In years characterised by a strong presence of the olive fly, it's important immediately start filtering the oils obtained from damaged olives: they result, as we have already seen in a recent article, more cloudy, unstable and subject to rapid oxidative and microbiological degradation. This is due to the high presence ofmicrodroplets of emulsified water, air, proteins, enzymes, pectins and mucilages.
These substances, while giving the oil a “veiled” appearance, they constitute an unstable component, capable of initiating processes of degradation already in the first few weeks.
The many defects that emerge
The residual water content can exceed 2000 ppm (parts per million), favoring the activity of hydrolytic and oxidative enzymesWhen olive pulps are damaged, they release a reactive mixture of enzymes and colloidal materials: among these, Lipases degrade triglycerides into free fatty acids and glycerol, increasing the acidity and reducing the stability of the oil..
At the same time, the lipoxygenase (LOX) and other oxidative enzymes (peroxidase, catalase, superoxide dismutase, ascorbate peroxidase), which in healthy olives perform an antioxidant function, after extraction they now become unstableIn the presence of oxygen and water, these enzymes accelerate the formation of peroxides and secondary compounds, responsible for rancidity and the loss of fresh aromas.
L'aqueous environment rich in organic residues It also promotes the proliferation of yeasts, bacteria and molds coming from damaged olives. These microorganisms produce metabolites such as ethanol, acetic acid and volatile aldehydes, which generate sensory defects known as scald, winey and sludge, compromising the product classification of the oil.
Cellular degradation also involves the loss of water-soluble polyphenols, such as hydroxytyrosol, dispersed in the aqueous fraction, and thealteration of the volatile compounds responsible for the green fruitiness and herbaceous notes. The result is a unstable oil, with a reduced shelf life and a tendency to develop defects just a few months after milling.
In oils obtained from olives affected by the fly or overripe, the sludge formation is particularly rapid, visible deposits can appear as early as 15–20 days. These residues activate fermentative and anaerobic reactions that lead to the appearance of the main sensory defectsThe quantity and speed of formation of muck depend on the variety, climatic conditions, and health of the olives: rainfall during harvest or prolonged drought influence the presence of solid particles and emulsified water.
Filtration: a preventive and qualitative measure
In this situation, filtration represents a fundamental intervention: removes water, enzymes and suspended particles, limiting degradation processes and reducing the microbiological load. small productions intended for self-consumption, can be a careful and timely transfer is sufficient, but for For professional productions, filtration should be carried out within 24–48 hours of extraction.
Filtration techniques: from traditional to technological
Techniques vary from the traditional method “Barese style”, which he uses cotton wool for static filtration, to more advanced systems, such as the layer filtration (plates), with stainless steel frames that house cellulose and mineral filter layers (examples: Seitz® – Sartorius, Becopad® – Eaton Filtration, SUPRApak™ – Pall/Sartorius). Depth filter layers, composed of cellulose, diatomite, and perlite, represent the technological heart of professional filtration today.
These materials act on two fronts:
- Mechanical restraint: Larger solid particles, pulp fragments, cell tissue, and sludge, are retained within the porous structure of the filter. The cellulose forms a three-dimensional mesh that traps impurities based on their size.
- Selective absorption: The smallest impurities, such as proteins, enzymes and mucilage, bind to the filter surface, allowing for targeted and effective cleaning.
This double mechanism produces a clear, stable oil free from reactive fractions, reducing the risk of sensory defects and improving shelf life. Closed module systems, such as SUPRApak™, also offer greater filter surface area, reduced oxidation, and improved hygiene.
Cross-flow filtration, based on polymer or ceramic membranes, is particularly gentle and suitable for oils with high aromatic quality. The operating pressure, 0,5–2 bar, and the flow rate must be regulated with peristaltic or centrifugal pumps, avoiding turbulence that could reactivate adsorbed particles or introduce oxygen.
Monitoring and benefits of early filtration
During filtration, it is useful to monitor turbidity by evaluating the peroxide value, free acidity, K232/K270 parameters, and total phenolic content. Numerous studies demonstrate that Early filtration reduces the peroxide index and the content of pro-oxidant metals, such as iron and copper; improves oxidative stability and shelf life; preserves lipophilic phenols, such as oleocanthal, oleuropein aglycone, and the volatile compounds responsible for the green fruitiness; it eliminates the potentially harmful fermentative microbiota.
When stored correctly, a filtered oil can maintain its extra virgin characteristics for over twelve months, while an unfiltered oil tends to develop defects after just 3–4 months.
In short, in difficult years, filtration completes the oil's qualitative process, protecting the work of the olive grower and the miller and guaranteeing the consumer a stable, healthy product consistent with its original sensory characteristics.
Filtering does not mean “stripping” the oil, but preserving its purity, integrity and its ability to last over time.
