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Reheated food – how safe is it? Microwaves and hot holding

We’ve all reheated food at one time or another. Perhaps it’s leftover curry. Maybe it’s the remnants of last night’s Chinese meal. Or soup or stew. We can reheat food in the oven or on the hob. However, we often use the microwave instead. After all, it’s fast, simple and convenient.

Manufacturers of ready-prepared meals give precise instructions on how to cook their products. Cooking times vary dependent on the wattage of the microwave. They will often require you to stir the product part way through the cooking process. And all will specify a standing time. Nevertheless, not everyone leaves the product to stand. Instead they tip it out onto their plate and set about consuming it.

microwave reheating foodWhy is microwave standing time important?

Like all reheated food, microwaved food needs to be piping hot when served. This is to ensure that any harmful bacteria which may have developed in the food is destroyed. Since we can’t see bacteria with the naked eye we can kill it off by cooking it. The recommended temperature is 75 degrees C or above for two minutes. For reheating a temperature of 82 degrees C is recommended.

For food to be safe, all parts of the product must reach this temperature. That’s why we stir sauces, soups and stews in pans, so that the heat is distributed evenly. The snag is that microwave ovens heat food unevenly. So even though it might be piping hot around the edges, the food could be much cooler in the middle. Bacteria could still survive in these cooler areas. It’s therefore always important to allow the heat to reach them. Stirring partway through assists in this, but the standing time is equally important in ensuring that the heat travels through the entire product.

chicken and rice salmonella food poisoningAn often-cited salutary tale concerns the microwave cooking of a frozen chicken and rice meal. 44 people contracted salmonella food poisoning in America. Although they had followed the cooking instructions on the packaging they had not left the product to stand. As a consequence, the product was not steaming hot throughout. Hence the Salmonella enteritis bacteria had not been killed. Standing time is part of the cooking process and should not be ignored.

High-risk foods

reheated pastaOf course, rice and chicken are particularly high-risk foods. Pre-cooked rice is prone to the Bacillus cereus bacterium. This is a persistent little critter. It can be destroyed by heating to a high temperature, but it also produces heat-resistant spores. To reduce the risk of the bacteria multiplying and producing spores, cool the leftover rice as quickly as possible and keep in a refrigerator. On no account leave it out overnight.

Other foods which have hazard potential if reheated incorrectly are:

  • Sauces containing milk or cream
  • Any cooked meats or cooked food containing meat e.g. lasagne, chilli, curry, casserole, stew
  • Seafood, including stews, fish patties, fish stock etc.
  • Cooked pasta and pasta dishes
  • Protein-rich foods such as eggs, beans, quiche, lentil burgers, nuts and so forth

Hot holding and reheated food in catering

Anyone who has had HACCP and Food Safety training will know that time and temperature are Critical Control Points in the quest to prevent microbiological contamination of products. We can control the growth of pathogenic bacteria by the heating, chilling or freezing of food. Ensuring that the temperature at which food is stored or held remains constant is extremely important.

We have already stated that food should be cooked to 75 degrees C or above (or at e.g. 70 degrees C for two minutes if higher temperatures are detrimental to the food’s quality). As far as chilling is concerned, the multiplication of most bacteria can be controlled at 8?C or below. However, best practice is to store food at 5 degrees C or below. Frozen products should be kept at -18 degrees C or below.

For hot-holding, the core temperature of the product must remain at 63 degrees C or above. Any reheated food product should have reached a temperature of 82 degrees C prior to being placed in a hot cabinet or bain-marie. The temperature of the cabinet or bain-marie must itself have reached 63 degrees C throughout before it can accept food.

How to monitor the temperature of hot-held food

It goes without saying that regular monitoring of the temperature of the food is essential for food safety. For food such as rotisserie chicken and other hot meats this can be achieved using a digital probe. It should be inserted into the thickest part of the meat to check the temperature. Stews, soups etc. should be stirred before being checked with a digital thermometer. Regular mercury/glass thermometers should never be used. If they should break the food would become contaminated.

For hot cabinets, the temperature can be monitored by reading the temperature dial. Probes can be used as a back-up. You should also set a maximum time limit on the display of products.

You can find out more about bacteria and microbiological contamination on our Level 3 Food Safety course. Understanding of hazards and Critical Control Points, monitoring, validation and verification is covered in detail on our Level 3 HACCP course.

Ah! I think I just heard the microwave ping. Better leave it a minute or so….

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