Although olives leaves are currently considered a waste material from oil mills, they have great potential to be transformed into by-products due to their high oleuropein content. Oleuropein is a glycoside precursor of hydroxytyrosol, which is the phenolic compound with the highest antioxidant capacity in nature and which is associated with multiple health benefits. For this reason, the demand for oleuropein is growing in the pharmaceutical, cosmetic and food sectors. The objective of this study is to determine the stability of oleuropein in olive leaves from oil mills in solid and aqueous forms under different conditions of temperature, relative humidity and lighting. The results indicate that the degradation of oleuropein conforms well to first-order kinetics. The rate constants at the temperatures tested in the aqueous extracts indicate activation energies from RT_l to 80 °C and from 7 °C to 14 °C, as the degradation reactions were different in these ranges. Furthermore, olive leaf powder stored at any temperature with an RH ≥ 57% showed greater stability after six months, which is an encouraging result for the storage and transformation of this waste in oil mills.

Olive leaves are rich in phenolic compounds, which give them antioxidant properties that are associated with a lower incidence of disease. Therefore, the aim of this work was to determine the phenolic content, antioxidant activity, and toxicity of the aqueous extracts of olive leaves of the main Spanish and Greek cultivated and wild genotypes. For these purposes, ‘Picual’ and ‘Arbequina’ leaves from Spain and ‘Koronoeiki’ and ‘Kalamon’ leaves from Greece were collected, as were wild olive leaves from both countries. The aqueous extracts of these genotypes were analyzed by HPLC-DAD, and the DPPH·, ABTS·⁺ Folin–Ciocalteu, and Microtox^® methods were also used. ‘Picual’ had the highest oleuropein values, followed by wild olive leaves from both countries and ‘Arbequina’. The latter was reflected in the antioxidant activity measured by DPPH· and ABTS·⁺, which positioned the leaves of ‘Arbequina’, ‘Picual’, and the wild genotypes as having the most antioxidant activity. As expected, these leaves also had the highest total phenol content, as measured by Folin–Ciocalteu. Regarding the inhibition of the bioluminescence of Aliivibrio fischeri of the aqueous leaf extracts measured by Microtox^®, the EC50₁₅ ranged between 11.82 and 82.50 mg/mL, demonstrating similar behavior to other herbal infusions.

Olive leaves are still considered waste in the oil industry; however, the leaves have a content of oleuropein and other bioactive compounds that gives them great potential to be transformed into by-products. The most appropriate moment along an agronomic cycle (November 2019 to October 2020) has been evaluated to take advantage of this potential of the olive leaves varieties of Picual, Cornicabra and Manzanilla. In addition, factors that affect the content of phenolic compounds such as absolute maximum and minimum temperatures, relative humidity, sunshine hours, rainfall, differential of temperatures and mineral nutrition have been studied. The results show that the pruning season was the best time to take advantage of the olive leaf due to its high oleuropein content, especially in Picual and Manzanilla. The variety was the factor that most affected all the phenolic compounds studied, while the absolute minimum temperature factor notably affected verbascoside. Particular mineral nutrients, such as Mg and Fe, turned out to be most useful for differentiating locations. The content of verbascoside and hydroxytyrosol was also affected by agronomic conditions (location/conventional or ecological).

Nowadays, the techniques for the analysis of glycosidic precursors in grapes involve changes in the glycoside structure or it is necessary the use of very expensive analytical techniques. In this study, we describe for the first time an approach to analyse intact glycosidic aroma precursors in grapes by high-performance liquid chromatography with a diode array detector (HPLC-DAD), a simple and cheap analytical technique that could be used in wineries. Briefly, the skin of Muscat of Alexandria grapes was extracted using a microwave and purified using solid-phase extraction combining Oasis MCX and LiChrolut EN cartridges. In total, 20 compounds were selected by HPLC-DAD at 195 nm and taking as a reference the spectrum of phenyl β-D-glucopyranoside, whose DAD spectrum showed a first shoulder from 190 to 230 nm and a second around 200–360 nm. After that, these glycosidic compounds were identified by High-performance liquid chromatography–quadrupole time-of-flight mass spectrometry (HPLC-qTOF-MS). Disaccharides hexose pentose were the most abundant group observed with respect to the sugars and monoterpendiols the main aglycones found.

Drought is perhaps the most important abiotic stressor affecting plants. Grapevine (Vitis vinifera L.) is a drought-tolerant species, and this feature makes it a traditional crop in semi-arid climate areas. However, not all cultivars respond to drought in the same way. Many studies on grapevine drought response have focused on physiological traits. This study mainly used agronomic indicators to assess the drought response of 13 red cultivars. Our results revealed high variability in must isotope ratios (δ¹³C and δ¹⁸O), yield components, and grape must quality. Bobal, Garnacha Peluda, Garnacha Tinta, Mazuela, and Moribel cultivars responded well to drought conditions, simultaneously maintaining high yields and must quality. By contrast, Garnacha Tintorera, Forcallat Tinta, and Tempranillo cultivars showed high water use efficiency but had low yield and must quality. Therefore, these cultivars can be considered poorly adapted to drought conditions. By knowing which cultivars perform well under drought conditions, viticulturists can reduce their reliance on water irrigation and continue to maintain vineyard sustainability in current and future semi-arid climatic conditions. This research also contributes novel information about the Castilla-La Mancha region, where there have been no previous similar assays.

Oleuropein and other phenolic compounds contained in olive leaves give it the potential to be transformed from residue to co-product in an oil mill. However, the moment of the agronomic cycle in which their potential transformation is higher is not known in detail. Therefore, for the first time, a monthly study of the evolution of such compounds throughout an agronomic cycle is made (November 2019 to October 2020). Arbequina olive leaves were collected from three plots and the interactive effects of agronomic conditions were investigated, such as crop management (conventional and ecological), plantation framework (intensive and super-intensive) and location under different climatic conditions. The results showed that the main compound throughout the cycle was oleuropein and the highest levels occurred around the pruning season (February/March). Crop management and location affected the content of verbascoside and hydroxytyrosol, while plantation framework only influenced the flavonoid content. All compounds were affected by relative humidity and differential temperature, although hydroxytyrosol showed the highest correlation with the maximum temperature. The absorbance measurements by ultraviolet-visible spectrophotometry showed trends parallel to the oleuropein concentration measured by high-performance liquid chromatography, which suggests that this method could be useful to easily study the evolution of oleuropein in the oil mill.