Why is cooked rice dangerous?
Rice is a well-known food implicated outbreaks of food poisoning. Uncooked rice may contain spores of Bacillus cereus which can survive high cooking temperatures. Time and temperature abuse can trigger germination of bacterial spores and production of exotoxins. Rice held in warm temperatures (e.g.10°C< to 5°C) can encourage the multiplication of B. cereus because it meets optimum pH range (6.3-6.5) and minimum water activity (0.93) for bacterial growth1.
Commons symptoms, such as nausea and vomiting, can occur 1-5 hours after ingestion of exotoxins in the food. Abdominal pain and watery diarrhoea may also occur 6 -24 hours after consumption of intact and viable bacteria because endotoxins form part of bacterial cell wall and released into the gut when the bacteria dies.
Sushi rice should be cooled to 5°C or below within 2 hours of cooking or acidified. Cooled rice is recommended for making sushi intended for longer term storage and consumption (e.g. 2 days). Whereas hot/warm rice mixed with cold fish should be served immediately to the customer2.
Follow a standardised recipe to ensure the correct volume of vinegar solution is added to a specific amount of rice. Consistency of ingredients is important to achieve a uniform pH throughout the sushi rice.
Acidify rice immediately after cooking with the vinegar, salt and sugar solution. Rice must be mixed thoroughly to reach a consistent pH and prevent pooling. Acidified rice can be kept at room temperature for up to 4 hours (recommended)2.
Take a small amount of rice from the batch of acidified rice and insert a clean and calibrated pH meter into the sample. Wait until the reading remains stable for at least 5 seconds. Rice must achieve a pH of 4.6 or below2. This acidity is enough to inhibit multiplication of pathogenic bacteria. Check consistency of acidification by taking several pH measurements from different areas of the batch. Do not ‘mix’ the samples before taking the pH reading. Avoid taking a measurement from a pool of vinegar solution as this will provide a false reading2. Always follow manufacturer’s instructions for use and calibration of the pH meter. Never use pH strips to check acidification of rice2. Acidified rice can be kept at room temperature safely for a maximum of 4 hours2. Warm rice (below 63°C) that does not achieve a pH of 4.6 or below must be discarded within 2 hours or cooled rapidly from 55°C to 20°C within 2 hours and refrigerated.
Documentation and Records
There must be a written and validated safe method in place for the production and storage of sushi rice. This document must include recipe specifications, production methods, using a pH meter, record keeping, correction actions and verification. Taking and recording pH measurements of multiple batches acidified rice will provide documentary evidence to show a consistent and uniform critical limit can be achieved. This critical information will help validate a consistent and safe method.
Records are important because they demonstrate compliance with the HACCP plan and support a due diligence defence. They must state time of preparation and the pH of the sushi rice and details of any corrective action taken2. Food handlers preparing Sushi must be trained and details of training recorded.
We discuss the improper handling of rice and other food safety hazards on all food safety and HACCP courses. Contact us on 01296 320247 or email Peter at email@example.com Guidance provided is for educational purposes only.
- Nezih Müftügil (2016). CHILLING RATE OF COOKED RICE AND RISK OF Bacillus cereus GROWTH IN RESTAURANT OPERATION. In Journal of Food and Health Science. Vol 2(4) pp.183-188.
- Food Safety Authority of Ireland. Safe production of sushi [ Online]. Available at https://www.fsai.ie/faq/safe_production_sushi.html (Accessed 31/03/2020)