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I get that keeping food temperature low will slow down chemical reactions and preserve food, but sometimes my sister preserves food by heating it every 6 hours or so (up to 1-2 days). Does this help preserving the food? If yes, how does it work?

If it matters, the food currently in question is this one, but I would appreciate a general explanation. In case anyone is wondering, she doesn't store it in the fridge because it is full.

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4 Answers 4

up vote 17 down vote accepted

YES! If food is quickly heated and kept above 140F/60C, microorganisms can't grow in the food. Professional kitchens use steam tables to keep food warm at this temperature throughout a day's service. If the product is cooled quickly (generally using an ice bath), then it can be used for more than one day.

Another approach is often used for stocks and soups: heat them to 165F/75C daily or every two days, to kill microorganisms, then cool them quickly with an ice bath to under 40F/5C. This can be done 2-3 times to extend the fridge life of the product.

The most important part of both approaches is to use a food thermometer to verify that food is fully heated/cooled, and to make sure that heating and cooling is done QUICKLY. For food safety, you do not want food to be in the 40F/5C to 140F/60C range for more than an hour if possible, or two hours tops.

Another trick to extend the fridge life of food is to immediately transfer it from a still-hot pot to a cool container, and then leave the top of the container open to allow steam to escape. This greatly reduces cooling time; even in a refrigerator, a sealed container may take over an hour to cool down to a safe temperature.

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Everything here is correct, and I'd like to draw attention to the fact that heating the food up to cooking temperature is critically important here. If you heat it up to any less than 60° C then you are literally incubating more bacteria. Also, doing this every 6 hours seems a bit silly as all food lasts 1-2 days in the refrigerator anyway. –  Aaronut Aug 14 '11 at 17:13
    
@Aaronut: An excellent point, I'll edit my answer to reinforce this. By the way, is there a good place I can collect all the food safety info? Like, maybe an FAQ or tag wiki? –  BobMcGee Aug 14 '11 at 17:15
    
The tag has both. FAQ and wiki. The wiki kind of sucks right now so feel free to edit away. –  Aaronut Aug 14 '11 at 17:23
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No.

Although it depends on what you call preserve.

Heating will kill off most of the bacteria that may have grown, but will destroy more and more nutrients each time you reheat. Also its important to reheat above 65C (maybe check that, its close to this figure).

For your sister, her pasta dish is perfect in the fridge for 2 days. In fact if she doesn't hav e fridge, its probably better to just leave it out than to reheat it every 6 hours. It wont grow anything dangerous in that time.

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the fridge is full –  Fitri Aug 14 '11 at 16:21
    
Dead wrong. Professional restaurants have steam tables specifically to keep food hot enough to prevent bacteria growth. If left in the steam table too long, the flavor and texture won't be as good, but it works quite well. –  BobMcGee Aug 14 '11 at 16:31
    
@BobMcGee - this is not dead wrong, the question he was asking was not about keeping food warm constantly but heating it periodically. My question deals with heating food periodically. It will keep the food but it will destroy it each time you heat it. –  Toby Allen Aug 16 '11 at 17:50
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Different foods will provide different mediums for bacterial growth, some being more hazardous to leave in the danger zone (40f - 140f) than others. The method described might not be as great a risk for something acid as it would be for say, rice. But it should be taken into account that the heat used to kill bacteria will not necessarily denature the toxins that they excrete, so it strikes me that this sort of preservation strategy would become progressively more dangerous the longer you pursue it.

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If the food has been acidified (ph < 4.6) then it will last indefinitely in the refrigerator anyway, unless there's mold growth. So while this answer is technically correct, it's focusing on the least important issue. –  Aaronut Aug 14 '11 at 17:16
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As people interested in cooking we should all be aware of food hygiene and take it very seriously. As such I would recommend against your sister's methods, they are quite dangerous.

While bacteria can't grow or reproduce at 60C many types are more than capable of surviving; their cellular processes will shut down placing them in a kind of stasis or hibernation. So in a restaurant freshly cooked food will stay bacteria free when kept at 60C, however that is not what your sister is doing.

The problem with these methods isn't the period when the food is hot, it's the period after its been heated as in practice it will stay warm for some time and this provides optimum conditions for bacterial growth.

Let's look at another piece of evidence, sterilisation. If one wanted to sterilise a babies bottle say it would not be enough to just place it in boiling water, instead it is boiled for some time. In fact a safer way is to use a machine which steams the bottle (steam can be far hotter). The only way to be sure something is totally sterilised is to autoclave it, this involves both steam and high pressure and the items are kept in these extreme conditions for some time. Ofcourse autoclaves are only used when one requires there to be 0 bacteria, say in a lab or hospital. The lesson: bacteria are hardy things which are particularly hard to kill.

Final remark, better safe than sorry, particularly when young children are involved.

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