Fish Preservation and Storage
This section covers Fish Preservation and Storage: chilling, freezing, drying, salting, smoking, canning, modified atmosphere packing and irradiation. The exam tests the principle behind each method and the storage condition that makes it work.
Core rules
- Preservation works by lowering temperature, lowering water activity, changing pH, excluding oxygen or destroying microorganisms. State the principle, not just the method name.
- Chilling uses ice or chilled seawater. Freezing generally uses blast freezing at about -35 to -40 degrees C, then storage at -18 degrees C or colder.
- Drying lowers water activity; a target below 0.60 is common for shelf-stable dried fish. Salting may be dry, brine or pickle; brine concentration must be stated as weight by weight.
- Canning uses a sealed container and a scheduled retort process. Low-acid fish products need a scientifically established thermal process.
Worked example
Make 100 kg of 20 percent w/w salt brine. Salt = 20 kg; water = 80 kg. Reason: percentage w/w is salt divided by total brine mass, not salt per 100 kg water. Adding 20 kg salt to 100 kg water gives 16.7 percent, not 20 percent.
Traps
- Refreezing thawed fish and calling it fresh frozen.
- Quoting brine percentage without the basis.
- Ignoring glaze weight when declaring net weight.
- Assuming sun drying is safe without control of insects, rain and rehydration.
- Treating a retort process as a fixed time for all can sizes.