The use of bioactive compounds has been maintaining its significance from nutritional aspects. Due to the increasing demand for them in potential markets, researchers struggle to create new sources and improve their methods. Plant materials possess plenty and a diverse range of these compounds. However, their availability strongly depends on the extraction techniques in addition to the sampling methods and the applicability of the method to the specific parts of the plant. Thus, it is crucial to develop a common, precise way which will enable to extract all the active components regardless of their origin and their location in the plant material. Besides, the new method ought to have the highest economic value in comparison to the present applications which means that the efficiency of the extraction should be acceptable on industrial scale as well. Even though numerous methods have been improved so far, it seems to be unlikely to achieve a standardized solution with high valorization for the extraction of bioactive compounds from plants until now. This review aims to discuss the novel extraction methods in addition to the conventional techniques focusing on the critical parameters such as the cost, time, yield, feasibility and eco-friendliness of the process.

The aim of this study was to obtain an extract rich in bioactive components from yellow onion peels, which are generally considered waste material. Accordingly, a three-factor three-level Taguchi (L9) experimental design with three factors, namely ethanol concentration (A; 50%, 75%, 100%), extraction temperature (B; 30, 40, 50 °C), and sonication time (C; 10, 20, 30 min) was used to optimize the ultrasound-assisted extraction process of onion peel powders. Through Taguchi optimization, the optimum extraction conditions were determined as A2B3C2 to obtain the extract with the highest total phenolic matter (TPM) content and antioxidant activity (DPPH (%)). In addition, the extract produced under A2B1C2 conditions was found to be the richest in total monomeric anthocyanin (TMA) content with the highest level of color pigments. In order to determine the overall optimization conditions and to reduce the three-response optimization process to a single response, the simple sum weighting (SAW) method was used as a multi-criteria decision-making method. As a result of the optimization, it was concluded that an extract rich in bioactive components with optimal TPM and TMA contents and DPPH (%) value could be obtained as a result of sonication at 30 °C for 20 min to onion peel powders mixed with a solvent containing 75% ethanol (A2B1C2). The extraction conditions of bioactive components from yellow onion peels were successfully optimized by the Taguchi-SAW hybrid optimization method.