杏吧原创

MENTION the words 鈥渟cience鈥� and 鈥�cooking鈥� in the same sentence and the phrase 鈥渕olecular gastronomy鈥� springs to mind. Scientific techniques have added zest to high-end cuisine, but you don鈥檛 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鈥檝e 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 say to sear 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 : as sugars and amino acids react, usually under heat, they produce a huge 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 鈥� 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 us, and whether the amounts we consume are dangerous. Still, to be safe, the UK鈥檚 Food Standards Agency says to limit acrylamide exposure by aiming for golden, rather than brown, with foods like roast potatoes and toast.

Cook pasta at a rolling boil

Many Italian chefs say 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 room stops the pasta sticking together.

It鈥檚 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掳C, well below water鈥檚 100掳C 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 鈥� 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 , author of , points out that most of the starch that thickens risotto comes from fine particles on the grains鈥� surface and it is the addition of liquid, not stirring, that mostly affects its release. (If you rinse the rice before you cook it, the starch comes off and you get a risotto with little creaminess.) Lopez-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 , who teaches a chemistry of cooking course at the American University in Washington DC. 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. 鈥淏ut if you control your ingredients just right from the outset, a no-stir method should give you the creaminess that you鈥檙e looking for,鈥� he says.

Avoid certain oils for frying

Don鈥檛 fry in olive oil. This common advice is based on its low smoke point of around 165 to 190掳C. When the oil is heated to this temperature, fat molecules get 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 , 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 in nut or sunflower oils.

Still, this consternation over cooking oils may be a storm in a teacup: just don鈥檛 reuse the oil too much, says of the Olive Center at the University of California, Davis. 鈥淢ost cooking oils are safe for cooking at high temperature until the oil gets very oxidised and starts breaking down.鈥�

Butter too has a low smoke point, around 190掳C. To avoid burning, some chefs say to add oil too. But it鈥檚 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掳C without it smoking, though you may sacrifice some flavour.

Some dirt won鈥檛 hurt

A bit of dirt won鈥檛 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鈥檚 why public health bodies recommend washing fruit and veg to remove soil, especially if you鈥檙e eating them raw. But don鈥檛 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 , 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 3 minutes on wood, but none died on plastic. When contaminated boards were left overnight, bacteria multiplied on plastic ones but couldn鈥檛 be recovered from wooden ones the next day.

One problem with plastic boards is that it鈥檚 easier for knives to leave grooves where bacteria can thrive. Hardwood boards are generally tougher, and their porous structure means , 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.

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鈥檚 best to use different boards for raw meats and any food that won鈥檛 require more cooking, such as raw salad ingredients.

Yolk spoils the whip

Egg whites鈥� ability to be whipped into an airy foam is 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鈥檚 because fats can bond to the amino acids, stopping them from interacting with each other to create the strong network. But you needn鈥檛 despair: one drop of yolk in 100 grams of egg white just means it will take longer to reach stiff peaks. Three drops, you鈥檝e 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鈥檛 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, affecting how much the amino acids want to avoid or contact water.

Egg whites aren鈥檛 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鈥檛 do much besides coating the surface, so there鈥檚 little benefit to hours of marinating. Acids like 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.

鈥淭here鈥檚 little benefit to hours of marinating. Many flavour molecules just coat the surface鈥�

What about brining turkeys? When meat cooks, muscle fibres contract, squeezing out liquid (See 鈥淪ear meat to lock in juices鈥�). Salt dissolves some muscle proteins, such as myosin, which loosens the fibres, letting them take on more water and ensuring they don鈥檛 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 gets 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鈥檛 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, chef J. Kenji Lopez-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. Lopez-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 get 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.

Souffl茅 is a recipe for failure

The word souffl茅 needn鈥檛 inspire terror. Souffl茅s 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 , 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 souffl茅 cools, the volume of gas in the bubbles decreases, and the water vapour condenses 鈥� this is why souffl茅s collapse if left before serving. Cooking your souffl茅 at a lower temperature for longer will lead to a less pronounced rise, but may help it resist collapse by enabling heat to penetrate and firm up the centre. It鈥檚 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 the UK in the 1970s and 80s, there was an increase in salmonella in chickens. The bacteria didn鈥檛 make the hens sick, but infected their eggs. 鈥淯nless these eggs were fully cooked, until the white and yolk were solid, the bacteria were liable to remain viable and cause disease in humans,鈥� says Barbara Lund at , a centre for food science and health research in Norwich, UK. No runny yolks, folks.

鈥淭oday, 90 per cent of eggs sold in the UK are from hens vaccinated against salmonella鈥�

Since then, the UK has required most hens to be vaccinated against salmonella. It worked. In 1997, there were . Within five years of the new strategy, that was halved. Today, 90 per cent of eggs sold in the UK, 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 the UK 鈥� and in countries such as the US, where hens are not vaccinated 鈥� 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鈥檚 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鈥檙e cooling something like a vat of piping hot soup. The US Department of Agriculture suggests using small containers that can chill quickly and .

That鈥檚 because leaving food out, especially in a warm room, is risky: between 20 and 50掳C, many bacteria can double every half hour. Improper cooling practices in restaurants contribute to a year in the US.

As for what you do with that saved portion of spaghetti, the UK Food Standards Agency recommends , 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鈥檚 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鈥檚 resistant to heat, so a blast in the microwave won鈥檛 ensure the rice is safe.

Thou shalt not refreeze

Whether you can safely refreeze food depends on how you defrosted it. It鈥檚 all about maintaining a safe temperature 鈥� . Above this, bacteria grow.

That鈥檚 why it鈥檚 safest to thaw food in the fridge. Provided it hasn鈥檛 started to spoil, it鈥檚 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 food scientist Barbara Lund. It can cause serious infection and deaths in vulnerable people, and is known to multiply at temperatures as low as 3掳C.

Another reason not to refreeze ice cream is that it just won鈥檛 be as nice. It鈥檚 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.

This article appeared in print under the headline 鈥淭he proof is in the pudding鈥�