At last, a different onion question!

Anonymous asks: "Would like to know if you can get sulphuric acid poisoning from rotting onions? Had a bag of onions in my cupboard for months and they had rotted now all I can smell is onions. Really strong at times. "

A few bits to this:

Firstly, sulphuric acid is not volatile, so you cannot smell it.   Heat it sufficiently and you will get sulphur trioxide fumes which you can definitely smell but I am talking 400degC, somewhat warmer that the average cupboard.

Secondly, it is corrosive rather than toxic.

But on to the onions.  When things rot, all manner of compounds are formed and released.  What compound and what level will be dependent on onion type and the organisms associated with the rotting.  Onions contain a lot of sulphur so the breakdown products will be smelly.  Unpleasant but not necessarily harmful.

Chuck them out and air the cupboard.  There is not much else you can do.
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The Mayo Clinic

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Don says:

"I had the wonderful privilege of touring Mullins Food Products, Makers of mayonnaise.. "(Mayonnaise) doesn't even have to be refrigerated. No harm in refrigerating it, but it's not really necessary." He (the food chemist) explained that the pH in mayonnaise is set at a point that bacteria could not survive in that environment.

(The food chemist) says that when food poisoning is reported, the first thing the officials look for is when the 'victim' last ate ONIONS and where those onions came from (in the potato salad?). Ed says it's not the mayonnaise (as long as it's not homemade Mayo) that spoils in the outdoors. It's probably the onions, and if not the onions, it's the POTATOES.

He explained, onions are a huge magnet for bacteria, especially uncooked onions. You should never plan to keep a portion of a sliced onion. He says it's not even safe if you put it in a zip-lock bag and put it in your refrigerator. "

Ye gods! Where do I start?

The easy bit first: see a previous post on the myth of onions being bacteria magnets. Make sure you read the comments, as the saga went on for a while.

Secondly, there is a popular myth (only in the US for some reason) that mayonnaise is a chief culprit in food poisonings. It isn't. Or at least commercial mayonnaise isn't, as it is made in strictly controlled conditions.

1. The pH of mayonnaise is kept low, at a level that bacteria cannot survive.
2. The water activity of mayonnaise is low, meaning that the moisture present in the mayonnaise is not available to the bacteria (or mould) to use and
3. the product is made in sterile conditions.

The consequences of that last item eludes many people. They think that bacteria just happen. That is as sensible as watching a field, hoping for corn to grow. If you don't plant it, it wont grow.

And "if not the onions, then the potatoes"?

Anything that grows in dirt can have bacteria on it. It will be in an environment where there is E-coli, Salmonella, Clostridium, lots of bugs with long and threatening names. Wherever birds poop and animals roam will have bacteria. You meet up with them every day of your life. And survive.

But if you take some potatoes and cook them there will be no bacteria alive on them.

Put them in a potato salad with mayonnaise and they will still have no bacteria on them.

The biggest problem is when you add bacteria to the salad yourself and then leave the salad sitting unrefrigerated for hours on end. At room temperature bacteria multiply tenfold an hour.

So where do these bacteria come from? Spices, raw unwashed vegetables and, most commonly, poor personal hygiene when preparing the food.

But from the mayonnaise? No.

From onions? No.

From cooked potatoes? No.
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Onions as bacteria magnets.

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Pauline asked: there is an email circulating claiming that cut onions are "magnets for bacteria" and should never be stored for later use (even in the fridge for a few days) as they will cause food poisoning. True?

Snopes (here) rates it as "undetermined".

I am willing to go out on a limb and say "false".

Reason 1: Nothing is a bacteria magnet. Firstly, bacteria have minimal mobility. They usually travel in water droplets, if at all. Sneezes, for example. Moulds can release spores which get blown around but bacteria usually grows in moist environments and are slimy, making getting airborne difficult. Secondly, if there was such a thing as a 'bacteria magnet' it would be enormously useful in the medical field for drawing bacteria away from the ill and infirmed. Not such use has been made of onions.

Contact with unclean hands can introduce bacteria to new surfaces but they need a surface that will support growth, otherwise they will just stay there without multiplying or die.

Reason 2: Bacteria like moist, neutral environments. Not many acidic things grow bacteria. That's why vinegar is used for preserving. The surface of a cut onion is acidic due to the production of sulphuric acid (this is what makes your eyes water). There are some moulds that will tolerate acidic conditions and grow on onions but they are not high risk, they are visible, and any normal person would cut them off or ditch the onion.

Reason 3: High risk foods are usually high in protein and available moisture. Onions are low protein, verging on nil, and what moisture they have is contained in their cellular structure. The surface, as well as being acidic, dries soon after cutting and will not support bacterial growth.

Reason 4: If onions are attracting bacteria, where are they coming from? Somewhere else in your fridge? Perhaps it is time to clean the fridge.

Reason 5: In the 20 odd years I worked in a food laboratory, onions were never mentioned as even a suspect in a food poisoning case.

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Footnote: we did have some onions brought to the laboratory as a food poisoning complaint once. A guy had eaten them and ended up in hospital. Only problem was, they weren't onions.

They were daffodil bulbs.
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Lady Fi's Onions and a bit on gold, for good measure.

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Lady Fi asks: Have you read my post about onions?
Why not tell us more about the tearful reactions they can trigger in lots of us?

Also - what's the deal about alchemy turning stuff into gold? How did it start? Why did people try? And is it really impossible?

Onions make you cry because, when you cut them, you release Propanethial S-oxide, a volaile compound that reacts with the moist surface of your eye to form sulphuric acid. Your eyes then water in response to this irritation.

Interestingly, onions do not contain propanethial S-oxide and it is thought that onions produce this tear inducing compound to protect the plant from herbivores. It is the product of a series of chemical reactions that occur once the onion has been damaged. Cells are broken open as the onion is cut and this releases the enzyme alliinase and water, which react with S-1-propenyl-L-cysteine sulfoxide forming a mixture of products including propanethial S-oxide.

Various suggestions are about to stop this process - cutting onions under water, chilling onions to reduce the volatility of the compound, heating the onion to kill the enzymes that create the compound, or what I sometimes do if the stove is free, cutting the onions under the exhaust fan on the stove.

And gold?

The alchemists have dreamed of turning lead into gold but, alas, had to fall back on other retirement schemes. Modern day nuclear physics has successfully transmuted lead into gold, but the expense far exceeds any gain. Ironically, it transpired that, under true nuclear transmutation, it is far easier to turn gold into lead than the reverse reaction, which was the one the alchemists had ardently pursued.


Reference:
http://www.chm.bris.ac.uk/motm/pso/psoc.htm
http://en.wikipedia.org/wiki/Nuclear_transmutation