Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: thedoc on 14/05/2016 21:50:01
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Caring Sharing asked the Naked Scientists:
Hi Chris
I live in the tropics in balmy Malaysia. I don't have a refrigerator. So, if I opened a can of tomato paste and transferred it onto a glass bottle and covered it with oil by pouring on top of it so that only the oil layer is in contact with air, is it going to be safe from botulism and other bacteria? How long can I preserve it that way?
Cheers
Mas
What do you think?
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The problem with the botulinum bacteria is that it can live and grow even in the absence of oxygen. But more than that, in the absence of oxygen it produces an exquisitely toxic substance (the botulinum toxin) which creates long term paralysis of muscles, including those you need to, say, breath. And oil can readily exclude oxygen, and fail to preserve food. So no, oil on its own is not sufficient to prevent botulism. Indeed, this is a common cause of botulism; people put things like garlic gloves in oil, but the bacteria can still grow under the oil.
Still, it might well be possible to do this, if the food has sufficient preservatives in it, such as enough salt, or sugar and if the Ph is correct so that botulinum and other bacteria can't grow.
Certain foods basically work that way; for example many ketchups are basically tomato sauce with oil, sugar and vinegar, and some (but not all) do not require refrigeration.
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and transferred it onto a glass bottle
Better to pour oil into the opened can rather than disturb the contents. Most canned products are pretty sterile as long as you keep them out of contact with air.
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and transferred it onto a glass bottle
Better to pour oil into the opened can rather than disturb the contents. Most canned products are pretty sterile as long as you keep them out of contact with air.
NO!
Canned fruits and tomatoes should be removed from the can as soon as it is opened. Allowing the can to stay in contact with the acidic juices once oxygen is allowed into the system will facilitate leaching of iron from the can into the food. A layer of oil on top will not keep the contents oxygen free. Transferring to a glass container would be preferable to keeping it in the can.
http://www.lifehacker.com.au/2012/07/why-you-shouldnt-store-open-tins-in-the-fridge/
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That, plus simply cracking the can exposes it to the bacteria in the air, and botulinum spores can be airbourne, so you immediately contaminate the contents when you open it, before you pour the oil in; and then the oil excludes the oxygen.
Techniques like canning, and preserves were invented to do this kind of stuff safely; it's extremely unwise to 'wing it'.