There are three modes of heat transfer: radiation, conduction and convection. All three heat transfer modes occur during biscuit baking. However they differ in importance and effect on the product, and their application depends on the oven design.
( Featured image Ashworth CB5 belt, Biscuit Baking Technology, 2nd Edition )
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 (other than a perfect “black body”), they are partially absorbed and partially reflected. Good emitters are also good absorbers of thermal radiation.
Electromagnetic waves vary in wavelength from infinitely short to infinitely long. They cover a spectrum from gamma rays (around 10-14 metres) to radio waves (around 102 metres). Visible light is a form of electromagnetic radiation with wavelengths of 0.40 – 0.71 micrometres (microns). Infrared radiation, which is important in baking, has wavelengths above visible light from 0.71 – 1.5 micrometres (near infrared) and from 1.5 – 4.0 micrometres (far infrared). Microwave baking uses longer wavelengths, normally around 12cm.
Thermal radiation is transferred from objects of a higher temperature to objects of a lower temperature. The electromagnetic radiation emitted by an object is directly related to its temperature. If the object is a perfect emitter (a black body), the amount of radiation given off is proportional to the 4th power of its temperature as measured in Kelvin units. (Kelvin units =Centigrade temperature + 273.15). For objects other than perfect black bodies, the power of radiation P is also proportional to the object’s emissivity e. This relationship is described by the Stephan-Boltzmann Law:
P = e x sigma x T4
where sigma = 5.67 x 10-8W / m2K4 and T is the temperature in Kelvin
Since the radiation is proportional to the temperature to the power of 4, a small increase in temperature will produce a very large increase in thermal radiation. The amount of radiation increases exponentially with a linear rise in temperature.
Most radiant energy emitted in a baking oven is within the infrared spectrum and most of the energy emission is spread over a relatively narrow spectrum. The maximum emission is inversely proportional to its temperature. A higher temperature produces a shorter wavelength. The relationship is known as Wien’s Law and is described by the following equation:
Maximum wavelength = C/T
where C is a constant equal to 2897 and T is the temperature in Kelvin units.
If we consider a Direct Gas Fired oven, the flame temperature will be approximately 1960oC when burning natural gas. The wavelength will be 2897(1960+273.15) = 1.3 micrometres. This wavelength (NIR) is close to the optimum penetration of biscuit doughs and will ensure excellent development of the structure and volume of the biscuits.
For an Indirect Radiant oven with the hot tubes or ducts at 3500C, the wavelength will be: 2897/(350 + 273.15) = 4.65 micrometres. This is a typical wavelength in an Indirect Radiant oven and it is in the infrared spectrum.
Finally the thermal energy radiated to an object is dependent on the object’s distance from the source of the radiation. The intensity of the thermal radiation received is inversely proportional to the object’s distance from radiating surface, (inverse square law).
Intensity = I/d2
where I is the intensity of radiation at the object’s surface and d is the distance from the radiating surface. For example if the distance is halved from 20 units to 10, the intensity will be multiplied by 4.
At 20 units distance: Intensity = 100/202 = 0.25
At 10 units distance: Intensity = 100/102 = 1.00
This is a very important factor in the design of an oven. The nearer the radiating surfaces are to the product, the more intense the thermal radiation will be and the more efficient the heat transfer.
Effect of radiation on the dough pieces
When infrared energy impacts an object, such as a dough piece, its primary effect is to set molecules in vibration. The temperature of the dough piece is a measure of the average kinetic energy of its molecules, which increases due to the radiation from the oven surfaces in the baking chamber.
Objects such as dough pieces will absorb part of the infrared energy impacting on them and will reflect part. The energy absorbed penetrates the dough pieces and rapidly heats the centre of the dough pieces as well as the surface. This is an important attribute of radiation. This attribute is readily seen in microwave baking where the centre of a cookie is baked more quickly than the outer surface.
The Law of Heat Conduction is known as Fourier’s law and this is shown in the equation below.
Q = – k T
The heat by conduction, Q , is proportional to the thermal conductivity k, multiplied by the negative temperature gradient (difference in the higher and lower temperatures) – T
Thermal conductivity k is the property of a material that indicates its ability to conduct heat. The thermal conductivity predicts the power loss in watts through a piece of material. This is important in calculating the optimum insulation required for the oven.
The conduction of thermal energy to the dough pieces during baking process is most relevant to baking on a solid steel band or a heavy mesh band such as a CB5 type. In these cases the dough pieces are deposited directly onto a hot oven band and the heat is conducted rapidly into the base of the dough pieces. Heavy mesh oven bands are normally pre-heated to a high temperature, 120oC – 150oC.
Solid steel bands and CB5 bands have a relatively large heat mass and a large area in contact with the dough pieces. The conduction of heat to the base of the dough piece is optimised and contributes significantly to the total heat transfer. It is ideal for products such as soda crackers requiring high bottom heat to achieve good volume and an open texture. Conduction is also of major importance in the baking of soft cookies and cake on steel bands; the band conducts heat directly into the base of the cookies causing the fats to melt and the cookie dough to flow on the band to its final shape.
Materials such as mineral wool, normally used for oven insulation, have a low thermal conductivity. Mineral wool is manufactured from molten rock, stone, glass or slag, which is spun into fibres. The mineral wool is supplied loose or compacted into mattresses. The mineral wool typically has a thermal conductivity, k, measured in watts per metre kelvin of 0.06 – 0.10 W/(m.K) at baking temperatures and is used at densities of at least 60 kg/m3.
Convection is described by Newton’s Law of Cooling, which states that “the rate of heat transfer is proportional to the differences in temperatures between the body and its surroundings".
The term convection is used in the biscuit industry to describe a heat transfer system employing jets of hot air blown directly on to the surface of the dough piece and the oven band from above and below. This hot air rapidly removes moisture from the surface of the dough pieces and increases the temperature of the surface of the dough pieces.
The convection system is very efficient at removing moisture during the middle part of the baking process. The hot, moving air constantly impinges on the surface of the dough pieces, rapidly evaporating moisture and removing it from the baking chamber through the extraction system. The convection baking system is usually controlled by altering the temperature of the air which is blown by the circulating fan into the baking chamber at a constant speed and volume.
It will be seen that this “convection” system only affects the surface of the dough piece and primarily the top surface as the bottom of the dough piece is fully or partly shielded by the oven band. In the case of a steel band or heavy mesh band, the heat transfer to the base of the dough piece is solely by conduction.
The effect of the impingement of the hot air jets on the surface of the dough pieces is to dry the surface rapidly and this will form a dry, hard skin which will then rise in temperature and increase in colour. Since a large volume of hot air is being blown evenly across the dough pieces, the colour will tend to be even and will not show highlights and patterns as achieved by a radiant heat transfer system.
The rapid drying of the surface of the dough pieces forms a hard skin and prevents the expansion and “lift” of the dough pieces so that this system is not used in the first part of the oven when baking crackers, most biscuits and cookies. In the first one third of the oven length, convection should be at a minimum to prevent the skinning of the dough pieces and allow the texture, volume and shape of the products to develop.
Radiation is the most important method of heat transfer for biscuit baking. It occurs mainly by electromagnetic radiation of infrared wavelengths from direct gas burners, the hot surfaces of the baking chamber and tubes or ducts carrying hot gases from the burners. This radiant heat is penetrative and efficient and occurs without adverse side effects, such as the rapid drying or skinning of the surface of the dough pieces.
We have noted that the radiation (transfer of thermal energy) is dependent on several factors:
1. Baking: 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 of the band in contact with the dough piece. With steel bands and heavy mesh bands this approximates to full contact. 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. Conduction is also important in baking soft doughs on steel bands.
2. Oven insulation: Heat from the baking chamber is conducted to the oven outer covers and contributes to the heat loss to the bakery. Insulation material with a low thermal conductivity is used to reduce this heat loss.
1. 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 efficiently 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.
2. The convection of air in the baking chamber, where hotter air will rise must be taken into account when controlling oven top and bottom temperatures independently.
3. Convection baking is favoured by oven manufacturers as it is relatively low cost to build and has a very simple control system. The burner power (total heat input to the baking chamber) is the main baking control in each zone. Most convection ovens have a fixed speed circulating fan and simple dampers to adjust the percentage of top to bottom airflow.
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