Dynamics of Espresso Basket Geometry and Its Effect on Flow Rate and Extraction Yield Distribution

Sugar Caramelization Temperature Ranges and Physicochemical Mechanisms During Coffee Roasting

In the green coffee roasting process, caramelization is more than just a phenomenon of sugar turning brown; it is a key non-enzymatic browning reaction that determines the sensory characteristics of coffee. In particular, the polymers generated through the decomposition and recombination of sucrose at high temperatures above 160°C form the body and complex sweet aromas of coffee. This paper analyzes roasting efficiency through the chemical mechanisms and thermodynamic modeling of this process.

1. High-Temperature Pyrolysis of Sugars and Reaction Pathways

Caramelization is a process where sucrose is sequentially polymerized into caramelan, caramelen, and caramelin through dehydration reactions. This reaction proceeds without the involvement of amino acids, and as the reaction temperature increases, volatile components such as diacetyl and furan series are amplified. The thermodynamic core of the reaction is governed by the following thermal energy balance equation:

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

Here, $Q_{rxn}$ represents the reaction enthalpy generated during the Maillard reaction and caramelization process. When the temperature reaches 200°C after the first crack, the exothermic reaction $(Q_{rxn} > 0)$ accelerates, accompanied by rapid changes in the internal structure.

2. Glass Transition Temperature $(T_g)$ and Cell Structure Dynamics

The phase transition of the cellulose matrix inside green coffee beans during roasting determines the porous structure of the coffee. According to the Gordon-Taylor relation, the glass transition temperature $(T_g)$ is highly dependent on the moisture content $(w)$.

$ T_g = \frac{w T_{gw} + k(1-w) T_{gs}}{w + k(1-w)} $

In the above equation, $w$ is the moisture content that acts as a plasticizer. During the initial drying stage, $T_g$ is low, keeping the cell walls in a rubbery state and promoting plastic deformation, but as moisture evaporates, $T_g$ rises exponentially. This causes the coffee bean to become brittle after the first crack and is a decisive factor in forming the micro-porosity $(\epsilon \approx 0.3 \sim 0.5)$ of the cell walls in the latter stages of roasting.

3. Heat Transfer Mechanisms According to Roasting Environment

The quality of the beans depends on the efficiency with which the external heat source is transferred to the center of the beans. Heat transfer within the roasting drum involves a complex interaction of three modes.

• Conduction: $q_{cond} = -k A abla T$, referring to contact heat transfer between the drum surface and the green coffee particles.
• Convection: $q_{conv} = h A (T_g - T_s)$, adjusting the roasting speed by controlling the heat transfer coefficient $h$ provided by the heated gas flow to the surface.
• Radiation: $q_{rad} = \epsilon \sigma A (T_{surr}^4 - T_s^4)$, where the radiant energy of the drum inner wall is absorbed by the bean surface, promoting the development of brown pigments.

Reaction Stage	Temperature Range	Physicochemical Characteristics
Drying Stage	100-150°C	Endothermic reaction (dominated by $Q_{evap}$), preparation for glass transition
Maillard/Caramelization	160-195°C	Generation of volatile components, increase in $Q_{rxn}$
End and Carbonization	200°C or higher	Acceleration of exothermic pyrolysis, maximization of $T_g$

In conclusion, to maximize caramelization efficiency, the Rate of Rise (RoR) following the sharp drop in $Q_{evap}$ after the drying stage must be precisely controlled. By properly adjusting convective heat transfer in the $170\sim 195^{\circ} C$ interval, it is possible to optimize sugar decomposition and successfully fix aromatic compounds within the porous structure formed inside the beans. Roasters should not simply focus on increasing temperatures, but rather establish strategies to control compound development by understanding physical $T_g$ changes and heat transfer equations.