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Olive oil is well known for its health benefits, many of which stem from its specific composition of antioxidants and fatty acids.
The fatty acids within olive oil (e.g. linoleic and linolenic acid) are dietary essentials, while monounsaturated fatty acids (MUFAs), such as oleic acid, have the potential to reduce the levels of low-density-lipoprotein cholesterol.
Antioxidants found in the oil (for example phenols and tocopherols), support the body in protecting itself from cellular damage of free radical species.
Variations in environmental conditions while olive fruits are growing have the potential to affect the oil quality and, therefore, its related health benefits.
Because of this, it is important that oil composition is analyzed under varying conditions to better understand the growth criteria needed to ensure fruit from each harvest meets commercial quality guidelines.
Researchers from the Polytechnic University of Madrid and the Instituto de la Grasa (a center for Food Service and Technology research), employed a range of analytical techniques - including high-performance liquid chromatography (HPLC), gas liquid chromatography (GLC), nuclear magnetic resonance (NMR), and optical spectroscopy – to thoroughly assess the impact that water stress had on the quality of oil from two olive orchards in 2012 and 2013.
Each orchard was given identical water irrigation treatment until late summer. Each orchard was then divided into four areas with differing levels of irrigation – T1, control; T2, reduced; T3, medium; T4 low. The water supplied to T2, T3 and T4 had an average of 68% (248.0 mm), 39% (210.5 mm) and 15% (183 mm) of T1 (290.5 mm), respectively.
The midday stem water potential (Ψstem, MPa) was gaged at 15 day intervals until the end of October, when the plants were harvested. Irrigation treatment was found to significantly modify Ψstem for all treatment areas in both years.
In 2012, average Ψstem values were found to be -1.24 MPa, -1.47 MPa, -1.86 MPa and -2.39 MPa for T1, T2, T3, and T4, respectively. In 2013, the average Ψstem values were found to be far lower; -2.35 MPa, -2.68 MPa, -4.06 MPa and -4.76 MPa for T1, T2, T3, and T4, respectively.
After being harvested, oil quality was then characterized via an assessment of its water and oil content, antioxidant concentration and fatty acid composition. This assessment involved fresh fruits being weighed before being dried for 48 hours and weighed again to determine their water content. Next, NMR was used to ascertain the total oil content - a MiniSpec MQ-10 from Bruker was used to do this.
This instrument can provide a bulk measurement with no sample preparation required. Additionally, opacity or color does not affect the data collected – a common issue when utilizing optical techniques.
In 2012, no significant difference was found in water and mean oil production between the four different treatment groups. In 2013, however, T1 produced considerably more oil (2267 kg/ha) when compared with the other treatments (T2, 1729 kg/ha; T3, 1552 kg/ha; T4, 1319 kg/ha).
Fatty acid composition was analyzed through the use of GLC, with no significant differences found between the control sample (T1) and the reduced irrigation treatments used the in the 2012 harvest.
In 2013, the T4 oil had a much lower concentration of oleic acid coupled with an increased concentration of linoleic acid, in contrast with T1. This difference implies that a higher stress environment can promote the desaturation of oleic acid into linoleic acid.
Concentrations of the antioxidants phenol and tocopherol were analyzed via reversed phase HPLC and HPLC, respectively. The concentration of phenolic molecules was found to be significantly affected for both seasons.
In 2012, it was found that as the water irrigation was reduced, concentrations of the abundant and complex phenolic molecules were lowered. An identical trend was seen in 2013, but this decrease was also seen in simple phenolic molecules.
A number of other critical parameters, which impact on the characterization of oil quality were also evaluated, including maturity index, taste and oxidative stability. No major differences were noted between the samples in 2012.
Results from 2013 revealed that the maturity index was considerably higher in T4 when compared to T1, while the oxidative stability was significantly higher in T1 (39.7 hours) in contrast with T4 (29.2 hours). Taste in T1 was also much higher for pungent and fruity intensities when compared to that of T4.
The researchers posited that the variation in oil quality between the two different seasons was primarily a result of the lower rainfall in 2013, coupled with the shadow effective rooting depth in the orchard used throughout that year.
These two factors, in conjunction with the irrigation treatments, led to an even higher stress environment for the plants growing in the 2013 orchard, with reduced Ψstem values, and substantial differences in oil quality between each of the treatment areas.
In summary, analytical techniques like GLC, NMR, and HPLC were used to highlight that olive oil quality is impacted upon by Ψstem, and that in cases where this value is under -2.39 MPa, substantial differences may arise.
In spite of the changes observed in the oil characteristics (especially for the 2013 season) all of the oil was found to be within the required standards of commercially available extra virgin olive oil.
The data from this study does, however, indicate that a minimum Ψstem value of -2.21 MPa should ideally be preserved during oil synthesis, in order to facilitate maximum oil production, phenol and fatty acid content alongside oxidative stability.
References
- 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.
- Oliveoiltimes.com. (2020). Olive Oil Health Benefits. https://www.oliveoiltimes.com/olive-oil-health-benefits
- Bruker.com. (2020). The MiniSpec MQ Series. https://www.bruker.com/products/mr/td-nmr/minispec-mq-series.html
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