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)