Coffee Roasting

Coffee roasting is the first step in making a great cup of coffee. There are many steps in between... and every one has a bearing on the end product... but if the roasting isn't right, nothing will save the final taste.

In coffee roasting, the roaster has to take into account that coffee beans have over 400 chemical compounds in at least trace form and the types and amounts vary in each origin coffee due to the soil conditions, bean varietal, climate, amount of rainfall and growing elevation. Numerous chemical reactions take place with these compounds, which makes the unique single origin fragrances/aromas a mystery.

Bean size, density, moisture content and cell structure affect the roast as well as local conditions, i.e. temperature, humidity and barometric pressure.

Properly stored green beans have 9-13% moisture content.  Anything higher than that invites mold growth. The targeted moisture content is achieved during processing in the origin country. The beans are shipped and stored in consumer countries at a targeted temperature of 70 F and controlled humidity of 40-60%.  Bean quality is usually stable for around a year in these conditions.

Coffee roasting can be as simple as roasting in a popcorn popper or shaking a skillet on a stove, but more control is needed to consistently bring out pleasing aromas while muting the undesirables.

Roasted beans are approximately double in size to the green beans and 12-20% lighter in weight. The larger size and lighter weight are a result of the bean structure expanding in the roast as moisture and CO2 are released and the resultant loss of some bean material.

Every specialty coffee roaster experiments with each origin bean to finally come up with the roast that s/he decides is the preferred taste.  Notes are taken through each experimental roast so that when the best perceived flavor is identified, it can be reproduced with each successive roast.  This becomes the profile for that particular origin bean.

In the coffee roasting process one or more of three types of heat are used:

• Radiant heat- Energy emitted from the hot drum surface through space to cooler beans.
• Convective heat- Energy from a hot air or gas stream to cooler bean.
• Conductive heat- Bean-to-bean contact and bean-to-drum contact. 

The two most common types of roasters are the fluid bed and drum. The fluid bed roaster uses a stationary roast chamber. The beans are kept in suspension by hot air blowing into the chamber.  The amount of air, temperature and bean quantity are all fixed, so there is no roast control.  It is a very simple design and process.

The advantages to this type of roasting are that personnel do not have to be trained (only looking for a certain bean color) and there are minimal moving parts.

Basically, in drum roasters (see pics), green coffee beans are loaded into a hot rotating drum and a gas flame on the exterior of the drum is controlled to produce a desired temperature rise over a selected period of time for a certain variety of bean. The rotating drum constantly lifts the beans and drops them through a hot air stream. The hot air temperature can be controlled during the roast to achieve the temperature rise of the bean.

The drum type roaster is the more popular coffee roasting machine.

Drum Roasting

Each drum roaster will have basic equipment to measure the drum and bean temperature and a cooling bin for the roasted beans to protect against overcooking.

Software is available on most machines for computer controlled roasting of installed profiles.

The burners employed are usually either atmospheric or infrared. Pre-mixed air heat burners are used on fluid bed and large drum applications.

An atmospheric burner's effluent gas heats the air and is the medium for heating the beans (mainly convective heat with 5-10% being radiant).

Infrared rays are a part of the electromagnetic spectrum that includes all known wavelengths (radio waves, microwaves, visible light, ultraviolet light, gamma rays and x-rays). Infrared is on the red edge of the visible light spectrum. 

Infrared burners provide radiant heat, so heat exchangers are employed to provide the convective heat needed for roasting.

For a batch, the average length of time for roasting coffee is between 12-20 minutes. As in a baking oven, the roaster is heated to a certain temperature before the beans are introduced into the drum. Once the beans are dropped into the roasting chamber, a temperature drop occurs for 1-1.5 minutes as a temperature equilibrium between the chamber and bean takes place.

As the temperature begins to rise (starting ~160 - 185 F), notes are kept on the degree of rise; measured every 30 seconds or every minute. The beans will start turning  a light yellow in 4-6 minutes followed by light to darker shades of brown through the rest of the process.

This part of the process is an endothermic reaction. The moisture in the beans is driven off, drying the bean. It also becomes lighter (by weight) and the color changes from green or blue-green to  a yellowish color. From here to the first crack the color progresses to light brown, cinnamon and on to darker shade of brown.

Around 9 minutes (~370 - 395 F), the first crack will occur. This is an exothermic reaction with the final moisture and ~50% of the CO2 gas escaping the beans. The beans double in size during this reaction.

It is ~1 minute after the first crack that the degree of roast measurement begins.  There are numerous names for each degree and the degrees are measured up to the point of the beans actually catching fire.

All roasts will go past the first crack before quitting because the targeted aroma/flavors begin to form after the first crack and just prior to the second crack.

Around 12-14 minutes into the roast a second crack begins.  This crack is harder to hear because it is the cell structure (cellulose) of the bean that is fracturing.

Once the second crack reaches its peak, the origin bean aromas are rapidly diminishing and the roast aromas are becoming more dominant.

After the second crack has finished, the unique flavor of the origin bean is no longer present and all coffees are taking on the same roast aroma. The oils have come to the surface of the beans giving a shiny appearance.

If the roast is continued, the only difference in taste is the difference between the common roast to a carbony taste.  The beans are now black and the shiny appearance has left because the cellulose (wood) structure of the bean is burning. Except for the shape of the bean, it looks like burnt wood in a fireplace.  At this point (~450 - 500 F), the beans are ready to catch fire.

If the beans are not cooled when dumped, they will keep cooking, so cooling bins are usually equipped to pull ambient air through the bin to assist in cooling or a quench water spray is used to cool the beans quicker. If water is used, care is taken to have the correct amount to only cool the beans so as not to add water back to the beans through absorption.

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