I read everywhere on the web that cooked food should not be reheated
more than once.

I live alone and prepare 'stew' type dishes in some quantity. I freeze
most of this, but do not always finish what I have put aside for
immediate consumption. This I reheat the next day, and sometimes there
are still leftovers. I reheat this the day after etc.

My question is this: if my food is reheated by boiling it for a few
minutes, why should it not be safe to eat even if reboiled a hundred
times? Surely even if it is saturated with the most appalling
bacteria, these will all be destroyed by being boiled thoroughly.

In fact, in theory, should not a piece of meat which is green and
mouldy be perfectly safe to eat if brought to the temperature at which
all pathogens are destroyed (not that I actually *want* to eat green
mouldy meat!)?

I have been told that even if the pathogens are destroyed, they can
leave toxic substances that are not destroyed by heat. But I can't
find anything on the web about this.

Hello yellowlion,

It seems that you were correctly informed that even if the pathogens
are destroyed, they can leave toxic substances that are not destroyed
by heat.  Moreover, recontamination is possible at each recooling and
reheating.

According to a guide from the US Food and Drug Administration (FDA),
passing food through the "temperature danger zone" of 41 degrees to
135 degrees Fahrenheit more than once carries greater "potential for
the growth of spore-forming or toxigenic bacteria" as well as "the
potential to be recontaminated with Listeria monocytogenes, which
could grow during refrigerated storage" (pp. 12, 18 of guide, pp. 16,
22 of PDF file).  This guide (p. 24 of guide, p. 28 of PDF file)
describes some dangers:

"Some bacteria form spores that survive cooking.  These spores can
germinate and grow if food is improperly held after cooking. 
Bacterial spores are likely to be present in most foods.  When a food
is expected to contain spores of toxigenic bacteria such as
Clostridium botulinum or Bacillus cereus, reheating may be
ineffective.  The emetic toxin of B. cereus, which has been largely
associated with outbreaks in starchy foods, is very stable to heat. 
While the toxin of C. botulinum may be destroyed with extended
reheating, the critical limit for reheating in the Food Code (165 ºF
for 15 seconds) will not be effective in ensuring the food's safety.

Staphylococcus aureus does not produce spores, only a heat-stable
toxin when present in large numbers. Time- or temperature-abused, RTE
[ready-to-eat], PHFs [potentially hazardous foods] that are touched by
bare hands or otherwise contaminated with the organism are at risk."

"Managing Food Safety: A Regulator's Manual For Applying HACCP
Principles to Risk-based Retail and Food Service Inspections and
Evaluating Voluntary Food Safety Management Systems" (Center for Food
Safety and Applied Nutrition, July 2005)
FDA
http://www.cfsan.fda.gov/~acrobat/hret3.pdf

Accordingly, you see advice like this:

"Potentially hazardous food that has already been reheated should not
be cooled and re-heated a second time, to avoid the food being at
temperatures that support the growth of pathogenic bacteria four times
(cooling, heating and re-cooling and re-heating).  If pathogens were
present in the food, they could multiply to dangerous levels."

"Safe Food Australia" (Australia New Zealand Food Authority, 2d Ed.
July 2001) [p. 65 of guide, p. 72 of PDF file -- see also pp. 62-63 of
guide, pp. 69-70 of PDF file]
Food Standards Australia New Zealand
http://www.foodstandards.gov.au/_srcfiles/complete_safefood.pdf

I believe that these documents answer your question.  If you need any
clarification, please let me know.

- justaskscott


Search strategy --

Searched for many terms on Google in various combinations, including:

reheat
re-heat
reheating
re-heating
"food safety"
staphylococcus
clostridium
listeria
bacillus
toxins
once
"more than once"
"only once"
"danger zone"
"temperature danger zone"
"through the temperature danger zone"

---

Clarification of Answer by justaskscott-ga on 31 Aug 2005 23:07 PDT

My feeling is that it's better to know than not know.  I think I'll go
buy a probe thermometer too ....

Many thanks Scott. I rather wish I hadn't asked that question. I'm off
to buy a probe thermometer...

Um I think I am going to go clean out my refrigerator.

you may want to concider the fact that stwewed food has many of the
nutrients cooked out of it too, re-heating repeats the rinsing away of
nutrients.
You may find youself becoming deficient in some vitamins if you
continously eat this way which will in turn increase your
susceptability to bacterial attack in the first place.  Some of the
most temperature sesitive vitamins are vitamin C and Vitamin D, I
suggest in the meantime you have some fresh fruit to supplement your
stews.

Are you not able to replace your stew-pot with a wok?
A few noodles, vegetables and some frozen chicken peices can make a
great meal in 5 mins or less.

I am touched by your kindness, Undone. But I do not survive
exclusively on 'stew' type dishes (which include north African
'tagines' and similar). I do actually eat plenty of fish, a lot of
veggies, preferably brushed with olive oil and grilled. It's just that
I adore (especially in the winter) the comforting and long-cooked
dishes of north Africa and France (sometimes even of England, of which
I am a native, although living in Switzerland for the past 35 years).
I never got 'into' the wok (!), but cook similarly in an ordinary
frying pan (I think).