Tuna undergoes IQF tunnel freezing, which is different from ordinary fish fillets and shrimp: it has high fat and hemoglobin content, making it more sensitive to freezing speed, temperature uniformity, and cold chain stability after freezing. The equipment selection and process control should be designed around the principles of "rapid passage through ice crystal zones from -1 ℃ to -5 ℃, stable center temperature compliance, low thawing weight loss, and stable color and flavor".
1. First distinguish the purpose: raw food/sashimi grade or cooked food/canned food/pre made dishes
Sasebo grade, raw frozen tuna chunks: High requirements for color, flavor, and histamine control, and usually need to enter the ultra-low temperature cold chain of -30 ℃ to -60 ℃ after quick freezing.
Tuna chunks/fish fillets for cooked food, canned food, and pre made dishes: can be transported through regular IQF tunnels, but it is also necessary to ensure that the center temperature quickly drops below -18 ℃.
Due to different purposes, the temperature level, packaging form, and subsequent storage requirements of the equipment will vary greatly.
2. The tunnel temperature should be low enough and the zoning should be stable
Ordinary IQF tunnels can usually achieve temperatures ranging from -35 ℃ to -45 ℃, suitable for fish fillets, shrimp, squid rings, etc.
If tuna is used as a high-value frozen product, it is recommended to pay attention to whether the equipment can be matched with a lower evaporation temperature or liquid nitrogen supplementary cooling section; If only used for cooked food chunks and fish fillets, conventional tunnel style IQF can generally meet the requirements, but the center temperature, thawing weight loss, and color stability must be verified.
3. It is necessary to quickly pass through the maximum ice crystal generation zone
Tuna muscle cells are more sensitive to ice crystal damage. When freezing, try to shorten the residence time between -1 ℃ and -5 ℃ to reduce the risk of large ice crystals puncturing muscle fibers.
During equipment acceptance, it is important to focus on whether the inlet temperature, mesh belt speed, wind speed, and outlet center temperature are stable, rather than just judging by "freezing and hardening".
Before feeding, it must be pre cooled and cannot be frozen with heat
Before cutting tuna into pieces or fish fillets and entering the tunnel, it is best to control the center temperature at 0-4 ℃.
If the temperature of the raw materials is too high, it will cause fluctuations in tunnel load and a decrease in freezing speed. The outer layer will crust first, and the internal moisture will not be able to come out. After thawing, it is easy to produce water and firewood.
5. The cutting thickness should be uniform
The greater the difference in thickness between tuna chunks, fish fillets, and diced fish, the poorer the freezing consistency.
It is recommended to control the thickness deviation of products from the same batch within a small range to avoid the center of thick blocks not being fully frozen and thin blocks being excessively dry and worn out. For IQF, consistent specifications also directly affect unit price, packaging specifications, and kitchen yield.
6. Surface moisture and blood should be controlled
Before freezing, it is necessary to drain, blow dry or lightly dehydrate to reduce surface free water.
Excessive surface water can lead to:
Frost formation intensifies inside the tunnel;
Products adhere to each other;
Uneven ice coating after freezing;
Dry consumption and weight loss increase.
Tuna has a lot of blood, so cleaning and draining should be gentle to avoid excessive flushing that can cause myoglobin loss and darkening of color.
7. The placement must be single layered to avoid stacking
The IQF tunnel relies on strong convective cold air to contact each individual unit.
Tuna chunks and fish fillets should be laid flat in a single layer, leaving gaps; Cannot be stacked or placed tightly. Otherwise, insufficient penetration of cold air can lead to external freezing and internal softening, as well as local water leakage after thawing.
8. Ice jackets should be controlled according to the customer and purpose
Golden spear fish often need to be wrapped in ice clothing after freezing to prevent drying and oxidation.
The thickness of ice clothing is usually controlled according to order requirements, usually in the range of 5% to 15%, but it needs to be verified according to customer standards, product form, and packaging methods.
The ice jacket is too thin, resulting in high storage dry consumption; Thick ice clothing can easily lead to disputes over net weight. It is recommended to control the temperature of the iced water at low temperature clean water to avoid overheating.

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