Loving your leftovers

If food isn’t cooked thoroughly in the first place, the food could still contain a few harmful bugs. Although these wouldn’t be enough to make you ill, they could grow in the food over time, especially if allowed to stay warm. Also, leftover food (whether well-cooked, undercooked or uncooked) can pick up contamination when it is handled, from touching work surfaces or from touching other foods, and so needs to be treated carefully.

The temperature ‘danger zone’ in which most bugs are able to grow starts at just 8°C, and food that is just left out after cooking or eating is likely to be warmer than that for some time. The longer leftover food stays warm, the longer any bugs present have to grow quickly. Food will cool more quickly if split into smaller portions or shallow dishes. And from what we know about the rate that bugs grow and how quickly food can cool, getting it in the fridge within 1 to 2 hours is a good safety net.

Leftovers should be used within two days. Prompt and effective cooling of food slows down the biological processes that bugs carry out to live and multiply; enzymes within the bugs and the metabolic processes the enzymes carry out work much more slowly in the cold. For many bugs this halts growth completely but some can continue to grow slowly, even at fridge temperatures below 5°C, so the recommended two days is determined from what we know about how quickly these cold-loving bugs grow.

In some instances, even effective cooking and reheating may not be enough to ensure safety. Uncooked rice can contain spores (resting cells) of a bacterium called Bacillus cereus, which are resistant to heat and survive normal cooking temperatures. If cooked rice is allowed to stay warm and not kept chilled, these spores can germinate, grow and produce a toxin (poison). This toxin is also resistant to heat, so if it has been formed in the food it won’t be destroyed by thorough cooking or reheating. Keeping cooked rice chilled and eating it within one day is critical for food safety.