Heat Transfer and Browning Foods

Yesterday, in one of my lazy moods, I succumbed to the microwave pizza dinner. Opening the box, I noticed the instructions mentioned placing the frozen pizza on a shiny, metallic disk but only if I used the microwave and not the oven. This got me thinking about the effect of the disk on the pizza and so I did a little research on the browning of foods.

But first, a look at cooking methods and an examination of how each method affects browning.

Cooking, at its simplest definition, is preparing food through the use of heat. The different methods of cooking or heat transfer are broken down into conduction, convection, and radiation. Every method of cooking involves one or more of these heat transfer methods.

Conduction

This is the exchange of thermal energy through direct contact between a heating element and the food. Different materials result in different heating times and temperatures. Please refer to the article “Equipment and Gear: Common Materials of Cookware” on CookingForEngineers.com for a complete breakdown of materials that directly affect conduction.

Pan-frying or sautéing are common forms of conduction. The pan heats up and, through direct contact with the food, cooks the food. Fat or oil used in the frying provides uniform contact with heat, lubrication to prevent sticking, and some flavor of its own. Oddly enough, cooking in oil is considered a dry technique because the oil acts more like a cooking material than anything else. The moisture in the food will still be contained because it will not mix with the oil surrounding it.

Convection

Whereas in conduction heat is transferred through direct contact, in convection, heat is transferred by the movement of molecules in either gas or liquid. The fast moving molecules of the convection medium collide with the slower molecules in the food and heat them up. Baking and roasting are common forms of convection cooking. The heating elements within the oven heat the air and that comes in contact with the food. Boiling and steaming are also forms of convection with water or steam acting as the convection fluid. In deep-frying, the oil envelops the food, like a fluid pan that completely encases the food and heats the surface evenly.

Convection relies much on the density of the fluid. Liquid convection, either through boiling, steaming, or deep frying, is a much more effective transfer of heat than gas convection. This is why boiling a potato is much faster than baking. The denser the fluid, the more often the molecules collide with the food and the fast the food heats up. Therefore in convection methods involving air such as baking, the temperatures must be much higher than in liquid convection. This is why you can stick your hand into a 500°F oven without burning yourself but you cannot stick your hand into a pot of boiling water at only 212°F.

Radiation

While conduction and convection are heating methods through molecule to molecule contact, radiation is the transfer of heat through waves of pure energy. Most of the heating energy comes from the infrared radiation below visible light. When you hold your hand near glowing coals or a stovetop burner, the heat you feel is infrared. Technically, everything emits thermal radiation including you and me, and so every cooking method has an element of radiation.

Grilling and broiling, the former with heat below the food, and the latter with heat above, are two methods of radiation cooking. Of course there is convection from the air in between the heat source and the food and conduction from the grate, but the heat is primarily radiated.

Microwaves are below infrared waves on the spectrum and so carry much less energy. Infrared waves have enough energy to heat up almost all types of molecules, but microwaves tend to only heat up polar molecules such as water, sugar, and fats. Foods containing water are heated by these microwaves which penetrate about an inch into the food’s surface. The interior of the food is still heating by conduction of the heat from the surface into the interior.

Cooking Method: Grilling/Broiling
Heating Method: Primarily radiation from heat source, secondarily conduction from grate and convection of air between food and heat
Wet/Dry: Dry
Browning?: Yes

Cooking Method: Baking/Roasting
Heating Method: Primarily convection of air, secondarily radiation from oven walls and conduction from baking pan
Wet/Dry: Dry
Browning?: Yes

Cooking Method: Boiling
Heating Method: Convection
Wet/Dry: Wet
Browning?: No

Cooking Method: Steaming
Heating Method: Convection of steam and condensation of vapor
Wet/Dry: Wet
Browning?: No

Cooking Method: Pan-frying/Sautéing
Heating Method: Conduction of pan and oil
Wet/Dry: Dry
Browning?: Yes


Cooking Method: Deep Frying
Heating Method: Convection of oil
Wet/Dry: Dry
Browning?: Yes


Cooking Method: Microwave
Heating Method: Radiation
Wet/Dry: Dry
Browning?: No

The Browning Reactions: Caramelization and the Maillard Reaction

Heating foods intensifies flavors already latent within the foods; however, browning creates new flavors that are intrinsic to the cooking process. This is why a poached salmon and a grilled salmon both tastes identifiably like salmon, but you can also easily distinguish one food as poached and the other as grilled. There is flavor within the cooking method itself created by caramelization and the Maillard Reaction.

Caramelization

We have all had caramel candies before, but how many of us realize that those sugary delights are not much more than sugar itself. The caramelization of sugar is the simplest browning reaction happening at around 330°F/165°C. Plain table sugar melts into a thick syrup, then gradually darkens into a light yellow and eventually a dark brown. The flavor begins sweet and clean, but develops acidity, bitterness, and a rich aroma. The chemical process itself is complicated, but the reaction products include organic acids, sweet and bitter derivatives, fragrant molecules, and brown polymers.

The Maillard Reaction

Named for Louis Camille Maillard, the French physician who documented these complex reactions around 1910, Mailliard Reactions are responsible for bread crusts, chocolate, coffee, dark beers, and roasted meats. The sequence begins at about 220°F/115°C when a carbohydrate molecule and an amino acid bind together in an unstable structure, producing flavorful by-products. The involvement of amino acids brings nitrogen and sulfur creating meaty and earthy flavors. These reactions create that crust on seared foods and the brown coloring of a good roast as well as multitudes of other browned foods.

Both caramelization and the Maillard Reaction require relatively high temperatures beginning above the boiling point of water 212°F/100°C. As a result, wet processes such as boiling and steaming will never be able to brown foods because the temperature of the food will only get as high as the 212°F with slight adjustment due to elevation and atmospheric conditions. Dry methods are able to reach much higher temperatures allowing the browning reactions to occur. This is why braised foods are usually seared first to create those flavors and colors that otherwise wont occur in a wet, low temperature setting.

There are notable exceptions to browning above the boiling point. Basic solutions, concentrated mixtures of carbohydrates and amino acids, and long cooking times can create the same reaction. Examples include reductions of stock to create demiglace and brewing beer.

Back to my microwave pizza. Metal placed in microwaves usually creates dangerous sparking through the buildup of electric fields. Very small amounts of metal however can be heated without creating a danger and when this metal is heated, it reaches temperatures far beyond the boiling point of water. This is the function of the metallic disk with my pizza. The disk is placed underneath the crust and so when it is heated by the microwaves, it subsequently heats the crust through conduction at temperatures high enough for Maillard Reactions to occur. This is how microwave pizza makers brown the crusts. The effect can also be seen in Hot Pocket brand stuffed sandwiches which utilize a microwave sleeve slipped around the Hot Pocket with similar metallic coating. The microwave sleeve heats up hot enough to brown the crust of the Hot Pocket.