The science of the kitchen: why onions make you cry and other everyday questions

Every cook has stood at the cutting board with streaming eyes, wondering why a vegetable would do this to a person. A few other kitchen habits come with their own mysteries: why the chicken never browns, why the roast must “rest”, why everyone salts the pasta water. Here are the well-established answers, and what to do with them on a Tuesday night.

Why onions make you cry

When you cut an onion, you break open its cells and different compounds mix. They react and produce a gas that drifts up and irritates the surface of your eyes. Your eyes respond by making tears to rinse it away. Nothing is wrong with you, and nothing is wrong with the onion.

What helps in practice:

• A sharp knife. A dull blade crushes cells instead of slicing them, which releases more of the irritant.
• Chill the onion. Twenty to thirty minutes in the fridge before cutting slows the reaction down for many people.
• Leave the root end for last. The root end holds a lot of the stuff you want to avoid, so trim it late.
• Ventilate. A fan or an open window blows the gas away from your face.
• Cut fast, then walk away. Do not hover over the board.

Wearing goggles does work. It looks silly and it is a fine choice.

Why food browns, and why yours does not

Browning is flavour. When protein and sugars on the surface of food meet high heat, they form hundreds of new savoury compounds. This only happens well on a dry surface in a hot pan, because water holds the surface at 100 degrees C until it has evaporated.

So:

1. Pat meat and vegetables dry with paper towel before they go in.
2. Preheat the pan for a couple of minutes, then add the oil. It should shimmer.
3. Do not crowd. If pieces touch, they steam in their own liquid. Cook in batches, or use a bigger pan.
4. Leave it alone. Food releases from the pan when it has browned. If it sticks, give it another minute before you poke.

Salt draws water out of meat, so for a steak or chop salt just before cooking, or at least 40 minutes ahead and then pat dry again.

Why meat needs to rest

During cooking, the juices in meat get pushed toward the centre. Cut it straight away and much of that liquid ends up on the board. Give it a rest and the meat relaxes a little, the juices settle, and each slice is moister.

As a rough guide, a steak or chop needs about five minutes, a whole chicken or a roast 15 to 20 minutes, loosely tented with foil. Do not wrap it tightly, or the crust goes soft. Resting also gives you time to make the gravy and set the table, so it is rarely wasted time.

Use a thermometer, not guesswork. Health Canada recommends cooking whole cuts of poultry to an internal temperature of 74 degrees C, and ground meat to 71 degrees C. Pork and beef cuts have their own guidelines, so check the current ones.

Why salt the pasta water

Pasta absorbs water as it cooks, and the water is the only chance to season it from the inside. Unsalted water gives you flat noodles that no sauce can fully rescue.

• Use about 10 grams of salt per litre of water (roughly two teaspoons of fine salt). It should taste like mild broth.
• Add it once the water boils.
• Use plenty of water, at least four litres for 500 g of pasta, so the temperature recovers fast and the pasta does not clump.
• Stir in the first minute or two. That prevents sticking. Oil in the water does not.
• Keep a mug of the cooking water before draining. A splash of it helps a sauce cling to the pasta.

A common myth says salt makes water boil faster. At the amounts used in cooking it makes almost no difference to the boiling temperature, so salt for taste only.

A host’s note

If you cook for guests, these four habits are where the biggest improvement comes for the least effort: dry the food, heat the pan, rest the meat, season the water. If something goes wrong, it usually comes down to crowding the pan or rushing the rest.

For more everyday technique, browse our home cooking section.

This article is general cooking and food-safety information, not medical advice. If you have a health concern, talk to your doctor or pharmacist.