The effect of filter press filtration on the shelf life of extra virgin olive oil (EVOO) is studied.
Olives were harvested and processed on the same day by a continuous industrial plant equipped with a hammer crusher, a vertical malaxator, and a decanter.
One part of the oil obtained was bottled in 0.5 liter dark-green bottles with headspace representing a quarter of the bottle’s volume (cloudy oil). The other part was filtered (using a filter press) directly after decanting and manually bottled using the same procedure (filtered oil).
All samples were stored in a purpose-built chamber and periodically analyzed. Samples were characterized by their water content, free acidity, peroxide value, specific extinction coefficients (K232 and K268), total chlorophyll, tocopherols, total phenolic compounds, HPLC phenolic profile, volatile compounds, fatty acid composition, and sensory analysis.
Immediately after filtration, the only differences that could be detected were related to turbidity and total solids. In the subsequent analyses, filtered oil retained superior nutritional and sensory characteristics than cloudy oil. More importantly, the filtered oil remained at the standard required for categorization as “extra-virgin olive oil” for longer.
Data show that the free acidity increases in the cloudy samples, whereas they remain constant in the filtered. After 6 months of storage, the difference is roughly 0.1% and became significant. Lipases act in the interface between water and oil with the following mechanism: the hydrophobic part of the enzyme bounds with the oil, whereas the active site aligns with the substrate and severs the ester bounds of the triglycerides. Hence, the free fatty acids increase is probably due to the water content of cloudy oils, which allows lipase enzymes to hydrolyze triglycerides during the storage period.
Oleuropein has been found at concentration of 11.7 mg/kg immediately after olive oil production. Usually, oleuropein is not detected or detected in trace in olive oils, few works report it at concentration higher of about 10 mg/kg in the olive oil, and in the literature, there is a lack of paper describing oleuropein trend during storage. Hence, its changes among the olive oil shelf-life will be briefly described.
After November, oleuropein increases up to 20.7 mg/kg in January and finally slightly decreases until the 5.3 mg/kg of July. No differences occur between filtered and cloudy oils.
The concentration of ethyl acetate, considered to be a winey marker was consistently higher in cloudy oils than filtered oils; moreover, it only increased over time in the case of cloudy oils.
The sensory evaluation revealed that filtered oils remained fruity longer than cloudy oils, although in one case, defects prevented quantification. The volatiles profile supports the sensory results: E-2-hexenal was constant over time in filtered oils, but only 2 months after production it was significantly lower in cloudy oils. In cloudy oils, the sensory defects prevented their detection, whereas in filtered oils, the intensity of the bitter and pungent remains substantially stable over time, in agreement with the content of the sum of oleuropein and its derivatives and ligstroside derivatives, respectively.
The results confirm that filtration is essential in order to maintain the nutritional characteristics of the oil and slow the onset of the main defects. On the one hand, filtration eliminates suspended solids and reduces the moisture content to below IOC limits; on the other hand, it does not affect the product’s sensory and nutritional properties.
The filtration system evaluated in this study uses a filter press and cardboard filters. The disadvantages of this method are the high cost of filter plates, the non-negligible percentage of oil that is trapped in the plates (and cannot be recovered), and high manpower costs.