Lesson 4: How Coffee is Roasted‏

Hot Air Roasting

Hot air roasting, also often referred to as fluidised bed roasting,
involves the application of hot air to the beans on a fluid bed
system. Air is normally heated by an electric element, whilst the
necessary air flow is produced by a fan. The fluid air keeps the
beans moving and thus negates any risk of scorching.

There have been opinions expressed, which I tend to lean towards,
that sometimes coffee can appear a little brighter than it might
have been had radiant heat roasting been used. Brighter in coffee
terms means more lively with any acidity accentuated. The acidity
referred to is not typically the bad sour type that immediately
comes to mind for most people.

This is good acidity, perhaps citric based or floral, and it
offers a little extra flavour and perhaps a slight kick in
contrast to the smooth tones often found in coffees. The issue is
just how prominent that acidity should ideally be.

Hot air roasting can prove difficult when controlling the roasting
profile for professional use (profiles are explained in Lesson 5),
though some roasters swear by it, so it is a case of using whatever
best suits individual needs. The fluid bed method is successfully
employed in some home roasters and it is very well suited to these
smaller machines.


Drum Radiant Heat Roasting

Drum roasters work like a large washing machine drum, turning the
coffee beans with a heat source below. I have found that gas
(either propane or natural) is better than electricity. By better,
I mean the flavour of the coffee differs when tasted in the cup and
I prefer coffee roasted using gas.

Roasting time is typically anywhere from 11 minutes to around 19
minutes, and temperatures can reach as high as 450°F. Anything
longer than 20 minutes and I beginto discern a baked taste within
the coffee.

When it has finished roasting, the coffee is then 'dropped' out of
the roaster drum and cooled in a large integral tray with stirrers.
Good quality roasters achieve cooling using air, though some
larger machines use water, which for me is not good for the coffee
bean.


What Happens To The Beans During The Roast Process?

For the first few minutes the beans are 'endothermic', taking in
heat. They remain greenish, before slowly turning a lighter
yellowish colour and giving off a slightly grassy smell.

The beans then begin to steam as their moisture content dissipates
and the steam becomes fragrant. Soon afterwards the 'first crack'
can be heard, as the beans expand and crack when the real roasting
process begins and the beans become 'exothermic', giving off heat.
Sugars begin to caramelise, the structure of the beans breaks down
and oils within them begin to rise to the surface.

After first crack the beans once again become endothermic, and the
coffee is actually consumable depending on taste preference and the
particular type of bean.

If left to roast further, caramelisation continues and oils become
more evident on the surface of the beans. 'Second crack' then
occurs as the beans once again become exothermic.

Most roasts will have been terminated somewhere between the two
cracks or shortly after the second. If left longer the roast will
become very dark, smoke increases and natural sugars disappear,
taking some of the potential sweetness from the cup. If charcoal
appears, the roast has definitely gone too far. Coffee101.co.uk