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Analyzing Oil Content of Olive Crops Using NMR (For Research Only)

For research only.

Olive oil is well known for its health benefits. It contains the essential fatty acids linoleic and linolenic acid as well as monounsaturated fatty acids, such as oleic acid, which help reduce levels of low-density-lipoprotein cholesterol. The antioxidants in the oil, including tocopherols and phenols, help protect the body from cellular damage of free radical species.

However, changes to environmental conditions, such as water irrigation, during the growth of olive fruits can impact on the oil quality and, hence, its health benefits.

Analytical techniques, such as nuclear magnetic resonance (NMR), are useful to assess the oil production of olive plants under water stress and help optimize the optimum growth criteria for olive orchards.

Researchers from the Instituto de la Grasa, a Food Service and Technology research center, and the Polytechnic University of Madrid separated olive orchards into different areas, each with varying levels of water irrigation. The changes to oil quality over two seasons in 2012 and 2013 was monitored.

The midday stem water potential (Ψstem, MPa) was measured every 15 days during oil synthesis. Irrigation treatment was found to significantly modify Ψstem for all treatment areas. Higher Ψstem values in the control area was observed compared to the reduced water treatment areas in both years, with lower Ψstem values overall in 2013 compared to 2012.

After harvest, the oil quality was characterized by assessing the oil content, fatty acid composition, and antioxidant concentration.

NMR was then performed on dried samples using a MiniSpec MQ-10 from Bruker to determine the total oil content. This equipment provides a bulk measurement, so no sample preparation was required, and the color or opacity did not affect the data, which can be an issue when using optical techniques. In 2012, there was no significant difference in mean oil production. However, in 2013, the control area produced significantly more oil compared to the other treatment areas.

The fatty acid composition was analyzed, and no significant difference was observed in the 2012 harvest. However, in 2013, the oil with the lowest water irrigation had a significantly lower concentration of oleic acid and a significantly higher concentration of linoleic acid, compared to the control, indicating a higher stress environment stimulates the desaturation of oleic acid into linoleic acid.

The concentration of the antioxidants tocopherol and phenol were analyzed. For both the 2012 and 2013 season, the concentration of phenolic molecules was significantly affected. In 2012, as the water irrigation decreased, the concentration of the complex and abundant phenolic molecules decreased. This same trend was observed in 2013 as well, but the decrease extended to simple phenolic molecules.

The significant changes in oil production and composition observed with NMR and other analytical techniques correlates to a reduction in Ψstem. From these data, the researchers recommend a minimum Ψstem value of -2.21 MPa should be maintained during oil synthesis to achieve maximum oil production, fatty acid, and phenol content and oxidative stability.

Reference:

  1. García J.M. et al. (2020). Deficit Irrigation During the Oil Synthesis Period Affects Olive Oil Quality in High-Density Orchards (cv. Arbequina). Agricultural Water Management. https://doi.org/10.1016/j.agwat.2019.105858.