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Since I only cook for one or two, I will tend to buy an onion and only use part of it, and then keep the rest in the fridge. Sometimes it will be a while before I use the rest. Is there any guidance on when I should finally just throw it out? What are the signs to look for? What degradations should I expect to trade off against constantly throwing out half an onion (decreased flavor, increased bitterness)?

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11 Answers 11

up vote 26 down vote accepted

When I use fresh onions, we store the unused parts for up to a week in our fridge in either a ziploc or a sealable rubbermaid-style container. For particularly pungent onions they go in the crisper drawer to keep the smell from being too strong in the rest of the fridge, but usually they're just on one of the shelves. Stilltasty says 2-3 days but my personal experience says otherwise.

Generally in our house, they don't last long enough for the flavor to degrade too much, but they will get a little dried and the smell will get stronger over time. After a week, they shouldn't be bad in the sense that you'll get sick - but you'll want to use them in a meal where the onion is the star, as the aroma will take over the dish.

Another option that I'm a big fan of is to dice or strip the leftovers (or, a 5 lb bag at once!) and freeze them on a cookie sheet (instead of in a bag, where they'll turn into a giant clump). Once fully frozen, dump them into a plastic freezer bag, and store in the freezer for easy access to a handful at a time. For most applications, you can defrost them right in the pan/pot/casserole as you cook, so they're very handy and accessible. We do the same with peppers in our house for convenience.

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+1 for alt suggestion: do you notice any kind of over heating of the onions due to the freezer-sautee pan? I.E. if I wanted to carmelize them, would I run into a difficulty due to them transitioning from cold to hot so quickly? –  mfg Sep 30 '10 at 13:44
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@mfg - so far so good with the freezer-to-pan transition. the only difference so far is a slight texture difference (a little less crispy) if i'm sauteeing them fast for something like a stir fry. sometimes to be safe i add them to a cold pan with the oil and let it all heat up together, but i've gone from freezer to a hot oiled pan with no problem as well. for caramelizing low and slow, they seem to do great, and even get softer a little faster! –  stephennmcdonald Sep 30 '10 at 13:56
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Freezing would actually be helpful for caramelizing onions. Part of the work that needs to be done is to collapse the cell walls; the freezer will have done that for you. –  Ray Jun 27 '12 at 11:41

I've had good luck with sealed glass containers ... including when storing sliced or diced onions. I find that plastic containers can start to pick up the onion smell if you leave onion in there for too long.

I've kept a halved, but otherwise in one piece, onion for over a week. If it dries out a little, you can always just take a slice off the cut side.

Cut up onion can keep for about a week, but it helps to have used a sharp knife -- a dull knive damages more cells and it'll start to break down faster. (and if it's sharp enough, you can slice it up, but keep the shape, so there's no extra exposed cut surface to lose moisture from). The more damage you do to the cells, the stronger the 'oniony' smells will get.

My judge on if it's still okay to use is to look at it -- if it's gone translucent, smells funny or looks funny (including dried out), I pitch it.

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Great advice on the sharp knife, I've noticed that's made a significant difference in how long it lasts without stinking up the fridge. I'd +2 if I could, as the glass container advice is spot on - I used a plastic container once, and had to make that the "onion container" until the end of its life. My only addition to that: If your glass container has a plastic lid, consider lining it with foil or plastic wrap before sealing so the lid doesn't pick up the smell. I learned that the hard way storing garlic; my one blue Pyrex lid still smells like garlic 3 months later. –  stephennmcdonald Sep 30 '10 at 14:42
    
@stephen : I haven't had a problem with the pyrex lids and onion, but I also tend to buy mild onions (red, sweet, etc.) unless they're for a specific purpose, and when I'm storing a 1/2 onion, store it cut-side down (against the glass). –  Joe Sep 30 '10 at 18:59
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Another thing to note is that the less cut it is, the longer it will stay fresh. A half onion with the root-end still attached will stay fresher than a tupperware full of pre-diced onion. –  heathenJesus Jun 27 '12 at 16:22

If you have no container, you can also use a piece of plastic clinging wrap to cover the cut part. If you notice that the cut dries out or does not look appetizing anymore, simply cut of one thin slice and presto, it's fresh and juicy again.

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Another option is to caramelize one or two onions in a frying pan, and then freeze the pre-cooked onions in smaller portions. (I tend not to like cooking with frozen onions, since the extra water will steam the onions rather than allow them to cook at the higher temperature you could achieve without the extra water.

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A couple of years ago I found a container designed to store cut onions. It looks like a plastic bowl with a rubber and metal lid. The smell is kept inside the container and the onion lasts several weeks.

I don't know how it works, but it most certainly does! Before I had it, I would wrap an onion in plastic and it wouldn't last quite a week.

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I tend to put them in a sealed container with just a small splash of vinegar or lemon juice.... Seems to help them to not dry out.. :-)

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I just wrap it up in foil. It will start to smell when it's going bad. You're not supposed to save onions, since they are so cheap. You can also buy already diced onions at Walmart. I believe they stay good for at least two weeks.

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Cut onion can be stored 7-10 days in a sealed container in the refrigerator. I've not experimented specifically with glass or plastic, but I can understand how glass would be better. I'll have to give that a try. Sharp knives are key, that was a spot on tip!

With regard to the comment about cut onion absorbing bacteria, that is false. Anyone interested in more on that can click here: http://onions-usa.org/faqs/onion-flu-cut-myths

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http://onions-usa.org/all-about-onions/how-to-select-cut-prepare-store-onions

straight from those who know what we are crying about..

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Summarizing the information would add value. –  SAJ14SAJ Dec 16 '13 at 23:51

I like to store them in a bottle, they will last at least a week.

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In a bottle alone? With some sort of liquid? What kind of bottle? In the fridge or on the counter? –  Laura Jun 7 '12 at 15:51

I also used to save unused onions for another day, until I read that a cut onion should not be reused in cooking as the peeled surface drinks up all the toxins and bacteria out of the atmosphere and into the onion.

One remedy for flu symptoms is to place a cut onion on the bedside table before sleeping. In the morning the onion will be black in colour having drawn up all the bacteria etc in the room. And the flu symptoms will have disappeared.

I would rather chuck out a half-used onion (after leaving on the kitchen surface overnight) knowing it had worked as a kind of disinfectant.

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"the peeled surface drinks up all the toxins and bacteria out of the atmosphere and into the onion" is the most ridiculous superstition I have heard of lately. –  rumtscho Jun 27 '12 at 11:59
    
it is not true that cut onions left in fridg is poisonous or that they drink up toxins and bacteria out of the air. I did research on it and I found it all to be false –  pamclapier May 3 at 23:34

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