Olive flower phenolics finish off free radicals

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  • Published: Dec 1, 2017
  • Author: Ryan De Vooght-Johnson
  • Channels: HPLC
thumbnail image: Olive flower phenolics finish off free radicals

Little is known about the phenolic species in olive flowers

Olives are a significant commercial trees in many countries, such as Tunisia, where large areas of land are devoted to them. Although there have been several studies on the composition of olive fruit and leaves, the flowers have been comparatively neglected. There is currently much interest in the possible health benefits of foods with significant antioxidant properties. There are a variety of antioxidant phenolic compounds present in olives, and it would be valuable to see how their levels change as the flowers develop. Olives can sometimes suffer from erratic flowering, which is detrimental to fruit production, so flower development is an economically important topic.

The Sfax researchers looked at the olive flowers from the popular Chemlali variety, examining how their phenolic compounds and antioxidant power varied through the season. Samples were taken from a number of trees in the Sfax region as the flowers developed between April and August.

Olive flower phenolics examined by HPLC and LC-MS

The flowers were broken up using a household blender, and the resulting paste-like material was extracted with 80:20 methanol:water over 24 hours. The solids were filtered off and the solution was washed with hexane to remove unwanted oil compounds. The total phenolic content was estimated by colourimetry in terms of gallic acid equivalents using the Folin–Ciocalteu reagent. Other colourimetric methods were used to determine total biphenols, total flavonoids and total flavonols. The radical scavenging ability of the extracts was evaluated using the stable free radical DPPH (1,1-diphenyl-2-picrylhydrazyl).

HPLC was carried out with an LC-10ATvp pump, a C18 Shim-pack VP-ODS column and an SPD-10Avp UV detector (all Shimadzu). Gradient elution was employed, using 0.2% aqueous acetic acid and methanol; the proportion of the latter was increased from 5 to 100% in a series of gradients. Each sample was analysed in triplicate. Four standards, oleoside, oleuropein, tyrosol and hydroxytyrosol, were used to prepare calibration curves. Other phenolic compounds were assayed using the most closely related standard. The calibration curves showed good linearity. The limits of detection (LODs) for the four compounds used as standards ranged from 0.006 to 0.048 μg/mL, while the limits of quantification (LOQs) ranged from 0.087 to 0.270 μg/mL.

LC-MS employed a Thermo Scientific Accela UHPLC system fitted with a Phenomenex Luna C18(2) column. Three mobile phases were used: methanol, water and acetonitrile (all with 1% formic acid), changing via a single linear gradient from 0:90:10 to 90:0:10, respectively. An LTQ Orbitrap XL mass spectrometer was used with electrospray ionisation in negative ion mode. 20 phenolic compounds were identified by comparison to standard samples or literature data.

The total phenolic content increased over the months examined, as did the radical scavenging ability of the extracts (a measure of antioxidant strength), with the relationship between the two being statistically significant. Overall, most of the individual phenolic compounds also increased over the flowering season, including oleuropein aglycon, which was the most abundant phenolic compound present.

Olive flower antioxidants increase over spring and summer

A seasonal variation in the phenolic species present in olive flowers was clearly shown by the HPLC and LC-MS results. A clear increase in total phenolics and antioxidant ability was found between April and August. The authors noted that further work is needed on the influence of phenolic compounds on the regulation of olive flowering. It would also be interesting to determine whether the levels of the compounds in the flowers could be linked to those in the fruits.

Related Links

Electrophoresis, 2017, Early View Paper. Rekik et al. Evaluation of phenolic composition and antioxidant activity changes in olive flowers during development using HPLC/DAD and LC-MS/MS.

Phytochemical Analysis, 2013, 24, 504-512. Taamalli et al. Characterisation of phenolic compounds by HPLC–TOF/IT/MS in buds and open flowers of ‘Chemlali’ olive cultivar.

The Journal of Nutritional Biology, 2002, 13, 636-644. Tuck et al. Major phenolic compounds in olive oil: metabolism and health effects.

Article by Ryan De Vooght-Johnson

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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