Comparative study of different particle sizes of added olive leaves for the content of target polyphenols in virgin olive oil

The addition of olive leaves during processing of olive oil has been studied and reviewed from different perspectives but there is a paucity of information on the roles of particle sizes of the added leaves in phenolic content of the oil. Dry ground olive leaves with a range of particle size fractions (0.07–3.0 mm) were added to crushed olives prior to the malaxation to compare their effects on: (i) the content of total and selected polyphenols (particularly oleuropein and verbascoside), (ii) antioxidant capacity (in vitro), and (iii) physicochemical quality and the yield of the extracted olive oil. Besides particle size, that was the main factor of study, olive pitting and malaxation time (30 and 60 min) were also considered as independent variables. Olive leaves with 0.3 mm followed by 0.15 mm favorably exhibited significant effects (p < 0.001) in all assays. Indeed, the oil samples with 0.3 mm leaves produced from the pitted olives showed maximum values when the malaxation time was (i) 30 min – for oleuropein (5.85 mg per kg oil), verbascoside (4.02 mg per kg oil), luteolin (15.44 and mg per kg oil), and total phenolic content (TPC) (368.01 mg per kg oil), and (ii) 60 min – for hydroxytyrosol (19.14 mg per kg oil) and tyrosol (16.89 mg per kg oil). These findings indicate that the particle size of added olive leaves can play a significant role in the content of principal polyphenols of the resulting olive oil; a topic that has not been approached in the literature.