Olive Mill Wastes: A Source of Bioactive Molecules for Plant Growth and Protection against Pathogens

Abstract

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.

Available Pathways for Operationalizing Circular Economy into the Olive Oil Supply Chain: Mapping Evidence from a Scoping Literature Review

“…The by-products of olive oil extraction process generated by the agro-industrial subsystem are solid and liquid olive mill wastes (OMWs), in varying amounts and physiochemical compositions depending on fruit-related factors (e.g., variety or ripening stage of the processed olives) or production year and microbial treatments [8], as well as on the extraction method employed (i.e., traditional discontinuous pressure system and three-phase, twophase or multi-phase decanter continuous centrifugation cycle). Apart from olive leaves and olive stones, that can be minor agro-industrial by-products (the former resulting from the initial washing phase, the latter obtainable by separation from pulp before or after oil extraction), the main OMWs are the following:…”

“…In fact, it involves resource-intensive activities (in terms of chemical, energy, and water requirements) and generates several by-products and residues, which usually end up as wastes. The waste streams deriving from the milling stage are particularly troubling to manage, being produced in huge amounts on a seasonal base and in a short period of time (typically from October to January) [8], presenting high phytotoxic and polluting properties, and causing severe environmental impacts if inappropriately disposed [9,10].…”

“…Currently, if not sent to pomace oil extraction facilities, which are not equipped to accept wet pomace by the way, exhausted pomace is commonly destinated to incineration, with consequent loss of organic matter [15]. By mechanically separating the pulp from kernels during the extraction process, the recently introduced multi-phase decanter technology produces two novel by-products: a wet destoned fraction, called patè olive cake, and a dry kernel-enriched fraction [8].…”

“…Olive mill wastewater (OMWW), which is a dark-brown liquid composed by 83-92% of water (vegetative and process water) and for the remaining part by an heterogeneous complex of organic matter constituted by olive oil residues and olive pulp [8,13]. This by-product is generated at the highest rates by the pressure system and above all by the three-phase extraction method (respectively, 600 and 800-950 kg per ton of processed olives), and in drastically reduced quantities (250 kg) by the two-phase extraction method.…”

“…This by-product is generated at the highest rates by the pressure system and above all by the three-phase extraction method (respectively, 600 and 800-950 kg per ton of processed olives), and in drastically reduced quantities (250 kg) by the two-phase extraction method. While containing valuable substances such as mineral nutrients and bioactive molecules, OMWW is also characterized by an high degree of phytotoxicity, important polluting charge, and low biodegradability [16], for which it is known as one of the most harmful effluents in agriculture, and it has been considered and treated for a long time only as an environmental problem and an economic burden, due to its high disposal costs [8,17]. The direct use of this by-product as a water source is limited: in order to be safely utilized in other production cycles, it requires a detoxifying treatment [8].…”