Mention the words “science” and “cooking” in the same sentence and the phrase “molecular gastronomy” springs to mind. Scientific techniques have added zest to high-end cuisine, but you don’t have to be using liquid nitrogen, spherification and the like to benefit. Home cooks can learn a lot from science, especially when it comes to knowing which bits of kitchen folklore are worth swallowing.

It turns out that many top tips make little difference, while others undermine flavour or even increase the risk of food poisoning. We’ve sifted through some of the most commonly promoted techniques, chewed over the science and put the tips to the test to figure out which should be saved and which should be thrown away.

Sear meat to lock in juices

Many chefs advise searing meat first at a high heat to trap moisture. But if you cook identical steaks to the same internal temperature, one that is roasted then seared is often juicier than one that is seared then roasted.

Why? Higher heat makes the muscle fibres contract more, forcing liquid out. A cold steak takes longer to sear in a hot pan than a steak that has been warmed in the oven, so loses more liquid.

To retain moisture, the most important step is to rest the meat after cooking. As muscle fibres cool, they widen, holding on to more juice.

Searing does boost flavour by browning the outside. This is caused by Maillard reactions: as sugars and amino acids react, usually under heat, they produce a range of flavour compounds that contribute to the distinctive tastes of everything from roast beef to popcorn. It is Maillard reactions that turn bread into toast and provide the savoury crunch of roast potatoes. But they also produce acrylamide – particularly in starchy foods cooked at high temperatures. In the body, acrylamide is converted into glycidamide, which can bind to DNA and cause mutations. In animal studies, consuming acrylamide has been shown to cause cancers.

There is less certainty about its effects on humans, and whether the amounts we consume are dangerous. Still, to be safe, Britain’s Food Standards Agency advises limiting acrylamide exposure by aiming for golden, rather than brown, with foods such as roast potatoes and toast.

Cook pasta at a rolling boil

Many Italian chefs will tell you that you must add pasta to a big pot of water at a vigorous boil. This is because it will return to the boil quicker and the extra space stops the pasta sticking together.

It’s true that adding a set amount of pasta to a smaller pot will lower the water temperature more than adding it to a bigger pot. But returning to a boil takes the same amount of energy and may even be quicker in a smaller pot because it has less surface area.

As for sticking, this only happens during the first minute or so of cooking, when surface starch granules swell and pop. Stir the pasta for the first minute and you can happily leave it to cook in your small pot. In fact, you can turn the heat off and leave the lid on and it will cook just fine, as starch gelatinises at about 82 degrees Celsius, well below water’s 100-degree boiling point.

Risotto: never stop stirring

To make risotto, you must add stock slowly and stir constantly, right? Traditionalists are adamant that doing so ensures even cooking and the perfect creamy consistency.

This creaminess comes from dissolving starch. Rice contains two forms: amylose, made of long, straight chains packed tightly together; and amylopectin, which has a branching structure. Constant stirring and the slow addition of stock are supposed to help dislodge amylopectin – by rubbing rice grains against each other and pushing liquid through gaps in the branches. The amylose stays tightly packed, so the rice retains some bite.

But chef J. Kenji López-Alt, author of The Food Lab: Better Home Cooking Through Science (2015), points out that most of the starch that thickens risotto comes from fine particles on the grains and it is the addition of liquid, not stirring, that most affects its release. (If you rinse the rice before you cook it, the starch comes off and you get a risotto with little creaminess.) López-Alt found that vigorous stirring for a couple of minutes at the end of cooking gives equally creamy results.

Still, traditionalists have a point, says Matthew Hartings, who teaches a chemistry of cooking course at the American University in Washington DC, in the United States. The higher the ratio of released starch to liquid in the pot, the thicker the risotto will be. Adding stock gradually gives you more control over that ratio. “But if you control your ingredients just right from the outset, a no-stir method should give you the creaminess you’re looking for,” he says.

Avoid certain oils for frying

Don’t fry in olive oil; this common advice is based on its low smoke point of around 165 to 190 degrees. When the oil is heated to this temperature, fat molecules are oxidised, producing aldehydes and other compounds that can be toxic and give an acrid flavour.

But studies have found that both extra-virgin and ordinary olive oils hold up well at high heat, producing fewer toxic aldehydes than other oils. That may be because the unsaturated fat in olive oil is more resistant to oxidation than the types found in nut or sunflower oils.

Still, this consternation over cooking oils may be a storm in a teacup: just don’t reuse the oil too much, says Selina Wang, research director of the Olive Center at the University of California, Davis. “Most cooking oils are safe for cooking at high temperature until the oil gets very oxidised and starts breaking down.”

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Butter, too, has a low smoke point, around 190 degrees. To avoid burning, some chefs say add oil, too. But it’s the milk proteins in butter that cause the smoke, and these will remain. If you skim them off to make clarified butter or ghee, you can heat it to 250 degrees without it smoking, though you may sacrifice some flavour.

Some dirt won’t hurt

A bit of dirt won’t do you any harm, will it? Well, soil can carry harmful bacteria such as E coli, outbreaks of which can be deadly. In fact, bugs on fruit and vegetables can be even more harmful than those found on raw meat. That’s why public health bodies recommend washing fruit and vegetables to remove soil, especially if you are eating them raw. But don’t wash uncooked meat: you risk splashing bacteria around and cooking will kill bugs.

Plastic is safer than wood

Plastic chopping boards are meant to be superior because bacteria can get into wooden ones and linger.

To test this, US food safety researcher Dean Cliver contaminated chopping boards with germs such as E coli and salmonella. He found that 99.9 per cent died within three minutes on wood, but none died on plastic. When contaminated boards were left overnight, bacteria multiplied on plastic ones but couldn’t be recovered from wooden ones the next day.

One problem with plastic boards is that it’s easier for knives to leave grooves where bacteria can thrive. Hardwood boards are generally tougher, and their porous structure means bacteria sink below the surface, where they get trapped, have little room to multiply and eventually die off. That said, when any chopping board gets too many grooves, food safety specialists recommend replacing it.

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Plastic boards should be quite safe after a run through the dishwasher as they can be sanitised at high temperatures. However you clean your board, let it dry thoroughly as bacteria need moisture to grow.

Regardless of material, to avoid cross-contamination it’s best to use different boards for raw meats and any food that won’t require more cooking, such as raw salad ingredients.

Yolk spoils the whip

Egg whites whipped into an airy foam are a wonder of kitchen chemistry. They are about 90 per cent water and 10 per cent protein. Some of the amino acids that make up these proteins are hydrophilic, meaning they are attracted to water. Others are hydrophobic. The proteins start curled up, with the hydrophobic regions tucked inside. As the beating causes them to unfurl, they form clumps with the hydrophobic bits sandwiched against each other and the hydrophilic parts pointing out towards the water. The resulting network of proteins around each air bubble gets stronger and stiffer as you keep beating.

Recipes warn us that a trace of yolk or grease will ruin a whole batch. That’s because fats can bond to the amino acids, stopping them from interacting with each other to create the strong network. But you needn’t despair: one drop of yolk in 100 grams of egg white just means it will take longer to reach stiff peaks. Three drops and you’ve got problems. The same goes for working in a bowl with oily residue.

In your meringue-making, you might have experienced liquid leaching out of the foam that won’t mix back in. This can happen when the proteins clump together too strongly, collapsing the foam and leaking water. Adding lemon juice or cream of tartar should help. These acids lower the pH (a measure of its alkalinity or acidity), affecting how much the amino acids are attracted to water.

Egg whites aren’t the only option for foamy meringues. It was recently discovered that the liquid from tins of chickpeas, known as aquafaba, whips up just as well.

Marinate meat for flavour

You need to marinate meat at least overnight, right? Maybe not. Only salt, small sugar molecules and some acids are able to penetrate more than a few millimetres. Aromatics and flavour molecules won’t do much besides coating the surface, so there’s little benefit to hours of marinating. Acids such as lemon juice denature proteins and can leave your meat mushy. So instead of giving meat a soak, it may be better to simply rub spices on the outside.

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What about brining turkeys? When meat cooks, muscle fibres contract, squeezing out liquid (See “Sear meat to lock in juices”). Salt dissolves some muscle proteins, such as myosin, which loosens the fibres, letting them take on more water and ensuring they don’t contract quite as much when they cook. The trouble is, that just leaves you with a bird pumped full of water. Dry salting can create a concentrated brine that becomes absorbed into the meat, letting it hold on to more of its original moisture without diluting the flavour.

Hot pan for perfect Yorkies

There are two rules for Yorkshire puddings: the pan must be smoking hot before the batter goes in and you shouldn’t open the oven door or the cold air will make them collapse.

It took an American to challenge the accepted wisdom on this British creation. While researching the best Yorkshire puddings, López-Alt found that a preheated pan makes little difference for small puddings in a muffin tin. (The advice is probably aimed at larger tins, which are harder to heat up.) And opening the oven door made no difference.

Much more important is letting the batter rest. López-Alt found this affects how much the puddings rise, with ideal results from batter made a day earlier. During cooking, proteins and starches break down and are rearranged into new compounds. These give the batter colour and flavour, and allow elastic-like gluten to develop from the flour, which results in larger air bubbles when baked. Making the batter in advance gives this process a head start.

These reactions occur slowly in the fridge or at room temperature, and speed up during baking. The pudding will rise more if the batter starts at room temperature. Cooking from cold will give you more cup-shaped puds as the edges heat faster than the middle.

Soufflé is a recipe for failure

The word “soufflé” needn’t inspire terror. Soufflés are like meringues: you beat air into the egg white, fold it into your flavoursome mixture, then watch it rise as air bubbles expand in the oven.

According to Harold McGee, a food writer who focuses on the chemistry of cooking, expanding air accounts for only a quarter of the rise. The rest comes from evaporating water. When the soufflé cools, the volume of gas in the bubbles decreases, and the water vapour condenses – this is why soufflés collapse if left before serving. Cooking your soufflé at a lower temperature for longer will lead to a less pronounced rise, but can help it resist collapse by enabling heat to penetrate and firm up the centre. It is also important to grease the dishes: if the mix sticks to the side, it might rise too little or unevenly.

Chill your eggs

Where should you keep eggs? Storing them in the fridge was once advised to reduce risk of bacterial growth. In Britain in the 1970s and 80s, there was an increase in salmonella in chickens. The bacteria didn’t make the hens sick, but infected their eggs. “Unless these eggs were fully cooked, until the white and yolk were solid, the bacteria were liable to remain viable and cause disease in humans,” says Barbara Lund of the Quadram Institute, a centre for food science and health research in Norwich, Britain. No runny yolks, folks.

Since then, Britain has required most hens to be vacci­nated against salmonella. It worked. In 1997, there were 33,000 infections with the bacteria. Within five years of the new measures, that was halved. Today, 90 per cent of eggs sold in Britain, labelled with the Lion Quality mark, are from vaccinated hens. Last year, the Food Standards Agency even declared runny yolks safe for pregnant women.

For the other 10 per cent of eggs sold in Britain – and in countries such as the US, where hens are not vaccinated – refrigeration reduces infection risk.

Leave leftovers to cool

Some people whack warm leftovers straight into the fridge, others leave them to cool to room temperature first. Who’s right?

The idea behind waiting is to avoid raising the temper­ature inside the fridge, potentially putting other contents at risk of bacterial growth. But this is only really a problem if you’re cooling something like a vat of piping hot soup. The US Department of Agriculture suggests using small containers that can chill quickly and refrigerating all leftovers immediately. That’s because leaving food out, especially in a warm room, is risky: between 20 and 50 degrees, many bacteria can double every half hour.

As for what you do with that saved portion of spaghetti, the Food Standards Agency recommends eating refrigerated leftovers within two days, since some bugs can continue to grow slowly at fridge temperatures. Frozen leftovers should last up to four months.

Rice, however, should be eaten in one day. That’s because Bacillus cereus, a bacterium found in uncooked rice that can cause diarrhoea and vomiting, forms spores that can survive cooking. These can produce a toxin that’s resistant to heat, so a blast in the microwave won’t ensure the rice is safe.

Thou shalt not refreeze

Whether you can safely refreeze food depends on how you defrosted it. It’s all about maintaining a safe temperature – below 4.5 degrees. Above this, bacteria grow.

That’s why it is safest to thaw food in the fridge. Provided it hasn’t started to spoil, it is then fine to refreeze. Thawing food in the microwave or in water is OK, but it should then be cooked immediately. Thawing food on the worktop is danger­ous as bacteria can multiply rapidly at room temperature. This is why you should never refreeze ice cream that has melted. Listeria monocytogenes is known to survive for prolonged periods in ice cream, says Lund. It can cause serious infection and deaths in vulnerable people, and is known to multiply at temperatures as low as 3 degrees.

Another reason not to refreeze ice cream is that it just won’t be as nice. It is usually churned as it freezes so that it forms small crystals separated by air. Melted and refrozen ice cream will instead form a dense block.

Perfect poached eggs

There are plenty of tricks for poaching eggs: the cling film method, silicon cups and the vortex, to name a few.

It is true that adding vinegar or salt to the water will help the whites coagulate at a lower temperature. Egg proteins mostly have negative charge, so adding positively charged particles can help them get close to each other and bond. But the best advice is to use fresh eggs. As an egg ages, carbon dioxide escapes through the shell and is replaced by air. The pH of the egg white increases, destabilising links between its albumin and lysozyme proteins. These proteins then dissolve, making the egg white runny. “Loose whites” put into hot water drift around, rather than clinging to the yolk. You can test the freshness of an egg in water: old eggs are more buoyant because of the air they have absorbed.

If you’ve only got old eggs, chef Heston Blumenthal has a trick to give you tidy whites. Break the egg into a wire mesh strainer. Liquid whites will pass through, leaving you with solid whites that won’t dangle off during cooking.

No more tears

As you chop onions, they release enzymes that lead to the production of syn-propanethial-S-oxide, the volatile compound that makes you weep. There are countless tips for avoiding this chemical assault. We put some to the test.

A) Freeze the onion for 10 minutes: the cold is supposed to reduce the activity of the enzymes. Verdict: quite effective.

B) Peel and soak the onion in water for 10 minutes, the eye-stinging compound is water-soluble. Verdict: the volatile chemical isn’t released until the onion is chopped, so this did little to help. It also made the onion slippery, so harder to chop cleanly and safely.

C) Chew gum while you chop, this is supposed to help by forcing you to breathe through your mouth. Verdict: the tear-producing glands that get irritated are in your eyes, so it’s unlikely to help. For us, it made no difference.

D) Wear glasses, contacts or goggles, this blocks syn-propanethial-S-oxide from getting into your eyes. Verdict: goggles work, though perhaps not for impressing dates.

Tomato catch-up

You say tomato, I say: “Not in the fridge!” Chilling unripe tomatoes reduces production of enzymes that generate taste and aroma compounds, and many don’t recover when back at room temperature. That is why supermarket tomatoes can be flavourless: they were probably picked some time ago and kept cold to extend their shelf life.

But what about tomatoes that are already ripe? According to Trevor Suslow and Marita Cantwell, at the University of California, Davis, the ideal temperature for firm, ripe tomatoes is seven to 10 degrees. Unfortunately, most of us don’t have a storage space in that range. So what’s better, the fridge or a warm kitchen?

In blind tests done by chef Daniel Gritzer, tasters noticed little difference between tomatoes stored at room temperature and ones that were refrigerated for two days. However, warmer parts of the fridge – the top shelf and near the door – are probably best.

New Scientist