There are two types of coffee roasting machines:

ROTATING DRUM
This type has an operating principle based on:

  • conduction Ð convection
  • roasting by direct contact with the drum walls, heated by flame with hot air flow.

Roasting time varies between 8 and 15 minutes.

FLUID BED
This type has an operating principle based on:

  • convection
  • hot air flow, allowing more even roasting
  • temperature adjustment and accurate timing

Roasting time varies between 8 and 15 minutes.

The fluid bed requires a continuous cycle and greater production quantities, determined by the achievement of a standard quality.

CHANGES CAUSED BY ROASTING
The heat used in roasting causes the coffee beans to undergo a veritable transformation:

  • reduction in moisture (from 12 - 8% to 5 - 1 %)
  • reduction in weight (by 15 - 20% due to water evaporation)
  • increase in volume (by 30 - 60% due to formation of carbon dioxide)
  • colour change (beans become darker) modification in content (+ 12/16% fats, -10% sugars, - 3% acids)
  • development of aromas

These changes are gradual and are determined by the increase in the roasting temperature.

  • 50 °C: internal modification of tissues commences
  • 60 °C: evaporation process commences
  • 100 °C: first colour changes come about
  • Between 150 and 180 °C: light roast stage
  • Between 200 and 230 °C: optimal degree of roasting
  • Between 215 and 220 °C: light roasting (more acidic and less bitter)
  • Between 220 and 225 °C: medium roasting stage
  • Between 225 and 230 °C: dark roasting (more bitter and less acidic)






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