How to Adjust Flavor Without Changing Roast Level - What's the Relationship Between Roasting and Flavor?

Professional coffee knowledge exchange. For more coffee bean information, please follow Coffee Workshop (WeChat official account: cafe_style)

Energy

Today, let's talk about energy and explore how to make roasting adjustments from this perspective.

In ancient Greek, energy means "activity, operation" - a physical quantity that can be observed indirectly, considered as the ability of one physical system to do work on another physical system. Work is defined as the spatial accumulation effect of force when an object moves along the direction of the force, and equals the product of force and displacement in the direction of the force.

The total energy contained in an object is based on its total mass. Like mass, energy can neither be created out of thin air nor destroyed out of thin air. Energy, like mass, is a scalar quantity. In the International System of Units (SI), the unit of energy is the joule, but other units such as kilowatt-hours and kilocalories are sometimes used, which are also units of work. Energy is a unified measure used to quantify the scale of all material movement.

(From Baidu Encyclopedia)

The Essence of Roasting

From the above, we can understand that coffee roasting is essentially energy transfer. Coffee beans receive energy from the roasting drum and heated air, producing a series of physical and chemical changes. The energy here is transferred in the form of thermal energy. When energy enters coffee beans, all molecules begin to move due to gaining energy. The breakdown of large molecules, the polymerization of small molecules, and the changes in color and volume of coffee beans are all part of energy's game.

The heating process of green beans is the heat conduction process from the roasting drum and the air inside it. Objects conduct heat because of energy differences. The low temperature of green beans means they contain less energy, so thermal energy conducts from the drum to the green beans, causing their temperature to rise. Their transfer only occurs under one condition: energy difference. Green beans heat up because the external temperature is higher. This process is irreversible, meaning heat can only flow from high to low.

Exothermic First Crack

We all agree with the concept that "first crack is exothermic," but from the theory above, this seems somewhat inappropriate: which has higher energy during first crack, the coffee beans or the roasting drum? Think again, if the coffee beans are heating the roasting drum, is their own energy decreasing, meaning are they cooling down?

Obviously, the energy of the roasting drum must be higher than that of the coffee beans, and coffee beans cannot both output energy and continue to heat themselves. Here we need to view coffee beans as a combination of water and coffee bean tissue and discuss them separately. During roasting, water absorbs energy and its temperature rises, eventually reaching boiling point to form steam, with volume increasing significantly. The coffee bean tissue obstructs this steam from expanding, so internal pressure continuously rises. Eventually, the soft bean tissue cannot block the steam's escape, breaking through the coffee bean's fibrous structure and carrying away energy, while the bean fibers continue to absorb energy. The characteristic is that the heating rate drops significantly at the start of first crack, and the air temperature rises - this is the essence of being exothermic.

As steam carries large amounts of energy causing drastic changes in the roasting environment, it's easy to lose control. This requires opening the damper or reducing heat to stabilize the energy balance inside the roasting drum. But don't forget that coffee bean tissue still needs energy for reactions, so you need to anticipate when the air temperature's energy is depleted and supplement with heat, otherwise you'll experience temperature loss.

The Essence of Roasting Adjustments

Here we introduce a specialized term in coffee roasting: AUC (Area-Under-Curve). Literally, it means calculating the area under the bean temperature curve, intended to represent energy usage during the roasting process. We need to use AUC when we want to adjust flavor without changing roasting time and coffee roast degree.

Example

Below is the roast curve from our Yangjia 600g semi-hot air roaster (Butterfly Geisha).

Butterfly A is what we consider the optimal flavor baseline for roasting, while B is the comparative roast.

It's visible that both Butterfly roasts have the same charge temperature, discharge temperature, and roasting time, but curve B's AUC is less than curve A's (the area between the two curves represents the energy difference). This means the coffee in roast B received less energy than in roast A, leading to less flavor development than A (as mentioned above, when molecules inside coffee receive less energy, fewer reactions occur). Cupping also intuitively reflects this - the Butterfly from curve B has poor flavor development due to insufficient energy supply and has grassy flavors.

Therefore, when we see insufficient energy supply in the roasting curve, we can increase heat to supplement the portion of energy that was less absorbed, thereby achieving roasting adjustments without changing the roast degree. As in curve C, we increase heat in the early stage to raise the temperature, then reduce heat at the browning point to flatten the temperature rise, achieving the same total energy absorption and reaching the goal of adjusting flavor without changing roast degree.

The adjusted curve C Butterfly possesses the same high-quality aroma and flavor as the baseline, and because it absorbed more energy than A during the vigorous Maillard reaction period (starting from 150°C), the enzymatic flavors (floral and fruit aromas) are more pronounced.

Of course, there's another adjustment method: extending roast B's time to make the overall AUC similar, but this would affect the roast degree, so we won't discuss it today.

Finally, I want to say that coffee roasting is a job of finding optimal solutions, aimed at best showcasing the regional flavor characteristics of coffee beans. There are quite many and complex adjustable parameters. A perfect batch might be accidental, but making every batch perfect is very difficult because "a slight difference can lead to a thousand miles of disparity." I hope all roasters dedicated to coffee beans can achieve their dreams~

FrontStreet Coffee (FrontStreet Coffee): A roastery in Guangzhou with a small shop but diverse bean varieties, where you can find various famous and lesser-known beans, while also providing online store services. https://shop104210103.taobao.com

Important Notice :

前街咖啡 FrontStreet Coffee has moved to new addredd:

FrontStreet Coffee Address: 315,Donghua East Road,GuangZhou

Tel:020 38364473