Weight is a continuous signal of the roasting process

During roasting, coffee beans don't just change in color and smell. Weight also decreases over time as water, gas, and volatile substances leave the bean's structure, alongside physical and chemical changes driven by heat. Research by Hernández and colleagues directly tracked temperature and mass during batch roasting, showing that mass can be recorded online alongside thermal variables.

This makes the mass curve a useful data source. However, the data only carries meaning once you know where, how, and under what raw-material conditions it was measured.

Mass loss doesn't stem from a single cause

In the early stage, water within the bean is heated and released. As heat continues rising, roasting reactions change the bean's structure, gas forms, and volatile substances can escape. So the mass curve is the result of several overlapping mechanisms.

Hernández and colleagues modeled heat and mass transfer in batch roasting while measuring temperature and mass loss. Bottazzi and colleagues also built a numerical model accounting for heat transfer, moisture evaporation, and roasting-related reactions. Both research directions show that a section of mass decline shouldn't be read as if it represents just one phenomenon.

Time and temperature shape the curve

If thermal conditions change, the rate of mass loss changes too. Research on the mass-loss kinetics of Arabica coffee shows the level of mass loss depends on the process's thermal conditions. In that study, the rate of mass loss was described with a kinetic model and varied with roasting temperature.

This doesn't mean a curve from one type of coffee can be applied to every other type. Initial moisture, bean size, variety, equipment, and roast configuration can all change the trajectory.

Roast level can be viewed from several simultaneous signals

Color, bean temperature, time, and mass lost can all be used together to describe roast state. Hernández and colleagues studied brightness and surface-area kinetics during roasting, showing color and physical variables can be modeled along the thermal process.

If mass alone is used to determine roast level, an operator might overlook cases where two batches have the same weight loss but different thermal histories. So the mass curve fits better in the role of a control signal when read alongside temperature and time data.

In-roaster mass loss needs to be separated from post-roast change

The moment beans leave the machine, the post-roast process hasn't fully ended. Gas keeps escaping and mass can keep changing. If a batch is weighed right after cooling and then reweighed after a different time interval, the result may no longer reflect the same state.

So when building a curve to compare batches, the weighing moment needs to be clearly defined. If the before-and-after weighing points aren't consistent, the difference can include post-roast condition differences alongside the in-roaster differences.

Reading the curve to control a batch

A useful curve can show how the rate of mass loss changes over time, which batch has an unusual pattern, and which point needs rechecking. If the same raw material and roast configuration produce one batch with a clearly deviated curve, this data can become an early signal to check temperature, airflow, time, or raw-material conditions.

But a fixed loss ratio shouldn't be used as a standard for every coffee. A value only carries meaning within the conditions under which it was established. The kinetic study by Vargas-Elías and colleagues was also conducted on Arabica with one specific system and roasting conditions.

Mass is most useful when turned into contextual data

To turn the mass curve into a control tool, it needs to be recorded alongside charge weight, raw-material moisture if available, time, temperature, equipment configuration, weighing moment, and cooling conditions. When this data travels together, the mass lost stops being a standalone number and becomes a trace of the process.

In practice, this rule should be written into a short operating guide so others can repeat it. Once measurement is standardized, the mass curve can be used to track long-term trends instead of only serving a one-off check.

A curve can only be compared if the time markers and sample state are defined in advance. You can choose charge weight as the starting point and post-cooling weight at a defined condition as the endpoint. If the way these two markers are determined changes between batches, the loss ratio will carry additional methodological error.

The measurement's start and end points need to be clearly defined

The curve can also help assess repeatability. When multiple batches of the same raw material are placed on the same time-and-mass coordinate system, the average shape and each batch's deviation can be observed. If a batch deviates from the usual range, thermal and operational data can be reviewed to find the cause. This use is more practical than searching for a single ideal loss ratio.

References

• Hernández et al. (2007). Analysis of the heat and mass transfer during coffee batch roasting. Journal of Food Engineering, 78, 1141–1148. https://doi.org/10.1016/j.jfoodeng.2005.12.041
• Bottazzi et al. (2012). A numerical approach for the analysis of the coffee roasting process. Journal of Food Engineering, 112, 243–252. https://doi.org/10.1016/j.jfoodeng.2012.04.009
• Vargas-Elías et al. (2016). Kinetics of mass loss of Arabica coffee during roasting process. Engenharia Agrícola, 36, 300–308. https://doi.org/10.1590/1809-4430-Eng.Agric.v36n2p300-308/2016
• Hernández, Heyd & Trystram (2008). Prediction of brightness and surface area kinetics during coffee roasting. Journal of Food Engineering, 89, 156–163. https://doi.org/10.1016/j.jfoodeng.2008.04.026