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Roasting Science

Matthew Poh

1 | Roasting Science

Roasting methods are based on a few Scientific Theories, which naturally complies to certain expected and measurable framework of change. A few are listed here as an introduction.

1.1 | ⚗️ Thermochemistry Effects

Loading en.wikipedia.org⁠

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Endothermic process is any process with an increase in the internal energy of the system. In such a process, a closed system usually absorbs thermal energy from its surroundings, which is heat transfer into the system.

Exothermic process (exo- : "outside") describes a process or reaction that shows the releases of

energy⁠

from the system to its surroundings, usually in the form of

heat ♨️⁠

light 💡⁠

electricity 🪫⁠

sound 🔊⁠

⁠

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1.2 | ♨️ Heat Transference

The Various Type of Heat Transference in which heat can be transferred are namely | Conductive Heat, Convective Heat and Radiation.

1 | Conductive Heat

Thermal Conductivity K = (QL)/(AΔT)

Conductive heat is the transfer of heat between two objects that are directly touching. When you touch something that’s hot and you burn your hand, that’s conduction.

In coffee roasting, conduction occurs if any type of heating element touch the drum, when beans come into contact with the surface of the drum and even when the beans touch each other inside the roaster.

Conductive heat transfer plays a lesser role during coffee roasting than you might think. In drum roasters, only about 30% of heat transfer comes from conductive heat transfer.

2 | Convective Heat

Convective heat is the transfer of heat through a liquid or gas. When you put your hands into a bowl of hot water and feel the heat, that’s convective heat.

There are two types of convection: natural and forced. Natural convection describes the natural tendency of hot air to rise and cool air to fall. Forced convection is the forcing of air via a fan or pump to transfer heat. Almost every roaster uses a fan to some degree to pull air through their roaster.

3 | Radiation

In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium.

1.3 | ⚛️ Maillard Reaction

⁠

The Maillard reaction⁠

(/maɪˈjɑːr/ my-YAR; French: [majaʁ]) is a chemical reaction between amino acids and reducing sugars that gives browned food its distinctive flavor. Brown Coffee Beans are derived with this as the main principle behind the Roasting Process.

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2 | Roasting Art

⁠

2.1 | The Roasting Process

Also known as the “DARK ART”, roasting coffee transforms the chemical and physical properties of

green coffee beans⁠

into roasted

coffee⁠

products. The roasting process is what produces the characteristic flavour of coffee by causing the green coffee beans to change in taste.

Unroasted beans contain similar if not higher levels of

acids⁠

protein⁠

, sugars, and

caffeine⁠

as those that have been roasted, but lack the taste of roasted coffee beans. The flavour & colour are naturally caused by

Maillard⁠

and other chemical reactions that occur during roasting.

Step 1: Loading and Drying

At the beginning of each batch, raw “green” coffee is loaded into the roaster’s charge system. Once the roaster’s internal air temperature reaches Charge Temperature of 180C-200C, the green bean batch is dropped into a spinning, hot drum to begin the roasting process.

This is the first stage of drying where internal temperature quickly drop to between 80C-100C. The moisture content will drop from 10-12% to 0%. Once the bean moisture is completely dried, the next stage begins.

Step 2: Yellowing

With moisture reduced to 0%, further heating will start to yellow the bean. For this stage, a high amount of heat is applied to bring the coffee up to roasting temperatures. The air temperature within the dome will rise from the low to about 180C.

During this period, the colour of the coffee shifts from green to yellow and the smell shifts from freshly cut grass to hay-like. The Silverskin will also buckles at this stage. This stage has to be done quickly enough to preserve some internal moisture but not so quickly that the outside of the beans are at risk of scorching. At a point where the bean cannot hold any more energy going into here body, she explodes signaling the next stage.

Step 3: First Crack

During this stage (depending on the type of coffee and roast degree) the coffee’s full flavour is developed. At around 180C the “first crack” occurs – a popping noise occurs as each bean expands under the pressure of carbon dioxide and water vapour produced from chemical reactions within the bean. At this point the internal pressure drops as these gases are allowed to escape, resulting in a bean with an ideal internal temperature, pressure and moisture for flavour development. There is a weight loss of about 5%. The bean size doubles and the beans are brown in colour.

Step 4: Development

Between 180C and 200F the sugars and amino acids in the coffee react at a rapid pace to create the final flavour of the coffee. The flavour of coffee roasted is derived from caramelisation of sugars and the reaction of sugars and amino acids known as the “maillard reaction”. There is a weight loss of approximately 13%.

During this period the coffee turns toward darker shades of brown and the aroma develops into a nutty, cocoa-like and malty character. This is the end of roasting time for light blends.

Step 6: Second Crack

In this second crack, the beans surface starts to have an oily sheen. This is also the end of roasting time for medium blends

Each coffee has a precise final temperature and time of roasting, both developed through a mixture of intuition and experimentation on the part of the roasting team. For example, a Yirgacheffe Ethiopian coffee might be roasted to 200C in 12:30, while a Colombian Espresso coffee will be roasted to 210C in 14:30.

Step 7: Dark Roast

Coffees roasted to temperatures above 210C are considered to be dark roasts. At these temperatures the aromas and tastes developed during the initial stages are combusted and new aromas that are toasty, smoky and spicy develop. Sweet and acidic tastes are replaced by the development of bitters. Most dark roasts develop to between 210C and 220C to achieve full development of these characteristics.

In addition, all the beans will be very oily. There is also a volume gain of 35% while concurrently a weight loss of 20%.

For more information on Roasting, please do not hesitate to contact us.

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