Technology continually improves nearly all aspects of our lives. This is especially true in kitchen appliances. One noteworthy advancement is the convection oven, which has surpassed the conventional oven as the appliance of choice in modern kitchens.

Convection ovens have been popular in Europe and in restaurants for years, but were introduced to the US residential market in 1978 by the Jenn-Air Corporation.

While more expensive than traditional ovens, these units offer tremendous advantages over non-convection models. They cook food in as little as half the time of conventional ovens, and do so at lower temperatures. As a result, foods stay moist and retain their nutrients.

Convection roasting is hard to beat for flavor, appearance, and tenderness. Restaurant meats such as prime rib are crusty and flavorful on the outside but still rich and juicy on the inside due to convection cooking. The crusty exterior seals the meat and traps the juices and flavor inside.

Convection occurs when another medium or fluid is used to convey the heat from the source to the food, as in water boiling in a pot, or hot air moving in an oven, which in turn heats the food.

In a convection oven, a heating element heats the air while a high speed fan blows it around the heating chamber, which speeds up the cooking process.

The Effects of Air Movement

As food is heated in an oven, an insulation barrier forms on the surface of the food, caused by the heat exchange between the hot air and the cold food and by the evaporation of moisture in the food. This insulation slows the cooking process.

A convection oven blows away this insulation layer and replaces it with fresh hot air, so the heat transfer is faster and more efficient. This process is best accomplished when the food is uncovered.

Another explanation of this process is that moving air speeds up the heat transfer that occurs when hot and cold air collide. In cold climates, this effect is called wind-chill factor. Air movement not only intensifies the feeling of coldness, but also makes it felt more quickly.

Still another illustration of this process is a fan blowing air over ice cubes; this causes them to melt faster by removing the layer of cool air surrounding them.

As in the above examples, convection cooking speeds up chemical reactions that occur when food is cooking. Baked goods release steam more quickly, roasted meats render fats more quickly, and sugars in vegetables caramelize faster.

How Heat is Distributed in Convection Ovens
In a conventional oven, air is warmest near the heat source. Food must be placed near the center of the oven because food placed on the bottom rack is more susceptible to overcooking or burning. Likewise, food placed near the top of the oven often overcooks because rising heat can accumulate there.

That is not the case in a convection oven. Convection ovens usually have both upper and lower heating elements to prevent hot spots. The internal temperature is more easily regulated because the fan constantly circulates the hot air.

A true convection oven will have a third heating element placed in the back of the oven near the fan to ensure that the fan will be blowing hot air. In some ovens, a baffle covers this element to draw air past it, into the fan, and back out to the oven. Ovens that lack this third element are apt to cook less evenly.

Various manufacturers call this configuration "true convection", "third-element convection", or "European convection" because it was first popularized there. Some add even more features to create new terminologies. For example, Dacor calls it "pure convection" because they add an air filtering system that traps odors and keeps them from being transferred from one food to another.

The shape of the food also affects convection cooking. Long, thin meats cook faster than bulky cuts of the same weight because more of their surface is exposed to moving hot air. The size of the pan can also be a factor. The same amount of food cooks faster in two small pans than it does in one large pan because two pans allow air to circulate more freely.

To allow for better air contact with the food, the cooking container should be open and not much higher than the food inside. For example, cookie sheets should be flat with no raised sides.

A convection oven of a given size will usually have a smaller internal capacity than a standard oven because of the space required for the fan and air handling equipment. But a convection oven will cook more food per cubic inch than a standard oven. Since the heat-circulating fan is not inside the oven cavity, the oven can be filled from top to bottom as long as an inch of space is left for the air to circulate between the food and the oven walls.

Most ovens are designed to let the user turn convection on and off as desired. Some allow the convection element to be used with just the broiler or just the baking element or with either of these heated to a lesser or greater temperature. Gaggenau has an oven with nine cooking combinations. Other manufacturers, including Thermador and GE offer ovens with a hidden radiant element in the bottom of the oven so that pizza and breads can be cooked directly on the heat source with convection too.

The Physics of Convection:
To understand the effects of heating and evaporation on food products during cooking, one must first understand the basic principles of heat transfer and mass transfer.

Heat is defined as the energy that is transferred as the result of temperature difference. Temperature difference is the driving force of heat flow. The greater the temperature difference, the faster the rate of heat flow.

During the cooking process, the temperature difference between the food surface and the core determines the rate of heat flow to the food core. The greater the surface-to-core temperature difference, the faster it will cook. Any change in the cooking process that raises surface temperature, will in turn affect the heating rate of the core.

The transfer coefficient includes such factors as the velocity of the air, its specific heat, specific weight, and thermal conductivity. These factors affect the turbulence generated at the food surface and the amount of heat that the air carries to it. Other factors are the shape and size of the food, its orientation to the moving air, and its proximity to adjacent objects and surfaces.

Convection heat transfer can be natural (free) or forced. Natural convection is the natural movement of air due to density differences caused by temperature variants in the system. Forced convection uses fans to increase the heat transfer rate, which in turn raises the transfer coefficient.

Forced convection also is beneficial for mass transfer reasons. Mass transfer refers to the evaporation of moisture that occurs at the food surface. Moisture migration within the product interior is dependent on the food's temperature, its composition, moisture concentration, and water-holding capacity. Surface evaporation depends on the relative humidity of the air surrounding the surface of the food.

If moisture is evaporating from the surface into the air stream, the air would quickly become saturated if it didn't move on. Once it is saturated, evaporation ceases, no matter what the temperature differential is. Convection cooking is essentially a high temperature drying process. When foods are cooked in a convection oven, heat is transferred from the air to the product, while at the same time moisture evaporates from the product into the air. This process is known as simultaneous heat and mass transfer.

Effects of Convection Cooking on Food
Meat, poultry, fish, and seafood all cook extremely well by convection. They all cook faster, nicely crusted, browned, and juicier than they would in a microwave. In fact, several different types of foods can be cooked in different pans on different racks at the same time and will still cook evenly without transferring tastes. Foods can even be reheated in a convection oven and still turn out as well as they did the first time around. Foods lose less moisture and nutrients as well.

Cooking pans and food can be placed right up to the walls of the oven without reducing air flow or slowing cooking time, and the food will still cook evenly.

For the same reason that pies, breads, and cookies are allowed to cool on racks that allow air circulation, pastries cooked in convection ovens have flakier crusts and lighter textures.

Cooking with Convection Ovens:
Although the same recipes will work on a convection oven as in a standard oven, there are some guidelines that should be followed for best results:

When baking, oven temperature should be reduced by 25° F. Times will be approximately the same to a few minutes less than the recipe or package recommends. Preheating is still recommended if the recipe calls for it. Alternatively, temperatures can be lowered slightly, and cooking times reduced proportionally.

Example: A recipe calling for a cake to be baked at 350 degrees for 30 minutes should be cooked at 320-325 degrees for 30 minutes, or alternatively 340 degrees for 25 minutes.

If the original conventional recipe baking time is less than 15 minutes, this time should be kept the same but the temperature should be reduced by 25-30 degrees F.

When roasting, temperatures stay the same, but the cooking time should be reduced. The time savings will depend on the size of food being prepared. Pans with high sides should not be used. The sides interfere with optimal air circulation and can decrease browning. (Preheating is not necessary for roasting.)

Example: A recipe calling for a roast to be baked at 400 degrees for 50 minutes should be cooked at 400 degrees for approx. 30-35 minutes.

As with any recipe, the food should be checked about 5-10 minutes before the expected finish time, due to variables such as the temperature of the food when first placed in the oven, quantity of food being cooked, individual preference, etc. The more food in the oven at one time, the longer the cooking time.

To maintain consistent air flow, the oven door must be kept tightly closed while it's in operation.

Note that convection cooking only works when the moving air comes in direct contact with the food. Meats cooked in a close roaster or casseroles will not cook much differently in a convection oven than they do in a standard model.

Shiny bright pans reflect heat, resulting in more delicate browning. Cookies should be baked on shiny, metal pans without sides. If multiple racks are used, one may be placed directly over another. This centers the cookies in front of the fan, which allows for more even browning.

When baking cakes, pies or other foods in pans that do have sides, pans should be staggered so that one pan is not directly above another pan. This promotes more even air flow and more even browning.

For multiple-rack baking, additional baking time may be needed for items on the middle rack. A rule of thumb for the middle rack is 30-60 additional seconds of baking time for thin foods such as cookies and 1 - 2 more minutes of additional baking time for thicker items such as biscuits.

Dark or dull pans absorb heat, resulting in darker browning. These pans should be used only when dark, crisp crusts are desired. Pans should be placed at least one inch apart and the same between the edges of the pan(s) and the oven walls.

The choice of when to use a convection oven and when to use a conventional oven with radiant heat often depends on the cooking container rather than the food. The whole point of convection cooking is to have the fan circulating the heat around the food.

• Cookie sheets - use convection oven
  
• Shallow pans (inch sides or so) - use convection oven
  
• Anything covered (including roasting bags) - use conventional oven
  
• Deep roasting pans - use conventional oven
  

Countertop convection ovens are extremely popular small appliances, but they're limited in size and features, and they also take up valuable counter space. They won't hold a large turkey or a sheet of cookies. But the same cooking method is available in full sized ranges and wall ovens. The most critical element of their design is size. If space is limited, a unit must be selected that will fit the space available.

Ranges
Gas, electric or dual fuel ranges with convection ovens are available in 30", 48" and 60" widths. Ranges can be equipped with any combination of ovens that a manufacturer offers.

Typically a 30" and 36" range will have a single oven; a 48" convection range will have two ovens (one larger and one smaller), typically with one convection and one non-convection oven; and a 60" convection range generally has two convection ovens.

Built-In Wall Ovens

Wall ovens can be recessed into the wall, housed in specialty tall oven cabinets, or installed in abase cabinet below a cooktop.

Wall ovens are available in widths from 24" to 36", and in single, double or triple (with microwave) configurations.

Some single ovens, often called convenience ovens offer a combination of a microwave and convection oven, and can be installed above the countertop anywhere in the house where quick food preparation is needed .

Multiple units can include any combination of conventional, convection, microwave, and warming drawer ovens.

Note: Not all configurations or sizes are available from all manufacturers

Many convection ovens have shallower interiors than their conventional counterparts to make room for the fan and air handlers. This should be considered when making a selection. Some additional features offered by various manufacturers are:

• Self-cleaning
  
• Computerized control panel with intuitive finger-touch keypads
  
• Stay cool exterior surfaces
  
• Adjustable racks that slide out very easily on rollers.
  
• Infrared broiler
  
• Choice of colors, e.g. stainless steel, black on black, white on white, and bisque
  
• Porcelain intertiors
  
• Displays that show interior temperature
  
• Timed and Delayed bake feature to control cooking cycle.
  
• Removable oven door for easy cleaning
  
• Pre-heat alarms that sound an alert when temperature has been reached
  
• Auto Convection Conversion automatically converts cook time and/or temperature
  

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