Agar-based gels enriched with Platostoma palustre extract: Effects of concentration and pH on structure, mechanics, and phenolic release

Nutritional food gels have attracted increasing attention for their ability to improve food quality and functionality. Agar-based gels and Platostoma palustre (Hsiantsao) are widely consumed as beverages or jellies in many Asian countries. The extract concentration and pH are critical factors influencing the mechanical, physical, and biochemical properties of these gels. This study investigated the structural, mechanical, and release characteristics of agar gels incorporating P. palustre extract (PPE) under varying concentrations and pH conditions. Phenolic compounds in PPE acted as organic cross-linkers and structuring agents, promoting the formation of stable and homogeneous gel networks. FTIR analysis confirmed hydrogen bonding and interactions between polysaccharides and polyphenols within the gel matrix. SEM observations and pore size analysis revealed that gels containing 20-30% PPE formed denser networks with smaller pores, resulting in slower phenolic release, with a peak at 80 min. In contrast, 10% PPE gels exhibited larger pores and faster release, peaking at 10 min. Gels containing 20-30% PPE at pH 5 exhibited the highest phenolic release (24.54 and 33.98 μg GAE/mL after 80 min) while maintaining structural integrity. Under acidic conditions (pH 3), the A20-P3 gel showed the highest compressive strength (72.87 ± 6.39 kPa). Additionally, PPE exhibited strong antioxidant activity (as measured by DPPH, ABTS + , and FRAP assays) and α-glucosidase inhibitory activity. Overall, the incorporation of P. palustre aqueous extract significantly influenced the structural integrity, mechanical properties, and release behavior of agar-based gels. These findings suggest that agar-based gels enriched with P. palustre extract could serve as functional food matrices capable of maintaining a stable structure and enabling controlled release, thereby enhancing their overall health benefits.