Utilising Olive Leaves to Enrich Olive Oil with Oleuropein Aglycone
Permanent link to Research Commons versionhttps://hdl.handle.net/10289/16290
Olive oil (OO) is an excellent source of minor bioactive compounds, such as oleuropein aglycone, which is gaining increasing attention due to its biological properties and benefits on human health. The European Food Safety Authority (EFSA) have approved a health claim for OO if it contains 250 mg/kg of these desired bioactive compounds. However, the concentrations of bioactive compounds can vary between batches of OO and may contain bioactive compounds that are below the threshold which provides health benefits. Enrichment of OO is commonly employed to achieve a higher concentration of these bioactive compounds. Often a waste product such as olive leaves is used because it contains a significant amount of the oleuropein aglycone precursor, oleuropein glycoside (8-14% dry weight). The present research aimed to analyse commercial extra virgin olive oils (EVOOs) in the New Zealand market, explore the effect of olive leaf drying on the composition of oleuropein in olive leaf tea (OLT) and the stability of OLT stored at different temperatures (−20, RT, 40 °C, 189 days). Lastly, an investigation was carried out to determine whether the oleuropein in OLT could be hydrolysed to oleuropein aglycone via an exogenous source of β-glucosidase (almond flour/meal) so that the resulting aglycone could be transferred into a low content OO producing an enriched marketable product with health benefits. A liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was validated and was used to analyse target analytes in New Zealand commercial EVOOs (n = 9). There was a large variation in the concentration of target analytes (oleuropein, ligstroside, oleuropein aglycone, oleocanthal, oleacein, hydroxytyrosol, tyrosol and elenolic acid) and none of the oils analysed met the health claim (250 mg/kg). The content of oleuropein in OLT increased under different leaf drying conditions in the order fresh<RT<70 °C while other target analytes tended to increase with drying at RT. The period leaves were left to dry was also deemed a crucial factor in the concentrations obtained. A stability experiment of OLT ii determined that the majority of target analytes were stable at −20 °C and RT, whereas precursors such as oleuropein and ligstroside degraded at 40 °C. Almond flour/meal proved to be an effective source of exogenous β-glucosidase to hydrolyse oleuropein in OLT; however, the mass transfer of aglycone into the oil was complex. Changing the source of enzyme to almond milk resulted in an aglycone concentration of 6 mg/kg in the oil but this also resulted in oleuropein aglycone to be quantified in the aqueous portion of samples (the highest being 613 mg/kg). The presence of a hydrophobic compound in the aqueous portion of samples was unexpected. It was thought this result may be caused by solubilisation through thermodynamically stable micelle formation within the emulsion formed during enrichment experiments. Salting out the reaction resulted in the highest amount of aglycone (30 mg/kg) as this process likely disrupted micelle formation. Since commercial EVOOs vary in concentration of analytes such as oleuropein aglycone, enriching an OO is desirable. OLT can be used as an effective source of oleuropein which can be hydrolysed via β-glucosidase present in almond flour/meal and milk. However, the mass transfer of the respective aglycone into olive oil to achieve an enriched product with a concentration at or above the health claim (250 mg/kg) was complex.
The University of Waikato
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