Roasting

Roasting

Green coffee beans are roasted and aromas, acids, oils and flavours are released and created.

Coffee is roasted at 200°C to 250°C, for 10 to 15 minutes.

The green beans slowly dry and become yellow at first, releasing a toasted popcorn smell.

In the second stage (first crack), gases are formed, causing the beans to double in size and become a light brown colour. The first crack occurs as the gas ruptures the beans.

In the next step the beans change from a light brown colour to medium brown colour. The roasting process continues when a darker colour is desired. The second crack occurs when medium to dark brown and it is much quicker sounding. This change of colour happens a lot faster and can be hard to control.

At whatever stage the roaster decides to stop roasting, the beans are poured out onto a cooling vat, which stirs the beans to quickly cool them in order to stop the cooking.

Medium-dark roasts are traditionally suitable for making espresso coffee. They have a low acidity and have a full body. The bean colour is an indicator of how long it has been roasted and the degree of roasting will determine factors such as richness, acidity, flavour and nuttiness of the brew.

A note on flavour: Describing the tastes of different roasts is as subjective as putting a wine into words. In both cases there's no substitute for your own personal taste.

Roast Level Aliases Roaster Watch Surface Flavour
Light Cinnamon roast, half city, New England After about seven minutes the beans "pop" and double in size, and light roasting is achieved. American mass-market roasters typically stop here. Dry Light-bodied and somewhat sour, grassy, and snappy
Medium Full city, American, regular, breakfast, brown At nine to eleven minutes the beans reach this roast, which U.S. specialty sellers tend to prefer. Dry A bit sweeter than light roast; full body balanced by acid snap, aroma, and complexity
Dark High, Viennese, French, Continental After 12 to 13 minutes the beans begin hissing and popping again, and oils rise to the surface. Roasters from the U.S. Northwest generally remove the beans at this point. Slightly shiny Somewhat spicy. Complexity is traded for rich chocolaty body; aroma is exchanged for sweetness.
Darkest Italian, espresso After 14 minutes or so the beans grow quiet and begin to smoke. Having caramelised, the bean sugars begin to carbonise. Very oily Smokey; tastes primarily of roasting, not of the inherent flavour of the bean.

Table courtesy http://en.wikipedia.org/wiki/Coffee_roasting.
 
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