Vacuum Flask Cooking - Precaution

Precaution

If a large part of the cooking time is spent at temperatures lower than 60 °C (as when the contents of the cooker are slowly cooling over a long period), a danger of food poisoning due to bacterial infection, or toxins produced by multiplying bacteria, arises. It is essential to heat food sufficiently at the outset of vacuum cooking. 60 °C throughout the dish for 10 minutes is sufficient to kill most pathogens of interest, effectively pasteurizing the dish. The best practice is to bring briefly to a rolling boil then put the pot in the flask. This keeps it hottest longest.

However the bacteria which causes deadly botulism, Clostridium botulinum, is widespread and can survive boiling and will multiply in certain foods at room temperatures in a sealed container. Foods that have not been autoclaved or pressure cooked at a temperature of 121°C+ (250°F+) for at least 3 minutes should be assumed to contain live Clostridium botulinum. If this food reaches temperatures of 3 °C to 43 °C (38 °F to 110 °F) it should be assumed to contain Clostridium botulinum multiplying and producing deadly botulin toxin. Remove the food before it cools to 43 °C (110 °F) and eat, keep hot, or refrigerate.

If there is any question of the possibility of botulinum toxin, the food should be reheated to 80°C+ (176°F+) for 20 minutes to deactivate the toxin. Note that this does NOT kill the spores, but just neutralises the toxin.

You can use a kitchen thermometer to check the temperature when you open the flask. The better the flask, the more food, and the less the time, the higher the final temperature will be. You can also cover the flask top with fluffy towels or other insulating material to increase the amount of insulation and the final temperature. Most flask tops are not vacuum insulated, so are not as effective insulators as the vacuum flask sides. See insulation values of common materials.

Growth of the Clostridium botulinum bacterium is prevented by high acidity (or high dissolved sugar), so dishes with a high proportion of tomatoes or lemons are usually safe.