Olive paste, derived from the milling and malaxing operations, is the input material for the separation step in the production of extra-virgin olive oil. Olive paste is a heterogeneous mixture of three phases. In order of decreasing density, they are:
- The ‘insoluble solids phase’, consisting of organic semisolid components and the woody fragments from the pit shells. It is 25–30% by weight of the olive paste, with 75% pit fragments and 25% cell wall fragments.
- The ‘aqueous phase’, consisting of water and water-soluble components (salts, simple sugars, simple phenolics and so forth). It is 50–60% of the total paste weight, with 92–95% water and 5–8% soluble solids.
- The ‘oil phase’ consists of 97–99% triglycerides and 1–3% minor components, the latter being a complex mixture of lipophilic, hydrophilic and amphiphilic components with critical roles in sensory and nutritional quality. It is estimated to be 10–20% by weight of the olive paste, depending on cultivar and fruit maturity.
The most important objective of the separation process is to recover as much oil as possible from the olive paste.
- The three-phase process
The first centrifugation step is carried out with a three-phase decanter in which the three phases are separated. This is based on their different densities, in the order of increasing density: the oil – the aqueous – and the insoluble solids phases. The insoluble solids are discharged as pomace and sent to further fractionation and utilization.
- The two-phase process
The wastewater problem has spurred research on new decanter design and operation. Three-phase LWC (Low Water Consumption) and two-phase decanters are the alternative solutions now available. The insoluble solids and vegetation water are discharged together from the decanter as a semi-liquid slurry.
The choice of a two or a three-phase decanter requires careful evaluation by the milling company, taking into account various technical, economic and environmental aspects.