When dough pieces for crackers are cut the dough sheet is only 1.3 -2mm thick. Baking to create the structure, texture and volume of the biscuits can only be achieved by infrared radiation. Heating through convection and conduction from the oven band primarily affects the the outside surfaces of the products.
The radiation also reduces the moisture content in the centre of the biscuit, reducing “checking”, hairline cracks in biscuits caused by the moisture gradient between the centre and surface of the product. Infrared radiation is effective in colouring the products, producing good colour contrasts.
Biscuit doughs absorb infrared radiation at wavelengths over 2.5 microns through a change in the vibration of water molecules which causes temperature increase. This radiation is absorbed by the biscuit ingredients.
Food component Absorption wavelengths
Water 2.7 – 5.0 microns
Sugar 3.25 – 3.7 microns
Proteins 2.83 – 5.92 microns
Lipids, fats 4.4 – 5.76 microns
A model of the baking of biscuits in an indirect fired oven extrapolated to a band oven baking process indicated a heat transfer profile of 45% by radiation, 35% by forced convection and 20% by conduction. Another study of heat and mass transfer in industrial biscuit baking ovens found 69% of heat transfer by radiation, 28% by convection and 3% by conduction. Most studies revealed that radiation was the predominant mode of heat transfer, varying between 50% and 80%.
Crackers require an open, flaky structure and light crispy texture. This requires a high heat input in the first one third of the oven. The heat input is provided by radiation from the Direct Gas Fired oven zones and conduction from the pre-heated Compound balanced Weave band.
The first zones have minimum extraction and no convection. Humidity is important and the initial temperature rise of the dough pieces is faster with a moist atmosphere. In a Direct Gas Fired oven about 30% of the humidity is from the products of combustion. Steam may also be applied at the oven feed end.
The surface of the dough pieces must remain flexible to lift and achieve the required volume and thickness of the cracker.
Cracker doughs have a high percentage of water, approximately 20 - 25 % of the total dough weight. The moisture is removed in the middle and final zones of the oven.
The moisture content will be reduced to 1.5 – 2.5% of the product weight. This requires the evaporation of approximately 200 g of water for every kg of cracker baked. The evaporation of the water requires a high heat input. The cracker oven will have a high heat rating based on the radiant heat transfer and conduction from the pre-heated oven band
With infrared radiant heat transfer the moisture is removed from the centre of the dough pieces achieving a more even moisture content from the centre to the surface of the product. This is important in reducing “checking”, cracking of the biscuits after packaging.
Heat transfer in the middle and final zones of the oven can be enhanced by the turbulence system.
Radiant baking in the final oven zones is essential to achieving colour contrast for the crackers to show darker blisters and a pale background.
Direct Gas Fired Oven with turbulence and pre-heated Compound Balanced Weave band.
Baking time: 2.5 min
Oven set temperatures: 300/300/280/250
Baker Pacific Direct Gas Fired oven
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