing 1 → Slicing (three pieces) → Trimming (Picking insects) → Descaling → Cleaning 2 (adding antioxidants) → Freezing → Fractionation → Ice glazing → Packaging → Storage of finished products.

1.3.2 Operation points Raw material pretreatment: including bloodletting, head removal, internal organs removal, and cleaning. Bloodletting is the key to guaranteeing the quality of frozen red fish fillets. It is required to release net fish blood, remove residual fish muscles and blood clots, and pay attention to not cut fish guts when bleeding and viscera are removed, so as not to affect product color and taste.

Frozen or frozen: If raw materials are not directly processed, the raw materials can be frozen and frozen.

Raw materials are released and thawed: the raw materials are taken from the frozen storage tank to the thawing field, and semi-thawing is performed in the thawing tank with ice water with 0.3% of antioxidant Vc-Ma color protection. Add ice to maintain 0 °C temperature.

Scale removal, cleaning: Immediately after thawing the raw material, remove the tail and go to the scale removal process, and quickly wash with running water.

Cutaway: Cut the raw material into 3 pieces, 2 pieces of fish, and one bone in the middle. Make sure that the fillets are intact when sectioning. Place 2 slices of fish relative to each other and preserve with ice.

Trimming (Picking insects): Trimming incomplete fillets and removing fish bones from fish. Redfish raw materials are generally parasites, born in the epidermis of fish, very obvious, without the help of inspection lighting, can be clearly seen with the naked eye, feel. Depending on the position of the worm, pick the worm from one side of the hide or from one side of the meat with a rake. When removing parasites, do not make the cut of the epidermis too large so as not to affect its appearance.

Thorn-thorn: Detachable products get higher prices in the market. Redfish usually has about 15 spines on one side, and it is pulled out in the direction of the spine with special equipment. This process should arrange for someone to check whether it is completely decontaminated.

Washing (adding anti-oxidants), loading plate: After trimming (de-thorning), the fillets are quickly washed with ice water, drained and washed with 0.3% iso-Vc-Na ice-water solution. When loading, the fillets should be arranged neatly. Incomplete slices, severely faded redfish are Grade 2 products. In addition, there may be dark spots on the surface of the fish body, and redfish with a dark spot diameter of more than 3 cm are generally used as the second grade.

Single freezing: Before the single freezing, the surface of the fish should be light and the fish should be smooth and even in color. General control single freezing temperature -32 °C, speed 300 ~ 400r/min. After a single freezing machine, it was quickly transferred to a plate freezer and continued to freeze until the fish's central temperature reached -15°C.

Grading, ice-plating, packaging: Weighing on a tablet-by-table electronic scale, fish fillets are categorized into: 20, 22, 25, 30, 40, and 5kg, etc. according to customer requirements. After glazing, press 5kg2 into a plastic bag.

Finished product storage: quickly loaded into the carton and quickly warehousing, clear identification of finished products, placed neatly.

2 Results and Discussion
2.1 Determination of Antioxidant IsoVc-Na Concentration

The decline in the freshness of the processing and inventory process will cause the redfish to fade. Faded redfish will greatly reduce its commercial value. The mechanism of fading is that redfish are rich in unsaturated fatty acids and are prone to oxidation and fade. In order to maintain the red color of the red fish, it can be treated with the antioxidant Vc-Na during processing. The strong reduction of isoVc-Na inhibits oxidation, thereby controlling discoloration. We treated 7 groups of redfish with 0%, 0.05%, 0.1%, 0.2%, 0.3%, 0.4%, and 0.5% of isoVc-Na, and compared the color protection effects in two cases: 1 Processing 2 hours + quick-frozen 4 hours); 2 Frozen storage over 45 days to determine the optimal concentration of antioxidant isoVc-Na. The result is as follows:

A. Processing (2 hours of processing + quick-frozen for 4 hours) Fading: The fading of the red fish skin of group 1 occurred. No fading was observed in the 2-7 groups, and the color protection effect was not clearly distinguished.

B. Post-freeze fade after 45 days (see Table 1).
Table 1 Effects of different Vc-Na concentrations on the fading of red fish skin

The discoloration of red fish skin at the different Vc-Na concentrations
10% epidermis fades, matte
2 0.05% epidermis fades, matt
3 0.1% fading eased slightly but still faded
4 0.2% epidermal fading is not serious, matt
5 0.3% Do not see fade, surface has no obvious luster
6 0.4% no visible fading, no obvious surface gloss
7 0.5% Do not see fade, glossy

From Table 1, it can be seen that the concentration of the anti-oxidant isoVc-Na is selected to be 0.3% or more, which can suppress the fading of the redfish during processing and storage. Integrating various factors, it was determined that the concentration of anti-oxidant isoVc-Na was 0.3%.

2.2 Measures for Maintaining Antioxidant IsoVc-Na Concentration During Processing In the process of processing frozen US red sashimi, the color protection effect is mainly determined by 0.3% of sodium erythorbate. Therefore, it is very important to maintain the concentration of antioxidant Vc-Na throughout the entire process. In addition to controlling the processing speed and processing temperature, the following measures can be taken:
1 Monitor the quality of process water: Test the residual chlorine concentration of the process water to avoid excessive oxidation of residual chlorine and reduce the concentration of antioxidant Vc-Na.
2 During the defrosting of raw materials, the concentration of antioxidants must be diluted by adding ice directly to maintain 0°C. Ice can be added after wrapping it with a film.
3 control cleaning process:
Cleaning 1: This process is required to be completed as soon as possible. Excessive, excessive washing will cause the loss of water-soluble isoVc-Na. However, there are more coliforms at the scale and tail junctions, requiring thorough scaling. In order to resolve this contradiction, it should be carefully managed in this step, both to control the time of washing, and to scale clean.
Wash 2: After washing with water, be sure to drain the water before washing with antioxidant-free ice water to prevent contamination and repeated use of diluted antioxidant concentrations. It is also possible to remove the isoVc-Na that was lost in the descaling process by hand cleaning the skin of the red fish with a different Vc-Na solution when the plate is not washed after the sectioning.
4 When the ice coat is applied again, it is treated with 0.3% of antioxidant isoVc-Na.

2.3 Control of Microorganisms in the Processing of Redfish Due to its excellent meat quality, fish has a high water content and are prone to spoilage. Therefore, microbiological control problems in redfish processing are also very important. The number of bacteria required to detect the raw material was between 1.0 and 5.0 105 cells/g, and the coliform bacteria was between 3.6 and 9.1 MPN/g. Listeria monocytogenes may sometimes be found in raw materials imported from redfish. In 2003, the Zhoushan Entry-Exit Inspection and Quarantine Bureau detected Listeria monocytogenes in three batches of imported redfish. However, it is not allowed to use a disinfectant to directly treat the fish body to control microorganisms throughout the processing. This is due to the fact that general disinfectants such as sodium hypochlorite use their oxidizing properties for disinfection. The direct use of sterilizing agents in products not only does not allow the regulation, but also directly oxidizes fish pigments to increase discoloration and reduce the highly protective effect of different Vc-Na color protection. Moreover, sodium hypochlorite at a concentration of 50 mg/L has little disinfection effect, especially when the fish body is wet and treated with isoVc-Na. It can be said that most of the control of microorganisms in the processing of redfish is performed at 0°C ice water. According to another experiment, redfish fillets are processed in sections, trimmings and other procedures. In the case of non-standard hygiene operations, the contamination rate of coliform bacteria will be high, reaching 250 MPN/g or more. Therefore, the scales used for the knives, cutting boards, hands, operating tables, and grading of each process are required to be disinfected once every hour using a 100-150 mg/L sodium hypochlorite solution.

Controlling the processing speed and processing temperature can reduce the detection rate of Listeria. In general, special treatment of raw materials may be necessary because raw materials may contain Listeria. This is because: 1 At present, the countries where imported products cannot detect Listeria monocytogenes are limited to the United States and Canada; 2 Listeria strains are relatively sensitive to acids and are completely inhibited at pH 4.4, while completely inhibiting the pH of Salmonella. The value must be less than 3.7. For products with specific requirements, acetic acid or citric acid (citric acid is often used as an antioxidant synergist, both protective effect) on the product bacteriostatic.

3 Product quality indicators
3.1 sensory index color: the epidermis is bright red, shiny, fish was beige; organization and morphology: the organization is tight, the body is complete, the fish skin is not damaged, the fish surface no dark spots more than 3cm in diameter.
3.2 Microbiological indicators The total number of bacteria is less than 1.0105/g and the number of coliform bacteria is less than 3 MPN/g. Salmonella and cholera bacteria should not be detected.
4 Summary US redfish as raw material, mainly through the process of cutting, picking insects, single frozen and other processes into a single color frozen for the international market, red snapper. Treatment with 0.3% anti-oxidant isoVc-Na during processing can provide ideal color protection.

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