Two-Stage Valorization of Olive Pomace: Phenolic Recovery and Chemometric Assessment of Alkaline Treatment for Cellulose Isolation

This study investigated a two-stage valorization strategy for olive pomace, in which phenolic compounds were first recovered and the remaining residue was subsequently subjected to alkaline pretreatment for cellulose-oriented processing. In the first stage, defatting and phenolic extraction parameters were optimized based on total phenolic content (TPC). The acetone/hexane mixture at a 1:5 solid-to-solvent ratio preserved the highest TPC in defatted olive pomace (581.29 ± 6.79 mg GAE/L). For phenolic extraction, the highest TPC was obtained using 70:30 water/ethanol at a 1:5 ratio for 3 h (363.78 ± 1.22 mg GAE/L), although the 2 h treatment under the same conditions showed no significant difference. In the second stage, conventional and autoclave-assisted alkaline pretreatments were compared using 1.25 M, 2.5 M and 3.75 M NaOH. The selected autoclave-assisted treatment with 5% NaOH increased the cellulose proportion to 50.3%, while lignin and hemicellulose decreased to 24.0% and 15.7%, respectively. PCA of FTIR spectra explained 81.88% of the total variance and separated the 5% NaOH autoclave-treated samples. SEM, XRD, color analysis and TD-NMR confirmed matrix disruption, cellulose exposure, higher lightness and treatment-dependent mobility changes. Overall, sequential phenolic recovery followed by autoclave-assisted alkaline pretreatment improved olive pomace valorisation and produced phenolic and cellulose-rich fractions.