The Science of Cooking: Heat vs. Temperature

J. Kenji López-Alt wrote . . . . . . . . .

This is an exclusive excerpt from my book, The Food Lab: Better Home Cooking Through Science, a grand exploration of the world of food science and how it can make everyday home cooking easier and more delicious. It’s on sale now anywhere books are sold, or online.

What Is Cooking?

I know you’re eager to jump right in and start cooking, but first answer this question: What is cooking?

Cooking is about transferring energy. It’s about applying heat to change the structure of molecules. It’s about encouraging chemical reactions to alter flavors and textures. It’s about making delicious things happen with science. And before we can even begin to understand what happens when we grill a hamburger, or even what equipment we might want to stock our kitchen with, we have to get one very important concept into our heads first, as it’ll affect everything we do in the kitchen, starting with which pots and pans we use. It’s this: Heat and temperature are not the same thing.

At its most basic, cooking is the transfer of energy from a heat source to your food. That energy causes physical changes in the shape of proteins, fats, and carbohydrates, as well as hastens the rate at which chemical reactions take place. What’s interesting is that most of the time, these physical and chemical changes are permanent. Once a protein’s shape has been changed by adding energy to it, you can’t change it back by subsequently removing that energy. In other words, you can’t uncook a steak.

The distinction between heat and temperature can be one of the most confusing things in the kitchen, but grasping the concept is essential to helping you become a more rational cook. Through experience, we know that temperature is an odd measure. I mean, pretty much all of us have walked around comfortably in shorts in 60° weather but have felt the ridiculous chill of jumping into a 60° lake, right? Why does one but not the other make us cold, even though the temperature is the same? Let me try to explain.

Heat is energy. Third-grade physics tells us that everything from the air around us to the metal on the sides of an oven is composed of molecules: teeny-tiny things that are rapidly vibrating or, in the case of liquids and gases, rapidly bouncing around in a random manner. The more energy is added to a particular system of molecules, the more rapidly they vibrate or bounce, and the more quickly they transfer this movement to anything they are touching—whether it’s the vibrating molecules in a metal pan transferring energy to a juicy rib-eye steak sizzling away or the bouncing molecules of air inside an oven transferring energy to the crusty loaf of bread that’s baking.

Heat can be transferred from one system to another, usually from the more energetic (hotter) system to the less energetic (cooler). So when you place a steak in a hot pan to cook it, what you are really doing is transferring energy from the pan burner system to the steak system. Some of this added energy goes to raising the temperature of the steak, but much of it gets used for other reactions: It takes energy to make moisture evaporate, the chemical reactions that take place that cause browning require energy, and so on.

Temperature is a system of measurement that allows us to quantify how much energy is in a specific system. The temperature of the system is dependent not only on the total amount of energy in that body, but also on a couple of other characteristics: density and specific heat capacity.

Density is a measure of how many molecules of stuff there are in a given amount of space. The denser a medium, the more energy it will contain at a given temperature. As a rule, metals are denser than liquids,* which in turn are denser than air. So metals at, say, 60°F will contain more energy than liquids at 60°F, which will contain more energy than air at 60°F.

*All right, Mr. Smarty-Pants. Yes, at high enough temperatures, metals will melt into very dense liquids, and yes, Mr. Even Smartier-Pants, mercury is a very dense metal that is liquid even at room temperature. Got that out of your system? OK, let’s move on.

Specific heat capacity is the amount of energy it takes to raise a given amount of a material to a certain temperature. For instance, it takes exactly one calorie of energy (yes, calories are energy!) to raise one gram of water by one degree Celsius. Because the specific heat capacity of water is higher than that of, say, iron, and lower than that of air, the same amount of energy will raise the temperature of a gram of iron by almost 10 times as much and a gram of air by only half as much. The higher the specific heat capacity of a given material, the more energy it takes to raise the temperature of that material by the same number of degrees.

Conversely, this means that given the same mass and temperature, water will contain about 10 times as much energy as iron and about half as much as air. Not only that, but remember that air is far less dense than water, which means that the amount of heat energy contained in a given volume of air at a given temperature will be only a small fraction of the amount of energy contained in the same volume of water at the same temperature. That’s the reason why you’ll get a bad burn by sticking your hand into a pot of 212°F boiling water, but you can stick your arm into a 212°F oven without a second thought (see “Experiment: Temperature Versus Energy in Action,” below).

Confused? Let’s try an analogy.

Imagine the object being heated is a chicken coop housing a dozen potentially unruly chickens. The temperature of this system can be gauged by watching how fast each individual chicken is running. On a normal day, the chickens might be casually walking around, pecking, scratching, pooping, and generally doing whatever chickens do. Now let’s add a bit of energy to the equation by mixing a couple cans of Red Bull in with their feed. Properly pepped up, the chickens begin to run around twice as fast. Since each individual chicken is running around at a faster pace, the temperature of the system has gone up, as has the total amount of energy in it.

Now let’s say we have another coop of the same size but with double the number of chickens, thereby giving it double the density. Since there are twice as many chickens, it will take double the amount of Red Bull to get them all running at an accelerated pace. However, even though the final temperature will be the same (each individual chicken is running at the same final rate as the first ones), the total amount of energy within the second coop is double that of the first. So, energy and temperature are not the same thing.

Now what if we set up a third coop, this time with a dozen turkeys instead of chickens? Turkeys are much larger than chickens, and it would take twice as much Red Bull to get one to run around at the same speed as a chicken. So the specific heat capacity of the turkey coop is twice as great as the specific heat capacity of the first chicken coop. What this means is that given a dozen chickens running around at a certain speed and a dozen turkeys running around at the same speed, the turkeys will have twice as much energy in them as the chickens.

To sum up:

  • At a given temperature, denser materials generally contain more energy, and so heavier pans will cook food faster. (Conversely, it takes more energy to raise denser materials to a certain temperature.)
  • At a given temperature, materials with a higher specific heat capacity will contain more energy. (Conversely, the higher the specific heat capacity of a material, the more energy it takes to bring it to a certain temperature.)

In this book, most recipes call for cooking foods to specific temperatures. That’s because for most food, the temperature it’s raised to is the primary factor determining its final structure and texture. Some key temperatures that show up again and again include:

  • 32°F (0°C): The freezing point of water (or the melting point of ice).
  • 130°F (52°C): Medium-rare steak. Also the temperature at which most bacteria begin to die, though it can take upward of 2 hours to safely sterilize food at this temperature.
  • 150°F (64°C): Medium-well steak. Egg yolks begin to harden, egg whites are opaque but still jelly-like. Fish proteins will tighten to the point that white albumin will be forced out, giving fish like salmon an unappealing layer of congealed proteins. After about 3 minutes at this temperature, bacteria experience a 7 log reduction—which means that only 1 bacterium will remain for every million that were initially there.
  • 160° to 180°F (71° to 82°C): Well-done steak. Egg proteins fully coagulate (this is the temperature to which most custard or egg-based batters are cooked to set them fully). Bacteria experience a 7 log reduction within 1 second.
  • 212°F (100°C): The boiling point of water (or the condensation point of steam).
  • 300°F (153°C) and above: The temperature at which the Maillard browning reactions—the reactions that produce deep brown, delicious crusts on steaks or loaves of bread—begin to occur at a very rapid pace.The hotter the temperature, the faster these reactions take place. Since these ranges are well above the boiling point of water, the crusts will be crisp and dehydrated.

Sources of Energy and Heat Transfer

Now that we know exactly what energy is, there’s a second layer of information to consider: the means by which that energy gets transferred to your food.

Conduction is the direct transfer of energy from one solid body to another. It is what happens when you burn your hand by grabbing a hot pan (hint: don’t do that). Vibrating molecules from one surface will strike the relatively still molecules on another surface, thereby transferring their energy. This is by far the most efficient method of heat transfer. Here are some examples of heat transfer through conduction:

  • Searing a steak
  • Crisping the bottom of a pizza
  • Cooking scrambled eggs
  • Making grill marks on a burger
  • Sautéing onions

Convection is the transfer of energy from one solid body to another through the intermediary of a fluid—that is, a liquid or a gas. This is a moderately efficient method of heat transfer, though in cooking its efficiency depends greatly on the way the fluid flows around the food. The motion of the fluid is referred to as convection patterns.

As a general rule, the faster air travels over a given surface, the more energy it can transfer. Still air will rapidly give up its energy, but with moving air, the energy supply is constantly being replenished by new air being cycled over a substance such as food. Convection ovens, for instance, have fans that are designed to keep the air inside moving around at a good clip to promote faster, more even cooking. Similarly, agitating the oil when deep-frying can lead to foods that crisp and brown more efficiently.

Here are some examples of heat transfer through convection:

  • Steaming asparagus stalks
  • Boiling dumplings in stock
  • Deep-frying onion rings
  • Barbecuing a pork shoulder
  • The top of a pizza baking in an oven
  • The top of a pizza cooks via convection and radiation.

Radiation is transfer of energy through space via electromagnetic waves. Don’t worry, that’s not as scary as it sounds. It doesn’t require any medium to transfer it. It is the heat you’re feeling when you sit close to a fire or hold your hand above a preheated pan. The sun’s energy travels to the earth through the vacuum of space. Without radiation, our planet (and indeed, the universe) would be in a lot of trouble!

An important fact to remember about radiant energy is that it decays (that is, gets weaker) by the inverse square law—the energy that reaches an object from a radiant energy source is proportional to the inverse of the square of its distance. For example, try holding your hand 1 foot away from a fire, then move it 2 feet away. Even though you’ve only doubled the distance, the fire will feel only about one-quarter as hot.

Here are some examples of radiant heat transfer:

  • Roasting a pig on a spit next to hot coals
  • Toasting garlic bread under the broiler
  • Getting a tan from the sun
  • Broiling some marinated salmon

Most of the time, in cooking, all three methods of heat transfer are used to varying extents. Take a burger on the grill, for example. The grill grate heats the patty directly where it is in contact with it through conduction, rapidly browning it at those spots. The rest of the underside of the patty is cooked via radiation from the coals underneath. Place a piece of cheese on the burger and pop the lid down for a bit, and convection currents will form, carrying the hot air from directly above the coals up and over the top of the burger, melting the cheese.

You might notice that these three types of heat transfer heat only onto the surface of foods. In order for food to cook through to the center, the outer layer must transfer its heat to the next layer, and so on, until the very center of the food begins to warm up. Because of that, the outside of most cooked foods will almost always be more well done than the center (there are tricks to minimizing the gradient, which we’ll get to in time).

Microwaves are the only other standard method of energy transfer we commonly use in the kitchen, and they have the unique ability to penetrate through the exterior of food when heating it. Just like light or heat, microwaves are a form of electromagnetic radiation. When microwaves are aimed at an object with magnetically charged particles (like, say, the water in a piece of food), those particles rapidly flip back and forth, creating friction, which, in turn, creates heat. Microwaves can pass through most solid objects to a depth of at least a few centimeters or so. This is why microwaves are a particularly fast way to heat up foods—you don’t need to wait for the relatively slow transfer of energy from the exterior to the center.

Phew! Enough with the science lesson already, right? Bear with me. Things are about to get a lot more fun!

Experiment: Temperature Versus Energy in Action

The difference between the definition of temperature and the definition of energy is subtle but extraordinarily important. This experiment will demonstrate how understanding the difference can help shape your cooking.

Materials

1 properly calibrated oven
1 able-bodied subject with external sensory apparatus in full working order
One 3-quart saucier or saucepan filled with water
1 accurate instant-read thermometer

Procedure

Turn your oven on to 200°F and let it preheat. Now open the oven door, stick your hand inside, and keep your hand in the oven until it gets too hot to withstand. A tough guy like you could probably leave it in there for at least 15 seconds, right? 30 seconds? Indefinitely?

Now place a pan of cold water on the stovetop and stick your hand in it. Turn the burner to medium-high heat and let the water start to heat up. Stir it around with your hand as it heats, but be careful not to touch the bottom of the pan (the bottom of the pan will heat much faster than the water). Keep your hand in there until it becomes too hot to withstand, remove your hand, and take the temperature.

Results

Most people can hold their hand in a 200°F oven for at least 30 seconds or so before it becomes uncomfortably hot. But let it go much above 135°F, and a pan of water is painful to touch. Water at 180°F is hot enough to scald you, and 212°F (boiling) water will blister and scar you if you submerge your hand in it. Why is this?

Water is much denser than air—there are many times more molecules in a cup of water than there are in a cup of air. So, despite the fact that the water is at a lower temperature than the air in the oven, the hot water contains far more energy than the hot air and consequently heats up your hand much more rapidly. In fact, boiling water has more energy than the air in an oven at a normal roasting temperature, say 350° to 400°F. In practice, this means that boiled foods cook faster than foods that are baked or roasted. Similarly, foods baked in a moist environment cook faster than those in a dry environment, since moist air is denser than dry air.

Source: Serious Eat

Call for “Paradigm Shift” to Fight Airborne Spread of COVID-19 Indoors

Queensland University of Technology (QUT) air-quality expert Distinguished Professor Lidia Morawska is leading an international call for a “paradigm shift” in combating airborne pathogens such as COVID-19, demanding universal recognition that infections can be prevented by improving indoor ventilation systems.

Professor Morawska led a group of almost 40 researchers from 14 countries in a call published in Science today for a shift in standards in ventilation requirements equal in scale to the transformation in the 1800s when cities started organising clean water supplies and centralised sewage systems.

The international group of air quality researchers called on the World Health Organisation to extend the indoor air quality guidelines to include airborne pathogens and to recognise the need to control hazards of airborne transmission of respiratory infections.

Professor Morawska, director of QUT’s International Laboratory for Air Quality and Health, said there needed to be a shift in the perception that we could not afford the cost of control, given the globally monthly harm from COVID-19 had been conservatively estimated as $1 trillion and the cost of influenza in the US alone exceeded $11.2 billion annually.

“We need to establish the foundations to ensure that the air in our buildings is clean with a significantly reduced pathogen count, contributing to the building occupants’ health, just as we expect for the water coming out of our taps,” Professor Morawska said.

“Mandated building ventilation standards need to include higher airflow, filtration and disinfection rates, and monitors that allowed the public to observe the quality of air around them.
“We should have virus-free air indoors.”

Professor Morawska said response efforts to combat airborne viruses were too weak because airborne infections were harder to trace than food or waterborne outbreaks.

“We’ve provided strong evidence that airborne transmission spreads infections, so there should be international ventilation standards that control pathogens,” she said.

“Most minimum ventilation standards outside of specialised health care and research facilities only control for odour, CO2 levels, temperature and humidity.

“Ventilation systems with higher airflow rates and which distribute clean, disinfected air so that it reaches the breathing zone of occupants must be demand controlled and thus be flexible.”

Professor Morawska said ventilation systems should also be demand-controlled to adjust for different room occupancies, and differing activities and breathing rates, such as exercising in a gym versus sitting in a movie theatre.

“For decades, the focus of architects and building engineers was on thermal comfort, odour control, perceived air quality, initial investment cost, energy use, and other performance issues, while infection control was neglected,” Professor Morawska said.

“Buildings consume over one third of energy globally, much of it expended on heating/cooling outdoor air as it is brought indoors.

“While building designs should optimize indoor environment quality in terms of health and comfort, they should do that in an energy-efficient way in the context of local climate and outdoor air pollution.”

“Wide use of monitors displaying the state of indoor air quality must be mandated too, because the general public currently have no way of knowing the condition of indoor spaces they occupy and share with others.

“Visible displays will keep building operators accountable for air quality. The public should be aware and demand safe environments.

“None of this means that every indoor space should become a biosafety facility, but a building should be designed and operated according to its purpose and activities conducted there, so that airborne infection risk stays below an acceptable level.”

While detailed economic analysis was yet to be done, Professor Morawska said estimates suggested necessary investments in building systems may be less than one per cent of the construction cost of a typical building.

“The cost of infections come from different pockets—building and operating costs, health care costs—but ultimately, society pays for all the costs so a cross-system reallocation of budgets must also be facilitated to mandate new ventilation standards,” Professor Morawska said.

“The benefits are beyond infectious disease transmission. Improved indoor air quality may reduce workplace absenteeism, ‘sick building syndrome’ and allergic reactions.

“The reduction in productivity losses alone may cover the cost of any ventilation changes.”
Last year, research by Professor Morawska and a group of 239 international experts published an open letter on the need to rethink popular advice on how COVID-19 is spread: It is Time to Address Airborne Transmission of COVID-19.

Source: Queensland University of Technology

What’s for Lunch?

Grilled Mackerel Set Meal at Uoshin (魚真) in Osaka, Japan

The price is 850 yen plus tax.

U.S. Expert Panel: Get First Colonoscopy at 45, not 50

Cara Murez wrote . . . . . . . . .

A lot of people think of age 50 as the magic number for getting a first colonoscopy, but earlier is better, a prestigious U.S. expert panel now says.

Based on evidence that younger people are being diagnosed with colon cancer and would benefit from screening, the U.S. Preventive Services Task Force (USPSTF) is moving the recommended age for colon cancer screening from 50 to age 45.

The recommendation is for all adults without symptoms, personal health history of colon polyps or family health history of genetic disorders that increase risk, the task force noted.

“Colorectal cancer screening saves lives and people ages 45 to 75 should be screened … to lower their risk of dying from this devastating disease,” said Dr. John Wong, chief scientific officer of the USPSTF. “There is new science about colorectal cancer in people younger than 50. That science has allowed us to expand our recommendation to people ages 45 to 49.”

Though the USPSTF is an independent, volunteer group of health experts in a range of specialties, its recommendations carry weight. For example, the Affordable Care Act linked USPSTF recommendations with its insurance coverage requirements.

The task force does not have enough evidence to show benefits to moving the screening age even lower, Wong said, but called for additional research.

The American Cancer Society was already recommending screening this younger age group, having altered its recommendations in 2018 to include those age 45 to 49.

The changing USPSTF recommendations will be mean less confusion about which recommendation to follow as well as insurance coverage for screening at an earlier age, said Robert Smith, senior vice president of cancer screening for the American Cancer Society.

“We want doctors and the public to recognize the value of starting screening at age 45, instead of putting it off until age 50 or even later, which frankly many people do,” Smith said. “It’s not as if everybody immediately begins screening at age 50. They commonly put it off until their middle 50s.”

By some estimates, one-quarter to one-third of people in the already recommended age group of 50 to 75 aren’t getting their screening on time, even though colon cancer is the third-leading cause of cancer death in the United States.

“This recommendation of screening for colorectal cancer is a critical preventive measure for all people ages 45 to 75 to help them live longer and healthier lives,” Wong said.

Noting Black adults have particularly high rates of colon cancer and are more likely to die from the disease, the task force encouraged doctors to reach out to their Black patients to be sure they’re screened regularly.

Also, people should talk to their doctors if they have a change in bowel habits, blood in their stools or darkening stool, which can be a sign of bleeding, Smith said.

Several types of tests can screen for colon cancer, Wong said. Some can be done at home. Some can be done in a medical office. Patients can talk to their doctor to figure out the right test for them.

The task force recommended both direct visualization tests such as colonoscopy and stool-based tests. The right test is the one that gets screening done, the task force statement said. The cancer society also recommends a stool test or direct visualization test. Visualization tests also include sigmoidoscopy or CT colonography.

“We can prevent this disease through screening and we can find it early,” Smith said.

There isn’t one answer for why more younger people are getting colon cancer, Smith said. An editorial accompanying the new recommendations — published May 18 online in the Journal of the American Medical Association — said risk can be reduced through diet and lifestyle changes.

Nearly 53,000 Americans will die of colon cancer this year, the task force estimates.

The USPSTF did not change its recommendations for older adults, ages 76 to 85.

“We recognize that the benefits and harms depend on an individual’s overall health, whether or not they’ve been screened before that and their personal circumstances and preferences. So we recommend that people in this age group should talk with their clinician about whether screening is right for them,” Wong said.

Evidence used to make the new recommendations included randomized controlled trials and USPSTF modeling studies.

Source: HealthDay

Spicy Fish Cakes

Ingredients

1 lb fillets of white fish, skinned
1-1/4 cups skim milk
1 bay leaf
2 tablespoons low fat margarine
1 small onion, peeled and finely chopped
1 green pepper, seeded, cored and finely chopped
1/2 teaspoon chili powder, or to taste
1/4 cup whole wheat flour
salt
1 egg, beaten
5 tablespoons whole wheat breadcrumbs
1/2 cup peanuts, crushed or finely chopped
1 tablespoon milk
2 tablespoons sunflower oil
celery stalk, to garnish

Sauce

3 scallions, finely chopped
2-1/2 tablespoons chopped mint
1-1/4 teaspoons lemon juice
2/3 cup plain unsweetened yogurt, chilled

Method

  1. Poach the fish in the milk with the bay leaf for 10 minutes, turning it once. Allow to cool slightly. Drain and chop the fish and reserve the milk.
  2. Make the sauce. Beat the scallions, mint and lemon juice into the yogurt. Chill in the refrigerator.
  3. Melt the low fat margarine and fry the onion and pepper over medium heat for 3 minutes, stirring once or twice. Stir in the chili powder and cook for 1 minute. Stir in the flour. Pour on the reserved milk, stirring constantly until the sauce boils. Simmer for 3 minutes. Beat the sauce thoroughly.
  4. Remove from the heat, beat in the fish and season with salt. Beat in half the egg. Leave the mixture to cool, then shape into 8 flat cakes. Mix together the breadcrumbs and the finely chopped peanuts.
  5. Beat the remaining egg with the tablespoon of milk. Dip the fish cakes in the egg mixture and then in the breadcrumbs and peanuts, to coat. Heat the oil in a non-stick skillet and fry the cakes for 3-4 minutes on each side.
  6. Garnish with a celery stalk and serve hot with the chilled sauce.

Makes 4 servings.

Source: Cooking Naturally


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