Roasting Green Coffee Beans

Author: Bliss Coffee Roasters  Date Posted:3 November 2020 


Green Bean Moisture

Prior to being shipped around the world from the plantations to roasters, green coffee beans have been through a drying process. The process varies depending on the region and methods used, but the goal is the same – to reduce the moisture content, but some moisture remains in the bean. Green bean moisture content is ideally between 10% to 12%, although the International Coffee Organization accepts a broader range, between 8–12.5%.

If moisture content in the green bean is too low (less than 10%), the result can be reduced quality in the cup and less flavour. Green beans with high moisture content (over 12%) puts the green coffee at risk of mould and a more significant weight reduction when roasting.

The green coffee beans will lose the majority of their moisture during the roasting process, and the weight of the coffee beans will drop because of this. Fun fact  - the coffee bean will increase in size as this happens, like popcorn!

It’s not uncommon for a batch of coffee to lose around 20% of its weight during a roast due to the reduction in moisture.

Roasting Green Coffee Beans

Roasting is the transformation from a hard, green seed to the aromatic and flavourful brown bean we all know and adore.

What happens during the process?

When coffee beans are roasted, they undergo significant chemical and physical changes.

The drying phase starts immediately after what we call ‘the turning point’. When the room temperature green beans are added to the hot drum of the roaster, the temperature inside the roasting drum falls before rising again. The point at which the temperature begins to rise again is ‘the turning point’ and this is when the drying process starts. During drying, water content begins to vaporize, and pressure starts to build inside the beans.

At around 150°C, the beans Maillard reaction has started and the beans start to turn brown.

During the Maillard reaction, gases including carbon dioxide, water vapour, and some volatile compounds are created. The internal pressure increases enough to break the cell walls of the beans, making an audible ‘popping’ sound, this is what is known as ‘first crack’.

Melanoids begin to develop during this stage. As well as changing the colour of the beans, they contribute to the final mouthfeel of the coffee.

‘First crack’ is generally considered to be the lightest roast that would be enjoyable in the cup, dependent on the bean itself. Drinking coffee roasted to jus this point will be very light and possibly floral in taste, but may be a little too ‘grassy’ in taste.

After first crack, the roast changes from an endothermic reaction (the beans absorb heat from the drum) to an exothermic one (the beans release heat).

During this stage of the roast, the physical transformations continue – beans increase in porosity, oils migrate to the walls of the cell, and the colour darkens further.

To get the most out of this process, the roaster must pay absolute attention to what is happening inside the roaster. In just minutes, or even seconds, the entire taste and flavour profile of the coffee can be completely changed.

Roasters must truly understand their roasting equipment like the back of their hand. Whilst many modern-day roasters are large and computerised, at Bliss Coffee Roasters, we still believe in the art form that is roasting. A hands-on process that relies on the roasters trained skills and senses. 

At Bliss Coffee Roasters, our roasting team certainly understands our classic 7kg roaster and knows it like the back of their hand!

Who knew so much science goes into making a cup of coffee!?

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