Rheology and Microstructure Investigation of Aiyu and PGA Gels

Jing-Ting Zhu^1,2*, Jung-Ren Huang²

¹Department of Physics, National Taiwan University, Taipei, Taiwan
²Institute of Physics, Academia Sinica, Taipei, Taiwan

* Presenter:Jing-Ting Zhu, email:a00501907@gate.sinica.edu.tw

Aiyu, the fig plant Ficus Pumila var. Awkeotsong, is a native plant of Taiwan, whose edible jelly forms via calcium-mediated gelation of its pectin-like mucilage. We performed a comprehensive, multiscale study combining rheology, spectroscopy (NMR and Raman), and time-evolving Small-Angle X-ray Scattering (SAXS) to elucidate the structural evolution of Aiyu gelation. Because polygalacturonic acid (PGA) constitutes the primary structural backbone of pectin, PGA gels were prepared and analyzed to serve as a structurally relevant control for comparative analysis.

Results on Aiyu Gelation Dynamics
Rheological data reveal the mucilage transitions from a viscous solution (G' < G'') to a gel (G' > G'') within 30-60 minutes. Simultaneously, time-resolved SAXS measurements probe the structural rearrangement, showing a gradual increase in the fractal dimension toward two in the low-q regime as the gel network forms. For high-q regimes, the fractal dimension decreases to zero as gelation progresses. NMR studies show the peaks between 2.4-3.0 ppm move toward higher chemical shifts and become sharper over time. The Raman spectrum shows a broad peak between 300-3000 cm^-1 (532 nm excited) that continuously decreases until the gelation is formed. These findings provide detailed insight into the kinetics of junction-zone formation and aggregation.

Key Observations from Comparative Analysis
The comparison with the PGA gel highlights several key differences from the natural Aiyu system:
1. Solution Environment: Aiyu mucilage can dissolve well at a moderately acidic pH ≈ 5, which is lower than the near-neutral pH ≈ 7 required for commercial PGA. This suggests that side chains and methoxylated groups help the Aiyu pectins dissolve in a low-pH environment.
2. Thermal Behavior: The Aiyu gel exhibits thermal irreversibility, remaining stable upon heating. In contrast, the PGA gel demonstrates thermal reversibility; when heated above 60°C, it becomes a solution state and returns to a gel state after cooling. This indicates the Aiyu network is stabilized by stronger or more complex cross-links than those in simple PGA systems.
3. Syneresis: Aiyu gels exhibit significant syneresis (expulsion of solvent) after reaching maturity (typically after one day). Conversely, the PGA gels remain stable and show no syneresis over long observation periods. This indicates a fundamental differences in network stability and long-term organization.

Keywords: Hydrogels, Rheology, SAXS