Optimizing the management and drying of olive leaves (Olea europaea L.) for upcycling into nutraceuticals: A study on Tuscan cultivars

Virgin olive oil supply chain generates large amounts of pruning residues such as olive leaves, rich in bioactive phenols, but still largely underutilized. Phenols are susceptible to enzymatic degradation, thus proper post-harvest management, especially drying conditions, is crucial to maximizing phenolic and oleuropein content. This study assessed the impact of different drying methods on leaves’ phenolic content, establishing a protocol to achieve oleuropein levels > 5% on dry weight, making them suitable for the foods and nutraceuticals industry. The effects of cultivar, harvest season, and storage time before drying were also evaluated by HPLC-DAD-MS. Oven drying at 105 °C was identified as optimal, providing oleuropein concentrations even greater than 10% DW. Other approaches like lyophilization, drying at 50 °C, and extraction from fresh leaves caused up to 93% oleuropein loss. Delayed drying after harvesting led to oleuropein degradation as well. The study contributed to clarify conflicting results in the literature regarding the impact of different drying methods and temperatures. It also provided suitable protocols for preserving oleuropein in olive leaves and obtaining dried material with oleuropein content greater than 5% w/w, summarized as follows: olive leaves of specific cultivars (e.g., Frantoio) harvested in late winter/spring must be oven-dried at 105 °C within three days after harvesting.