Physics of Water Absorption and Expansion of Coffee Grounds during Espresso Puck Pre-infusion

In-depth Exploration of Roasting Science: Thermal Decomposition of Trigonelline and Physicochemical Dynamics Inside the Bean

Coffee roasting is a highly non-equilibrium dynamic process occurring due to the superposition of numerous physical phase transitions and chemical reactions within the bean's cellular structure. In particular, trigonelline (1-methylpyridinium-3-carboxylate), an alkaloid representing approximately 1.0–1.2% of green beans, undergoes thermal decomposition during roasting to be converted into highly bioavailable niacin (Vitamin B3), while simultaneously acting as a primary precursor for pyridine-based volatile aroma compounds. This process is closely linked to the internal energy balance and glass transition phenomena of the bean, extending beyond simple temperature changes.

1. Glass Transition Temperature $(T_g)$ and Dynamic Transitions of Cellular Structure

Green beans consist of a dried porous cellulose matrix. During the roasting process, moisture content $(w)$ acts as a plasticizer that determines the flexibility of the polymer matrix. The glass transition temperature $(T_g)$ of the cell wall can be described by the Gordon-Taylor relation:

$ T_g = \frac{w_1 T_{g1} + k w_2 T_{g2}}{w_1 + k w_2} $

In the early stages of roasting, when moisture content is high, $T_g$ is low, placing the bean in a "Rubbery" state. In this state, the cellular structure is prone to plastic deformation in response to external pressure and water vapor expansion. However, as it passes through the browning stage and moisture content drops to 2–3%, the free volume of the polymer chains decreases, leading to a sharp rise in $T_g$. The bean then becomes fixed in a "Glassy" solid state with high brittleness. This phase transition occurs after the first crack and serves as a physical metric that ultimately determines the final morphology of the porous structure $(\epsilon \approx 0.3 \sim 0.5)$.

2. Thermal Energy Balance Model and Chemical Conversion of Trigonelline

The internal temperature of the bean is determined by the combined action of multiple heat transfer modes in the roaster: conduction $(q_{cond})$, convection $(q_{conv})$, and radiation $(q_{rad})$. The transient heat energy balance equation governing this is as follows:

$ m C_p \frac{dT_s}{dt} = q_{cond} + q_{conv} + q_{rad} - Q_{evap} + Q_{rxn} $

The physical meaning of each term in the above equation is clear. In the drying stage, the latent heat of water vaporization $(Q_{evap} = \frac{dm_w}{dt} \Delta H_{vap})$ is overwhelmingly large, suppressing the rate of rise (RoR) of the bean temperature. However, near the first crack, the evaporation rate decreases, and as the thermal decomposition of trigonelline and the Maillard reaction become active, $Q_{rxn}$ shifts to an exothermic reaction with a positive (+) value.

• Demethylation Reaction: Trigonelline undergoes demethylation due to thermal energy in the range of approximately 180°C–230°C to produce nicotinic acid (niacin). The higher the roast intensity, the greater this conversion rate, which enhances the functional and nutritional aspects of coffee.
• Generation of Pyridine Compounds: Pyridine, methylpyridine, and vinylpyridine as thermal decomposition byproducts have low sensory thresholds, forming the toasted aroma and sharp smokiness of coffee. The rate of this process is exponentially dependent on temperature according to reaction kinetics.

3. Engineering Approach to Roasting Control

In modern coffee science, flavor optimization depends on the control of pyridine generation. If the RoR is excessively high, rapid thermal decomposition occurs before the Maillard reaction, resulting in an imbalance where carbonized pyridine odors are emphasized. Therefore, in high-temperature ranges, the convection coefficient $(h)$ must be precisely adjusted to minimize the temperature gradient between the particle surface and the center. The table below summarizes the changes in physical properties according to roasting temperature ranges.

Process Stage	Temperature Range	Physicochemical State
Drying	100-150°C	Rubbery, dominated by $Q_{evap}$
Browning	150-190°C	$T_g$ rise, initiation of Maillard reaction
Exothermic	190°C~	Glassy, acceleration of exothermic $Q_{rxn}$

In conclusion, controlling heat release in the latter half of the roasting process is essential to ensure efficient niacin conversion of trigonelline and the complexity of pyridine aromas. From an engineering perspective, it is clear that designing roasting airflow that accounts for the bean's thermal conductivity and optimizing heat energy transfer by considering particle structure porosity are essential.