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Can you reheat food without additionally cooking it? Is there a threshold temperature that begins the re-cooking process?
Matthew Stevens
Sydney, Australia
Cooking is the application of heat to food. So, applying heat to cold leftovers will cook them further. One very important process in cooking is a chemical reaction known as the Maillard reaction, in which sugars become linked with proteins. This results in the mouth-watering scents of cooked foods, particularly meats. Although heat hastens the reaction, it continues in the fridge, albeit slowly. This explains why many foods taste even better the next day after a night in the fridge. In short, any amount of added heat will cook food.
Mike Follows
Sutton Coldfield, West Midlands, UK
Reheating food without re-cooking it is a delicate balance that requires care to avoid the risk of food poisoning. Reheating can unintentionally alter the texture, flavour or appearance of food, making it less appetising. It can also cause dryness or lead to the degradation of certain nutrients, such as vitamin C.
Cooking involves structural and chemical changes in food that are triggered at specific temperature thresholds. For example, proteins in eggs and meat begin to denature (i.e. cook) between 49°C and 71°C (120°F to 160°F) while starch gelatinisation in foods such as rice and pasta occurs between 60°C and 82°C (140°F to 180°F).
Ideally, reheating should restore food to a desirable eating temperature without causing further significant chemical changes. To achieve this, the process needs to be done quickly and evenly, to avoid partial heating or cold spots where bacteria might survive.
In most cases, food should be reheated thoroughly to 74°C (165°F), as recommended by the US Department of Agriculture, in order to kill harmful microbes and reduce the risk of foodborne illness. In such cases, some degree of re-cooking is inevitable. The temperature range between 4°C and 60°C (39°F to 140°F) is considered optimal for the rapid growth of bacteria, so it is important to minimise the time food spends in this so-called danger zone. However, if food intended for reheating has been cooled rapidly and refrigerated within 2 hours of initial cooking, it may be worth reheating it only to between 49°C and 60°C. This can help reduce the extent of additional cooking while still bringing the food to a safe and palatable temperature. For foods with a liquid or semi-liquid consistency, stirring during reheating helps to distribute heat evenly and eliminate cold spots. Using a food thermometer can confirm that the desired temperature has been reached.
Sous vide (French for ‘under vacuum’) is arguably the most effective method for reheating food without overcooking
Sous vide (French for “under vacuum”) is arguably the most effective method for reheating food. It involves placing the food in a vacuum-sealed bag and immersing it in a precisely controlled water bath using a device known as an immersion circulator. This method allows for accurate temperature control and even reheating, significantly reducing the risk of overcooking or bacterial growth when carried out correctly. To prevent bacterial growth, food should be reheated only once.
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