Why −18 °C is not enough for tuna
The red colour of tuna comes from myoglobin, a pigment in the muscle. Over time it oxidises to metmyoglobin, which is brown. In frozen storage, this browning continues slowly and speeds up as the temperature rises. Patent and research literature on tuna preservation describes browning that cannot be prevented at −18 °C, which is why sashimi-grade tuna loses its value in an ordinary freezer.
Research on frozen tuna has found that colour loss is greatly suppressed at around −45 °C and below, and industry practice for premium tuna commonly uses storage around −50 °C to −60 °C. Starting freshness matters too: fish frozen in poor condition will not be restored by cold storage.
Who needs a −60 °C freezer
- Sushi and Japanese restaurants that buy frozen tuna loins or saku blocks for raw service.
- Hotels with speciality restaurants holding premium seafood for weeks rather than days.
- Seafood importers, distributors and processors handling ultra-low frozen product.
- Premium seafood retailers selling frozen sashimi-grade fish.
The chain must stay ultra-low
An ultra-low freezer only protects colour if the fish arrived ultra-low. Product that warms to −18 °C during transport or sits in a standard freezer for days has already started to brown. Ask your supplier how the fish is shipped and at what temperature, check product temperature on delivery, and move it into the −60 °C freezer immediately. Avoid moving blocks between freezers at different temperatures.
Storage freezer, not a quick freezer
Most −60 °C cabinets are storage freezers. They hold product that is already frozen; they are not designed to freeze fresh fish quickly. Slow freezing forms large ice crystals that damage the muscle and cause drip loss and soft texture on thawing. If you freeze fish yourself, use equipment designed for fast freezing, then transfer to ultra-low storage.
Raw fish and parasites
Food-safety authorities in several countries set freezing requirements for fish intended to be eaten raw, to control parasites. Requirements differ by country and change over time, so follow FSSAI's rules and your own food-safety plan, and ask your seafood supplier how their product has been treated. Ultra-low storage protects quality; it does not replace the handling requirements your regulator sets.
What to look for in the freezer
| Feature | Why it matters |
|---|---|
| Holds −60 °C at your room temperature | Rated performance means little if tested in a cool room and installed in a hot store |
| Thick insulation and heavy lid gaskets | Limits heat gain and frost at the lid |
| Inner lids or baskets | Lets staff retrieve one block without exposing the whole load |
| Temperature display and alarms | Warns of a rise before colour is lost |
| Accessible drain and defrost guidance | Ultra-low cabinets frost quickly and need periodic defrosting |
Handling and thawing
- Vacuum-pack or tightly wrap blocks so the surface does not dry out (freezer burn).
- Label each block with date received and rotate first in, first out.
- Open the lid briefly and know where each item is before you open it.
- Thaw using the method your supplier recommends; thawing affects colour and drip as much as storage does.
- Take out only what will be used in the next service.
Site, power and running cost
A −60 °C freezer uses considerably more energy than a −18 °C one of the same size and rejects more heat into the room. Place it in a ventilated, preferably air-conditioned space with the clearance its maker specifies. Give it a dedicated earthed supply, ask about voltage range and stabilisers, and plan generator backup if outages are frequent. A closed, well-filled ultra-low chest holds its temperature for some time during a short outage, but keep the lid shut.
Questions to ask the supplier
- Does the freezer hold −60 °C at the room temperature in my store, and how long does it take to pull down from warm?
- Is the refrigeration single-stage or cascade, and who services it in my city?
- What alarms are included, and can they send remote alerts?
- How often will it need manual defrosting, and how is the meltwater drained?
- What is the rated energy use per day, and at what room temperature was it measured?
Frequently asked questions
- Can I keep tuna at −60 °C indefinitely?
- No. Ultra-low storage slows quality loss greatly but does not stop it. Follow your supplier's guidance on storage life and rotate stock.
- Is −40 °C good enough?
- It is far better than −18 °C. Research suggests colour loss is strongly suppressed from about −45 °C downward, which is why premium tuna is usually stored around −50 °C to −60 °C.
