foods Article A Cross-Flow Ultrasound-Assisted Extraction of Curcuminoids from Curcuma longa L.: Process Design to Avoid Degradation Arianna Binello 1 , Giorgio Grillo 1 , Alessandro Barge 1 , Pietro Allegrini 2, Daniele Ciceri 2 and Giancarlo Cravotto 1,* 1 Dipartimento di Scienza e Tecnologia del Farmaco, University of Turin, Via P. Giuria 9, 10125 Turin, Italy; [email protected] (A.B.); [email protected] (G.G.); [email protected] (A.B.) 2 INDENA S.pA., Viale Ortles, 12, 20139 Milan, Italy; [email protected] (P.A.); [email protected] (D.C.) * Correspondence: [email protected]; Tel.: +39-011-6707183; Fax: +39-011-6707162 Received: 9 May 2020; Accepted: 2 June 2020; Published: 4 June 2020 Abstract: Rhizomes of Curcuma longa L. are well known for their content of curcuminoids, which are compounds with interesting biological activity against various inflammatory states and diseases. Curcuminoids can degrade during processing. This piece of work investigates fast, efficient and cost-effective metabolite recovery from turmeric under ultrasound-assisted extraction (UAE). An analytical evaluation of curcuminoid stability under sonication in different solvents is reported for the first time. HPLC and quantitative 1H-NMR were used. Under the applied conditions, EtOAc was found to be the optimal extraction medium, rather than EtOH, due to its lower radical generation, which facilitates better curcuminoid stability. Kinetic characterization, by means of the Peleg equation, was applied for single-step UAE on two different rhizome granulometries. Over a time of 90 min, maximum extraction yields were 25.63% and 47.56% for 6 and 2 mm matrix powders, respectively. However, it was observed that the largest portion of curcuminoid recovery was achieved in the first 30 min. Model outcomes were used as the basis for the design of a suitable multi-step cross-flow approach that supports and emphasizes the disruptive role of cavitation. The maximum curcuminoid yield was achieved over three steps (92.10%) and four steps (80.04%), for lower and higher granulometries, respectively. Finally, the central role of the solvent was further confirmed by turmeric oleoresin purification. The EtOAc extract was purified via crystallization, and a 95% pure curcuminoid product was isolated without any chromatographic procedure. No suitable crystallization was observed for the EtOH extract. Keywords: Curcuma longa L.; curcuminoid stability; multi-step extraction; ultrasound-assisted extraction; extraction kinetic 1. Introduction In recent years, non-synthetic and biologically active compounds from vegetal sources have gained increasing interest because of their important role in health-care systems worldwide; pigments have found use as additives or supplements in food, pharmaceutical and cosmetic industries. One of the most widely studied sources of natural pigment plants is Curcuma longa L., a perennial rhizomatous shrub belonging to the Zingiberaceae family. Also known as turmeric, this plant is commonly used as a coloring and flavoring agent in the food industry and, in particular, it is known as the ‘golden spice of life’, and constitutes the main component of curry [1]. However, it is also appreciated in traditional medicine for its biological properties, which are mainly related to its curcuminoids; chemical Foods 2020, 9, 743; doi:10.3390/foods9060743 www.mdpi.com/journal/foods Foods 2020, 9, 743 2 of 19 components that include curcumin (CUR), demethoxycurcumin (DMC) and bisdemethoxycurcumin Foods 2020, 9, x FOR PEER REVIEW 2 of 20 (BDMC) (Figure1)[2]. Figure 1. Chemical structures of curcuminoids: (a) curcumin (CUR), (b) demethoxycurcumin (DMC), (cFigure) bisdemethoxycurcumin 1. Chemical structures (BDMC). of curcuminoids: (a) curcumin (CUR), (b) demethoxycurcumin (DMC), (c) bisdemethoxycurcumin (BDMC). 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