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Why is Cooked Rice considered High Risk?

In response to various questions we have had over the years on the subject, we thought it would be worthwhile to dedicate this post to explain in very simple terms, why it is that cooked rice is a high risk product.

For most of us, eating a leftover takeaway for breakfast is not generally acceptable but for some others, it’s a positive treat.  A piece of pizza or the scraps of rice and bhuna from a left-over Indian meal are, to some, a delicious breakfast which will set them up for the day.  When it comes to rice however, it’s probably fair to say that most people tend to over order on their takeaways or overestimate when cooking and end up with huge volumes of the stuff (we never learn!), but is there a danger lurking in this staple of so many people’s diets?

Rice in its raw form often contains bacterial spores of a pathogen called Bacillus Cereus.  The spores are harmless all the while the rice is uncooked but, it is once the cooking process has been completed that the risk arises as the spores are activated by warmth.  Bacillus Cereus spores will often survive the cooking process. This is not an issue, so long as the rice is either consumed when cooked, hot-held above 63ºC, or cooled rapidly and chilled subsequent to cooking. The problem arises when the rice is left out post cooking and enters the temperature rage commonly referred to as the ‘danger zone’ (5ºC – 63ºC, but particularly 20ºC – 50ºC). Once the temperature is favourable, the spores will then begin to germinate, and will release exotoxins in the rice. It is these toxins which cause food poisoning.  The symptoms of Bacillus Cereus food poisoning are often vomiting and diarrhoea and in most cases generally last for about 24 hours; unpleasant and unwanted. It is also worth mentioning however, that there is a second type of Bacillus Cereus which produces an enterotoxin within the intestine. The incubation period for this is often slightly longer than the first type (12 – 24 hours), with the symptoms primarily being abdominal pain, diarrhoea and fever.

Control measures

Freshly and thoroughly cooked, steaming rice should be safe.  The problem comes when rice is left to cool slowly and the bacteria go into overdrive, specifically between the temperatures of 28ºC and 35ºC.  The longer the rice remains left out of temperature control once cooked and is not adequately cooled and refrigerated, the greater the risk.  Avoid rice that has been left out for too long, it really isn’t worth chancing it!

If you are not intending to eat the rice as part of a hot meal but want to eat it cold, we would strongly recommend cooling it quickly and placing it in the fridge within one hour, keeping it at a temperature of 4ºC or lower.  This should make the rice safe to eat when cold; after all, a nice rice salad is an essential part of any good buffet.

If you’ve overestimated on the amount of rice you’ve cooked or purchased and don’t like to see things go to waste but plan on having it the next day as part of another hot meal, the same cooling process should be followed.  Cool it and place it in the fridge within one hour.  When it comes to reheating, make sure you heat it thoroughly (> 75 ºC), so that the rice is steaming, piping hot throughout.  It is recommended that you reheat rice once only and within 24 hours.  If you still have some left over it is best to discard it.

At this point it is important to note that cooked rice, purchased as part of a takeaway meal, would probably have already been reheated. The initial cooking would normally take place at the ‘mise-en-place’ stage in the restaurant’s preparation.

Following these simple guidelines should help you to avoid any food poisoning incidents.  Some rice (not intended for immediate consumption) can be refreshed and cooled instantly under cold running water. However in the absence of a blast chiller and especially for rice dishes with other ingredients and flavours incorporated (such as the base of a risotto or rice salad) a helpful tip when cooling is to decant the rice into a number of separate, shallow  containers; thus spreading out the surface area, enabling  it to cool down faster so that it should be cool enough to place it in the fridge within the hour.

Perhaps we associate food poisoning mostly with undercooked or poorly reheated meat or poultry, but this simple food grain poses just as high a risk and we need to be just as careful.  With that in mind, it is worth thinking twice about that pile of leftover rice on the side, inviting though it seems, and discard it completely.  If you’ve got a food bin, you won’t have to worry about it going to waste as it’ll be taken away for recycling, leaving you to rest in the knowledge that it will be utilised safely and productively and nobody will suffer any ill effects: surely the preferable outcome!