Porous Expansibility and Component Solubility Properties of Decaffeinated Coffee Beans

Decaffeination of coffee is a sophisticated biochemical separation process that selectively captures and removes the alkaloid caffeine (C₈H₁₀N₄O₂) while minimizing the loss of core organic compounds (chlorogenic acid, sugars, amino acids, etc.) within the green bean. The supercritical carbon dioxide (scCO₂) extraction method and the Swiss Water Process (SWP), which are widely used in the modern coffee industry, possess distinct physicochemical equilibrium mechanisms. Decaffeinated green beans processed in this way exhibit unique thermal reaction kinetics during roasting due to thermodynamic structural changes.

1. Supercritical Carbon Dioxide (scCO

₂) Extraction and Phase Equilibrium Partition Coefficient

Carbon dioxide reaches a supercritical fluid state, possessing both the solvent power of a liquid and the diffusivity of a gas, above its critical temperature (T_c = 31.1°C) and critical pressure (P_c = 73.9 bar). Mass transfer of caffeine within supercritical carbon dioxide is determined by the phase equilibrium Partition Coefficient (K) between the bean matrix and the supercritical phase:

K = C_scCO2 / C_bean

Here, C_scCO2 is the equilibrium concentration of caffeine in the supercritical carbon dioxide phase, and C_bean is the caffeine concentration inside the green bean cellular matrix. The K value is directly influenced by changes in the density (ρ) of the supercritical carbon dioxide as a function of system temperature and pressure. As pressure increases, fluid density increases, sharply raising the solvent power and exponentially increasing the partition coefficient K. In this process, a pre-treatment step that absorbs approximately 30% moisture into the green beans to induce cell wall swelling is essential, as water acts as a polar co-solvent, weakening the hydrogen bonds between caffeine molecules and promoting diffusion into the supercritical fluid.

2. Chemical Equilibrium and Osmotic Control in the Swiss Water Process (SWP)

The Swiss Water Process is an eco-friendly method that uses only water and activated carbon, excluding chemical solvents. The core of this process is the chemical equilibrium of Green Coffee Extract (GCE). GCE is an aqueous solution in which all water-soluble solids (sugars, organic acids, amino acids, etc.) of the green bean, excluding caffeine, are saturated. When caffeine-containing green beans are immersed in GCE, due to the difference in the chemical potential of the system, only caffeine molecules are eluted out of the bean to reach diffusion equilibrium driven by the concentration gradient (ΔC):

J = -D × (dC / dx)

According to Fick's 1st Law, the diffusion flux J is proportional to the concentration gradient. Since the concentrations of sugars and organic acids between the inside of the green bean and the external GCE are already at equilibrium, no net transfer of these substances occurs, and only caffeine is selectively eluted. During this process, the infiltration of water causes the porous structure of the green bean to expand, permanently reducing the mechanical strength of the cell walls.

3. Selective Caffeine Capture Mechanism of Activated Carbon

Caffeine eluted into the GCE is adsorbed and removed as it passes through a caffeine-selective activated carbon filter layer. While general activated carbon has a large surface area and adsorbs all organic matter, the activated carbon used in the SWP is coated with carbohydrates or calcium chloride in its micropores, physically blocking the adsorption of larger molecules like carbohydrates and organic acids, and selectively adsorbing only caffeine molecules with a hydrophobic planar structure. This adsorption equilibrium is described by the Langmuir adsorption isotherm:

θ = K_ad × C / (1 + K_ad × C)

Here, θ is the caffeine occupancy on the activated carbon surface, K_ad is the adsorption equilibrium constant, and C is the caffeine concentration in the solution. The GCE, now completely stripped of caffeine, is recycled back into the green bean tank to induce continuous decaffeination.

4. Roasting Kinetics of Decaffeinated Green Beans

Decaffeinated green beans have a porosity (ε) that has risen sharply from approximately 0.2 in regular beans to over 0.

35. Furthermore, during the decaffeination process, the hemicellulose structure constituting the cell walls is degraded, lowering the glass transition temperature (

T_g) of the amorphous polymer by approximately 15°C to 20°C. This results in the following specific thermodynamic reactions during roasting:

• Accelerated Heat Conduction: Due to the high porosity and low T_g, the thermal penetration depth decreases, causing a sharp increase in thermal conductivity. Heat is rapidly transferred to the core of the bean at the start of roasting, causing Maillard and caramelization reactions to proceed more than 1.2 times faster than in regular beans.
• Premature First Crack: The accumulation rate of internal carbon dioxide (CO₂) and water vapor pressure due to pyrolysis increases, causing the First Crack to occur at a much lower temperature (approximately 5°C to 8°C lower compared to regular beans).
• Need for Rapid Rate of Rise (RoR) Control: Because the heat absorption rate is high and the physical brittleness of the cell wall is increased, applying a standard roasting profile easily leads to scorching on the bean surface or a phenomenon where the outside burns while the inside remains under-roasted. Therefore, it is essential to lower the charging temperature and precisely modulate the heat supply around the First Crack.