Integrierte Vermeidung und Verminderung der Umweltverschmutzung

Chapter 2
There are four types of ovens; direct heating, indirect heating, electric and infrared ovens. All
oven types can be batch or continuous in operation. In batch ovens, the walls and the base are
heated. In continuous ovens, radiators are located above, alongside and below the conveyor belt.
Batch ovens incur higher labour costs than continuous ovens. Another disadvantage can be the
non-uniformity in baking times, caused by the delay in loading and unloading the oven.
In direct heating ovens, air and combustion gases are recirculated by natural convection or by
fans around the product to be baked. The temperature in the oven is controlled by adjusting the
air and fuel flowrates to the burner. The fuels normally used are natural gas, propane and
butane. The gas is burned in ribbon burners above or sometimes below the conveyor belt and
product. The advantages of direct heating ovens are their short baking times, high thermal
efficiencies, rapid start-up and good temperature control. Good management and care is
necessary to prevent contamination of the food by undesirable products of combustion.
In indirect heating ovens, the air in the baking chamber is heated via a heat-exchanger, by steam
or by burning a fuel. The air in the baking chamber is typically recirculated through the baking
chamber and the heat-exchanger. Other methods include passing the combustion gases through
radiator tubes in the baking chamber, or burning the fuel between a double wall whilst
exhausting the combustion gases from the top of the oven.
Electric ovens are heated by induction, heating radiator plates or bars.
In infrared ovens for baking vegetables, the raw material is washed and sliced or diced. The
vegetable pieces are then flash blanched for 60 to 80 seconds, in 100 % saturated steam in a jet
stream oven at 200 to 300 °C. The steam is delivered to the oven at a rate of approximately
500 to 540 kg/h. The air speed in the oven is 17 – 25 m/s. The product is transferred next into a
second jet stream where it is steam cooked at 270 to 300 °C in 50 % saturated steam for
65 to 85 seconds, without changing the steam and air speed conditions. The product is then
transferred to an oven for infrared baking for 3.5 to 5 minutes. The air temperature in the oven
increases from 240 to 350 °C as the product travels through the oven on a conveyor belt. In this
process, the vegetables lose approximately 30 to 60 % of their water content, depending on the
vegetable.
2.1.5.5 Roasting (E.5)
2.1.5.5.1 Objective
The aim of the process is to cook the food to make it edible, to produce dry products and/or to
enhance the structure of raw products.
2.1.5.5.2 Field of application
Typical ingredients that are roasted are coffee, nuts, cacao, chicory, fruit, cereals and preprocessed
cereals.
2.1.5.5.3 Description of techniques, methods and equipment
The raw product is usually exposed to hot air at temperatures of over 100 °C. Sometimes the
raw product is pre-dried. If so, first the water is evaporated from the product to decrease the
moisture content of 8 to 20 % to less than 1 %. If the product reaches a sufficiently high
temperature, i.e. over 120 °C, reactions take place in the product. These so-called Maillard
reactions are important in the formation of aromas in coffee and cacao. The duration of this
roasting process depends on the product and the specific aromas that are required. Roasting
times for coffee range between 1 and 20 minutes, while for cacao and other products this can be
up to 180 minutes. When the product temperature reaches the required level, the Maillard
reactions are stopped by either cooling the product with air or by quenching the product with
water followed by cooling with air.
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