Oven

An oven is the most important processing step in the baking industry for several reasons:

1. It is the workhorse of the bakery. The production output of a bakery is usually controlled by the capacity of the oven.²
2. Heat and mass transfer phenomena both take place simultaneously inside this piece of equipment, triggering physicochemical and biochemical changes in the product.
3. Baking in the oven is the step that imparts the final characteristics to the products (e.g., shelf life, flavor, texture, color, aroma).
4. It provides a crucial kill step that prevents pathogens from thriving within the product.

Origin

The fundamental oven model, with stone floor and dome structure, has been around for many years. It was the Egyptians who initially utilized a handcrafted oven cavity made from clay. The lower section formed the firebox in which pieces of dried wood and charcoal were burned. The upper section, accessible from the top, was the baking chamber.³

Several centuries later, the Roman Empire began to create more refined ovens with better baking quality and higher efficiency ovens. Brick and stone ovens were constructed inside the bakery’s premises, with thermostable materials for the baking chamber, and other high-temperature-resistant materials to insulate the oven and keep the warm environment for prolonged time frames. These ovens were powered by burning wood directly inside the heating chamber, removing the ashes, and then putting the dough/batter pieces inside for baking.³

Types of ovens

Depending on their mode of operation and heat transfer mechanism, ovens can be classified as either batch or continuous equipment and as using either direct or indirect heat exchange.

The type of oven that suits a bakery’s operation may be a function of production capacity, product specifications, floor space, available energy sources, operation efficiencies, construction materials, and maintenance needs.¹ The following are the common type of commercial ovens.

1. Direct-fired oven (DFO)

DFOs place combusting gas (energy source) inside the baking chamber to heat the air and the products. The heat transfer in a direct gas-fired oven is primarily carried out by radiation from the flames (ribbon burners placed above and below the oven band), top, base and walls of the baking chamber. Direct-fired ovens are very efficient because they convert most of the fuel to heat and process the products, and this lowers fuel consumption and operating costs.⁴

2. Indirect-fired oven (IFO)

IFOs indirectly heat the baking chamber by using exchangers. This oven is suitable for sensitive bakery products (e.g., cakes, pastries) since the byproducts of combustion remain inside the heat exchanger structure and do not come into direct contact with the dough pieces. This eliminates the risk of contamination and of impregnation of off-odors in the products.

This type of oven is less often used nowadays because of its limited power for heat transfer and energy efficiency (amount of fuel burned in a given time versus water loss (evaporated moisture) of the products during baking).⁴

3. Electric oven

Electric ovens have construction features similar to those of DFOs, and operate similarly in terms of heat transfer mechanism to bake the products. This type of oven uses electrical resistances in place of the traditional gas burners of DFOs.⁴

Electric-fired ovens have limited use in the baking industry due to their power consumption and costs per kWh. They also face scale-up challenges that require further research and industry application.

4. Peel brick oven

The peel brick oven was one of the first constructed baking units in human history. It consists of a massive brick material chamber. The chamber is connected to a refractory tile floor that holds the dough pieces. Coal and wood are used as fuel (combustion source).⁵

Because of their construction features (insulation capacity of materials and thickness of the walls), these ovens are able to steadily transfer radiant heat to the products, and also maintain high temperatures inside the baking chamber for prolonged periods of time. The ovens are operated manually and require special skills from the baker.⁵

5. Rack oven

A rack oven is a batch vertical oven into which racks full of sheet pans can be wheeled for baking. This unit can hold 8 to 20 sheet pans per baking cycle. Some units make use of electric or fuel sources, and place fans inside the baking chamber (generation of convection drying) to speed up baking times and to develop special features in the products.

This oven is suitable for retail operations due to its floor space economy, and medium to long baking cycle times. The products are baked upon customer order, and are often offered directly (unpackaged) for immediate consumption.² These ovens usually have programmable (saved) recipes so that the operator can change baking time and temperature, intensity of air ventilation, and steam impingement frequency.

6. Reel oven (also known as revolving tray oven)

A reel oven is an oven in which trays or shelves are placed on platforms rotating on a central horizontal axis. A high baking chamber is required to accommodate the reel structure, thereby saving floor space. Reel ovens are normally directly fired with gas or electricity, with the heating source located centrally across the floor of the chamber. This type of oven is mostly designed for retail bakeries or baking plants with small-scale production.⁶

Reel ovens often do not generate uniform distribution of heat transfer due to their revolving nature and interfering structure for radiant heat transfer. Products placed on sheet pans or trays continuously rotating may present uneven coloring or poor final moisture distribution.

7. Conveyorized oven (also known as traveling tray oven)

Conveyorized ovens replace the reel ovens concept with two parallel endless conveying chains that carry trays of products through the length of the baking chamber, so the dough pieces continually enter and leave the oven.⁵ Their main advantages are simplicity of design, and uniformity of baking as the products travel the same path through the baking chamber. A motor drive directly controls band speed, thereby determining baking cycle time.

Conveyorized ovens may be single-lap or double-lap. In single-lap ovens, the trays containing the products travel a single pass (back and forth). The trays in a double-lap oven travel through four heat zones instead of the two zones of the single-lap oven.⁵

8. Tunnel oven

Tunnel ovens are continuous mode operation baking units, and are commonly used in large-scale bakeries. This unit typically has a long baking chamber (usually more than 80 meters in length), which goes from one side (loading point) to another (unloading point) in a straight conveying band. The conveyor band material may be built of wire mesh or carbon steel sheets.

Tunnel ovens are commonly powered by fuels such as natural gas (used for baking), and electricity (for powering air circulation and conveying system). The baking chamber may be divided into several baking zones. This makes the application of a temperature sequence possible, which provides the baker more flexibility in baking conditions and more complexity for controlling baking parameters.⁷

9. Hybrid oven

Hybrid ovens combine the three modes of heat transfer and take advantage of their synergistic effect on products.⁴

This type of oven usually requires a high degree of automation since its construction, control systems, and energy sources are too complex to be manipulated manually.

Maintenance of ovens

Oven maintenance focuses on two major goals:

1. Prevent food safety hazards (physical, biological, and chemical) from occurring by reducing the likelihood of foreign material contamination, under-processing, and contamination with lubricants. These hazards may pose a food safety risk to customers, not to mention the loss of a good reputation and money.
2. Prevent mechanical, electrical, and thermal equipment failures that could negatively impact normal oven operation; and increase downtime, which could trigger significant economic losses.

Inspection and maintenance of equipment and calibration of measuring and instrumentation devices are vital for smooth oven operation. Special attention must be paid to: welded components, drive chains and belts, motors and drives, steam lines and fittings, air lines and fittings, seals and gaskets in piping, bearings, conveyor belts, bands, temperature indicators and controllers, in-line humidity meters and humidity exhaust systems, electrical control systems, and fans (axial or centrifugal).⁸

References

1. Manley, D. “Biscuit Baking.” Manley’s Technology of Biscuits, Crackers and Cookies, 4th ed., Woodhead Publishing Limited, 2011, p. 485.
2. Gisslen, W. “Baking and Pastry Equipment.” Professional Baking, 7th ed., John Wiley & Sons, Inc., 2017, p. 45.
3. Sheppard, R., and E. Newton. The Story of Bread. Routledge & Paul, 1957, pp. 107–120.
4. Davidson, I. “Oven Designs.” Biscuit Baking Technology: Processing and Engineering Manual, 2nd ed., Elsevier Inc. , 2016, pp. 73–90.
5. Stear, C. A. “Types of Oven and Oven Design.” Handbook of Breadmaking Technology, Elsevier Science Publishers LTD, 1990, pp. 602–791.
6. Walker, C. E. “Grain-Based Products and Their Processing: Oven Technologies.” Encyclopedia of Food Grains, 2nd ed., vol. 3, Elsevier Ltd., 2016, p. 328.
7. Conforti, F. D. “Industrial Preparation and Baking of Cakes.” Bakery Products Science and Technology, 2nd ed., John Wiley & Sons, Ltd, 2014, pp. 582–583.
8. Davidson, I. “Appendix 2: Oven Maintenance.” Biscuit Baking Technology: Processing and Engineering Manual, 2nd ed., Elsevier Inc. , 2016, pp. 302–315.