Noni fruits (Morinda citrifolia) are a source of phenolic bioactive compounds (scopoletin, alizarin, and rutin), which have antioxidant, antimicrobial, anticancer, and anti-inflammatory activities. In this study, subcritical water was applied to determine the extraction yields and kinetics of phenolic compounds from noni fruits. The scopoletin and alizarin yields increased with the increase in temperature from 100 to 140 °C, while that of rutin increased up to 120 °C and then decreased at 140 °C. The yields of all the compounds rapidly increased from 1 to 2 mL/min and then slightly up to 3 mL/min of water flow rate. The extraction kinetics were assessed using two mathematical models. The two-site kinetic desorption model had a better fit for all experimental conditions throughout the extraction cycle and best described the extraction kinetics of phenolic compounds from noni fruits. The diffusion coefficients of scopoletin and alizarin at 140 °C and 3 mL/min were 3.7- and 16.2-fold higher than those at 100 °C and 1 mL/min, respectively. The activation energies of alizarin were 2.9- to 8.5-fold higher than those of scopoletin at various flow rates. Thus, subcritical water could be an excellent solvent with higher extraction yields and shorter extraction times using an environmentally friendly solvent.

The present study investigated the recovery of antioxidative compounds obtained from Pseuderanthemum palatiferum (Nees) Radlk. and their biological activities using subcritical water in comparison with hot water, Soxhlet, and methanol extraction. In this study, high-performance liquid chromatography (HPLC) and three commonly used assays, that is, 2,2-diphenyl-1-picrylhydrazyl assays, 2,2\'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid), and ferric reducing antioxidant power, were employed to measure the number of valuable compounds and their antioxidant capacities, respectively. Additionally, the antimicrobial ability of the various extracts against seven pathogenic bacteria strains was assessed. The results demonstrated that the extracts obtained from subcritical water extraction (SWE) contain the highest amount of saponin (33.82 ± 2.08 mg diosgenin/g), phenolic (34.87 ± 0.29 mg CE/g), and protein (104.66 ± 7.63 mg BSA/g), which are obtained at 170 °C, 190 °C, and 230 °C, respectively. The highest amounts of total sugar and flavonoids (211.73 ± 1.58 mg glucose/g and 20.71 ± 0.42 mg RE/g, respectively) were obtained at lower temperature (130 °C and 130 °C, respectively), and these values were higher than those obtained from hot water, Soxhlet, and methanol extraction. HPLC analysis indicated that the highest amount of apigenin (3.46 ± 0.03 mg/g) and kaempferol (2.43 ± 0.03 mg/g) were obtained by SWE at 170 °C and 190 °C, respectively. Furthermore, the extracts from SWE exhibited higher antimicrobial activity against five of the seven pathogenic bacterial strains tested compared with those obtained from conventional extraction methods. Therefore, subcritical water could be utilized as a cost-effective and green solvent to extract valuable compounds from P. palatiferum (Nees) Radlk. leaf. PRACTICAL APPLICATION: Subcritical water was able to recover more bioactive compounds from the sample, such as phenolics, flavonoids, saponin, protein, and polysaccharides, than conventional solvents. The mixture of polyphenolic-polysaccharide-protein conjugates could be used in further steps, isolation, and purification, and applied to functional food.