Chemical characterization of Opuntia sp. by-products

The species from the Cactaceae family are important invaders in the Mediterranean. The seeds are usually discarded and proper utilisation of these waste products could lead to an important source of bioactive compounds. The seeds have been described to be rich in health-promoting polyunsaturated fatty acids and may potentially be included in animal and human diets [1]. Moreover, seeds could also be recovered for their high nutritional value, as sources of sugars, tocopherols, dietary fiber and polyphenols instead of being discarded, as it currently occurs. In this study, the composition in soluble sugars, fatty acids, organic acids, tocopherols and phenolic composition were determined in seeds of Opuntia macrorhiza (Engelm.) and Opuntia microdasys (Lehm.). The individual sugars profile was determined by HPLC-RI, fatty acids by GC-FID, organic acids by HPLC-DAD, tocopherols by HPLC-fluorescence and phenolic compounds by HPLC-DAD-ESI/MS. In both cases, the overall concentrations of soluble sugars in seeds were quite lower than those detected in other Opuntia sp. parts. Quinic (0.30 g/100 g dw) and oxalic (0.32 g/100 g dw) acids were the major organic acids detected in O. microdasys and O. macrorhiza, respectively. The fatty acid profiles detected in the seeds of both species were similar, with linoleic acid (C18:2n6) as the major fatty acid (71% in O. microdasys; 74% in O. macrorhiza). γ-Tocopherol was the most abundant tocopherol vitamer in both species (7.4 mg/100 g dw in O. microdasys; 5.8 mg/100 g dw in O. macrorhiza). Regarding the phenolic profile, eight compounds were identified: seven phenolic acid derivatives and one flavonoid, and O. macrorhiza seeds (1016 μg/g extract) revealed a higher concentration than O. microdasys (450 μg/g extract). Feruloyl di-hexoside was the most abundant phenolic acid in both species (225 μg/g extract in O. microdasys and 718 μg/g extract in O. macrorhiza). The seeds of Opuntia sp. presented several bioactive phytochemicals, which may allow considering this botanical part as highly interesting by-products.The authors are grateful to the Foundation for Science and Technology (FCT, Portugal) and FEDER under Programme PT2020 for financial support to CIMO (UID/AGR/00690/2013) and L. Barros contract; to the Interreg España-Portugal for financial support through the project 0377_Iberphenol_6_E.