Vol. XXVIII · The Cuisine of Japan Plate 5 of 11sashimi, sushi, 刺身, 寿司, sushi rice, 酢飯, shari, awase-zu, 合わせ酢, rice vinegar, pH, acidified rice, Bacillus cereus, parasites, freezing, Anisakis, sushi-grade, nigiri, food safetyS

Sashimi and Sushi.

Plain rice left out for an afternoon is a classic cause of food poisoning. Seasoned sushi rice, at pH 4.2, can sit on the counter through a lunch service.

Raw fish brings two hazards (XIX.6, XIX.7). Parasites, such as Anisakis larvae in wild marine fish, are killed by freezing — at −20 °C or below for 7 days, or −35 °C until solid and then 15 hours — which is why most fish served raw, apart from certain farmed salmon and tuna judged low-risk, has been frozen. Bacteria are controlled by freshness and cold: fish bled and iced at once, kept at 0–4 °C, cut with clean knives on clean boards, served soon. "Sushi-grade" is a trade description, not a legal standard; what matters is a seller who handles fish for raw use and says how it was frozen.

Sushi rice (酢飯, shari) is short-grain rice cooked a little firmer than usual (XXVIII.3) and, while hot, folded with awase-zu (合わせ酢): rice vinegar with sugar and salt, typically 60–70 mL of vinegar to 300 g of raw rice. The rice is fanned as it is folded so that it cools quickly, turns glossy and absorbs the seasoning. The vinegar also acidifies it. Cooked rice is the classic food of Bacillus cereus (XII.4), whose spores survive cooking and grow and make toxin if rice is left warm. At pH below about 4.6 they cannot grow, which is why food codes allow properly acidified sushi rice to be held at room temperature for a service — under a written plan and with its pH measured. The workshop seasons rice.

Below: the pH of sushi rice seasoned with the vinegar you choose per 300 g of raw rice (teaching curve calibrated to measured sushi rice, pH 4.0–4.6), and the growth of Bacillus cereus in it over the hours you choose at 25 °C (course growth model with pH), against plain rice.

What you will learn
Workshop
pH (teaching): 6.4 − 2.6 × (1 − e^(−v ÷ 35)) for v mL of 4.5 % rice vinegar per 300 g of raw rice; measured sushi rice is usually pH 4.0–4.6. Growth: course cardinal model for B. cereus (minimum pH 4.9, ICMSF), from 100 per gram (log 2), at 25 °C. Food codes: acidified rice held without temperature control needs a variance and a HACCP plan, usually with pH 4.6 or below.

Freeze the fish, acidify the rice

In the teaching curve, 60 mL of vinegar per 300 g of rice gives about pH 4.27, safely acidified; a timid 20 mL leaves it near 5.27, which does not stop B. cereus. Plain rice at 25 °C, in the course growth model, gains about 1.9 log of B. cereus in 8 hours and 3.4 in 12 — and the emetic toxin it makes survives reheating (XII.9). Acid is a barrier, not a licence: restaurants that hold sushi rice warm measure its pH, label the time, and discard it at the end of service.

Sushi rice for 300 g raw rice: wash and soak (XXVIII.3), cook with about 330 g water and a 5 cm piece of kombu. Warm 65 mL rice vinegar with 25 g sugar and 6 g salt until dissolved. Turn the hot rice into a wide wooden or glass bowl, pour the vinegar over a paddle, and fold with cutting strokes, fanning to cool it to body temperature; cover with a damp cloth. For sashimi at home, buy fish sold for raw use and previously frozen (or freeze it yourself at −20 °C for 7 days in a freezer that holds that temperature, XIX.6), keep it on ice, cut it just before serving, and eat sushi within a couple of hours.

At the stove

Season half a batch of rice with awase-zu and leave the other half plain. If you have pH strips, test both.

The first sushi was narezushi: fish packed in rice for months so that lactic fermentation preserved it, the rice thrown away. Edo-period Tokyo cut that to minutes, flavouring fresh rice with vinegar and topping it with fish from the bay — nigiri, the fast food of the 1820s.

Terms

酢飯 / shari
Sushi rice seasoned with vinegar, sugar and salt.
合わせ酢
The seasoned vinegar for sushi rice.
Acidified rice
Rice brought below about pH 4.6 to stop B. cereus.
Sushi-grade
A trade term, not a legal standard.
Raw fish: frozen to kill parasites (−20 °C for 7 days, XIX.6), kept cold, cut clean, served soon. Sushi rice: about 60–70 mL vinegar per 300 g rice brings it to pH ≈ 4.2, below the 4.6 that stops B. cereus.

Sources

  1. U.S. FDA Food Code 2022, 3-502.11: acidifying a food such as sushi rice so that it can be held without temperature control requires a variance and a HACCP plan; an equilibrium pH of 4.6 or below is the usual critical limit, and many guidelines aim for 4.2–4.4.
  2. 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.
  3. ICMSF, Microorganisms in Foods 5: Characteristics of Microbial Pathogens, Blackie, 1996 — growth limits (minimum temperature, pH and water activity) and typical heat resistance; the course rounds them.
  4. Bacterial growth: cardinal-temperature model (L. Rosso et al., J. Theor. Biol. 162, 1993) with cardinal temperatures and optimum rates rounded from ICMSF (1996); teaching values that put each organism in its known range.
  5. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.