Onion Peel: Turning a Food Waste into a Resource

Abstract

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.

Keywords: dietary antioxidants,; flavonols; onion skin; sustainable agriculture; traditional varieties.

Figure 1

(A) UHPLC-UV profile (365 nm) of “Rossa di Tropea” onion skin (solid line) and “Ramata di Montoro” onion skin extract (dashed line); (B) UHPLC-UV profile (520 nm) of “Rossa di Tropea” onion skin.

Figure 2

α-glucosidase inhibitory activities of “Ramata di Montoro” and “Rossa di Tropea” extracts at different concentrations. Value represents the mean of three replicates. Different letters denote significant differences among tested concentration within each extract by analysis of variance [ANOVA]. Statistical significance was defined as p < 0.05, using Tukey’s post hoc test for mean separation.

Figure 3

Effects of “Rossa di Tropea” and “Ramata di Montoro” extracts and main compounds on HDFA cell growth. Cells were treated for 48 h with DMSO (Ctrl) or with the indicated concentrations of different extracts. Viability was assessed by MTT assay and was expressed as a percentage of control. Values represent means ± SD. Different letters denote significant differences among tested concentration within each extract by analysis of variance [ANOVA]. Statistical significance was defined as p < 0.05, using Tukey’s post hoc test for mean separation.