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How is coffee roasted?

A roast profile is a set of parameters that define how a coffee should be, or has been, roasted. Below you will find generic roasting information (credit: MTPAK)

A roast profile is a set of parameters that define how a coffee should be, or has been, roasted.

Most roasters are fitted with a probe to measure the temperature of the beans in the drum, a probe to measure the exhaust gas, and an adjustable heat input. Specialty coffee roasters can use these three data points, along with time, to build a reliable roast profile.

Roast profile logging software, such as Cropster or Artisan, is used to capture the data and display it on a graph in real time, which allows roasters to make necessary adjustments.

According to coffee roasting expert Scott Rao, a typical roast profile generally follows an “S” curve in which bean temperature drops for 70 to 90 seconds, bottoms out, and then rapidly increases.

Before starting the roast, it’s important to work out a starting point for your roast profile by analysing various aspects of the beans. Factors such as bean density, hardness, moisture content, screen size, and batch weight can all have a profound influence on the final roast profile.

Rafael Silva works for Entre Cerros Coffee Roasters in El Salvador. He tells me that knowing this information will help roasters understand how their coffee will develop during the roast, how much heat and airflow will be needed to control the roast, and what the initial temperature, or charge temperature, should be before the beans are dropped into the drum.

“It’s vital we know the density of the beans before roasting,” Rafael explains. “Coffees with a low density and hardness will need a lower initial charge temperature, as they can easily become overroasted.

“Higher density coffees need a higher initial temperature to dry out the coffee faster and experience better development.”

He also says that variety plays a crucial role in how the coffee is roasted.

“All varieties are of different densities, no matter the altitude and location of the farm,” he says. “Some varieties are rounded shapes which makes them easier to roast, while others are elongated or pointed. These are extremely difficult to roast as we need to be careful not to burn the tips of the beans.”

As well as density and variety, processing methods can have a significant effect on the roast profile. Natural (dry) processed coffees and carbonic maceration methods both leave a lot of residual sugar in the beans, which, if not treated accordingly, can easily take on too much heat and burn the coffee.

Measuring Roast Profiles

    Before the appearance of roasting software, roasters would have to rely on certain physical changes within a bean to determine how much heat was needed for their coffee to reach full development.

    However, technological advancements mean that roasters are now able to put together exact timings and temperature points along with a bean’s physical markers.

    Depending on the style of a roast, you will want these physical markers to happen at different times to either eliminate or highlight certain flavours from the bean.

    Daily Coffee News states that a good place to start with profile translation analysis is to examine ranges that are marked by easily observable physical changes.

    Examples of ranges include the start of the roast through to the start of the colour change from green to yellow and yellow to brown, and from the start of first crack (when the temperature reaches around 196°C) to either the end of the roast or to the start of second crack.

    Other important, measurable elements that make up a roast profile are the turning point, rate of rise, and end roast weight loss.

    Using roast profile logging software, roasters can track and measure results within these ranges and markers, and make changes accordingly. Essentially, the more data you can capture during the roasting process, the more accurate and consistent your roast profile will be.

    The physical changes that happen inside a bean during the roast are complex, and understanding what’s happening at different stages can require a lot of trial and error. However, knowing at what temperature certain chemical reactions happen and how they affect the outcome of a roast is crucial when refining a roast profile.

    Using Roast Profiles to Develop Flavour

    Through small adjustments made during the roasting process, each roaster can craft their own distinct flavour of coffee.

    Whether it’s body, acidity or sweetness, they must decide which characteristics they want to highlight, and which they want to hide. However, with more than 300 flavour components in every green bean, achieving a desired profile is a complex process that requires lots of testing and dialling in.

    “You can roast to enhance acidity, to improve sweetness, to hide bad flavours, or just go as dark as you like,” Rafael explains.

    Rafael says that when sampling coffees, he goes in search of specific characteristics that he knows will appeal to his customer base. However, he warns that even if using the same batch of coffee, it’s important to constantly assess the outcome of the roasts to ensure the best results.

    “You need to be carefully watching over your roasts each time because coffee is constantly changing. It can gain or lose humidity, it can start fading off or degrading.

    “I would advise cupping your roasts all the time to see how the coffees are developing over time, and whether you need to make any adjustments.” Roast profiles can take a while to get right; but with reliable software and a firm understanding of the various factors at play during roasting, you should be able to develop an initial roast profile quickly, providing you with a good base from which to refine how you roast a coffee over time.

    More about roasting

    The roast profile is defined by temperature and roasting time. Central components are temperature rise (when / how intense do i heat the beans) and reduction of roast temperature over time. This roast profile varies depending on the type of beans used and leads to different taste characteristics. The roast time (from small batch roasters) is between 15-30min. and takes place at temperatures between 180 and 250 ° C.

    The roast profile used by the roasting master determines to a high degree of what flavours and characteristics of the coffee are emphasized at the end. Next to the roast profile, origin, type and processing of the beans influences the taste – vice versa these factors determine the type of roasting (in addition to moisture of raw beans, roast batch size, air flow, drum rotation, cooling duration…).

    The different roast levels of beans are referred to depending on the roasting temperature. A widely used method to determine the actual degree of roast of a bean is the visual classification of the color. During the roasting process, the beans change color – from yellow to darker browns.

    In the later stages of roasting oil begins to appear on the surface of the beans. However, the purely visual assessment is usually not accurate enough, therefore a combination from bean temperature, smell, color and sound is used to control the roasting process. Noise is also a key indicator during the roasting process.

    There are two temperature limits which are referred to as “Cracks” which get special attention by roast masters: The first Crack“ takes place at 200-202 ° C and is similar to the crack of popcorn (only slightly quieter). The second Crack“ is reached at 224-226 ° C. These “cracks” are caused by gases released during the roast process.

    Light Roast:

    • Cinnamon Roast 196 °C: A very light roast level – immediately at first crack. Light brown color with prominent acidity
    • New England Roast 205 °C: Moderate light brown color. Complex acidity

    Medium Roast:

    • American Roast 210 °C: Medium Brown – first crack. Slightly sweeter than light roast, full body with acidic notes
    • City Roast 219 °C: Medium shade of brown – balanced body, noticeable acidity.

    Dark Roast:

      • Full City Roast 225 °C: Medium dark brown with occasional oil sheen. At the beginning of second crack. Roast character is prominent.
      • Vienna Roast 230 °C: Moderate dark brown with light surface oil, more bittersweet, caramel flavor, acidity muted.
      • French Roast 240 °C: Dark brown, shiny with oil, acidity diminished. Bittersweet, caramel, Suppressed acidity.
      • Italian Roast 245 °C: Nearly black and shiny, burnt tones become more distinct, acidity nearly eliminated.

      Coffee Roasting Process

      THE MAILLARD REACTION

      Coffee is roasted between 180 ° C and 250 ° C. While inducing such temperatures the „Maillard-Reaction“ occurs which affects the flavor, texture and smell. To date, not all the reaction products of these “Maillard reaction” identified. The green coffee beans are supplied to the roaster in jute bags or containers, depending on the size of the roaster (Small to industrial roaster). The beans are cleaned once more by sieving, removing the remaining residues and then transported to the roasting machine.

      THE ROASTERS

      Essentially one can differentiate between two roasting machines:

      • Drum Roaster
      • Hot Air (Fluid Bed) Roaster

      DRUM ROASTER

      Drum roasters are mainly used by small to medium-sized coffee roasters plants and can roast between 5 kg to 120 kg per batch. Roast Masters, who want to achieve the highest quality work with drum roasters which offer a slower, more gentle roasting process.

      The principle is independent of the size of the drum: horizontal rotating drums are externally heated and the all raw beans are evenly moved inside. The beans themselves never come into direct contact with the flame. A shovel is attached at the front of the roaster to control of the roasting process. The shovel allows taking samples of beans during the roasting process. Thus the roast master can continuously monitor the color and sensory elements of his roast. When the beans have reached the desired degree of roasting, they have to be cooled. This is done by opening a flap on the roaster through which the beans fall on to a cooling sieve and will be cooled there by air suction and agitation. Finally, the beans still have to de-gas for some time until they can be packed.

      HOT AIR (FLUID BED) ROASTER

      Simply put, the Air-Roasting process involves coffee beans being suspended and roasted on a bed of hot air. The vortex within the roasting chamber created by the air, roasts each coffee bean at the same rate from all directions. The Air-Roaster also removes dust, chaff and smoke from the roast chamber ensuring a clean, pure and balanced roast. Air-Roasting also allows continuous and first-hand interaction with your coffee beans during the entire roast process using sight, sound and smell as well as to guide you through the roast from start to finish and to enhance your roasting experience altogether. So, with a continuous stream of fresh hot air coming into the roast chamber and everything else but the roasting coffee beans being removed, the end product has a very clean taste that is intensely aromatic, together with enhanced flavour notes, minus the acids and bitter tars that are usually produced by conventional roasters, it produces an exceptional cup of coffee.