What's in our food: Bugs in the kitchen can stand the heat

If you think food is safe because it has been cooked, think again.

Food scientist Rosemary Whyte says some bugs can withstand boiling temperatures as hibernating spores, then turn back into bacteria which multiply when food is cooled too slowly before eating.

The problem is most commonly found in restaurants and takeaway bars which cook large quantities of casseroles or rice and let them cool in large containers where the cold air never reaches the middle of the lump of food.

The bugs are rarely reported because the symptoms are mild.

"You need to rush to the toilet once or twice and you're okay after that," Ms Whyte says.

One of the bugs that lives in rice, Bacillus cereus, also makes people feel sick and vomit one to six hours after eating.

But the victims are back to normal within 12 to 24 hours.

In Christchurch in 2002, 40 people got sick with another bug, Clostridium perfringens, after eating at a restaurant which had cooled a 20-litre casserole in a bucket in a walk-in chiller.

"You just can't cool that volume of meat down in a bucket," Ms Whyte says.

She says the restaurant should have spooned the casserole into shallow containers with the food no more than 5cm deep.

It should then have been reheated to at least 74C before serving.

She believes too many cafes use pie warmers or heat lamps to warm food, rather than for their proper use of keeping food warm after it has been cooked.

"All they are doing is incubating the bugs, not killing them off," she says. "They should be heating pies in the oven."

In homes, she says, the major risk is refrigerators that are too warm.

They should be between 1C and 5C - as cold as you can make them without your lettuce turning to ice.