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Roasting
Objective
The aim of the process is to dry the product and to enhance its aroma, and/or to enhance the structure of raw products.Field of application
Typical products that are roasted are: coffee, cereals and pre-processed cereals, nuts, cacao, chicory, and fruits.Description of techniques, methods and equipment
The raw product is usually exposed to hot air (at temperatures over 100°C). Sometimes the raw product is pre-dried. In this instance, first the water is evaporated from the product. The moisture content is thus decreased from 8 -20% to less than 1%.If the product reaches a sufficient low moisture content by 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 product 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.
Roasting can be done either batch-wise or continuously. Typical equipment for batch roasting are: a drum roaster, a column roaster (cacao), a rotating disc roaster, a fluidised bed roaster, a spouting bed roaster, etc. Common to all equipment is that the product is heated and agitated at the same time. The product can be in direct contact with the hot air (convective heat transfer) or in contact with a heated surface (conductive heat transfer). Usually it is a combination of both.
The cooling takes place in separate equipment. This can be a cooling sieve where air is pulled through or a spouting bed cooler or any other equipment where the raw product is in contact with fresh air. Quenching with water can take place in the roasting chamber and sometimes in the cooling equipment. Cyclones are used as an integral part of the process to remove particulate matter/dust (mainly consisting of product residues and skins (chaff)) from the air before it is exhausted to the atmosphere. The recovered material is then reprocessed. The cooling air is also emitted to the atmosphere.
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