Profiling of phenolic compounds and evaluation of bioactivities in almond hull extracts via subcritical water extraction

Almond hulls (AH), a major byproduct of almond production, are rich in bioactive phenolic compounds. However, conventional extraction methods often face limitations in efficiency and selectivity. To address this issue, the present study employed subcritical water extraction (SWE) to improve extraction performance. AH extracts were prepared using SWE at 10 MPa, with temperatures ranging from 110 to 190 °C and treatment times of 5–20 min. The highest total phenolic content (181.11 mg GAE/g dried AH) and total flavonoid content (97.11 mg CE/g dried AH) were obtained at 130 °C for 20 min. LC–MS/MS analysis was performed and identified key phenolic compounds including protocatechuic acid, catechin, chlorogenic acid, caffeic acid, and procyanidin B. These compounds contributed to the extracts’ strong antioxidant properties, as confirmed by ABTS (111.2 mg AAE/g dried AH), FRAP (80.21 mg AAE/g dried AH), and DPPH (77.24 mg AAE/g dried AH) assays. The extract also exhibited notable anti-melanogenic activity in B16F10 cells and antimicrobial activity against Staphylococcus aureus, demonstrating its bioactivity. These findings highlight SWE as an environmentally friendly and sustainable technique for recovering valuable bioactive compounds from almond byproducts, supporting their potential applications across various industries.