Spores will remain dormant in dry conditions, they require moisture in order to germinate. They also need a warm temperature, an ideal temperature being 37°C. All rice must be considered to contain B cereus spores. As the raw, uncooked rice is dry, the spores are harmless. If the rice is cooked, the spores remain intact as the temperature of the cooking rice is too high for germination and food safety is not affected. If the rice is drained and served immediately or retained above 63°C, the spores will not germinate. During digestion, the spores will not germinate due to the temperature of the food and the acidity of the stomach.
If rice is to be served cold or used the next day, for example in stir fried dishes, it must be cooled as quickly as possible after cooking. After draining the hot, cooked rice, it must be held under cold running water until cool to the touch. The rice is then drained and refrigerated until required for use. The spores have not had enough time to germinate as the rice was brought through the temperature danger zone quickly and food safety is not compromised.
Many takeaways and restaurants do not, however, adopt this approach. After cooking, the rice remains in the kitchen area to cool down overnight. During the slow cooling period, the spores will start to germinate at 37°C. As there is moisture and protein in rice the germinated spores will vegetate or grow. During growth, B cereus release two exotoxins into the rice. The exotoxins are heat stable and cannot be destroyed by cooking. When the rice is used the next day it contains B cereus bacteria and exotoxins. If the rice is reheated, the bacteria turn back into spores, which will not be a food safety hazard, but the poisons will remain, which will cause food poisoning.
Even if the rice is produced correctly by a takeaway, food poisoning can still result. For example, you have bought some special fried rice, but could not eat it all and want to finish it the next day. Always ensure the rice is refrigerated, not left out overnight, as the spores will germinate as above.
Toxins are defined as organic poisons i.e., they are produced by living organisms such as animals, plants, and bacteria. Bacteria can produce two types of toxins, namely exotoxins and endotoxins. S aureus and C botulinum produce exotoxins, which can prove to be a serious food safety problem. Exo means produced outside the body. Therefore an exotoxin is a poison produced by bacteria and then ingested.
Another type of poison is known as an endotoxin which is produced inside the body from dying bacteria or from bacteria that produce spores. Gram negative bacteria have material called LPS in their external membrane which produces poisoning. An example of gram negative bacteria that release endotoxin are Salmonella spp.
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