During the recent research conducted by a team from DAGRI, Dipartimento di Scienze e Tecnologie Agrarie, Alimentari, Ambientali e Forestali, Università degli Studi di Firenze, Florence, Italy, a novel approach to post-harvest handling of olives was assessed: densimetric sorting (sorting olives by density).  This technique could allow producers to control the quality of extra virgin olive oil (EVOO) by separating olives based on their density. We delve into their findings and examine how sorting olives by density can affect the chemical composition and sensory attributes of extra virgin olive oil (EVOO).

The Role of Olive Sorting in EVOO Production

The cultivation of olive trees is crucial for many economies around the world, particularly in Mediterranean countries. Olive oil production involves various steps, including olive harvesting, sorting, and processing, all of which directly affect the final product. High-quality Extra Virgin Olive Oil is sought after for its health benefits, containing essential phenols, antioxidants, and healthy fats.  Sorting olives after harvesting may become a key factor in achieving high-quality oils.  Many producers in Australia aim to achieve high-expression olive oils.

Post-harvest densimetric sorting aims to classify olives by density to optimise and enhance oil quality. By separating olives into low- and high-density categories, producers can create oils with distinct sensory and chemical properties. The study, conducted at the Università degli Studi di Firenze, utilised a saline solution with varying concentrations to divide olives into two groups, and the oil was subsequently extracted for analysis.

The Phenomenon of Density-Based Sorting

Densimetric sorting is a method where olives are separated based on their ability to float or sink in a saline solution. The process of density-based sorting is influenced by factors like the olive’s ripeness, moisture or water content, and fruit development stage. In EVOO production, this method can either be beneficial or detrimental depending on the end goal for oil quality.

The researchers at DAGRI, Università degli Studi di Firenze categorised the sorting process into two main groups:

• Low-density olives: These olives floated in the saline solution and were generally riper with a higher maturation index and pulp-to-pit ratio.
• High-density olives: These sank in the solution, often indicating that they were less ripe and exhibited lower maturation indices.

The study focused on evaluating whether this sorting technique could enhance the chemical composition and sensory profile of EVOO.

Causes of Density Variation and Its Impact on Oil Quality

• Natural Variability in Olives: The variation in density among olives is largely due to natural factors such as water content, ripeness, and the physiological state of the fruit. Olives with higher water content tend to be less dense, while those that are more mature and contain more oil are denser. This natural variation can lead to differing oil qualities.
• Environmental Factors: External factors such as temperature, sunlight exposure, and irrigation practices also affect the density of olives. Olives exposed to more sunlight and grown under optimal water conditions are likely to develop better oil quality, whereas those from less ideal conditions might have lower oil content and higher water content.
• Mechanical Factors: The handling of olives post-harvest, including how they are transported and processed, can affect their density. Mechanical damage or bruising during transportation can alter the olives’ water retention, thereby impacting their density and oil quality.

The Science Behind Densimetric Sorting

At the core of the densimetric sorting process lies the relationship between the olive’s density and its chemical properties. The study showed that oils extracted from low-density olives (those that floated) had higher concentrations of phenolic compounds, such as oleuropein and ligstroside derivatives.  These compounds are known for their antioxidant properties and contribute to the oil’s bitterness and pungency, both desirable traits in high-quality EVOO.

The oil from high-density olives exhibited a higher peroxide value (PV), an indicator of oxidation, which could negatively affect the shelf life and flavour of the oil.  Additionally, the volatile organic compounds (VOCs) responsible for the oil’s aroma were more abundant in oils from low-density olives.  For instance, (E)-2-hexenal, a compound linked to fruity and green notes, was present in higher concentrations in oils from low-density olives.

Remedies and Practical Applications

Several practical implications emerged from the study on densimetric sorting, which could benefit olive oil producers:

• Fertilisation and Harvest Timing: To optimise the density of olives, producers can focus on balanced fertilisation and the right harvest time. By harvesting when the olives have reached optimal ripeness, it is possible to increase the proportion of low-density olives.
• Irrigation Management: Proper irrigation can reduce the water content in olives, thus increasing their density and potentially improving oil quality. Over-irrigation, on the other hand, can lead to waterlogged, low-quality olives.
• Growth Regulators and Pruning: The use of growth regulators to manage fruit set and density could also be explored, alongside strategic pruning practices to ensure balanced growth and better fruit quality.

Conclusion

The research conducted by DAGRI, Università degli Studi di Firenze on densimetric sorting of olives offers a promising avenue for enhancing the quality of extra virgin olive oil. By utilising simple saline solutions to sort olives based on their density, producers can effectively differentiate their oil products, catering to various market demands. Oils from low-density olives were found to contain higher levels of beneficial phenols and VOCs, contributing to better sensory attributes and longer shelf life. Further research is recommended to expand this technique across different olive varieties and production scales. With continued innovation, densimetric sorting could become a valuable tool in the pursuit of superior-quality EVOO.

Read full paper:  Densimetric sorting of olives to control olive oil quality

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References:

• Spadi, A., Guerrini, L., Zanoni, B., & Parenti, A. (2024). Densimetric sorting of olives to control olive oil quality. European Journal of Lipid Science and Technology, 126(7), 2200130. https://doi.org/10.1002/ejlt.202200130

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