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Food industry produces a large amount of onion wastes. Due to the high amount of bioactive compounds in onion by-products an idea for their reuse, could be use them as source of high-value functional and health ingredients. In this study, outer dry layers of coppery onion "Ramata di Montoro" were used as source of bioactive compounds. Firstly, the chemical profile of secondary metabolites of exhaustive extract, obtained by ultrasound assisted extraction was established by UHPLC-UV-HRMS/MS analysis. Subsequently, the supercritical fluid extraction was used as alternative and green method to recover flavonoids from onion skin. Main parameters such as pressure, temperature and composition of solvent modifier were optimized in order to improve the extraction efficiency of SFE technique, by using a response surface Box-Behnken design.

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Food waste is a serious problem for food processing industries, especially when it represents a loss of a valuable source of nutrients and phytochemicals. Increasing consumer demand for processed food poses the problem of minimizing waste by conversion into useful products. In this regard, onion (Allium cepa) waste consisting mainly of onion skin is rich in bioactive phenolic compounds. Here, we characterized the flavonoid profiles and biological activities of onion skin wastes of two traditional varieties with protected geographical indication (PGI), the red “Rossa di Tropea” and the coppery “Ramata di Montoro”, typically cultivated in a niche area in southern Italy. The phytochemical profiles of exhaustive extracts, characterized by ultra-high-performance liquid chromatography coupled with ultraviolet (UV) detection and high-resolution mass spectrometry, revealed that flavonols and anthocyanins were the characteristic metabolite classes of onion skins. Quercetin, quercetin glucosides and their dimer and trimer derivatives, and, among anthocyanins, cyanidin 3-glucoside, were the most abundant bioactive compounds. The potential of onion skins was evaluated by testing several biological activities: ABTS/oxygen radical absorbance capacity (ORAC) and in vitro alpha-glucosidase assays were performed to evaluate the antioxidant and anti-diabetic properties of the extracts and of their main compounds, respectively, and proliferative activity was evaluated by MTT assay on human fibroblasts. In the present study, by observing various biological properties of “Rossa di Tropea” and “Ramata di Montoro” onion-dried skins, we clearly indicated that this agricultural waste can provide bioactive molecules for multiple applications, from industrial to nutraceutical and cosmetical sectors.

Extra virgin olive oils (EVOOs) containing more than 5 mg/20 g tyrosol, hydroxytyrosol, and their secoiridoids can be recognized by health claims related to the protection of blood lipids from oxidative stress. Therefore, a reliable, accurate, and standardized analytical procedure is needed to determine these markers of EVOO quality. In order to overcome the limitations of current methods, a detailed investigation of sample preparation and chromatographic conditions was performed by UHPLC-UV-HRMS. The use of a C18 fused-core column and nonacidified gradient elution provided single, sharp peaks for oleocanthal and oleacein, allowing their reliable quantitation in UV profiles. Positive- and negative-UHPLC-HRMS/MS characterization of methanolic extracts revealed the presence of dimethyl acetal, methyl hemiacetal, and monohydrate derivatives of all secoiridoids. These artifacts were formed in aqueous methanol, which is usually employed to extract and analyze EVOO phenols, making the HPLC profiles more complex and the measurements less accurate and reproducible. Acetonitrile proved to be a suitable solvent to avoid the formation of secoiridoid dimethyl acetals and methyl hemiacetals and to efficiently extract EVOO bioactive phenols. Finally, the phenolic contents of Italian EVOO samples were determined by UHPLC-UV analysis of acetonitrile extracts before (direct method) and after acid hydrolysis (indirect method). The results indicated that the use of tyrosol and hydroxytyrosol as reference standards allowed more accurate quantitative data to be obtained. Direct and indirect methods provided comparable levels of EVOO phenols, highlighting the usefulness of acid hydrolysis in routine analyses. The improved procedure defines the most reliable conditions to provide an analytical method with suitable accuracy and repeatability in the analysis of healthy and functional EVOO phenols.