Why Do You Let a Steak Rest After Cooking?
― Even After the Heat Is Off, the Centre Keeps Getting Hotter
A cook doesn't cut a steak the moment it's done. They cover it with foil and leave it for a few minutes. This isn't to cool it down. Even after the meat leaves the heat, warmth is still moving inside it, and the centre keeps cooking a little.
You've cooked a steak at home. The surface is nicely browned, and it feels just right when you press it with a finger. You can't wait, so you cut into it straight away.
A red liquid spreads across the board. The centre of the cut is redder than you expected.
Yet in a restaurant kitchen, setting cooked meat aside to "rest" is the normal thing to do. Professionals take it off the heat a little before it's done. What happens in those few minutes?
There are two reasons to rest meat
Right after cooking, only the outside of the meat is hot and the centre is still lukewarm. Even with the heat off, warmth keeps flowing from hot to cold. So the centre keeps getting hotter for a while.
Right after cooking, the outer layer is very hot, and the water in it moves freely. Once the outside cools a little, less juice is thought to run out of the cut surface.
Both come from the same fact: a dish isn't finished the instant it leaves the heat. Let's take them in order.
With the heat off, warmth keeps moving inside
When you pan-fry meat, heat seeps from the surface inward bit by bit. Meat is full of water and passes heat on very slowly. So the moment it's cooked, the area near the surface can be close to 100℃ while the centre is in the 40s.
Take the meat off the heat, and the surface warmth escapes in two directions: out into the air, and in toward the middle of the meat. The air around meat is poor at carrying heat away, so most of it heads inward. The centre keeps getting hotter even though no flame touches it. Cooks call this "carryover cooking".
Look at Figure 1. It shows the temperature across a 2.5 cm steak, sliced through from the side. Just after cooking, only the two surfaces are hot, and the middle sags into a dip. Slide the control to lengthen the resting time, and the peaks at both ends crumble while the middle rises.
As the figure shows, the centre is hottest at around 5 minutes, and after that the whole piece slowly cools. How much it rises depends on the thickness of the meat and how it was cooked. A steak rises by a few degrees, and a large roast can rise by nearly 10℃.
That is why professionals take meat off the heat a little before the target doneness. They count on the resting time to carry it to just the right point. If you stop at exactly the right doneness, it will be overcooked by the time you eat it.
Why does less juice run out?
When meat cooks, its fibres shrink in the heat and squeeze out the water inside. The outer layer is close to 100℃ right after cooking, and the water there moves freely. Cut at that moment, and the squeezed-out juice rushes out of the cut surface.
After a short rest, the outer layer cools. The temperature evens out across the meat, and the juices settle. In comparison tests by food writers, meat cut straight away is reported to have left more liquid on the board.
Still, juice and steam keep escaping slowly during the rest. Saying that "resting puts all the juice back" goes too far, and cooks and researchers still disagree about how much it really helps. The first reason, carryover heat reaching the centre, is the one we can be sure of.
The time for heat to reach the middle grows with the square of the thickness. Twice as thick means about four times as long. That is why a thin steak is rested for a few minutes, while a large roast such as roast beef is rested for 15 minutes or more. Thin slices hardly need resting at all.
A steak can be eaten with a red centre because, in a whole piece of meat, bacteria sit mainly on the surface. In minced meat and chicken, bacteria may have worked their way inside. The usual guideline is to heat the centre to 75℃ for at least 1 minute. Don't rely on carryover heat alone; cook them through properly.
Summary
We rest cooked meat because heat stored in the outer layer flows into the centre, so it keeps cooking for several minutes after the heat is off. Professionals allow for this and take the meat off a little early. On top of that, once the outside settles, less juice is thought to run out when you cut.
The moment the heat goes off is not the moment it's done.
Inside the meat, heat is doing the final finishing.
Why cooked meat smells so good is explained in "Why does grilled food smell so good?", and how heat makes proteins set is explained in "Why does a raw egg set when you boil it?". Whether the old saying that "searing at high heat first locks in the juices" is true is checked in "Is it true that searing meat on high heat seals in the juices?".
- Get a cooking thermometer, cook a thick piece of meat, and measure the centre temperature right after taking it off the heat.
- Leave the thermometer in, set the meat on a plate, and write down the temperature every minute.
- See after how many minutes it peaked and by how many degrees it rose. Compare a thick piece with a thin one and the difference is easy to see.
A baked potato or a thick Japanese omelette will show the same rise at the centre. Be very careful with hot pans and oil.
For those who want more ― terms, formulas and links to textbooksEach part is labelled by level, from middle-school science to university courses
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school physics
- High school+Advanced high-school material, or a textbook sidebar
- UniversityUniversity-level courses (heat transfer engineering, food engineering) not taught in high school
- ResearchNot yet taught as settled fact, even at university; researchers are still investigating
Middle schoolTerms: this phenomenon has names
- Heat conduction: heat travelling through a material from the hotter part to the colder part.
- Carryover cooking: cooking that continues to the centre from stored heat after the food leaves the heat. It is kitchen jargon.
- Thermal diffusivity: a measure of how fast a change in temperature spreads through a material. Meat is close to water, and hundreds of times lower than metal.
Middle schoolHigh schoolCheck it with a formula: how high does the centre rise?
Think of the hot outer layer and the lukewarm inner part mixing during the rest and approaching the same temperature. It is the same idea as mixing hot and cold water.
| In symbols | T ＝ ( m₁ × T₁ ＋ m₂ × T₂ ) ÷ ( m₁ ＋ m₂ ) |
| In words | Evened-out temperature ＝ (outer mass × outer temperature ＋ inner mass × inner temperature) ÷ total mass |
| Where it comes from | Conservation of heat. The heat lost by the outside equals the heat gained by the inside (every part of the meat is treated as having the same specific heat) |
| T | Temperature after evening out (℃) |
| m₁, T₁ | Mass (g) and average temperature (℃) of the hot outer layer |
| m₂, T₂ | Mass (g) and average temperature (℃) of the inner part |
| Mass of one steak (2.5 cm thick) | 250 g |
| Outer layer (5 mm from each side): mass and average temperature | 100 g, about 70℃ |
| Inner part: mass and average temperature | 150 g, about 47℃ |
| Centre temperature just after cooking | about 45℃ |
| Heat held by the outside | 100 × 70 ＝ 7000 |
| Heat held by the inside | 150 × 47 ＝ 7050 |
| Together | 7000 ＋ 7050 ＝ 14050 |
| Evened out over the total mass (℃) | 14050 ÷ 250 ＝ 56.2 |
| Upper limit of the centre's rise (℃) | 56.2 − 45 ＝ 11.2 |
If no heat escaped into the air, the centre would rise by at most about 11℃. In reality heat keeps leaking from the surface, so in the Figure 1 calculation the peak came at 5 minutes, with a rise of about 8℃. This fits the kitchen experience of "a few degrees to nearly 10℃".
| In symbols | t ≒ x² ÷ α |
| In words | Soak-in time ≒ distance travelled squared ÷ thermal diffusivity |
| Distance from the hot layer to the centre (m) | 0.0075 |
| Thermal diffusivity of meat (m²/s) | said to be about 0.00000014 |
| Distance squared | 0.0075 × 0.0075 ＝ 0.00005625 |
| Divide by diffusivity (s) | 0.00005625 ÷ 0.00000014 ≒ 402 |
| Convert to minutes | 402 ÷ 60 ≒ 6.7 |
The estimate is about 5 to 7 minutes, which matches the rule of thumb to "rest a steak for about 5 minutes". Because distance enters as a square, a 5 cm roast takes four times as long, close to 30 minutes.
High schoolHigh school+How heat flows and how temperature evens out
High schoolHeat flows from the hot side to the cold side wherever there is a temperature difference, and the amount that flows is proportional to the steepness of the temperature slope. Just after cooking, the slope near the surface is very steep. So the flow inward is largest right after the heat goes off, and the centre warms quickly in the first few minutes.
High school+The heat escaping from the surface into the air is proportional to the temperature difference between the surface and the air (Newton's law of cooling). Covering the meat with foil limits air movement and evaporation of moisture, so less heat escapes outward. More heat goes inward instead, and the carryover rise is larger.
UniversityThe heat equation and the Biot number
How temperature changes inside meat is described by the heat equation (diffusion equation). It says that the rate of change of temperature equals the thermal diffusivity times the curvature of the temperature profile (the second derivative). Thermal diffusivity is the thermal conductivity divided by the product of density and specific heat. Figure 1 solves this equation numerically by slicing the thickness into thin layers. At the surface, the Biot number, the ratio of how easily heat leaves to how easily it travels inside, matters. For meat in air the Biot number is small, so heat goes inward more readily than outward. The "time proportional to distance squared" in ③ comes from the nature of this equation.
📖 Derivations and further reading: Heat equation (Wikipedia, in Japanese) ／ Thermal diffusivity (Wikipedia, in Japanese)
ResearchWhat is still not clear
- How much does resting help the juices? Some experiments show less liquid runs out when you cut, while others argue that once you add what escapes during the rest, the difference is small.
- What holds the juice in? Explanations include proteins and gelatin dissolved in the juice thickening as it cools, and the fibres relaxing slightly, but which matters, and by how much, has not been sorted out.
- How to include evaporation in the calculation. Water evaporating from the surface takes away a lot of heat. Water also moves through the meat carrying heat with it, so simple heat conduction alone cannot predict the temperature accurately.
In other words, this article too is "an explanation of what is known so far". The juice story in particular may change with future experiments.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: how heat travels (conduction, convection, radiation) | Heat from the outside travels to the centre |
| High school | Physics: conservation of heat, specific heat | Calculating the evened-out temperature |
| High school+ | Newton's law of cooling | Foil reduces heat escaping outward |
| University | Heat transfer and food engineering (heat equation, Biot number) | Figure 1's numerical calculation, the time estimate |
| Research | Food science (water-holding capacity of meat) | How much juice stays in |
| ― | Link to daily life | Stopping before the target doneness; resting thick cuts longer |
- Harold McGee, On Food and Cooking (Japanese edition: マギー キッチンサイエンス, Kyoritsu Shuppan)
- J. Kenji López-Alt, The Food Lab: Better Home Cooking Through Science (W. W. Norton, 2015)
- The Japan Society of Mechanical Engineers (日本機械学会), Heat Transfer Engineering Data (伝熱工学資料) (Maruzen)
- Heat equation ― Wikipedia (in Japanese)
- Ministry of Health, Labour and Welfare (厚生労働省), "Information on heating meat (centre at 75℃ for at least 1 minute)"
※This article is a general-audience science explainer. The figures given are rough guides to help you understand the mechanism. How fast the centre warms depends on the type of meat, its thickness, how it was cooked and the room temperature. Follow the guidelines from bodies such as Japan's Ministry of Health, Labour and Welfare when cooking meat.