Roasting Phase Transformation

In anticipation of the upcoming Intermediate Roasting Course on February 23-24, let's take a closer look at the important physical transformations that take place during the roasting phase.

It's clearly no secret that a coffee bean changes dramatically when roasted. From the green and unripen fruit we now know so well, it transforms into that aromatic and flavoursome explosion that we have come to love so much. Roasting is a very powerful yet delicate process enabling a profound physical change to the bean: today we’ll focus on these changes.

By nature, the raw (green) coffee bean is dense and compact, difficult to break and crush, and this is one of the reasons why coffee is roasted - to make it more fragile and permeable to water.

How the colour of coffee changes

The most noticeable physical change to the bean during roasting is undoubtedly the colour. In fact, before roasting, a coffee bean has a distinct green colour. Green coffee is so named not only because it is a fruit that has yet to be processed, but because it in fact has that distinctive light green hue. During roasting, the production of melanoidins changes the colour of the coffee until it develops a deep brown colouring. Melanoidins are polymers that form when sugars, but especially amino acids, combine chemically due to intense heat.

Change in moisture and mass within the bean

Roasted coffee beans contain about 2.5% water but, before the roasting phase begins, water makes up about 10-12% of its weight. This is a rather considerable difference and this overall mass reduction, after roasting, is therefore due to the loss of liquid and the partial transformation of solid into gas (CO2). Generally speaking, the roasted bean weighs about 20% less once it leaves the roasting drum.

Volume and porosity of the coffee

When coffee is roasted, the increase in temperature and the subsequent phase transformation of water into gas creates high levels of pressure. These conditions change the structure of the cell walls of the individual coffee beans, which in fact go from stiff to chewy. The nature of this change is due to the presence of polysaccharides - carbohydrate molecules firmly linked together - which push outwards, thus leaving a more dilated and porous centre to the bean. The bean expands in volume while reducing in mass, becoming less dense and therefore lighter and more soluble.

Oils that migrate from roasting

Coffee beans contain natural oils and lipids within them. During roasting, internal pressure causes these oils to “migrate” – push out - from the centre of the bean to the surface. However, lipids are essential to keep aromatic compounds inside the cell. These compounds are considered "volatile" and are synthesized into a chemical substance under high pressure helping to create the recognisable flavour and aroma of coffee. Naturally, the longer the roasting is, the more pronounced and evident the chemical and structural transformations of the beans will be.

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