Leakage of Organic Fatty Acids and Rancidity Rate According to Green Bean Cell Membrane Permeability During Storage

Physicochemical Integrity of Green Bean Cell Membranes and Storage Models Based on Glass Transition Temperature $(T_g)$

Preserving the quality of specialty coffee goes beyond simply shielding it from external environments. The cellulose matrix constituting the cell wall of the green bean and the phospholipid bilayer within it undergo a phase transition depending on temperature and water activity $(a_w)$. To understand the physical changes during storage, the influence of moisture as a plasticizer on the glass transition temperature $(T_g)$ of the green bean matrix must be modeled using the Gordon-Taylor equation.

From a polymer physics perspective, the glass transition temperature $(T_g)$ of the cellulose matrix inside the green bean is closely related to its moisture content $(w)$. As moisture increases, the intermolecular distance of the substrate within the green bean increases, securing molecular mobility and causing $T_g$ to decrease, which can be approximated as follows:

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

Here, $w_1$ and $w_2$ are the mass fractions of cellulose and moisture, respectively, and $k$ is a constant representing plasticization efficiency. When the storage temperature of the green bean environment exceeds this $T_g$, the green bean cell membrane transforms from a solid glassy state into a flexible rubbery state. In this state, membrane permeability increases rapidly, leading to the leakage of free fatty acids, which were previously sequestered within the phospholipid bilayer, into the cytoplasm. This results in an increase in substrate concentration for oxidative catalytic reactions.

Bean Physics Heat Transfer Models and Energy Balance in the Roasting Process

When stored green beans are introduced into the roasting process, the thermal energy balance during the initial drying stage is defined by transient thermodynamic equations. In the early stages of roasting, the evaporation of bound water inside the green bean causes a significant endothermic reaction $(Q_{evap})$. The energy balance equation governing the Rate of Rise (RoR) of the bean surface temperature is as follows:

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

The physical significance of each term in this equation is as follows:

• $q_{cond} = -k A abla T$: Conductive heat transfer due to direct contact between the drum wall and green beans
• $q_{conv} = h A (T_g - T_s)$: Convective heat transfer between hot air and the green bean surface
• $q_{rad} = \epsilon \sigma A (T_{surr}^4 - T_s^4)$: Absorption of radiant heat inside the roaster
• $Q_{evap} = \frac{dm_w}{dt} \Delta H_{vap}$: Latent heat loss due to moisture evaporation

As the temperature increases during roasting, $T_g$ rises sharply due to the evaporation of moisture, which acts as a plasticizer. Consequently, the green bean matrix transitions back into a glassy state, and after the first crack, structural brittleness increases, forming a porous structure $(\epsilon \approx 0.3 \sim 0.5)$. The pyrolysis reaction $(Q_{rxn})$ occurring during this process shows a behavior where it shifts from endothermic to exothermic at approximately $200^{\circ} C$, which serves as a critical inflection point for roaster control.

Conclusion: Establishing Storage Strategies

Regulating cell membrane permeability is the core of green bean storage. Only when the storage temperature is maintained below $T_g$ can the crystallinity of the phospholipid bilayer be preserved, physically limiting the rate of rancidity reactions. Since green beans with higher moisture content have a lower $T_g$, hot and humid environments cause fatty acids within the green bean to leak into the cytoplasm, leading to rapid degradation of flavor. Therefore, cold storage at $15 \sim 18^{\circ} C$ is the most scientific method to preserve the internal substrate of green beans in a high-glassy state, thereby maximizing the physical integrity of the cellular structure.