A green extraction method of phenolic compounds from olive leaves (Olea europaea L.): evaluation and prediction using multiple linear regression

For the preparation of phenolic extracts, heating is one of the treatments applied. The effect of this type of heat treatment on the phenolic composition and the antioxidant power of olive leaves extracts were studied. This study aims to assess the use of solar heating method, a revolutionary green technology for the efficient extraction of several phytochemicals from food by-products. Olive leaves (Olea europaea L.) were used as the source material for this study. Solar heating method has been used for the extraction of polyphenols and flavonoids compounds. The extracts of solar heating method, decoction, and maceration were characterized in terms of total phenolic compounds, total flavonoids compounds, and their antioxidant activity based on 2,2-diphenyl-1-picrylhydrazyl radical scavenging activity. The results showed that solar heating method extract provided a very satisfactory extraction yield in polyphenols, flavonoids compounds, and antioxidants (189.58 mg GAE/g DW, 87.53 mg QE/g DW, and 91.16% respectively), which was higher than that attained by decoction and maceration methods. Moreover, HPLC analyses showed an increase in rutin, oleuropein, hydroxytyrosol, vanillin, coumaric acid, quercetin, and caffeic acid levels in the solar heating method extract. Results indicated solar heating method as a good choice for green recovering polyphenols and antioxidants from the olive leaves. There was a positive linear correlation between antioxidants, polyphenols, and between antioxidants, flavonoids for solar heating method extract (coefficient r = 0.89 and 0.77, respectively). These results suggested that the polyphenols and flavonoids contributed significantly to the antioxidants of the investigated olive leaves.