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Role of Processing Techniques in Extraction of Polyphenols from Olive Leaves

Olive leaves, the by-products of olive oil industry, contain a diverse range of bioactive compounds, including polyphenols such as oleuropein and hydroxytyrosol. These components enable protection to promote health and reduce chronic diseases, via exhibiting antioxidation which highlights their ability to inhibit the formation/activities of free radicals. The growing awareness of the beneficial effects of these natural components has resulted in the growth of the idea of reusing these residues as functional food products. Additionally, valorisation of olive leaves is likely to i) help promote circular economy and minimise food waste/cost, and ii) provide a natural/affordable high added-value product. Synthetic antioxidants such as butylated hydroxyl anisole (BHA), traditionally used for food enrichment, are likely to be efficient in developing nutrition and extending the shelf-life of the products. However, the global demand for the replacement of synthetic additives by natural components has nourished the industrial desire to initiate adaptation of green/novel processing approaches.    

The challenging work of identifying the ideal method to recover endogenous phytonutrients, phenolics in particular, from olive leaves is somewhat inherent in their chemical/molecular complexities that are influenced by the parameters involved in pre/post-harvest practices. In this respect, these valuable food residues, while having good potential for dietary applications, have not reached the market/commercial use, or have been insignificantly commercialised.  To minimize the processing challenges involved in the green exploitation streams, it is important to find advanced technological solutions that can optimally accommodate nutritional, economic, and environmental requirements. Indeed, the ability to develop an efficient processing operation has a significant effect on the potentiality to perform sustainable utilisation of by-products. Optimised processing designs have partly developed to efficiently re-use agro-industrial by-products. These include identification of increased recovery/yield of bioactive components from olive leave by-products through improved mechanical extraction systems such as ultrasound-assisted, microwave-assisted, steam explosion, and supercritical fluid extractions.

Optimisation of processing designs, aiming at decreasing i) the need for toxic/organic solvent, ii) energy use/expenditure, iii) operation time, and iv) manufacturing cost, continue to be among the most sought-after research topics. Other examples include the efficiency of individual operation/processing steps, involved directly or indirectly in the extraction process, such as thermal treatments/drying operations that have relatively progressed to deliver sustainable processing, e.g., by using renewable energy with minimum drying time. In terms of functional applications of olive leaf extracts, research studies have shown valuable outcomes that potentially enable the integration of isolated antioxidants from olive leaves into i) a diverse range of food products, such as vegetable oils which are inherently susceptible to lipid oxidation – to promote oxidative stability/nutritional properties, ii) pharmaceutical supplements – to protect the body against inflammatory, hypertensive, atherosclerotic, and carcinogenic diseases, and iii) cosmetics. In view of the increased appreciation of using natural antioxidants, together with the growing interest in researching and finding the ideal solutions to recover these components, it is likely that olive leaves will find globally more extensive industrial applications in the future.

The information on this page is a summarised description. More details are available in the review article: “Fereshteh Safarzadeh Markhali, José A. Teixeira and Cristina M.R. Rocha., 2020. Olive Tree Leaves—A Source of Valuable Active Compounds. Processes, 8(9)” 

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