1. Raw Material Reception & Preliminary Inspection

• Raw material requirements: Fresh Pacific mackerel (scientific name: Scomber japonicus) that has been caught within a short time (usually ≤ 24 hours) and kept in low-temperature storage (0–4°C for short-term or -18°C for long-term during transportation) to avoid spoilage.

• Inspection items:

• Sensory check: Observe the fish’s appearance (bright eyes, red gills, elastic flesh, no off-odor), touch the body (no stickiness, firm texture), and check for signs of damage, parasites, or contamination.

• Safety testing: Sample and test for heavy metals (e.g., mercury, lead), microbiological indicators (e.g., total bacterial count, coliforms), and histamine (a key indicator of mackerel spoilage, as mackerel is a high-histamine fish).

• Temporary storage: Qualified raw fish are immediately transferred to a 0–4°C cold storage to maintain freshness; unqualified batches are rejected to prevent quality risks.

2. Thawing (If Raw Material Is Frozen)

• Thawing methods: Industrial production mainly uses controlled low-temperature thawing to minimize nutrient loss and bacterial growth:

• Refrigerated thawing: Place frozen fish in a 0–4°C cold storage for 12–24 hours (depending on fish size) until the core temperature reaches -1–0°C (flesh is slightly soft but not fully thawed).

• Air-blast thawing: Use low-temperature (2–5°C) and high-humidity (85–95%) air to circulate around the fish, accelerating thawing while preventing surface dehydration (“freezer burn”).

• Taboo: Avoid room-temperature thawing or hot-water thawing, as these methods cause rapid bacterial reproduction and loss of moisture/nutrients.

3. Pre-Cleaning & Evisceration

1. Rinsing: Rinse the mackerel with chilled freshwater (0–4°C) to remove surface blood, mucus, and seawater residues.

2. Evisceration: Use a mechanical eviscerator or manual operation to:

• Cut open the fish belly (from the anus to the gill cavity).

• Remove the intestines, stomach, gallbladder (avoid breaking the gallbladder, as bile will make the flesh bitter), and other internal organs.

• Trim off damaged or discolored parts of the belly wall.

3. Secondary rinsing: Rinse the eviscerated fish again to remove residual blood, viscera fragments, and bile stains, ensuring the cavity is clean.

4. Filleting & Deboning

1. Heading & Skinning (Optional):

• Some processes first cut off the head (at the gill base) using a heading machine; others keep the head temporarily and remove it during filleting.

• Skinning is usually done after filleting (easier to separate skin from flesh), but some lines skin the whole fish first for efficiency.

2. Filleting:

• Start from the head (or tail) and cut horizontally to separate the flesh from the vertebrae.

• Avoid cutting through the bones to ensure the fillet is intact.

• Use a band saw filleting machine or manual filleting knife to cut along both sides of the mackerel’s spine:

• Each mackerel produces 2 whole fillets (left and right sides).

3. Deboning:

• Pacific mackerel has small “pin bones” (intramuscular bones) along the center of the fillet. Use a pin bone remover (mechanical or manual tweezers) to pull out these bones one by one.

• Inspect the fillets under bright light to ensure no bones are left (a key quality control point to prevent choking hazards).

5. Trimming & Quality Sorting

• Trimming: Use a sharp knife to trim:

• Uneven edges, broken parts, or fatty tissue on the fillet surface.

• Discolored areas (e.g., brownish spots caused by oxidation) or blood clots.

• Remaining skin fragments (if skinning was incomplete).

• Sorting: Classify fillets by size, weight, and quality (usually 3–4 grades):

• Grade A: Intact shape, uniform color (pale pink to off-white), no defects, weight ≥ 100g per fillet.

• Grade B: Slight edge damage, minor color variations, weight 50–100g per fillet.

• Lower grades: Used for processed products (e.g., mackerel mince) instead of whole fillets.

6. Pre-Treatment (Optional: Brining/Glazing)

• Brining: Soak fillets in a low-concentration saltwater solution (2–3% NaCl) for 10–15 minutes. This:

• Enhances the fillet’s tenderness and flavor (prevents blandness).

• Inhibits partial bacterial growth (salt has a mild preservative effect).

• After brining, rinse briefly with chilled water to remove excess salt.

• Glazing: A common step for frozen fillets to prevent freezer burn. After freezing (see Step 7), spray a thin layer of chilled water (0–2°C) on the fillet surface. The water freezes quickly to form a transparent ice film (glaze), which:

• Reduces moisture loss during frozen storage.

• Protects the fillet from oxidation (maintains color and nutrition).

7. Quick Freezing (Core Step to Preserve Freshness)

• Freezing equipment: Industrial production mainly uses air-blast freezers or plate freezers:

• Air-blast freezer: Circulate -35–-40°C cold air at high speed around the fillets (placed on conveyor belts or trays). The freezing time is 30–60 minutes, and the fillet core temperature drops to -18°C.

• Plate freezer: Clamp fillets between cold plates (-30–-35°C) for 20–40 minutes. Suitable for thin fillets, as it ensures uniform freezing.

• Key requirement: The fillet core temperature must reach ≤ -18°C within 2 hours (meets international frozen food standards, e.g., FDA, EU No. 1069/2009).

8. Packaging

• Inner packaging: Use food-grade polyethylene (PE) plastic bags (transparent or opaque) that are moisture-proof, oil-resistant, and airtight. For individual portions:

• Pack 1–2 fillets per bag (e.g., 150g/bag) and vacuum-seal (removes air to prevent oxidation and freezer burn).

• Outer packaging: Use corrugated cardboard boxes (printed with product information) or insulated cartons (for long-distance transportation). Each box contains multiple inner packages (e.g., 10kg/box).

• Labeling: Each package must be labeled with:

• Product name (“Frozen Pacific Mackerel Fillets”), ingredient list (if brined), net weight, production date, shelf life, storage temperature (-18°C or below), manufacturer information, and certification marks (e.g., FDA, HACCP, BRCGS).

9. Frozen Storage & Transportation

• Frozen storage: Store packaged fillets in a -18°C ± 2°C cold storage with:

• No temperature fluctuations (avoid repeated freezing and thawing, which causes texture degradation and bacterial growth).

• Proper stacking (avoid crushing packages) and separation from strong-odor foods (mackerel absorbs odors easily).

• Transportation: Use refrigerated trucks or containers with temperature monitoring systems, ensuring the internal temperature remains ≤ -18°C throughout transportation. Avoid prolonged exposure to room temperature during loading/unloading.

Key Quality Control & Safety Standards

• HACCP (Hazard Analysis and Critical Control Points): Identify and control risks (e.g., histamine formation, bacterial contamination, bone residues) at critical steps (e.g., raw material inspection, freezing, storage).

• Microbiological monitoring: Regularly test fillets for pathogenic bacteria (e.g., Salmonella, Listeria monocytogenes) to ensure compliance with national/international standards.

• Traceability: Record every step (batch number, raw material source, production time, inspector) to enable product recall if quality issues arise.