Roasting green coffee at home

Most people buy their coffee already roasted. Some buy the beans green and roast small batches themselves.

This page covers what green coffee is, what happens in the roaster and how much weight the beans lose. It draws on published coffee research.

What green coffee is

Green coffee is the unroasted seed of the coffee cherry. Two ways of preparing it are the natural process and the washed process. In the natural process, the cherries dry in the sun for several days before the husk and fruit are removed. In the washed process, the fruit comes off first, and water helps remove the sticky layer underneath.

World Coffee Research keeps a catalogue of Arabica and Robusta varieties. Arabica's native home is Ethiopia. Robusta comes from the forests of western sub-Saharan Africa.

What happens in the roaster

Coffee researchers describe a roast in stages. In the first, the beans dry out and change colour from a greenish yellow to a yellowish brown. Next come the first popping sounds, which roasters call first crack. Some roasts carry on to a second round of popping, called second crack.

CoffeeResearch.org puts first crack at about 205°C and second crack at about 225 to 230°C. Coffee researchers note that there is no single standard way to measure how dark a roast is. Colour, density, weight lost and the time after first crack all describe part of it.

How much weight the beans lose

Beans lose weight in the roaster as water and other compounds leave them. Weighing a batch before and after roasting gives one simple measure of how far it has gone.

In one study of Ethiopian coffees, batches that lost 8.9% and 11.6% of their weight would count as underdeveloped by specialty coffee standards. Extraction yields peaked in batches that had lost around 12 to 14%, which generally meant lighter roasts. Darker roasts lost more than that.

The arithmetic works the same way for any batch. A 500g batch that loses 14% comes out at 430g, so a roaster who wants 430g of roasted coffee starts with 500g of green beans.

Resting after the roast

Freshly roasted beans go on releasing carbon dioxide for some time. Researchers who weigh coffee after roasting track this as a slow, continuing loss. In the study above, the roasted coffee had a ten-day degassing period before it was measured.

Roasting also loosens coffee silverskin, the thin outer skin of the bean. It comes away as the beans expand in the heat.

Storing green beans

Storage changes green coffee over time. Researchers say green beans keep best at a moisture content of about 10 to 12%, in a dry, cool place away from direct light.