The Science of Temperature in the Kitchen
Great cooks go to great lengths to control the temperature of their productions. As Shizuo Tsuji tells it, one Tokyo restaurant built an entirely new kitchen on the second floor in the very center of the building, so that soup can reach all the diners in less than 30 seconds. But temperature control is much more than challenging your guests’ ability to elegantly sip hot soup without making too loud a slurping sound. As temperature changes, food goes through physical and chemical changes, which lead to changes in texture and flavor. The iron chef and average home cooks could be separated by a couple of degrees on either side of a critical temperature.
The physics of temperature
What is temperature and how do we measure it? In the early 18th century, a German scientist named Fahrenheit defined 0⁰F as the lowest temperature he could achieve with a mixed solution of water, ice, and salt, and 96⁰F as the temperature of a healthy adult. This definition is not precise and the math is unnecessarily cumbersome, which is why most of the world eventually adopted the Celsius scale. The Swedish scientist Celsius in 1742 defined 0⁰C as the boiling point of water and 100⁰C as the freezing point of water (The scale was reversed the next year). This definition is easier to replicate but is not complete. It’s only true under normal atmospheripressure. Scientists don’t like ambiguity, so in 1954 the anchor points of the Celsius scale were changed. On one side is absolute zero, a theoretical value reached by extrapolating the ideal gas law. On the other side is the triple point of water: The single combination of pressure and temperature at which pure water, pure ice, and pure water vapor can coexist in a stable equilibrium. The problem with this definition is that it’s difficult to prescisely and consistenly measured in labs.. In 2019, the International System of units underwent its biggest revolution since the meter–kilogram–second (MKS) system was established with the 1875 Convention du mètre. Instead of depending on human artifacts or measurements, the SI became wholly derivable from unchanging natural phenomena based on fundamental physical constants. Specifically, the new definition of Kelvin is calculated from the energy equivalent as given by the Boltzmann's equation. Here is the exact mathematical formula:
- k: the Boltzmann constant
- h: the Plank constant
- ΔνCs: the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom
This brings us to what temperature is. At a microscopic level, all atoms and molecules are moving all the time (except at absolute zero, which the third law of thermodynamics tells us is unreachable). The Austrian Physicist Boltzmann invented the science of statistical mechanics, which models a system in terms of the average behavior of the particles making up the system. Temperature is the macro indication of the micro-movements of atoms and molecules. Mathematically, the temperature is proportional to the square of the average speed of particles. These days this shouldn’t be too much of a stretch of the imagination, but Boltzmann's work was under constant attack in his lifetime. He became depressed and committed suicide just before experiments verified his theories.
Building blocks for food
To really understand what happens to food at different temperatures, we have to go down to the molecular level. There are four basic food molecules: water, fat, carbohydrate, and protein. The water molecule consists of one oxygen atom and two hydrogen atoms. An atom has a nucleus with electrons running circles around it. The orbits of electrons can not be pinpointed like the orbits of the earth around the sun. Scientists can only calculate the probability of where the election might be in the atom. One of the mathematical tools they use is the Schrödinger equation. I remember reading about it in my college physics textbook. The authors declined to show the equation because “the math is too advanced”. I swore if I ever wrote a book I would not leave my readers hanging like that. Here’s one special case of the Schrödinger equation:
Well, I guess it’s not totally a bad idea to leave it out.
Conceptually, you can imagine the region around the nucleus where the electron might be as a cloud. When atoms come together to make molecules, electrons from different atoms have to share their cloud spaces. Sort of like when two bubbles come together they both burst and form one new bubble. This new bubble is not the sum of the old bubbles. It’s a different bubble. Chemists call this interatomic linkage a covalent bond. The bonds of paired electrons have different shapes, depending on their energy level. Some look like a ball, some look like a dumbbell, and some look like cloverleaves. The way these bonds form decides the chemical and physical properties of the resulting compound.
It seems odd to bring up electron clouds and quantum mechanics in an article about cooking, but bear with me, the behavior of electrons is the ultimate reason of many culinary phenomena. Here is an example: when you eat a hot chili, what do you do to relieve the burning sensation in your mouth?
Hot is not a taste like sweet, sour, or Umami. Chilis contain a chemical called capsaicin, which triggers the same sensors in our mouth that sense pain and heat. Our normal reaction is to reach for ice water. But it doesn’t work because the root cause is not heat. Capsaicin does not dissolve in water and gets washed away, because the chemical bonds in capsaicin are formed differently than those in water (Technically one is non-polar and the other is polar. See “what every cook should know about water” about what polar water molecules mean). What works? Milk, for instance. Milk fat does a good job of dissolving capsaicin.
The other three food molecules, Fat, carbohydrate, and protein, are all organic compounds. Organic compounds are built upon a carbon backbone. Each carbon atom has 4 electrons on its outer layer. Each of these electrons wants to pair with an electron from another atom and form a bond. A carbon atom will not be happy until it has formed 4 bonds, because that’s the state where it has the lowest energy. The simplest organic compound is methane, where the carbon atom forms 4 bonds with 4 hydrogen atoms. Butane is the fuel in the blow torches commonly used by home cooks to make creme burlee. It has a chain of four carbon atoms.
It is tedious to draw all the little dots for electrons, and all the hydrogen atoms and carbon atoms for complicated organic molecules. A common way to simplify the drawing is to:
- Use a short line segment to represent a bond.
- Omit hydrogen atoms and all the hydrogen/carbon bonds.
- Stop drawing the carbon atoms. It’s represented by a bend and a black dot in the carbon chain.
Fat
Structurally, fat consists of three fatty acid molecules attached to one molecule of glycerol, hence the name triglyceride.
Triglyceride molecules are non-polar. That’s why the non-polar capsaicin dissolves in milk fat.
On the carbon back bond of the fatty acid, if two adjacent carbon atoms share only one pair of electrons, we say they have a single bond. If they share more than one pair of electrons, they have a double bond or triple bond. When they have more than one bond, the extra bond(s) could in theory break up and each side bonds to a Hydrogen atom, respectively. If all the bonds on the backbone are single bonds, the fatty acid is saturated. If only one bond is a double bond, it’s a mono-unsaturated fatty acid. If more than one bond is a double bond, then it’s a poly-unsaturated fatty acid.
The saturated fatty acids can be packed tightly together in space. But whenever there is a double bond, there is a bend in the chain of Carbon atoms. The reason is this: In a single bond, only one pair of electron clouds need to merge. But in a double bond, two pairs of electron clouds have to merge and coexist in the same general neighborhood between two nuclei. To make matters worse, these electron clouds have different shapes. This quantum game of Twister results in extra space between molecules of unsaturated fatty acids to accommodate the bend. That’s why molecules of unsaturated fat are farther apart from each other.
Because the molecules of saturated fats can be packed together tightly, there is greater intermolecular attraction. Therefore it takes more energy to separate them, hence higher melting points. Coconut oil is solid at room temperature, while olive oil is liquid, because a higher percentage of Coconut oil is saturated fat.
Cocoa butter, the fat in Chocolate, exists in one of six possible crystalline forms, depending on how its fatty acid chains are packed together. Only phase 5 and phase 6 have melting points between 28⁰C and 37⁰C. The art of making great chocolate is to precisely control the temperature profile of the process so that only the right crystals will form. That’s how M&M’s “melt in your mouth, not in your hand”.
In naturally occurring unsaturated fat, the hydrogen atoms on either side of a double bond exist on the same side of the bond, as in the left side of figure 4. When hydrogen atoms are forced onto the carbon backbone of vegetable oil to make Margarine, some double bonds are changed into single bonds. The remaining double bonds “flip”, so that the Hydrogen atoms are on opposite sides of the double bond, as in the right side of figure 4. Chemists use the word cis and trans, which in Latin means “this side of” and “the other side of”, respectively. That’s where the name trans-fat comes from.
Protein
Chemically, proteins are polymers (large molecules consisting of similar units bonding together) of amino acids. Each amino acid has an amino (NH₂) group. They bond into a long chain to form proteins. Figure 5 shows one of the 20 amino acids that make up proteins: glutamic acid. The sodium salt of glutamic acid is known as MSG (Monosodium glutamate).
As is obvious from the formula of amino acids, they have many polar O-H and N-H bonds. Different amino acids group among the protein chain thus form bonds among themselves, which causes proteins to fold into a delicate structure. When protein molecules are heated, these bonds are broken. The long chains unfold, exposing their reactive side groups. This is the process of protein denaturation. Now one part of one protein molecule might bump into another part of another protein molecule and form a new bond. Protein coagulation is the process where denatured proteins begin to bond with each other and form a gelatinous mass.
Muscle fibers are largely made of protein cells. The fickleness of protein under heat is why meat is more challenging to cook well than vegetables and grains. Let’s look at what happens to a piece of steak when the temperature rises.
At 50⁰C, the myosin proteins begin to denature and subsequently coagulate. Myosin is the protein that are responsible for muscle contraction in live animals. Lumps that scatter light are formed in muscle cells, and meat starts to appear opaquely white. At 60⁰C, the protein collagen, which forms the sheaf outside muscle fibers, begins to shrink and squeeze the muscle fibers. The water in the space between protein molecules is squeezed out and the meat suddenly loses a lot of juice. Around the same temperature, the meat color changes from red to tan, when the iron atom in the red myoglobin (another protein in muscle cells) loses an electron. The result is the tan-color hemichrome. For a tender cut like the rib eye, this is about the highest temperature you want it to reach.
But for tough cuts with a lot of connective tissues like the brisket and the oxtail, leaving the collagen alone won’t make it tender. You need to melt the collagen to get the “falling off the bone” tenderness. Collagen starts to dissolve into gelatin around 70⁰C. However, this process doesn’t happen instantly. Even when collagen is at the right temperature, it takes time for it to melt. That’s why good barbecue takes a long time. When all the collagen melts away, not only are meat fibers not bound together anymore, the liquid gelatin makes the meat appear moist and succulent.
In general, the best temperature to cook meat is related to the body temperature of the live animal. Cows have body temperature of around 38⁰C, so the ideal temperature to cook beef is about 20⁰C higher than that. Chicken’s body temperature is around 42⁰C. So it should be cooked to a higher temperature than beef. Many recipes say the chicken is done when the juice runs clear. The juice of raw chicken is pink/red due to the color of myoglobin. As temperature rises, myoglobin, like all proteins, denatures and changes its color. Exactly at what temperature this color changes and what color it changes to depend on several other factors. So the color of the juice is not an accurate gauge. The best way to check the temperature of your roast chicken is a thermometer.
Fishes are cold-blooded. They should be cooked at lower temperatures. Personally, I prefer fish cooked to an internal temperature around 50⁰C, or even lower. However, undercooked fish does represent a food safety risk, depending on how much you trust your fishmonger. For the record, meat should be cooked at 60⁰C for 12 minutes to kill all parasites, per the FDA food code. Food safety is a complicated topic that can not be reduced to a couple of numbers. The quests for safety and the perfect texture are sometimes incompatible goals. Here I would just point out that high temperature alone is not enough to kill bacteria. Food has to be held at a high temperature long enough.
Eggs are another important source of protein. Egg yolks turn solid at around 65⁰C, but a common misconception is that egg whites solidify at a lower temperature. If you cook an egg in a 65⁰C water bath, you will see the egg white is still watery while the egg yolk has solidified. That’s why the Japanese onsen-tamago( 温泉蛋) is the way it is. The science of cooking a perfect soft-boiled egg will be explored further here.
Carbohydrate
Between 60⁰C and 80⁰C is when starches begin to gelatinize. Starch is probably the most consumed carbohydrate in many people’s diets. Chemically. starch is a polysaccharide. Saccharide comes from a Greek word meaning “sugar”. The most simple sugars are glucose and fructose. They are monosaccharides. The molecule of table sugar (sucrose) consists of one glucose and one fructose, so it’s a disaccharide. If we have a small number (2 to about 8) of simple sugar monomers chained together, we call them oligosaccharides. More than that it’s a polysaccharide. How sweet a substance tastes depends on the shape of its molecules. You can think of the sugar molecules as keys that unlock the sweet taste cells. Fructose tastes sweeter than glucose, with the same calorie content. Chemically, high fructose corn syrup is very similar to honey. So nutritionally there is nothing inherently evil about high fructose corn syrup compared to cane sugar or honey. What’s evil is perhaps the corn subsidy that makes it so cheap.
Starch is not the only polysaccharide in our diet. Cellulose, another polysaccharide, is the main building block of plant cell walls. Due to the different ways the glucose monomers are linked together in cellulose (see figure 8), it can not be broken down by human enzymes, hence indigestible. But it does have nutritional value. One contribution it makes is interfering with the digestion of simple sugar. It could warp up the high-calorie food ingredients and escort them out of your digestive tract. Insoluble fibers are also food for your gut microbiome, which is the focus of a lot of active research. Plant cell walls begin to break down at temperatures between 88⁰C and 92⁰C. Vegetables become mushy if boiled too long. But if you are trying to make creamy mashed potatoes, that’s the internal temperature you need to get the potatoes up to.
A common starch in the kitchen is rice. Starch molecules come in two configurations: amylose and amylopectin. Amylose is a long straight chain while amylopectin is highly branched. Long grain rice typically has the highest percentage of amylose while the high amount of amylopectin makes short grain rice sticky, because the branches of amylopectin are very good at trapping water molecules.
Starches exist as granules in rice. These starch molecules in the granules form crystalline regions under ambient temperature, giving rise to the hardness of uncooked rice. At room temperature, water can not get in between the molecules. You can submerge rice in water forever, and it will not cook. Under the heat, the intermolecular bonds between starch molecules break down, allowing water molecules to come in and attach to the binding sites. The granules swell and dissolve in water. This is called gelatinization.
The gelatinization temperature depends on the exact type of starch. In the context of temperature control, the important points are:
- The right temperature has to be reached for gelatinization to happen. For instance, a technique in Chinese cuisine is to marinate protein in cornstarch. When you stir fry the meat, you need to let them come up to temperature so the starch gelatinizes before you start the stirring as in “stir fry”.
- Starches are commonly used as a thickener. Be careful not to add too much. All movements in liquid slow down at lower temperature. Your sauce will become thicker as it cools down between the time when it’s removed from the stove and when it’s served at the dining table.
When starch cools down, the dissolved starch molecules start to develop cross-links among them and rearrange themselves in crystalline structures. This is called retrogradation. It doesn’t return the starch to the uncooked state, but this process can not be completely undone by reheating either. That’s why the best fried rice is made with leftover rice. Without the heating/cooling cycle, rice does not attain the right texture for fried rice.
Other critical temperatures
Water boils at 100⁰C. It’s a useful and convenient visual cue for the cook to tell the temperature without a thermometer. However, a lot of time you are not cooking with just water. When you are cooking a very thick liquid like tomato sauce, normal convection is impeded. Different parts of the liquid can have significantly different temperatures. The top may be lukewarm while the bottom is scorched. That’s why you need to keep stirring.
If you add alcohol to water, the resulting mixture has a new boiling point between the boiling point of water and pure alcohol. It’s incorrect to say the alcohol boils off first. The vapor of this mixture contains both alcohol molecules and water molecules. At a temperature lower than the water’s boiling point, this vapor does have a higher percentage of alcohol, that’s why distillation works.
An obvious, but often forgotten fact is: anything cooked in water is cooked at a temperature under the water boiling point, 100⁰C. You can call it browning, searing, or stir-frying. But as long as there is water around, you are just boiling or steaming. This brings us to another important phenomenon for cooks: the wet bulb temperature.
If you put two thermometers in an oven, one by itself, and the other wrapped in a wet cloth, they will give different temperature readings. The dry thermometer will read the air temperature inside the oven, while the “wet bulb” thermometer will read a lower temperature. That’s because it takes heat to evaporate water. We feel warmer in a humid climate than in a dry climate because under higher relative humidity, less water evaporates from our skin, and we don’t get as much benefit from the cooling effect of evaporation. When food is not dried out, they are not cooked under the temperature of the medium, whether it’s hot air or hot oil (yes, fried food experiences wet bulb temperature too), but under the wet bulb temperature.
Many flavor-producing chemical reactions happen above 100⁰C. They will not take place until all the water has evaporated. The first of these reactions, and considered by many the most important, is the Maillard reactions. They are a series of reactions between amino acids and simple sugars. Maillard reactions do happen at lower temperatures. Traditionally the Chinese century eggs (皮蛋) are made by coating the egg in an alkaline mixture of wood ash, calcium oxide, and salt. The egg white and yolk undergo Maillard browning to arrive at their unique colors.
But you don’t want to wait a century. Maillard reactions accelerate around 120⁰C to 130⁰C. The results are hundreds of new compounds, with strong and pleasing flavors and aromas. Maillard reactions happen in almost all kinds of meats, resulting in slightly different aromas from their specific combination of amino acids and sugars. Due to their chemical structure, most products of Maillard reactions appear brown.
Maillard reaction is sometimes called the browning reaction, because the products of Maillard reactions have rings or a collection of rings in their molecular structure, which reflect light in such a way that makes them appear brown.
Exactly what happens during Maillard reactions is not completely understood yet. But as a cook, these are what you should know about Maillard Reactions:
- Maillard reactions do happen at lower temperatures, but it accelerates around 130⁰C. This means it can not happen fast enough in the presence of water.
- It’s a reaction between protein and simple sugar. You need both for it to happen. That’s why meats brown better with butter, which has both protein and sugar (lactose). Most meats do brown without added sugar though, because they contain the sugar glycogen. This is most notable in scallops.
- It happens faster in an alkaline environment. That’s why some bagle makers dip their dough in a lye solution before cooking them. And that’s why there is baking soda in the recipe for the caramelized carrot soup in Modernist Cuisine.
The Maillard reactions are not the only reactions that produce brown pigments, though. At 170⁰C, sugars begin to break down, resulting in a sweet nutty flavor and brown color. This is called caramelization, which is often confused with the Maillard reaction.
It’s tempting for a cook to always strive to induce the Maillard reactions and caramelization. However, I would urge you to think of ways to accentuate and present the unique flavor of the ingredients, instead of always falling back on the familiar and the commonplace.
Next at 186⁰C, sucrose melts. Some people take advantage of this fact to calibrate the temperature of their ovens. I have never seen an oven that can get to a temperature and stay there. Their temperatures fluctuate around the set point, and the temperature differences among different spots in the oven are significant even after a long preheating period (Immersion circulators do a much better job controlling the temperature of water baths). All times and temperatures in recipes that involve ovens are just suggestions. To control what comes out of the oven, I would only trust multiple thermometers and carefully designed experiments.
Around 190⁰C, olive oil starts to smoke. We don’t care so much about the boiling point of cooking oils, because their fat molecules decompose into other molecules at temperatures way below their boiling point. Smoke is the gaseous product of decomposition. We discussed earlier when atoms come together, their electrons pair up and the electron clouds merge. They prefer that arrangement because it puts them in a lower energy, more stable state. Some of the products of smoking are molecules with unpaired electrons. They feverishly look for other chemicals to react with so their single electrons can be in a stable relationship. These species are called free radicals. Free radicals are the opposite of antioxidants, which is to say they are not good for your health.
If you heat the pan to 200⁰C, and splash a few drops of water on it, they will slide along the hot surface like little balls instead of being vaporized. This is the Leidenfrost effect. The temperature of the hot surface is so much higher than the boiling point of water, that an insulating layer of vapor is formed upon initial contact. The remaining water in the droplet just rests on this air cushion like a hovercraft.
To avoid having the food stuck to the pan, you should wait until the pan is really hot before dropping the food. The sizzling sound you hear is the water on the surface of the food vaporizing, creating the steam cushion.
The highest temperature for most home ovens is around 260⁰C/500⁰F. But commercial pizza ovens routinely reach 500⁰C. You want to bake your pizza crust at a temperature of at least 315⁰C/600⁰F to get the most crispy yet fluffy crust. High heat for a limited amount of time is the secret ingredient of great pizzas.
Speaking of high heat, the temperature of the charcoal grill can be as high as 650⁰C/1200⁰F. In comparison, a propane grill can only get up to about half the peak temperature. According to the Stefan-Boltzmann law (yes, that Boltzmann), the radiated energy is proportional to the 4th power of the surface temperature. That’s why charcoal grills can produce better crust.
There is a lot more to be said about cooking and temperature. But hopefully this discussion has convinced you of the importance of precise and consistent temperature control. No amount of scientific knowledge can replace the experience and intuition about the state of food that professional cooks acquire from practice. Better tools need to be developed for home cooks to make up for some of that deficiency. In the meantime, frequent tasting during cooking helps, but be careful not to burn yourself.
Ideas for a better thermometer
One problem all recipe writers warn about is: time in the recipe is just a suggestion. The ingredients, your oven, the climate, etc, all make a difference. A lot of home cooks learn that the hard way, and a lot of recipes never achieve their full potential in home kitchens throughout the world.
In order to predict when food will reach the right temperature as we are cooking, we need to get the spacial and temporal distribution of temperature inside the food, so that we can numerically solve the heat equation (as discussed in Link: How to cook a perfect soft boiled egg).
Ideally in one thermal probe, temperatures at multiple points are measured so a model for finite element analysis can be built. Obviously, if the ingredient can not be reduced to a two dimension model, you want to have multiple probes inserted at different points so you can have a three dimension model.
The readings of these probes over time have to be stored. At least one of the probes should be able to sense humidity so the wet bulb temperature can be calculated.
All the calculations and predictions probably can not be, and should not be done on the probes. Besides, just to find out their positions relative to each other, the probes have to be able to communicate. Practically, all probes should have wireless links to each other and a base station. They will send data wirelessly to the base station, where finite element analysis is done and predictions based on trained models are carried out.
Enjoyed this article? 🌟 Help spread the knowledge by forwarding it to a friend and don’t forget to follow me on Medium for upcoming posts. Your support is much appreciated — thank you!
