The most puzzling food and drink mysteries solved

Culinary conundrums

Ever wondered why prawns turn pink when cooked, or why bananas ripen everything in your fruit bowl? How about why chopping onions makes you cry? Read on for the answers to all those “I wonder why..?” questions about your favourite foods.

Why does honey never go bad?

You can store a jar of honey in your cupboard for years and it'll be fine. In fact, it's one of the few foods that doesn't spoil thanks to high acidity (a pH of around 4), low water content and low water activity. The latter refers to the proportion of water in a food that isn’t bound to food molecules and therefore is available for bacteria, yeasts and moulds to grow. Sugar molecules bind to water, so even though honey does contain moisture, it has a low amount of "free water" and can’t support bacteria growth.

Why does skimmed milk make more froth than whole milk?

If you're perfecting your barista skills at home, here's a tip for you. When it comes to adding a frothy top to your coffee, skimmed milk will give you the most foam with larger air bubbles. The absence of fat means the proportion of protein is higher, and this allows the creation of a stiffer, more abundant foam to top your latte or cappuccino. However, whole milk will create a thicker, creamier foam to give your drink more body.

Why does Swiss cheese have holes?

Swiss cheese (Emmental) gets its famous holes from bacteria known as P. shermanii that are added to the milk before formation of the curds. As the cheese ages, the P. shermanii release carbon dioxide that expands and causes the holes. These bacteria are naturally present in some other cheeses and can cause small holes and cracks that may be considered flaws. But in Swiss cheese they are added deliberately to achieve its characteristic look.

What causes brain freeze?

Brain freeze happens when blood vessels in the roof of your mouth rapidly constrict when cold food or drink, like ice cream, touches it, and then dilate again rapidly due to a heat stimulus (usually warm air on a hot day). These rapid changes near sensitive nerves create a fleeting but very painful headache.

What’s the difference between baking soda and baking powder?

Both baking soda (bicarbonate of soda) and baking powder are leavening agents, used to make baked goods rise. Baking soda is simply sodium bicarbonate – if you’re using this in baking you’ll need to add lemon juice or another acidic ingredient for it to work its magic. Baking powder, however, is already mixed with a source of acid (such as monocalcium phosphate) so it will help your cake rise all on its own.

Why do cut apples go brown?

Oxygen in the air activates enzymes (catalysts of biochemical reactions) in apple flesh. These enzymes trigger natural plant chemicals called polyphenols to start producing melanins, an iron-containing compound that gives the flesh a brown, rusty colour. The reaction happens quickly, so a sliced apple can turn brown in just a few minutes. But different apple varieties will go brown at different rates because they contain differing amounts of the initial enzyme and the polyphenols.

Why do bananas make other fruit go off quicker?

Bananas release a gas known as ethylene that begins the ripening process. This hastens the ripening of other fruit in the bowl and can also cause cut flowers in the vicinity to die quicker too. Many fruits produce ethylene (including kiwifruits which produce it in larger amounts), but we only tend to notice the effect with bananas as they usually come in large bunches.

Why do eggs yolks sometimes get a grey ring around them?

While this discolouration around hard-boiled egg yolks might look unattractive, it’s completely harmless. It's caused by iron in the egg yolk reacting with sulphur in the egg white to form iron sulphide. Hard-boiling eggs for a long time increases the likelihood of the green/grey colour forming, but plunging the eggs into cold water quickly after cooking decreases it.

And why do hard-boiled eggs sometimes smell so bad?

If your hard-boiled eggs smell... well, eggy, you've probably overcooked them. As with the grey ring around the cooked yolk, it's also down to a reaction between sulphur in the egg whites and iron in the yolk, which can produce hydrogen sulphide – the stinky gas associated with smelly flatulence.

What’s the white stuff that comes out of salmon when you cook it?

It looks pretty gross, but don’t worry – the white stuff that comes out of salmon during cooking is totally harmless. It’s essentially coagulated protein (or muscle fibres), known as albumin, that exists close to the surface of the fish when raw but seeps out when cooked. And the amount of albumin that seeps out depends on your cooking method: the higher the heat, the more albumin will appear on the surface.

Why is months-old, air-dried ham safe but fresh meat goes off in a few days?

Reduced water activity (the proportion of water in a food that isn’t bound to food molecules) is the reason why salted, cured meats can be safe to eat at several months-old having never been kept in a fridge. In the same way sugar molecules bind to water in honey, the salt in cured meat binds with any free water, so that moisture is no longer available to support microorganism growth.

Why does bread dry out but biscuits go soft?

A loaf of bread will harden over time due to a process called starch retrogradation, which involves the starch giving up the water it absorbed during the baking process. The same happens in biscuits too, but in this case it’s outweighed by the high content of sugar absorbing moisture from the atmosphere, making them soften. Cakes can go soft or hard depending on the recipe.

Why don't mushrooms turn to mush when cooked?

While veg like courgettes and broccoli lose their structure after cooking, mushrooms stay 'meaty' and 'fleshy'. It's all down to a polymer (a large molecular structure) called chitin in the mushrooms. It protects them from both breaking down or getting tough and chewy.

How does salt increase blood pressure?

Salt attracts water, so when you eat a salty diet it increases the amount of water retained by the body. This in turn can increase the volume of the blood, raising the pressure in blood vessels. Both the Dietary Guidelines for Americans and the UK's National Health Service say around a teaspoon of salt (6g/0.2oz) is the recommended daily allowance. It’s worth remembering that most of the salt in your diet comes from processed foods, not salt added during cooking or at the table.

How can seedless grapes reproduce?

Seedless grapes aren’t actually completely seedless, but have naturally mutated to have fewer and smaller seeds, without a tough coat. Obviously these "seeds" aren’t viable, so new plants are cloned through grafting, which involves cutting off and rooting a small piece of the grape vine.

Why do hot and cold water sound different when you’re pouring them into a glass?

Hot water produces a more high-pitched sound than iced water, which is down to the fact that warm water is slightly less viscous (or more runny) than cold. This slight change in viscosity changes the sound of the water against the side of the glass when poured.

Are canned tomatoes healthier than fresh?

Processing tomatoes through canning or cooking makes them a better source of lycopene – the red antioxidant that has been linked with a lower risk of heart disease and stroke. Heat softens the cell walls, allowing the lycopene to escape. Adding a little oil to tomatoes can help make them healthier too, as lycopene is fat soluble.

Can you eat mouldy cheese?

If it’s a hard cheese like Parmesan or Cheddar, it’s fine to just cut away the mould from the edge and eat the good cheese underneath. That's because harder cheeses don’t have enough moisture in them for the mould to penetrate far below the surface. But if a soft cheese like cream cheese or goats' cheese goes mouldy, you should throw it out.

What's the liquid on top of yogurt?

Pull back the lid on a pot of yogurt and it’s likely you’ll find watery liquid floating on the top. But don’t let it put you off as there’s nothing wrong with it. The liquid is just whey left behind from the yogurt-making process when the milk was curdled and strained. Stir it back in as it's full of nutritional goodness.

How do microwaves cook food?

When microwaves pass through foods, they make the water molecules in it vibrate, creating the heat energy which cooks the food. The microwaves don’t have the same effect on the molecules in air which is why the microwave itself doesn't get hot inside like an oven (this is also the reason you can’t create a crisp brown crust on a microwaved pie, for example).

Why do British people call sausages bangers?

Sausages supposedly started being called bangers in Britain during the First World War. Meat was scarce and sausages contained a high amount of fillers, including water, that expanded rapidly under heat, making the sausages explode open when cooked.

Can I still cook minced beef if it's gone brown?

Yes – if it’s been kept in the fridge, is within its use-by date (for food in the UK) and smells good, it’s fine to cook and eat. A brown colour in the middle of a pack of mince is simply due to a lack of oxygen, which causes a chemical change in the meat's myoglobin (a protein in an animal's muscle cells). It doesn’t mean that the meat is old, stale or unsafe to eat.

Why are nitrates in bacon a cancer risk?

Nitrates and nitrites are the chemicals that preserve bacon and ham. But they are also one of the factors that make processed meat a colon cancer risk. The reason is that nitrites in processed meats are in close proximity to proteins (specifically amino acids). “When cooked at high temperatures this allows them to more easily form nitrosamines, the cancer-causing compound,” explains Kate Allen, executive director of science and public affairs at the World Cancer Research Fund.

But the nitrates in beetroot, spinach and carrots are supposedly healthy?

Possibly, yes. There's evidence to suggest that polyphenols or chlorophyll in vegetables can prevent the formation of nitrosamines. This would favour nitrites being turned into nitric oxide instead – a compound with a blood pressure-lowering and performance-enhancing effect in the body. But other research suggests it isn’t a good idea to overload on nitrate-rich beetroot juice.

Is it true nutmeg is a mind-altering drug?

Nutmeg contains a chemical called myristicin that has been shown to result in psychotic episodes (hallucinations and delusions) after ingesting two tablespoons. It can start to have toxic effects and make you feel very unwell at consumption as low as a teaspoon though, so you should only use a small sprinkle.

Why do onions make you cry?

Cutting through the cells in an onion allows previously separated enzymes and amino acids in the onion to come into contact with each other, triggering a cascade of chemical reactions. It results in the formation of something called syn-propanethial-S-oxide, a chemical which easily vaporises and irritates the eyes. The only sure-fire way to prevent onion tears is to wear swimming goggles!

Why does asparagus make your pee smell funny?

Asparagusic acid, found only in asparagus, is the culprit here – when it's digested it creates sulphur by-products in your pee that evaporate almost immediately, causing an unpleasant scent. If you don’t think your urine smells strong after eating asparagus, that doesn’t mean you are metabolising the vegetable differently from other people – you have a genetic makeup that means you can’t smell it.

Why do prawns go pink when cooked?

The colour-changing transformation from raw to cooked prawns is down to a pigment called astaxanthin, a member of the carotene family. Astaxanthin is present all the time in the prawns, but when they are raw it’s linked to protein molecules and looks blue/grey. However heat disrupts the astaxanthin protein bond by “denaturing” the protein (making it uncoil and change shape) and this is what allows the pink colour to reveal itself.

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