Author : Nur Lailatul Rahmah

Summary : Piper betle L. (PBL) leaves are widely cultivated in Malaysia. PBL leaves contain abundant of phenolic compounds with many classes of phenolic acids such as caffeic acid (CFA), ferulic acid (FA), p-Coumaric acid (p-CA), gallic acid (GA) and chlorogenic acid (CGA) as antioxidant sources. To reduce the issues associated with organic solvent usage for extraction, subcritical water extraction (SWE) is recommended to recover the phenolic compounds and phenolic acids from PBL leaves. This study aims to extract the phenolic compounds, quantify the phenolic acids of PBL leaves using SWE and analyze the kinetic model. The first part of this study is to evaluate the effect of SWE process conditions (temperatures ranging from 100-275°C and times of 1-30 min) on the total phenolic content (TPC) and antioxidant activity (AA) of PBL leaves. Temperature and time of SWE had a significant effect on TPC and AA (p < 0.05), with the highest TPC (7.89 ± 0.26 mg GAE/g extract) and AA (92.98 ± 1.79%) obtained at 200°C for 5 min with 15% w/w solid loading. The relationship between phenolic acids content, TPC, SWE process conditions and subcritical water (SW) properties were analyzed. Temperature and time (p &lt; 0.05) significantly influenced the recovered phenolic acids at different process conditions due to different SW properties. FA (0.454±0.008 mg/g extract) is the main phenolic acid in the PBL leaves, with the maximum content achieved at 200°C for 5 min, whereas the highest content of other phenolic acids were extracted at different process conditions. The highest total phenolic acids (TPA) (0.582 mg/g extract) and TPC were obtained under similar process conditions at 200°C and 5 min. This result aligns with a significant correlation between TPA, SWE process condition, SW properties, TPC, and AA. SWE is a better method to extract phenolic acids, as evidenced by its higher TPA content than the Soxhlet method (0.291 mg/g extract). The kinetic model of TPC and phenolic acid were analyzed to understand the SWE process and predict its content. The two-site kinetic desorption first-order (TSDF) and Weibull models fit well with the experimental data on TPC extraction from PBL leaves utilizing SW. The models suggest that the extraction of TPC by SWE involves two primary processes: desorption (fast and slow) and mass transfer, similar to the conventional solid-liquid extraction systems. The kinetic study of phenolic acids extracted from PBL leaves using SWE reveals varying trends in the extraction rate constant with temperature. The temperature influenced the polarity of subcritical water in the system, which altered the solubility of phenolic acids. High temperature decreases dielectric constant, viscosity, and surface tension, hence enhancing the extraction rate constant. Phenolic acid's structure and the presence of glucose also affect the extraction rate. The TSDF model is more suitable for describing the extraction of phenolic acids compared to the Weibull model. Therefore, SWE is an effective method for the recovery of phenolic acids as antioxidant sources from PBL leaves, with the TSDF model being the most appropriate for explaining the process and predicting the content.