Knowing how long sushi can safely be left at room temperature can be quite handy, especially with the number of sushi restaurants providing takeaway and delivery services. Also the emergence of kaiten-sushi chains or restaurants serving sushi dishes on a conveyor-belt, where the food will likely be at room temperature for an extended period of time, make this a question many people ask themselves.
By definition, sushi rice is short-grain white rice treated with a type of rice-wine vinegar which alters the PH level and creates a hostile environment for bacteria. Treating the rice in this way means it remains safer to eat for longer as the amount of harmful bacteria growing on it is restricted.
Interestingly, rice is one of the food groups particularly susceptible to bacterial contamination due to its moisture, temperature and high starch content being a perfect breeding ground for bacteria such as Bacillus cereus, making it a big contributer to the number of reported food-poisoning cases.
Food safety guidelines recommend that raw fish is not left at room temperature for more than 2 hours, vegetable, cooked meat and prawn for 3 hours, and the rice itself for 4 hours (from time of cooking). Bacteria probably won't reach dangerous levels for some time after these limits- but these are the best practice guidelines enforced by most councils and environmental health officers in the UK.
Most professional restaurants these days adopt the HACCP (or Hazard Analysis at Critical Control Point) approach to food hygiene which involves probing the temperature of food, ensuring best practice is being followed, keeping to strict regulations for storing and handling food, and also logging and documenting the entire procedure. This helps to eliminate the risk of bacterial contamination and provides reassurance to the customer, which means we can enjoy our sushi without worrying about nursing an upset stomach the next day!