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I just chopped a particularly strong onion, I noticed some slight irritation but no tears. I realized then that onions haven't made me cry for years, decades even. I use all kinds of onions and take no special precaution to prevent tears. I don't refrigerate them, remove the core, use goggles, light a candle, anything like that. Granted, I've gotten better (faster) at it over the years, but I remember hating chopping onions in my youth because of the eye irritation. It doesn't seem that speed alone could account for the difference in my reaction.

Can I chalk it up to a similar phenomenon that gives me asbestos fingers and my tolerance for extremely hot (spicy) food?

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Equally as likely -- it's possible that there are changes in the onions. (factory farming & breeding for size and quantity has led to decreased vitamins in some plants; it might also decrease the sulphur). Also see cooking.stackexchange.com/q/17252/67 –  Joe Oct 8 '13 at 20:43
    
Over 25 years, many varieties across a great portion of the planet? –  Jolenealaska Oct 8 '13 at 22:46
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25 years ago, I never saw sweet onions in the store. I remember 15 years ago, it was a dig deal that someone was going down to Georgia and was going to be bringing back a bunch of Vidalias. With companies like Monsanto getting three generations a year in Hawaii, and much more foreign produce imports ... I'd say there's a good chance that the onions you get in the stores today are not the same as ones 25 years ago. –  Joe Oct 9 '13 at 2:36
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4 Answers 4

Given that the tear-causing effect of onions is due to a straight forward chemical process, it seems highly unlikely that you would develop any kind of actual immunity in terms of the effect. Scientific American describes it:

Peeling, cutting or crushing an onion's tissue releases enzymes called allinases, which convert these molecules to sulfenic acids. The sulfenic acids, in turn, spontaneously rearrange to form syn-propanethial-S-oxide, the chemical that triggers the tears. They also condense to form odorous thiosulfinates, coincidentally evoking the pungent odor associated with chopping onions and eliciting the false accusation that it is the odor that causes the weepy eye. Incidentally, sulfenic acid in garlic takes a different chemical route, sparing the eyes. The formation of syn-propanethial-S-oxide peaks at about 30 seconds after mechanical damage to the onion and completes its cycle of chemical evolution over about five minutes.

[ ... ] Free nerve endings [on the cornea] detect syn-propanethial-S-oxide on the cornea and drive activity in the ciliary nerve--which the central nervous system interprets as a burning sensation--in proportion to the compound's concentration. This nerve activity reflexively activates the autonomic fibers, which then carry a signal back to the eye ordering the lachrymal glands to wash the irritant away.

It turns out that newer science indicates the conversion is not spontaneous, but due to an enzyme for this purpose, now named lachrymatory-factor synthase.

What is likely, especially for an experienced cook are:

  • Improved knife skills and speed, lowering the amount of time in close contact with the irritating gasses produced by chopping onions
  • Increased tolerance for the specific sensation
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Consider the last sentence of my question. Asbestos fingers? Tolerance of hot food? Certainly I don't think for a second that my nerve endings have receded or that capsaicin changes its properties on my tongue. I will buy "increased tolerance for the specific sensation" as an explanation for my lack of onion induced tears, my ability to pluck pasta right out of boiling water and being able to eat the entire bowl of Pork Kimchi Soup from that little shop in Itaewon. Immunity might be an imprecise word, but it does seem appropriate to me as a layperson - at least pending other responses. –  Jolenealaska Oct 8 '13 at 19:41
    
Also consider the knife ur using and had been using when they made you cry. A sharper knife causes less damage to the onion and thus the enzymes wouldnt be released as much. I know when i need to sharpen my knife when my eyes start to irritate. And i could go through 20 onion without shedding a tear if im using a sharp knife. –  Charlotte's cook Oct 8 '13 at 22:40
    
Certainly my knives have become sharper over the years. So have my skills. –  Jolenealaska Oct 8 '13 at 22:49
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@Charlotte Here's a "yo" for your "ur." –  JYelton Oct 14 '13 at 19:04
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I don't know how old you are, but the most likely explanation is simply biological changes. Thermo receptors in the human body, most noticeably at hands and feet, become less sensitive as we get older, or, to put it another way, the threshold at which we experience these sensations is much higher, and that's an ongoing process as we grow and mature. There's a clue in this, for example - the reason why you're advised to test the baby's bath water with your elbow, even if you are in your twenties, is because the baby's receptors are on full alert and the skin is much more vulnerable than older skin, so your hands, at 25, won't experience great heat when a baby's will. There's a complicated scientific explanation regarding nerve fibres, but I won't go into that. I'm assuming this may also apply to the tongue, particularly given that we lose taste buds over the years roughly as follows - you may start out with thousands (some people have higher numbers at birth), but around age 80, the average human has about 80 left.

Regarding the lack of tears - I don't cry with onions any more either, although I still get the burning sensation. The reason I don't is because I've got something called Sjogrens, and am now rarely able to actually produce tears, no matter how much I may feel like it. I doubt this is the explanation for you, or your tongue would be too sore to put up with hot, spicy food, but there are other conditions that cause less fluid to be available to the tear ducts, again related with ageing, so your eye may, first, be less sensitive due to nerve fibre changes, and secondly, the tear response is not so readily activated even if a perceived irritant is detected.

Not very cheerful I know, and yet more proof that the obsolesence factor is built in...

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After you peel make sure you remove any of the transparent film left and rinse under tap before chopping this will reduce the processes which makes you tear up but to become immune - not sure about that, tolerance maybe.

Red onions I find not so bad.

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