bacteria formation

[Aa01]

does bacteria form on food wen it's hot, or when it's cooling down? i apologise for the abiguous question.

[Litchick]

I believe it's when the food is cooling down...

[Buenchico]

(Answer in 2 parts to get round the 2000 character limit):

Different bacteria thrive under different conditions but, in general, most bacteria like 'warm' conditions i.e. neither very hot or very cold (which explains both why we freeze/refrigerate food and why processed meal instructions always say 'ensure food is piping hot before serving').

Bacteria don't really 'form' at any temperature - they have to arrive from somewhere before they start multiplying. If you take a piece of meat out of the fridge it will already contain some bacteria. Some of these will have been within the animal when it was slaughtered and others will have been picked up subsequently but, since very few bacteria can succesfully multiply at low temperatures they don't present any major threat to human health. If you immediately cook the meat, the temperature will rise rapidly so that the meat is only 'warm' for a very short period before becoming too hot for the bacteria to divide. (Additionally, some, but not all, bacteria will be killed by the high temperatures of cooking).
If you eat the meat straight away, there will only be very low amounts of bacteria on the meat and you shouldn't suffer any ill effects. Equally, if the meat is quickly cooled and placed in a fridge, the meat will be safe to put in your sandwiches because, once again, it has only spent very limited time at 'warm' temperatures.

[Buenchico]

(2nd part):

Let's now assume that the meat was left out of the fridge for quite some time (at 'warm' room temperature) before cooking. In this case the bacteria already on the meat (plus others which might be picked up from the air or the surface on which the meat was placed) will multiply rapidly. Although the cooking process will kill off many of the bacteria there are going to be two problems when you come to eat it. Firstly, since there were more bacteria on the meat prior to cooking than there ought to have been, more are going to survive the cooking process but, even it was possible to kill all of the bacteria during cooking, the meat still wouldn't be safe to eat. This is because food poisoning is often caused, not directly by bacteria, but by the poisonous waste products of bacteria. All of the bacteria which were multiplying when the food was left out for too long were creating chemical poisons. Cooking might kill most of the bacteria but it won't remove the toxins left behind which can lead to food poisoning.

So, please note, in the above case, the bacteria were not multiplying either when the meat was hot or when it was cooling down. The damage was done much earlier.

Lastly, let's assume that that the meat is cooked straight from the fridge but then left out at room temperature. In this case, the freshly cooked meat is fairly free of bacteria (although some will have survived cooking) and it won't have toxins caused by bacteria (because it hasn't been through a 'warm' phase). Because it's then left out in 'warm' conditions, however, those bacteria which survived cooking (plus other bacteria from the air and from the surface which the meat is placed on) will then start to multiply and create more toxins. If the meat is now eaten cold, both the bacteria and toxins will be present. Even if it is re-heated, the bacteria may be largely destroyed, but the toxins will remain present.

Chris

[LitChick]

Excellent explanation Chris!