Subcritical Water Extraction for Valorisation of Almond Skin from Almond Industrial Processing

Almond skin (AS) is an agro-industrial residue from almond processing that has a high potential for valorisation. In this study, subcritical water extraction (SWE) was applied at two temperatures (160 and 180 °C) to obtain phenolic-rich extracts (water-soluble fraction) and cellulose fibres (insoluble fraction) from AS. The extraction conditions affected the composition and properties of both valorised fractions. The dry extracts obtained at 180 °C were richer in phenolics (161 vs. 101 mg GAE. g^-1 defatted almond skin (DAS)), with greater antioxidant potential (1.063 vs. 1.490 mg DAS.mg^-1 DPPH) and showed greater antibacterial effect (lower MIC values) against L. innocua (34 vs. 90 mg·mL^-1) and E. coli (48 vs. 90 mg·mL^-1) than those obtained at 160 °C, despite the lower total solid yield (21 vs. 29%) obtained in the SWE process. The purification of cellulose from the SWE residues, using hydrogen peroxide (H₂O₂), revealed that AS is not a good source of cellulose material since the bleached fractions showed low yields (20-21%) and low cellulose purity (40-50%), even after four bleaching cycles (1 h) at pH 12 and 8% H₂O₂. Nevertheless, the application of a green, scalable, and toxic solvent-free SWE process was highly useful for obtaining AS bioactive extracts for different food, cosmetic, or pharmaceutical applications.