Olive oil production generates high amounts of liquid and solid wastes. For a long time, such complex matrices were considered only as an environmental issue, due to their polluting properties. On the other hand, olive mill wastes (OMWs) exert a positive effect on plant growth when applied to soil due to the high content of organic matter and mineral nutrients. Moreover, OMWs also exhibit antimicrobial activity and protective properties against plant pathogens possibly due to the presence of bioactive molecules including phenols and polysaccharides. This review covers the recent advances made in the identification, isolation, and characterization of OMW-derived bioactive molecules able to influence important plant processes such as plant growth and defend against pathogens. Such studies are relevant from different points of view. First, basic research in plant biology may benefit from the isolation and characterization of new biomolecules to be potentially applied in crop growth and protection against diseases. Moreover, the valorization of waste materials is necessary for the development of a circular economy, which is foreseen to drive the future development of a more sustainable agriculture.

Almond blanched skins (BS) and blanch water (BW) are underutilized agro-industrial by-products, despite their richness in polyphenols and other bioactive compounds. This study investigates the reuse of BW as an extraction solvent to recover polyphenols from BS, avoiding the need for additional water or organic solvents. BW showed a total polyphenol content (TPC) of 475.2 ± 10.7 µg GAE/mL and a Total Antioxidant Capacity (TAC) of 840.4 ± 41.8 µM TE. Thermal extraction at 115 °C and 1.7 atm for 1 h (1:10 BS/BW w/v) produced an enriched blanch water (EBW) with a TPC of 809.2 ± 12.1 µg GAE/mL and a TAC of 2188.8 ± 3.6 µM TE. Subsequent microfiltration and nanofiltration further concentrated the bioactive fraction, with the nanofiltration retentate exhibiting a 2.8-fold increase in TPC and a 2.1-fold increase in TAC compared to the EBW feed. Spray-drying generated a fine, homogeneous, and storage-stable powder without loss of antioxidant capacity. UHPLC-QqTOF-MS/MS analysis identified catechins and their derivatives, procyanidins, and flavanol glycosides as the main polyphenols contributing to the strong antioxidant capacity of the powder. Overall, this workflow provides a scalable and water-saving strategy to convert almond-processing residues into high-value functional ingredients for food, cosmetic, and pharmaceutical applications.

Olive oil is a core component of the Mediterranean diet known for its nutritional properties and health benefits. Olive industry is moving to novel extraction systems for higher oil yield and quality and for waste reduction, which is a relevant problem in the process due to its toxicity and high disposal costs. Multi-Phase Decanter (DMF) is a modern two-phase system performed without adding water during the process. Using DMF, a wet by-product indicated as pâté and consisting of the fruit pulp and vegetation water (VW) is recovered. The pâté has a high content of potentially bioactive molecules that may be exploited to promote plant resistance against microbial pathogens. In this work, to identify by/products of biological interest, the VW recovered from the pâté by centrifugation was subjected to fractionation by tangential-flow membrane filtration (TFMF), combining microfiltration (MF) and ultrafiltration (UF). High-resolution NMR spectroscopy indicated the presence of bioactive molecules such as flavonoids, hydroxytyrosol and oleuropein with known antimicrobial activity. High-Performance Anion Exchange Chromatography with Pulsed Amperometric Detection (HPAEC-PAD) was performed to detect the presence of pectic oligosaccharides in the fractions, showing the enrichment, in the UF-concentrate fraction, of oligogalacturonides (OGs), well known for the ability to elicit defense responses and protect plants against pathogen infections. Arabidopsis thaliana plants treated with TFMF fractions displayed induction of defense responses and exhibited resistance against microbial pathogens without adverse effects on growth and fitness. This study shows that pâté by-products can potentially be exploited in agriculture as sustainable plant phyto-protectant. Graphical (image created with BioRender)