Heat is transferred from a substance if it is exposed to something that has a lower temperature

Understanding how heat transfer can affect your cooking can be important in deciding the method used in cooking. Most food is cooked using one of several methods: baking, frying, boiling, steaming. Although new methods allow for combinations e.g., combi ovens.

What is Heat?

All matter is made up of molecules and atoms that are always in different types of motion (translation, rotational, vibrational). It is this motion of atoms and molecules that creates heat or thermal energy. The more motion the atoms or molecules have the more heat or thermal energy they will have.

What is temperature?

Temperature is an average value of energy for all the atoms and molecules in a given system. Temperature is independent of how much matter there is in the system. It is simply an average of the total energy in the system.

What are the differences between temperature and heat?

The units of temperature is degrees (Centigrade or Fahrenheit). Another primary difference is that energy can be transferred without the temperature of a substance changing. An example would be latent heat -- where ice water remains at the freezing point even as more energy is brought into the system to melt the remaining ice. In general however as heat energy increases, the temperature will increase. The more massive a body is the more heat must be added to increase it's temperature.

How is heat transferred?

Heat can travel from one place to another in three ways: Conduction, Convection and Radiation. Both conduction and convection require matter to transfer heat. As long as there is a temperature difference between two systems heat will always find a way to transfer from the higher to lower system.

What is the role of specific heat capacity on heat transfer?

Heat capacity is the ratio of the heat added to (or removed from) an object to the resulting temperature change. Water has a high specific heat which means it will take a long time to heat up. The high specific heat of is because water is from its tight bonding structure. 

What are the three fundamental modes of heat transfer?

CONDUCTION--

Conduction is the transfer of heat between substances that are in direct contact with each other. The better the conductor, the more rapidly heat will be transferred. Metal is a good conduction of heat. Conduction occurs when a substance is heated, particles will gain more energy, and vibrate more. These molecules then bump into nearby particles and transfer some of their energy to them. This then continues and passes the energy from the hot end down to the colder end of the substance. Conduction occurs when we put a steak onto a very hot cast iron pan. The better the pan is in holding the 

CONVECTION--

Convection occurs when warmer areas of a liquid or gas rise to cooler areas in the liquid or gas. Cooler liquid or gas then takes the place of the warmer areas which have risen higher. This results in a continuous circulation pattern. Water boiling in a pan is a good example of these convection currents.

A convection oven is an oven that has fans to circulate around the air. Conventional ovens, which do not have fans, rely primarily on radiation from the oven walls, and to a lesser extent, on natural convection caused by temperature differences. The fans in convection ovens allow more heat to be transferred via convective heat transfer.

RADIATION--

Radiation is a method of heat transfer that does not rely upon any contact between the heat source and the heated object as is the case with conduction and convection. Heat can be transmitted though empty space by thermal radiation often called infrared radiation. Examples of radiation is the heat from the sun, or heat released from the filament of a light bulb. In traditional ovens transfer of heat is via radiation. 

TYPES OF COOKING 

BOILING - is the method of cooking food in boiling water, or other water-based liquids such as stock or milk. Simmering is gentle boiling In poaching the cooking liquid moves but with minimal size of bubbles. Boiling relies on convection of liquids for transfer of heat.

BAKING- is a method of cooking food that uses prolonged dry heat, normally in an oven, but also in hot ashes, or on hot stones. The primary source of heat is radiation. Fans within the oven will increase cooking times via convection of the air.

BROILING- is cooking by exposing food directly to radiant heat. Broiling differs from roasting and baking in that the food is turned during the process so as to cook one side at a time.

STEAM--Steaming works by boiling water continuously, causing it to vaporize into steam; the steam then carries heat to the nearby food, thus cooking the food. The food is kept separate from the boiling water but has direct contact with the steam, resulting in a moist texture to the food.

PAN FRYING- Pan frying is a form of frying characterized by the use of minimal cooking oil or fat (compared to shallow frying or deep frying); typically using just enough oil to lubricate the pan. In pan frying or sauteing the intensity of the flame and type of pan used is critical.

DEEP FRY-- Cooking in oil has many advantages and is a fast method. Oil not only reaches much higher cooking temperatures than water but also it has a lower "specific heat capacity than water". This will allow it to heat up much quicker than water.

PRESSURE COOKING--Pressure cooking is the process of cooking food, using water or other cooking liquid, in a sealed vessel, known as a pressure cooker. Pressure cookers are used for cooking food faster than conventional cooking methods, which also saves energy. The increased pressure under a sealed environment allows for higher temperatures.

MICROWAVE COOKING-A microwave oven cooks food because the "water molecules inside it absorb the microwave radiation" and thereby heat up and heat the surrounding food. 

COMBI OVEN-- is an oven with three functions: convection, steam and combination cooking.Combi ovens in the convection mode, the oven circulates dry heat. The steam mode injects water into the oven to keep the food moist

SOUS VIDE- In sous vide cooking food is vacuum sealed in a plastic bag and then cooked in water that is a specific temperature. Heat is transferred from the water to food via conduction. Sous vide devices that have the ability to circulate the water (convection) are more effective. 

The heat source for our planet is the sun. Energy from the sun is transferred through space and through the earth's atmosphere to the earth's surface. Since this energy warms the earth's surface and atmosphere, some of it is or becomes heat energy. There are three ways heat is transferred into and through the atmosphere:

• radiation
• conduction
• convection

Radiation

If you have stood in front of a fireplace or near a campfire, you have felt the heat transfer known as radiation. The side of your body nearest the fire warms, while your other side remains unaffected by the heat. Although you are surrounded by air, the air has nothing to do with this transfer of heat. Heat lamps, that keep food warm, work in the same way. Radiation is the transfer of heat energy through space by electromagnetic radiation.

Most of the electromagnetic radiation that comes to the earth from the sun is invisible. Only a small portion comes as visible light. Light is made of waves of different frequencies. The frequency is the number of instances that a repeated event occurs, over a set time. In electromagnetic radiation, its frequency is the number of electromagnetic waves moving past a point each second.

Our brains interpret these different frequencies into colors, including red, orange, yellow, green, blue, indigo, and violet. When the eye views all these different colors at the same time, it is interpreted as white. Waves from the sun which we cannot see are infrared, which have lower frequencies than red, and ultraviolet, which have higher frequencies than violet light. [more on electromagnetic radiation] It is infrared radiation that produce the warm feeling on our bodies.

Most of the solar radiation is absorbed by the atmosphere and much of what reaches the earth's surface is radiated back into the atmosphere to become heat energy. Dark colored objects, such as asphalt, absorb radiant energy faster than light colored objects. However, they also radiate their energy faster than lighter colored objects.

Learning Lesson: Melts in your bag, not in your hand

Conduction

Conduction is the transfer of heat energy from one substance to another or within a substance. Have you ever left a metal spoon in a pot of soup being heated on a stove? After a short time, the handle of the spoon will become hot.

This is due to transfer of heat energy from molecule to molecule or from atom to atom. Also, when objects are welded together, the metal becomes hot (the orange-red glow) by the transfer of heat from an arc.

This is called conduction and is a very effective method of heat transfer in metals. However, air conducts heat poorly.

Convection

Convection is the transfer of heat energy in a fluid. This type of heating is most commonly seen in the kitchen with a boiling liquid.

Air in the atmosphere acts as a fluid. The sun's radiation strikes the ground, thus warming the rocks. As the rock's temperature rises due to conduction, heat energy is released into the atmosphere, forming a bubble of air which is warmer than the surrounding air. This bubble of air rises into the atmosphere. As it rises, the bubble cools with the heat contained in the bubble moving into the atmosphere.

As the hot air mass rises, the air is replaced by the surrounding cooler, more dense air, what we feel as wind. These movements of air masses can be small in a certain region, such as local cumulus clouds, or large cycles in the troposphere, covering large sections of the earth. Convection currents are responsible for many weather patterns in the troposphere.

It is not the heat you feel but ultraviolet radiation from the sun that causes sunburns that lead to skin cancer. The warmth of the sun does not lead to a sunburn.

From the American Academy of Dermatology, sunlight consists of two types of harmful rays that reach the earth - ultraviolet A (UVA) rays and ultraviolet B (UVB) rays. Overexposure to either can lead to skin cancer. In addition to causing skin cancer, here's what each of these rays do:

• UVA rays can prematurely age your skin, causing wrinkles and age spots, and can pass through window glass.
• UVB rays are the primary cause of sunburn and are blocked by window glass.

There is no safe way to tan. This includes radiation from artificial sources, such as tanning beds and sun lamps. Every time you tan, you damage your skin. As this damage builds, you speed up the aging of your skin and increase your risk for all types of skin cancer.

Even on cloudy days, ultraviolet radiation can pass through clouds and cause a sunburn if you remain outdoors long enough.