An earlier question in The Last Word gave warnings about eating leftover rice. I have seen this advice before, saying it is not safe to keep cooked rice for more than a short time, even in the fridge. Yet supermarkets sell a lot of chilled meals that include rice and these can have 鈥渦se by鈥� dates up to a week away. What is different about the cooked rice in these meals?
鈥� The belief that cooked rice shouldn鈥檛 be stored comes from its high surface area and easily accessible energy, water and nutrients. These can lead to rapid growth of bacteria. However, in a closed container, there are far fewer bacteria, so you can keep rice for longer.
Additionally, food producers can fill containers with oxygen or other gases that slow microbial growth, whereas a typical home fridge is more likely to be filled with wet air and bacterial spores. Your ready meals are steam cooked at well over 100掳C, so have less bacteria to start with, then they are filled with protective gas and sealed.
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鈥淩ice has a high surface area and easily accessible nutrients, which can lead to bacterial growth鈥�
Keeping home-cooked rice cold helps, as does avoiding bacterial contamination after cooking. The type of rice influences the shape of the surface and also which nutrients other than starch are present. These in turn affect how fast things grow. Simply keeping the rice in a sauce is effective. For example, rice pudding takes longer to become unsafe because microorganisms mostly grow on the top where they have air.
In my experience, it is quite easy to tell when rice is not good because it starts to smell similar to bad socks.
Neil Shirtcliffe, Rhine-Waal University of Applied Sciences, Kleve, Germany
鈥� Rice and other cereals carry various microorganisms, particularly Bacillus cereus, which can form spores. These spores aren鈥檛 destroyed by regular cooking, so if the rice dish is kept at between 5掳C and 63掳C for too long, they will germinate.
B. cereus can produce a toxin that is released into the rice and can cause food poisoning, usually diarrhoea and vomiting. The risk of this is greatest with home-cooked rice or informal mass catering when temperature control is poor.
In the food industry, this risk is controlled by rapidly cooling rice-containing meals so that the spores don鈥檛 have time to germinate, and then maintaining the low temperature throughout the supply chain.
Ready meals are cooled to less than 90掳C, then chilled with very cold air in a blast chiller. Air is blown through a refrigeration unit to cool it and then over the food on a conveyor belt. Its temperature falls to below 3掳C in less than 90 minutes.
Once the chilled meal has been taken home, it should be reheated to at least 70掳C before it is eaten and it shouldn鈥檛 be left to cool gradually because the risk of spore germination and thus contamination is still present.
There is advice about cooking and storing rice on the UK .
鈥淭he food industry controls the risk of rice spoilage by rapidly cooling rice-containing meals鈥�
Richard Marshall, Associate Lecturer in Food Science, Bath Spa University, UK
鈥� It is true that leftover rice can trigger food poisoning due to a bacterium called Bacillus cereus that can survive the cooking process. If the rice is cooled slowly and left at room temperature, the bacteria can multiply quickly and produce a toxin. Because the bacteria can withstand cooking, reheating the product won鈥檛 kill the bacteria. What鈥檚 more, the rice won鈥檛 look or smell any different to normal rice.
One method of overcoming these issues is to make sure the rice doesn鈥檛 cool slowly. To prevent this, cook small batches, which will cool faster than large batches. The rice can also be separated into clean shallow dishes or run under cold water to accelerate cooling. Then it can be covered and stored in a refrigerator.
Producers of ready meals are careful to keep their factories bacteria-free: all staff members have to be 鈥渟terile鈥� and wear protective suits to prevent contamination.
Inside the factory, rice is treated with steam to kill bacteria before they get a chance to germinate. The steam doesn鈥檛 actually cook the rice; it only sterilises it. Now bacteria-free, the rice can be cooked as at home but on a large scale, and is packed into sterile, sealed containers.
Once sealed, nitrogen is injected into the containers to ensure that any residual bacteria cannot grow. These protocols allow us to safely eat pre-cooked rice straight from the microwave.
Lorna Beattie, Glasgow, UK
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