03 September 2008

Roasting
  • The coffee bean be roasted to bring out flavour and aroma.
  • When the bean temperature reaches and exceeds 165 °C the roasting begins, accompanied by a popping sound similar to that produced by popcorn.
  • It takes about 10-15 minutes to complete roasting with about 25-75% efficiency.
  • Continuous fluidized bed roasting only takes between thirty seconds and four minutes, it also operates at lower temperatures which allows greater retention of the coffee bean aroma and flavor.
Grinding
  • Reduces the beans to a size of anything between 0.5 and 1.1 mm in order to allow the coffee to be put in solution with water for the drying stage.
  • Sets of scored rollers specially designed to cut rather than crush the bean are used.

Extraction
  • The coffee roasted and ground must be put into solution with water.
    The water is added in 5-10 percolation columns at temperatures of between 155 to 180 °C, this concentrates the coffee solution to about 15-30% coffee by mass.
Drying
This step is a very important! There are two different methods used in plants all over the world, freeze and spray drying, each has its own advantages and disadvantages.

Freeze Drying
The basic principle of freeze drying is remove the water by sublimation.
It’s more expensive than other methods of drying, but results in a higher quality product.

The Freeze Drying Process

  1. Agglomerated wet coffee granules are frozen. Freezing too fast leads to large ice crystals and a very porous product and can also affect the colour of the coffee granules.
  2. Frozen coffee is placed in the drying chamber, often on metal trays.
  3. A vacuum is created within the chamber. The strength of the vacuum is critical in the speed of the drying and therefore the quality of the product. Care must be taken to produce a vacuum of suitable strength.
  4. The drying chamber is warmed, most commonly by radiation but conduction is used in some plants and convection has been proposed in some small pilot plants. A possible problem with convection is uneven drying rates within the chamber, which would give an inferior product.
  5. Condensation - the previously frozen water in the coffee granules expands to 10⁷ its volume, the removal of this water vapour from the chamber is vitally important, making the condenser the most critical and expensive components in a freeze drying plant.
  6. The freeze dried granules are removed from the chamber and packaged.

Spray Drying

  • This method of drying is preferred coz its cost effectiveness, short drying time, usefulness when dealing with such a heat-sensitive product, and the fine, rounded particles it produces.
  • The particles it produces are too fine to be used effectively by the consumer; they must first be either steam-fused in towers similar to spray dryers or by belt agglomeration to produce particles of suitable size.

Spray drying produces spherical particles of size roughly equal to 300 µm with a density of 0.22 g/cm³ (ref 2). To achieve this, nozzle atomisation is used. Various ways of nozzle atomization can be used each having its own advantages and disadvantages. High speed rotating wheels operating at speeds of about 20,000 rpm are able to process up to 60,000 pounds (27 t) of solution per hour (ref 3). The use of spray wheels requires that the drying towers have a wide radius to avoid the atomised droplets collecting onto the drying chamber walls.

Typical Spray Drying Characteristics :

  1. Completed in 5-30 seconds (dependent on factors such as heat, size of particle, and diameter of chamber).
  2. Moisture content change, IN = 75-85% OUT = 3-3.5%
  3. Air Temperatures, IN = 270 °C OUT = 110 °C

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