Crocus sativus L. has been cultivated throughout history to obtain its flowers, whose dried stigmas give rise to the spice known as saffron. Crocetin esters, picrocrocin, and safranal are the main metabolites of this spice, which possess a great bioactivity, although the mechanisms of action and its bioavailability are still to be solved. The rest of the flower is composed by style, tepals, and stamens that have other compounds, such as kaempferol and delphinidin, which have an important antioxidant capacity, and these can be applied in foods, phytopharmaceuticals, and cosmetics. The aim of this work was to provide an updated and critical review of the research on the main compounds of Crocus sativus L. flower, including the adequate analytical methods for their identification and quantification, with a focus on their bioactivity and bioavailability.

BACKGROUND Oil mills could benefit by preparing their own aqueous extracts from olive leaves. Accordingly, the present study aimed to measure the bioactive compounds richness of such extracts, especially oleuropein. A water‐based microwave extraction procedure was developed and a selective and precise high‐performance liquid chromatography with diode array detection (HPLC‐DAD) method was validated for the determination of oleuropein and others bioactive compounds from olive leaves. RESULTS The water solubility of oleuropein was determined to be 9.5 g L−1. The extraction procedure was optimized in terms of power, olive leaf weight/water volume ratio and time of extraction, and the results revealed that 2 mg mL−1 and a microwave irradiation at 800 W for 30 s resulted in the greatest efficiency. Oleuropein was determined by the new validation method, which showed good linearity (r2 = 0.996), precision (% relative standard deviation < 10%), recovery (118.6%), and limits of detection (17.48 mg L−1) and quantification (21.54 mg L−1). Good correlation (r2 = 0.979) was obtained between oleuropein of the olive leaf extracts determined by HPLC‐DAD and by UV‐visible spectrophotometry. CONCLUSION A simple extraction method was developed and validated to obtain aqueous extract from olive leaves by microwave extraction, determining for the first time oleuropein water solubility. Validation of the method showed that oleuropein in olive leaves could be quantified when it is at least 1% of dry weight by means of HPLC‐DAD. UV‐visible spectrophotometry can be useful in oil mills because it enables the content of oleuropein and other bioactive compounds content to be determined in situ in such leaf aqueous extracts. © 2020 Society of Chemical Industry

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 RTl 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 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).

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.