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Cooking kills bacteria. How about toxins excreted by bacteria?

Discussion in 'Lovely Grub' started by Lush, Jun 2, 2008.

  1. Wayland

    Wayland Hárbarðr

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    Ah.. I had always wondered about the time difference for onset. That makes good sense.

    Any idea how long it takes Staph aureus to produce dangerous levels of toxins after handling food?

    That sounds quite plausible, it could also have been used for masking bad flavours in hot countries, where spices are often cheap and readily available I guess.
     
  2. hedgepig

    hedgepig Nomad

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    The use of salt as a preservation agent relies on the inability of food spoilage organisms and food pathogens to grow in a high salt environment. What is actually happening is that the osmotic pressure in the high salt environment "sucks" water out of the bacteria ... and like any organism that relies on water to survive and reproduce, their growth is limited (I'm not going to say they are killed because this isn't strictly true). Osmotic pressure is also what is happening in fruit preserves and honey - the high sugar content prevents any bacteria in the food from obtaining enough water to grow and divide.

    It is important to note that in both these cases (curing with salt and preserving with sugar) that the growth of the food spoilage and food pathogenic organisms are prevented in the first place. Rubbing salt (or honey) on rancid meat will not make it safe to eat. This is because the bacteria have already grown and salt is not a strong bactericidal agent (neither is sugar). So I'm inclined to think that with spices that are rubbed on food prior to cooking that these are purely for flavouring and not for "food safety". Spices that are rubbed on freshly butchered meat that will be stored for a while ... perhaps this might be for preservation, but I would be surprised. One of the reasons that I would be surprised is that spices sold in the UK are tested (and sometimes found positive for) food pathogens such as Salmonella spp, E. coli, etc. Rubbing contaminated spice on meat for storage is a really bad idea.

    Gary, this is a "how long is a piece of string" question unfortunately. There is no definite answer on how long it would take as the answer will vary depending on initial level of contamination, the food type, the storage conditions, how the food was handled/prepared at the time of initial contamination and how the food was handled directly prior to final preparation for eating.

    Lets look at some of those in more detail:
    Initial level of contamination: obviously the more bacteria present initially then the sooner the level of toxin reaches critical.
    Food type: dry foods will be less prone than wet foods
    Storage conditions: cold inhibits food pathogen growth
    Food prep at initial contamination: a piece of meat kept as a single piece will be safer than a piece of meat ground up for burgers.
    Food handling prior to cooking: washing food directly before cooking will reduce the number of bacteria on the surface.

    Having said all that, I'm sure that someone has done the work to show what level of toxin produces the reaction. I can still remember my immunology lecture at university when we looked at superantigens and if I was going to guess (and it is just a guess), I would say that given the nature of them, it would be a relatively low amount (although again this is a guess and the levels would vary from person to person). Its also interesting to note that it is Staph superantigens that cause toxic shock syndrome!

    Again, to reiterate the bottom line. Basic food hygiene principles are easy to learn and follow (although I have seen lapses in people who should know better!).
     
  3. crazydave

    crazydave Settler

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    okay slightly at a tangent but I think it still holds up - the egyptians first used sugar as an antiseptic to pack wounds which we now know works because sugar is dessicant so doesnt poison the bugs but dries them out to kill them which is why the nhs now use sugar dressings for mrsa cases. the chinese worked a similar technique out but used salt.

    salt cures work only as long as the shell/crust remains unbroken and bacteria can get in which is why we can buy 50 year old parma ham. :)

    similar once sugar becomes diluted with dead bacteria and water it ceases to be effective so needs changing. honey is different as there's a bit more in it besides sugar and bee snot.
     
  4. hedgepig

    hedgepig Nomad

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    The wound care and food hygiene situations are not directly comparable for a number of reasons. For a start the human body has a number of powerful mechanisms (complement cascade, T-cells etc) to deal with infections and the sugar/salt dressings are only really designed to assist the body in dealing with the infection. The efficacy of salt or sugar as a bactericidal agent is poor, however as bacteriostatic agents they are quite effective. What they are doing is holding back the growth of the bacteria so that the body's defences are not overwhelmed. It is this holding back of growth that make sugar and salt good as preserving agents and their lack of bactericidal activity which make them useless in making contaminated food safe.

    As mentioned, the action of salt and sugar is through osmotic pressure, which is why (as you mentioned) they become less effective as they become more dilute (the osmotic pressure on the bacterial cell reduces as the salt balance reduced). If you were to prepare a bacterial colony in the lab and put it in some honey or syrup, then nothing would happen. However, if you were to then dilute the syrup down the resulting solution would be great bug food and the solution would become turbid with bacteria within a day or two. The point is the honey/syrup/salt doesn't kill the bugs, it just provides a hostile environment for them in which they can't get a foothold and grow up to significant numbers. It's the same with freezing - this doesn't kill the bacteria* but it does stop them from growing right up to the point at which you take the food out of the freezer and it warms up to room temperature. That's why the recommendation is that you don't refreeze food - during the thawing and refreezing process, enough bacteria can grow to cause a problem the next time you thaw the food.


    *It will kill some through disruption of the cell membranes, but significant numbers will survive (more than enough to cause a problem). Freezing is used in the lab as a method for keeping stocks of bacteria for future use. -20 celcius for short term, -70 celcius for longer term, liquid nitrogen for very long term. Freeze drying is also a method of producing bacterial stocks - I mention this as it shows that drying bacteria out does not have a 100% kill, however, it does stop them growing until you put them back in an environment that is conducive to growth.
     
  5. Lush

    Lush Forager

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    Learning something new every day :). This is why I love forums!
     
  6. firecrest

    firecrest Full Member

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    At the risk of going off topic, honey, as well as being antiseptic, is the only food that is truly imperishable. There is honey pots in egpytian tombs. The honey has become a hard mass but you can heat it up and it becomes honey again! why is that?
     
  7. andy_e

    andy_e Native

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    I seem to recall being told while I was doing microbiology at uni. about one bacteria sometimes found on food that would release toxins only if fried - may have been another one of the Clostridia, don't think it was C. botulinum - seemed strange to me at the time. A quick search hasn't brought up any corroboration, though thinking about it boiling wouldn't kill Clostridia spores but frying would probably burst them.
     
  8. Lush

    Lush Forager

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    wait.... I will make it on topic again, hahaha
    "honey, although almost imperishable, can be dangerous to infants, because of botulism". Every year a number of infants die because of eating honey.

    I think the answer to your question is obvious Firecrest. All you have to do is read this whole topic. especialy when osmosis is mentioned ;)
     
  9. Lush

    Lush Forager

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    ...true, it's Clostridia. It can survive boiling for 1 hour, amazing
     
  10. hedgepig

    hedgepig Nomad

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    Hehe, what a diverse topic!

    Honey:
    • the reason it hardens and then softens on heating is that it is a saturated sugar solution. If it was a totally pure, homogeneous solution then it would always stay liquid. However, there are always little particles present (bits of pollen, bee feet etc) that cause a process called nucleation. Crystals start to form round these nuclei and eventually all the sugar crystallises. Of course heating raises the saturation point and so the sugar goes back into solution.

    • Honey normally carries a warning against feeding it to children under 12 months because of the risk of infant botulism (as pointed out by Lush). The good news is that is it very rare anyway and so personally I wouldn't panic if a child of mine ate some honey - this is of course a personal view and not a recommendation!

    Bacteria, spores and heat:
    • Some bacteria form spores - including Clostridium spp as Andy mentioned. These spores are a survival strategy and are as tough as old boots. Clostridium botulinum used to be a problem with canned food because depsite the heat treating process killing off all the other microorganisms the Clostridia spores survived. There are other bacterial species that form spores too, Bacillus spp for example. Bacillus cereus is the cause of "fried rice" food poisoning. The spores survive the cooking and then germinate when the cooked rice is improperly refrigerated and reheated - the toxin (there are actually 3 toxins I think) is heat stable too so reheating doesn't solve the problem. The effects of this form of the poisoning appear quickly (like the Staph toxin) and cause vomiting. However, another presentation involving vomiting and diarrhoea can appear after a day or so.
    • Some bacteria, called thermophiles, LOVE the heat. They were found in hot springs, living quite happily in very hot water. It was from one of these bacterial species that one of the most important enzymes of the modern age was extracted - Taq polymerase - the enzyme that catalyses the Polymerase Chain Reaction ... which can multiply up a very small amount of DNA into lots of DNA. This of course is a very powerful tool in the lab. I spent some time researching and developing PCR based assays for TB diagnostics (published the work too). The police tend to use it for (sometimes dubious) scene of crime investigations. I could go on ... but I won't :)
     
  11. andy_e

    andy_e Native

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    Great post hedgepig, thanks - it may well have been Bacillus cereus that I was thinking of - it was a long time ago that I originally heard about it.
     
  12. crazydave

    crazydave Settler

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    camels milk is another super long term product I watched a feature on survival many moons ago in egypt and they had 30 year old bottles of milk which needed a shake because it had seperated but was still drinkable. apparently when water is short the cow camels strengthen the milk so the calf ends up drinking evap :)

    according to red dwarf - dogs milk is another long life product because it tastes the same after its gone off :yuck: bit like marmite I suppose ;)

    :)
     
  13. Wayland

    Wayland Hárbarðr

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    Great thread.

    Here's a related question while we have a few experts on hand. I've never really understood why certain foods should not be reheated.

    Sure, if it's reheated and left for a while I can see that would cause problems but why can't we reheat it if it's going to be eaten straight away?

    (I'm thinking of things like pork and chicken which I was always warned about reheating in the past.:confused:)
     
  14. Tadpole

    Tadpole Full Member

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    I re-heat chicken and pork, hell i re-heat just about everything, part from chips and roast spud, as they taste foul. when I cook I often cook extra and make up meal when they are cool I freeze them. Sort of homemade TV dinners for when I have no time later on in the month. Chicken pork beef fish rabbit they all get made and used, without any problems. Or If there isn't enough to make a meal I'll shove it in the fridge and re-heat it with some pasta or rice or even in a stirfry. I've never had a problem. My father who is an ex environmental health inspector does the same as I. I have found that as long as you reheat it in such away that any bugs on the outside are killed you should be ok.
     
  15. hedgepig

    hedgepig Nomad

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    Its not that foods shouldn't be reheated, its just that they should be reheated in a certain way. As in so many things the advice became "do not..." to avoid the possibility that people do it the wrong way.

    In terms of food safety what you want to avoid is having the food at a temperature for long periods where the bugs can grow up to a problematic level. The safest way to cool food is quickly to below 5 celcius. When reheating the same thing applies - reheat it quickly and ensure that all of it is piping hot. I think the food standards people actually provide recommendations on time and temperature ... just going to google to see if I can find this ...

    ... Yep, here we go although this is actually from Australia
    http://www.foodstandards.gov.au/_srcfiles/Cool%20_and_reheat_food.pdf

    Of course, the implication is that you shouldn't do this cooling and reheating more than once!
     

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