Microwave-Assisted Extraction of Phenolic Compounds from Broccoli (Brassica oleracea) Stems, Leaves, and Florets: Optimization, Characterization, and Comparison with Maceration Extraction

Abstract

Microwave-assisted extraction (MAE) to obtain phenolics from vegetable wastes has been of recent interest. Broccoli is one of the most globally produced vegetables, and around 43% of the harvest is considered waste. Thus, given the significant quantity of broccoli waste generated, the objective of this work was to optimize the MAE, to maximize the total phenolic content (TPC) from broccoli by-products (leaves and stems) and broccoli florets. The Response Surface Analysis was used in the optimization model to evaluate the impacts of methanol concentration, time, and temperature, and their interactions on the TPC of the broccoli extracts. The optimal MAE conditions were found to be 74.54% (methanol), 15.9 min, and 74.45 °C for broccoli stems; 80% (methanol), 10 min, and 73.27 °C for broccoli leaves; and 80% (methanol), 18.9 min, and 75 °C for broccoli florets. Under these conditions, the broccoli leaves exhibited the highest TPC (1940.35 ± 0.794 µg GAE/g DW), followed by the florets (657.062 ± 0.771 µg GAE/g DW) and stems (225.273 ± 0.897 µg GAE/g DW). The antioxidant activity of the broccoli extracts was evaluated under the optimal conditions by DPPH and ABTS assays, and the same behavior was observed in both studies, the broccoli leaves exhibited the highest antioxidant activity, among florets and stems. In addition, vanillic, sinapic, caffeic, chlorogenic, ferulic, gallic, neochlorogenic, and p-coumaric acids in the broccoli extracts were identified and quantified using HPLC. Furthermore, MAE was found to increase the phenolic yield up to 45.70% for broccoli leaves, 133.57% for broccoli florets, and 65.30% for broccoli stems, in less time compared with maceration extraction. MAE proved to be an efficient and sustainable technique to obtain phenolics from broccoli by-products, which can constitute a viable solution for valorizing broccoli wastes.