To what extent can the acidification of olive paste with organic acids increase the phenolic fraction of EVOO?
The main components that characterize the antioxidant capacity and physiological functions of extra virgin olive oil (EVOO) are secoiridoid phenolic compounds derived from oleuropein and ligstroside. Among these, oleacein, oleocanthal, and their aglycone precursors constitute the core of the phenolic fraction.
In this study, using the Picual variety, we examined the effects of acidification by adding ascorbic acid or malic acid to olive paste on extraction efficiency, phenolic composition, antioxidant capacity, pigments, and volatile components. Fruits with a maturity index (MI) of 3.7 to 5.5 were processed using the Abencor system, and conditions of 25°C, malaxation for 40 to 80 minutes, and acid concentrations of 0.02 to 0.08 mol/kg were analyzed using response surface methodology.
The superiority shown by ascorbic acid
Overall, ascorbic acid yielded better results than malic acid. Extraction efficiency reached 75.8–80.0% and increased with maturity, malaxation time, and acid dosage.
On the other hand, no significant deterioration due to acidification was observed in free acidity, peroxide value, K232, or K270. In other words, the basic chemical quality indicators remained stable while the phenolic fraction was altered.
Differences also appeared in pigment retention, with the ascorbic acid treatment recording a maximum of 18.9 mg/kg of carotenoids and 28.9 mg/kg of chlorophyll. With malic acid, the pH drop was more pronounced, which is thought to have promoted the conversion of chlorophyll to pheophytin.
Oleacein and oleocanthal show different behaviors
Particularly interesting is the selective action on secoiridoids. Oleacein increased with medium to high acid addition, reaching a maximum of 207.25 mg/kg. Meanwhile, oleocanthal reached a maximum of 191.43 mg/kg with low acid concentration and long malaxation time.
Molecular structure is involved in this difference. Oleacein, which has a catechol structure, is highly reactive to oxidation and easily decomposes during malaxation, whereas oleocanthal, which has a single phenolic hydroxyl group, is relatively more stable against oxidation.
Secoiridoid fraction determining antioxidant capacity
Antioxidant capacity measured by the FRAP method showed a maximum of 1177.9 μmol Trolox eq./kg. Furthermore, an extremely strong linear correlation of R² = 0.9818 was confirmed between antioxidant capacity and the total molar concentration of oleuropein derivatives.
This indicates that when considering the antioxidant capacity of EVOO, not just the ‘total polyphenol content’ but the phenolic profile itself—which secoiridoids are present and at what concentrations—is important.
The biggest variable is ‘maturity’
The factor that had the greatest impact in the study was the maturity of the fruit. At MI 3.7–5.5, total phenols, secoiridoids, photosynthetic pigments, and volatile components all decreased as maturity progressed. However, acidification, especially treatment with ascorbic acid, partially suppressed this decline.
It has been shown that medium to long malaxation and relatively low acid concentrations for immature to medium-ripe fruits, and higher acid concentrations and shorter malaxation times for ripe fruits, may be effective for phenolic retention.
However, there is an important point to note here. Under current regulations, oil to which organic acids have been added during the extraction process cannot be treated as standard EVOO. Therefore, this study demonstrates the potential for developing high-value-added functional lipid matrices with high concentrations of phenolic bioactive substances, rather than an improvement of traditional oil extraction methods.
Future work must include verification at an industrial scale, application to other varieties, antioxidant capacity evaluation using methods such as ORAC, effects on the lipoxygenase (LOX) pathway, and evaluation of sensory characteristics including bitterness and pungency.
The next focus of oil extraction technology is not simply to ‘increase polyphenols.’ It lies in integrally controlling maturity, pH, redox environment, enzyme activity, and malaxation conditions to design the desired phenolic profile.
Reference: Teatro Naturale