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Baking by infrared radiation

Three modes of heat transfer are used in baking biscuits: radiation, conduction and convection. The most important is infrared radiation, which has the following advantages:

Penetrative heat transfer: Infrared radiation penetrates biscuit dough by approximately 4mm, (depending on wavelength and moisture content). It is the only heat transfer mode to truly bake the product from the centre. This is the key advantage of baking by infrared radiation.

Biscuit structure: because radiation penetrates the dough pieces, it is essential to achieving good structure with optimum volume and texture and is always the main mode of heat transfer in the first part of the baking process

Even moisture content: radiant baking ensures a low moisture gradient from centre to the outer surface of the biscuit. It is the best heat transfer mode to avoid “checking” (cracks appearing in the biscuit after baking, due to the moisture gradient).

Efficiency: heating of the surrounding air in the baking chamber is not necessary, which lowers energy consumption.

Colouring: radiation enables highlighted colour contrasts for crackers and rotary moulded products, whereas convection gives an overall, bland, even colour.

Versatility: infrared baking is suitable for all types of biscuit

baking by infrared radiation


Radiation

All objects above a temperature of absolute zero radiate energy to their surroundings. This energy or radiation is emitted as electromagnetic waves which travel at the speed of light. The waves may travel through a vacuum or other medium. When they impact an object, they are partially absorbed and partially reflected. Good emitters are also good absorbers of thermal radiation.

Infra-red radiation is in the wavelength band of 0.7-300 microns (above visible light). Higher temperatures produce shorter wave lengths. Typical wavelengths in a radiant oven are around 4.6 – 6.4 microns, which provides good heat penetration of the dough pieces.

Radiation spectrum
Infrared radiation for baking is emitted principally by the DGF burner flames and by the radiant tubes in an Indirect Radiant oven.

Since the radiant energy is proportional to the temperature to the power of 4, a small increase in temperature will give a large increase in radiation. This contributes to the fast response of DGF ovens and high efficiency.

Direct Gas Fired oven bakes mainly by radiation from the burner flames
Direct Gas Fired oven bakes mainly by radiation from the burner flames
 
Indirect Radiant oven zone: baking by infrared radiation from the radiant tubes above and below the oven band 
Indirect Radiant oven zone: baking by infrared radiation from
the radiant tubes above and below the oven band 


Conduction 

Conduction transfers heat from the oven band directly to the base of the dough pieces. The heat transfer is dependent on the temperature and heat mass of the oven band and the surface area of the band in contact with the dough piece. With steel bands and heavy mesh bands this approximates to full contact and is very effective.
Ovens with band pre-heat can quickly transfer heat into the base of the dough pieces and achieve rapid development of the biscuit structure and texture; this is particularly valuable for cracker baking.
 
pre-heat burners 
Pre-heat gas burners raise the temperature of the oven band to around 150°C 


Convection 

Convection baking uses hot air jets which impinge directly on the top of the dough pieces and the underside of the oven band. This system effectively dries and colours the surface of the dough pieces. However it produces a hard, dry skin on the dough pieces and will prevent good expansion and “lift” of the product if used at the start of the baking process. Also it is a cause of a moisture gradient between the surface (very dry) and the centre of the biscuit (more moist). This may result in “checking”, (cracking of biscuits after baking), unless the moisture gradient is reduced after baking.

Direct convection system
 

 

Baking by infrared radiation: Direct Gas Fired Ovens

DGF ovens bake with infrared radiation. High rate burners enable rapid baking of crackers with excellent “lift”, open and flaky texture. For cracker baking pre-heat burners also increase conduction to the dough pieces in the first oven zone. 
 

Baking by infrared: Indirect Radiant oven 


Indirect ovens provide fuel security as gas or oil can be used. The burners fire into a heat exchanger and the hot air is circulated through radiant tubes above and below the baking band. Baking is by infrared radiation from the hot radiant tubes above and below the baking band. The heating system is essentially a closed system and the energy required from the burner is only sufficient to maintain the baking temperature.The burner draws in air for combustion and the excess is exhausted by a natural flue. This hot air / burnt gas in the flue can be used in a heat recovery system and this ensures a high efficiency.
 

Ind radiant oven diag 


Oven efficiency

Heat Recovery System


All gas burners draw in a large amount of air for combustion. 1.0 m3 of gas requires 3.0 m3 of oxygen (approximately 15 m3 of air) for complete combustion. This air is exhausted through the extraction system of a DGF oven and through the natural draught burner flue of an Indirect oven.
The hot air and burnt gas in the burner flues of an Indirect Radiant oven is at high temperature, typically over 200°C and this hot air can be recovered and used for baking in a Heat Recovery System.
The hot air can be diverted from the flues to a collection pipe running along the top of the oven. The hot air is drawn down the length of the oven by a fan which blows the hot air into a final radiant zone. This zone does not require a burner and is heated by radiant ducts above and below the band.

Oven efficiency with Heat Recovery System


Independent tests were carried out on 3 ovens in the same factory producing identical rotary moulded biscuits with the same baking time. The tests measured the oven efficiency by calculating the energy usage (gas) in kWh (kilowatt hours) to produce one kilo of baked biscuit.

Oven efficiency table

The Baker Pacific Indirect Radiant oven was 18% more efficient than the DGF/cyclotherm oven and 6% more efficient than the DGF/convection oven. The savings in gas consumption per 8 hour shift (23 tonnes of biscuits) are approximately 212 m3 of gas compared to the DGF/cyclotherm and 62 m3 compared to the DGF/convection oven.

 

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