How to Tell if An Onion is Bad [Definitive Guide]

Onions are synonymous with a multitude of favourite dishes; French onion soup, onion bhajis, onion and bacon quiche, to name but a few. 

The variety of onions available also offers a wide choice for different cooking purposes; salads, braises, soups and sandwiches, roasts, and baked goods, etc.

Onions are one of the most beloved vegetables, and a lot of people keep them in their pantries for daily use.

But before inspiration strikes, especially if it’s been a while since visiting your onion hoard, how do you tell if your onions have gone bad? This article will cover everything you need to know.

How to Tell if an Onion is Bad 

Regardless of which onion you buy, store, and use they all have a shelf life and will go bad eventually. So how do you tell if an onion is bad? There are several key signs that you can look for.

First, inspect the onion itself; is it solid or soft? Is the papery outer skin crisp and dry, or does it feel soft or squishy? Are there any weird colours or powdery residues? Lastly, is there anything sprouting from it?

Your onions should always have crisp and dry outer skins, and the flesh should feel firm when squeezed. The flesh should be moist, not dried out in any way. There also shouldn’t be any discolouration, or powdery mould growing, and there definitely shouldn’t be any green growing from it!

If there is only a small amount of silver-green powdery mould, this can be removed and the onion still used. The onion is ageing but is still safe to eat – as long as the mould is fully removed.

Secondly, how does the onion smell? A rotten or spoiled onion has a distinct odour; like rotting compost. If the onion smells of anything other than that zingy onion smell, it may be heading to your bin and not your chopping board.

To recap; spoiled onion identification:

  • Mould or discolouration of the skin
  • Wet skin, or dried out flesh
  • Soft or squishy flesh
  • Green growth from the top
  • Weird compost-like smell

Any one of these identifiers could indicate your onions are going bad, more than one and you should probably compost that onion as soon as possible.

How to Tell if an Onion is Bad After Cooking  

Much like in its raw state, a cooked onion has a couple of key signs of ageing. While cooked onions will usually last a few days in the fridge, they can go bad within a day or two if stored improperly.

If you’re unsure about the onions in your fridge, the quickest and easiest way to tell is to use your senses: smell and sight – do not taste! Rotten, or spoiled, onions will have a noticeably ‘off’ aroma, much like rotting compost.

The sweet hint of caramelisation will be gone, in its place will be a sour decaying smell. This smell is unavoidable and breath-takingly pungent.

Cooked onions that have gone bad will also look different, depending on their stage of decomposition. They can appear tighter and slimy, or loose and with a liquid around them that has strands of white through them. Onions with this appearance are breaking down and should be discarded immediately.

How Long Do Onions Last

Depending on whether they are raw or cooked, onions will keep in your fridge or freezer for quite some time. Cooked onions will last an average of 3 to 5 days in an airtight container or resealable bag.

Whole, raw onions will keep for longer in the fridge, however, the cold environment isn’t great for the skins – remember, they need to be dry and crisp. Whole unpeeled onions will safely keep for up to 2 weeks in the fridge, after that close inspection of the onion is recommended.

Cut raw onions will last about a week in an airtight container or bag, but ideally should be used within a couple of days, and are better used in cooked dishes as a precaution. Equally, peeled onions will keep for a few days, but should be used within 3-5 to be safe.

But, all of these time limits are extended dramatically if you have a freezer! Frozen onions, whether raw or cooked, can last for months without issues. Onions, kept in freezer-proof bags or containers, can last up to 10-12 months but do need to be cooked and eaten as soon as they are thawed.

Note: whilst it is okay to eat freshly thawed raw onion, the physical structure of the onion will have changed due to the freezing process. Once thawed, the onions will be super-soft and break apart easily. For this reason, frozen onions are better for cooked dishes rather than uncooked ones.

The worst place you can store your onions is on your kitchen counter. The added heat and light that is in most kitchens will increase the ageing of your onions dramatically. Your onions can become completely inedible and bad within 7-10 days if left at room temperature.

How to Store Onions

If you’re not planning on freezing your onions, or have limited fridge space, storing onions can be a little tricky. So, what’s the ideal way to store your onions?

Onions are best kept at between 45-55˚F/7-13˚C; it’s the same temperature range you should keep a nice bottle of white wine at. Ideally, you should keep your onions in a cool and dark place, like a basement or cellar.

Realistically, for most of us, that’s going to look more like under a kitchen sink (this cupboard area is usually dark and cold), and well ventilated in an open basket or box.

Onions kept in plastic bags will become damp, and encourage mould growth before quickly going bad.

Types of Onions

Red vs white vs brown vs green, are all onions created equal, or are some better than others? It’s easy to think of onions as either raw or cooked when it comes to eating them, but depending on which onion you choose, the end result can vary a lot.

Red onions

For many years have been hailed as the best. Their deep red skin and flesh are easy to spot, and they’re usually more expensive than their anaemic cousins. But, the truth is that they’re just sweeter than brown onions, and this sweetness adds a more noticeable flavour.

Traditionally, they’re used raw in salads as fine slivers, where they lend a sharp tang and eye appeal with their ruby-red skin. Alternatively, they’re great for pickling or caramelised for topping meat or including in quiches.

White onions

These are softer and subtler in their flavour, and have a white outer skin and flesh. Just like red onions, white onions are great for salads, and can be left chunkier than other onions owing to their milder flavour.

These onions are also great as a topping vegetable; scattered on a stack of ribs or steak, as a cook-in-the-bowl Vietnamese soup called Pho, or on your garden salad.

Brown onions

These are the most common of the allium clan; they’re in everything! The flesh is not brown, but a creamy white-ish colour, with a papery brown outer skin. This is the onion you want for cooking; soups, stews, braises, roasts, or fried.

Brown onions have the strongest flavour and are usually too powerful to eat raw. Instead, they should be cooked over enough heat to draw out their sweeter side, and develop what’s known as the Maillard reaction – or caramelisation.

Green onions

These are the subtlest in their flavour, and are often used in salads or as a garnish, rather than a base ingredient to be cooked. Sold as either Scallions or Spring onions, they have a white root end and a green stalk, and are completely edible – no peeling required!

Other alliums

There’s still more. There are also pearl onions, which are often sold pickled, or used in casseroles. Finally, Eschalots/shallots/French shallots, look like a cross between a red onion and a garlic clove.

They can have either red or brown papery skin, and either red or creamy-white flesh. These are the sweetest of all the onion family and are usually found in traditional French recipes.

Conclusion

Whilst onions are one of the most frequently used vegetables, they can go bad quite quickly if stored incorrectly. To identify whether or not your onions have gone bad, use sight and smell as your best judge.

Inspect each onion to assess whether it has a dry and crisp outer skin, and firm flesh underneath. Any onion that has damp or limp outer layers or a soft and squishy body should be discarded. Other signs can also include spots or discolouration, and a powdery silver-green mould.

Ideally, store your onions in an open basket, in a dark, cool and dry place, or freeze them for even longer storage.

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