Critical Variables Influencing the Ultrasound-Assisted Extraction of Bioactive Compounds—A Review

Ultrasound-assisted extraction (UAE) is a technique that belongs to the group of novel extraction methods belonging to green chemistry, like microwave assisted extraction (MAE), enzyme assisted extraction (EAE) or high-pressure assisted extraction (HPAE), because it promotes the extraction of compounds of interest, with a decrease in the consumption of resources, such as solvent and energy, and achieving remarkably higher extraction yields [1]. UAE has been mainly used to plant matrices to obtain extracts rich in bioactive compounds, such as phenolic compounds, pigments, polysaccharides, amino acids, among others. This methodology is based on the principle of cavitation, which leads to cell collapse of the matrix and allows the release of their inner substances. Several variables are relevant for the performance of UAE, including the solid-liquid ratio, the type of solvents used, the extraction time and the ultrasound power applied [2]. Besides, power and extraction time are closely linked to a fifth important factor: temperature. In practical terms, a correct optimization of these variables is essential to obtain an excellent performance, resulting in a maximal extraction yield. In addition, temperature is very important, as it can affect the integrity ofbioactive compounds, since most of them are thermolabile. Thus, large amounts of power linked to long extraction periods may lead to sample damage, so temperature control is essential for acorrect design of the cooling reactor and the optimization of of UAE extraction protocols. Keeping all this in mind, this critical review is focused on the influence of all the variables that affect ultrasound-assisted extraction, in order to analyze the critical factors involved in the optimization of this technique. Acknowledgments The research leading to these results was supported by MICINN supporting the Ramon y Cajal grant for M.A. Prieto (RYC-2017-22891) and the FPU grant for Anxo Carreira Casais (FPU2016/06135); by Xunta de Galicia for supporting the program EXCELENCIA-ED431F 2020/12, the pre-doctoral grant of A.G. Pereira (ED481A-2019/0228) and the program BENEFICIOS DO CONSUMO DAS ESPECIES TINTORERA- CO-0019-2021 that supports the work of F. Chamorro; by University of Vigo for supporting the predoctoral grant of M. Carpena (Uvigo-00VI 131H 6410211); and by the Bio Based Industries Joint Undertaking (JU) under grant agreement No 888003 UP4HEALTH Project (H2020-BBI-JTI-2019) that supports the work of P. Otero and P. Garcia-Perez. The authors are grateful to Ibero-American Program on Science and Technology (CYTED—AQUA-CIBUS, P317RT0003). The JU receives support from the European Union’s Horizon 2020 research and innovation program and the Bio Based Industries Consortium. The project SYSTEMIC Knowledge hub on Nutrition and Food Security, has received funding from national research funding parties in Belgium (FWO), France (INRA), Germany (BLE), Italy (MIPAAF), Latvia (IZM), Norway (RCN), Portugal (FCT), and Spain (AEI) in a joint action of JPI HDHL, JPI-OCEANS and FACCE-JPI launched in 2019 under the ERA-NET ERA-HDHL (n° 696295). References 1. Chemat, F.; Zill-E-Huma; Khan, M.K. Applications of Ultrasound in Food Technology: Processing, Preservation and Extraction. Ultrason. Sonochem. 2011, 18, 813–835, doi:10.1016/j.ultsonch.2010.11.023.2. Chemat, F.; Abert Vian, M.; Fabiano-Tixier, A.S.; Nutrizio, M.; Režek Jambrak, A.; Munekata, P.E.S.; Lorenzo, J.M.; Barba, F.J.; Binello, A.; Cravotto, G. A Review of Sustainable and Intensified Techniques for Extraction of Food and Natural Products. Green Chem. 2020, 22, 2325–2353, doi:10.1039/c9gc03878g.