Vol. XIX · Fish and Seafood Plate 6 of 9raw fish, sashimi, sushi, ceviche, gravlax, poke, parasites, Anisakis, freezing rule, −20 °C, sushi-grade, cured fish, salmonS

Raw and Cured Fish.

"Sushi-grade" is not a legal standard. What makes raw fish safe is a freezer colder than most home freezers, for longer than most people think.

Many sea fish carry the larvae of roundworms such as Anisakis, curled in the belly flesh; eaten alive they can burrow into the stomach wall and cause severe pain, and they trigger allergies in some people. Cooking to 60 °C kills them; so does freezing hard. Food rules for fish to be eaten raw therefore require freezing: in the US Food Code, −20 °C or colder for 7 days, or −35 °C until solid and then 15 hours; in the EU, −20 °C for at least 24 hours or −35 °C for 15 hours in every part of the fish. A home freezer at −18 °C does not meet either. Fish sold for sushi has usually been frozen this way at sea or by the supplier.

Curing changes fish without heat. Ceviche's lime juice and gravlax's salt and sugar unfold the proteins, so the flesh turns opaque and firm as if cooked (VI.1), but neither reliably kills parasites or bacteria: the fish for them must be as fresh and as frozen as for sashimi. Farmed salmon raised on pellet feed rarely carries parasites, and some rules exempt it, but the advice for anyone at higher risk (XVII.6) is to avoid raw fish altogether. The workshop freezes a 30 mm fillet.

Below: the centre temperature of a 30 mm salmon fillet put into the freezer you choose (thermal bench, with the latent heat of freezing), the hours until every part is at −20 °C or below, and whether the days you choose meet the US and EU rules for fish to be eaten raw.

What you will learn
Workshop
Thermal bench: 30 mm salmon fillet from 4 °C in still freezer air at the temperature shown; freezing point and latent heat from the course's food model. Rules: FDA Food Code 3-402.11 (−20 °C for 7 days, or −35 °C for 15 h) and EU 853/2004 as interpreted by EFSA (−20 °C for 24 h or −35 °C for 15 h in all parts).

Freeze it hard, freeze it long

In this model a 30 mm fillet in a −35 °C blast freezer is at −20 °C right through after about 5.0 hours; in a −20 °C chest freezer its centre approaches −20 °C only after about 12 hours; in a −18 °C home freezer it is at about −16 °C after twelve hours and can never go below −18 °C, so it never reaches the line. That is why the rules are written for commercial freezers, and why the safe course at home is to buy fish that the supplier states was frozen for raw consumption, or to freeze it in a freezer you have checked with a thermometer reads −20 °C or below, for a full week.

Raw fish also needs the freshness and cold handling of XIX.2 — bacteria are not killed by freezing (XIII.3) — and the knife and board hygiene of IV.7. Sushi rice is seasoned with vinegar partly so that it can sit at room temperature for a few hours (XXVIII.5). Gravlax: bury fresh, frozen-and-thawed salmon in equal parts salt and sugar with dill for two to three days in the fridge. Ceviche: frozen-and-thawed fish, lime juice for 15–30 minutes. Pregnant women, young children, the elderly and the immunocompromised should eat fish cooked.

At the stove

Put a thermometer probe into a piece of fish in your freezer for a day and read it. If it is above −20 °C, your freezer cannot make fish safe for raw eating.

Anisakiasis was first described in the Netherlands in the 1950s, from lightly salted herring; Dutch law then required herring to be frozen before salting, and the disease nearly vanished there. Japan, where raw fish is eaten most, still reports thousands of cases a year.

Terms

Anisakis
A roundworm whose larvae live in many sea fish.
Freezing rule
The temperature and time that kill parasites in fish to be eaten raw.
Ceviche
Raw fish "cooked" by lime juice: denatured, not made safe.
Gravlax
Salmon cured with salt, sugar and dill.
Raw fish is made safe from parasites by freezing: −20 °C for 7 days (US) or 24 hours in all parts (EU), or −35 °C for 15 hours. A home freezer at −18 °C is not enough. Acid and salt cures change texture, not safety.

Sources

  1. FDA Food Code 2022, 3-402.11: fish to be eaten raw is frozen at −20 °C or below for 7 days, or at −35 °C until solid and stored at −35 °C for 15 hours, or at −35 °C until solid and stored at −20 °C for 24 hours.
  2. EFSA Panel on Biological Hazards, "Scientific Opinion on risk assessment of parasites in fishery products", EFSA Journal 8(4):1543 (2010): freezing at −20 °C or below for at least 24 hours, or −35 °C or below for at least 15 hours, in all parts of the product kills Anisakis larvae; EU Regulation 853/2004 requires such treatment for fish to be eaten raw.
  3. H. McGee, On Food and Cooking: The Science and Lore of the Kitchen, 2nd ed., Scribner, 2004.