Red vs White Onion (Pyaz): How to Know if an Onion Is Rotten

That black powder under the onion (pyaz) skin? It’s a common mould called Aspergillus niger, it usually sits only on the dry outer layers, and a firm onion is fine once you peel those layers away and rinse. This guide shows how to know if an onion is rotten, when to use red, white or small onions, what onion benefits are real, and the “black onion side effects” mix-up that confuses half the internet.

Red vs white onion: which to buy

In most Pakistani and Indian kitchens “pyaz” means a red or pink onion, and that’s the right default. It fries to a deep brown for biryani and qorma, and its sharpness mellows in the pan.

White onions are more common in some markets than others. They’re juicier and taste cleaner raw, which is why some people prefer them in salad, raita and burgers. Their thinner skin means they bruise and rot a little faster, so buy them in smaller amounts.

Want a red onion milder for salad? Slice it and soak it in cold water for 10 minutes, then drain. Some of the sharp compounds wash out.

How to choose onions at the store

1. Squeeze: press the whole onion, then the neck. It should be rock hard everywhere. Any soft spot is the start of rot.
2. Check the neck: thin, dry and tight is good. A thick, damp or green neck means it wasn’t dried properly and won’t keep.
3. Look at the skin: dry, papery, crackly. A few loose outer scales are normal.
4. Sniff: a good whole onion barely smells. A strong or sour smell through the skin means one is rotting inside.
5. Check for sprouts: a green shoot at the top means old stock. Still edible, but it won’t store.
6. Pick size by use: medium onions (about 120 to 150 g) for everyday cooking; a 2 kg bag lasts a family of four roughly a week.

How to know if an onion is rotten

Peel it and look. Rot usually starts at the neck or at one side.

• Soft, slimy or see-through layers: cut well past the damage if it’s one small patch; throw it out if it runs through the middle.
• Brown or grey wet rings inside: neck rot. Bin the whole onion.
• Sour, rotten smell after peeling: bin it. A fresh onion smells sharp, never sour.
• Sprouted with a soft middle: bin. Sprouted but firm: cut out the shoot and use soon.

That black powder on onion skin

It’s Aspergillus niger, a mould found in soil everywhere. It grows on onions in warm, humid storage, usually between the dry outer skin and the first fleshy layer. It looks like soot.

Think of the dry skin as a paper wrapper. If the mould is only on the wrapper and the onion is hard, peel off the dusty layers plus one fleshy layer, rinse the onion under running water, and cook as normal. If you see black under several fleshy layers, or the onion is soft, throw it out.

Peel dusty onions away from your face, not over a pile of other food, and wipe the counter after. People with asthma, mould allergy or a weakened immune system (for example during chemotherapy) should skip heavily dusted onions altogether.

“Black onion side effects”: which black onion?

Many people searching this actually mean “black onion seeds”, which is a trade name for kalonji (nigella seeds). Kalonji isn’t an onion or onion seed at all. It’s a different plant. Its safe use as a spice on naan and in pickles is a separate topic from mouldy onions, so don’t mix the two up when you read online warnings.

How to store onions so they last

Whole, well-dried onions like it cool, dry, dark and airy. A basket or mesh bag in the coolest room works. Kept that way, a good red onion lasts weeks, sometimes a couple of months. Don’t store them in plastic, and don’t keep them right next to potatoes; we cover that pairing, and summer sprouting, in aloo pyaz storage in summer.

• Whole onions in the fridge: not needed, and the damp makes them soft. Exception: sweet salad onions, used within a week or two.
• Cut or peeled onion: airtight box in the fridge, used within about a week.
• Chopped onion in bulk: freezes well for cooking. Freeze flat in a bag, break off what you need.
• Fried onion (birista): fried crisp and fully cooled, it keeps about 2 weeks in an airtight jar, longer in the freezer.

Onion benefits: the honest list

A medium onion has about 40 to 45 calories, a little vitamin C, some B6 and fibre. Onions contain quercetin and other plant compounds, and red onions have extra red pigments (anthocyanins) in their outer rings. Peel thinly, because the outer fleshy layers are the most colourful.

The fibre in onion includes fructans, which feed gut bacteria. That’s a plus for many people and the reason onion gives others gas.

What’s overstated: raw onion “preventing” loo, onion juice “reversing” baldness, onion as a blood-sugar medicine. Our raw onion in summer piece takes the loo claim apart, and onion juice for hair fall covers the scalp question with its patch test.

Three things to make

Laccha pyaz (serves 4)

Slice 2 red onions into thin rings and soak 10 minutes in iced water. Drain well. Toss with the juice of 1 lemon, ½ tsp salt, ½ tsp red chilli powder and a handful of chopped mint. Serve with kebabs or dal chawal.

Crisp birista at home

Slice 500 g onions evenly thin. Fry in 1 cup hot oil on medium, stirring, for 15 to 20 minutes until golden brown. Lift out while slightly lighter than you want; they darken as they cool. Spread on paper. Keep the onion oil for biryani.

Simple onion tomato raita

Mix 1 cup dahi, 1 small chopped onion, 1 chopped tomato, ¼ tsp roasted zeera and salt to taste. Fine for most people; if onion gives you gas, use half the amount.

In desi homes

Onion myths

Myth: a cut onion left overnight turns poisonous

It doesn’t absorb germs from the air. A cut onion is just like any cut vegetable: covered and refrigerated, it’s fine for days. Left warm and uncovered, it dries and can grow bacteria, so fridge it.

Myth: crying less means a weaker onion

Tears come from a gas released when you cut the cells. Chilling the onion for 30 minutes and using a sharp knife cuts the tears. The onion tastes the same.

Surprising onion facts

• Onions are field-dried before storage: pulled and left to dry in the sun so the neck seals shut. That drying is why a good onion keeps for months.
• Kalonji is sold as “black onion seed” but comes from a completely different plant.
• Lasalgaon in Maharashtra is known as one of Asia’s biggest onion markets, which is why Indian onion prices are often quoted from there.

For picking the rest of the masala base, see our desi vs hybrid tomato guide, and browse more in Fruits and Vegetables.

Onion questions, answered