FISH TECHNOLOGY: STORAGE OF FISH
Diagram
Fresh fish and frozen fish
Fresh fish
Fish that has never been frozen. It is kept chilled on ice and must be used within days.
↓
Freezing
The fish is frozen fast, often at sea, so ice crystals stay small and the flesh is hurt less.
↓
Frozen fish
Fish held below freezing. It keeps for months, but it must be thawed before cooking, and thawing lets out drip.
Figure 1: Fresh fish is chilled and short lived; frozen fish is held below freezing and keeps for months. Drawn in our own style from the freezing chapters of the Handbook of Food Preservation, CRC, 2007, and the comparison of freezing with other preservation methods in Mallett, Frozen Food Technology, Springer, 1993, p. 18.
Introduction
Fresh fish and frozen fish are the same fish in two states, but they store, thaw and cook differently. Fresh fish is fish that has never been frozen and is kept chilled on ice, while frozen fish is fish held below freezing, often frozen fast at sea. Mallett compares freezing with other preservation technologies, and the Handbook of Food Preservation covers freezing in its own chapters.
The core difference is temperature and time: fresh fish must be used within days, and frozen fish keeps for months, but freezing changes the texture of the flesh, and thawing lets out moisture.
Quick comparison
Table 1: Fresh fish compared with Frozen fish
| Aspect | Fresh fish | Frozen fish |
|---|---|---|
| State | Never frozen, kept chilled on ice | Held below freezing, often frozen at sea |
| Shelf life | Days, even on ice | Months, while it stays frozen |
| Storage temperature | Chilled, near 0 C | Frozen, well below 0 C |
| Texture | Firm and fresh, with no ice damage | Can soften, because ice crystals damage the flesh |
| Moisture loss | Little, while it is fresh | Thawing lets out drip, so the fish loses moisture |
| Flavour | Fresh, delicate flavour at its best | Good, but can be a little duller after long storage |
| Thawing | None needed: cook it straight away | Must be thawed before cooking, slowly in the fridge |
| Cooking | Cooks fast and evenly | Must be fully thawed first, or the outside cooks before the inside |
| Convenience | Needs quick use, so it suits buying and cooking the same day | Can be kept in the freezer and used when wanted |
| Quality surprise | Fish sold as fresh may be days old by the time it reaches the shop | Fish frozen at sea can be fresher, in effect, than fresh fish that has travelled for days |
Scientific explanation
Definition
Fresh fish is fish that has never been frozen, kept chilled on ice from the catch to the shop. Frozen fish is fish that has been frozen, usually fast, and held below freezing until it is thawed for use. Mallett’s Frozen Food Technology compares freezing with other preservation technologies (p. 18), and the Handbook of Food Preservation covers freezing in its own chapters.
Fresh does not always mean just caught: fish sold as fresh may have spent days on ice in transport, while fish frozen at sea is frozen within hours of the catch, so the frozen fish can be the fresher of the two in effect.
Working principle
Freezing works by turning the water in the fish into ice, so bacteria cannot grow and enzymes slow almost to a stop. Fast freezing makes small ice crystals that hurt the flesh little, while slow freezing makes large crystals that tear the muscle fibres, which is why thawed fish can feel softer and let out drip.
Fresh fish keeps only by chilling: the cold slows bacteria and enzymes, but it does not stop them, so the fish spoils within days. That is the trade: fresh fish has the best texture, and frozen fish has the longest life.
Apparatus
Fresh fish needs ice, chilled boxes and cold rooms. Frozen fish needs a freezer: a blast freezer or plate freezer that freezes fast, and a cold store that holds the fish below freezing. The books used here describe the methods rather than naming one standard instrument for the comparison itself.
Measurement procedure
For fresh fish, the fish is gutted, washed and packed in ice, and kept chilled until it is cooked. For frozen fish, the fish is frozen fast, packed to keep out air, held below freezing, and thawed slowly in the fridge before cooking, so the drip loss stays small.
There is no single standard measurement procedure for this comparison in the books used here: whether fish is fresh or frozen is judged by its history and its state, not by a test result.
Factors affecting readings
The speed of freezing matters most for frozen fish: fast freezing gives small crystals and a better texture, slow freezing gives large crystals and more drip. The time in storage matters too: frozen fish slowly loses quality even below freezing, as its fats go rancid. For fresh fish, the time since the catch and the coldness of the ice matter most.
Formulas and calculations
The books used for this article give no formula for this comparison, so none is given here. The comparison is qualitative: it rests on definitions, materials and method, not on a calculation.
Industrial applications
- Fresh fish is used in fish markets and restaurants, where it is bought and cooked the same day.
- Fresh fish is used for sashimi and raw dishes, where only the freshest fish will do.
- Frozen fish is used in shops and freezers at home, where it is kept and used when wanted.
- Frozen fish is used for fish fingers, battered fillets and ready meals, where a steady supply matters.
- Frozen fish is used for fish caught far away, frozen at sea so it can travel the world.
Advantages and limitations
- Fresh fish, advantage: the best texture and flavour, with no thawing and no drip loss.
- Fresh fish, limitation: spoils within days, so it must be bought and used quickly.
- Frozen fish, advantage: keeps for months and can be stored at home until it is wanted.
- Frozen fish, limitation: freezing and thawing soften the flesh and let out moisture, so the texture is never quite the same.
FAQ
Can frozen fish be as good as fresh fish?
Yes, often: fish frozen fast at sea can be better than fresh fish that has spent days on ice in transport. The quality depends on how fast it was frozen and how long it was stored.
Why does thawed fish let out water?
Because freezing forms ice crystals that tear the muscle fibres, and when the ice melts the water leaks out as drip. Fast freezing makes smaller crystals, so there is less drip.
Should I thaw fish in warm water?
No: thaw it slowly in the fridge, so the outside does not warm up while the inside is still frozen. Slow thawing also keeps the drip loss smaller.
Can I refreeze fish after thawing?
It is not good practice: each freeze and thaw damages the texture more and gives bacteria more time to grow. Cook thawed fish rather than freezing it again.
How do I tell fresh fish in a shop?
Look for clear eyes, bright red gills, firm flesh that springs back when pressed, and a fresh sea smell, not a strong fishy smell. Those are the signs of fresh fish, and they fade as the fish ages.
References and further reading
- Handbook of Food Preservation, CRC, 2007 (freezing chapters)
- C. P. Mallett, Frozen Food Technology, Springer, 1993
